- A paleoclimate rainfall reconstruction in the Murray‐Darling Basin
(MDB), Australia: 2. Assessing hydroclimatic risk using paleoclimate
records of wet and dry epochs
- Abstract: Estimates of hydrological risk are crucial to enable adequate planning and preparation for extreme events. However, the accurate estimation of hydrological risk is hampered by relatively short instrumental records in many parts of the world. Information derived from climate‐sensitive paleoclimate proxies provide an opportunity to resolve hydroclimatic variability, but many regions, such as Australia's Murray‐Darling Basin (MDB), currently lack the suitable in situ proxies necessary to do this. Here, new MDB rainfall reconstructions are presented based on a novel method using paleoclimate rainfall proxies in the Australasian region spanning from 749 BCE to 1980 CE. Our results emphasize the need to develop additional reconstructions and, with the companion paper, demonstrate how this information can be used to benefit water resource management. This study shows that prior to the 20th century both dry and wet epochs have persisted for longer periods than observed in the instrumental record – with the probability of both dry and wet periods exceeding a decade at least 10 times more likely prior to 1883 than suggested by the instrumental records. Some reconstructed rainfalls exceeded the instrumental range (i.e. drier dry epochs and wetter wet spells) despite a systematic underestimation of extremes due to a combination of proxy quality and model bias. Importantly, the results demonstrate that the instrumental record does not cover the full range of hydroclimatic variability possible in the MDB. Therefore hydroclimatic risk assessments based on the instrumental record likely underestimate, or at least misinterpret, the frequency, duration and magnitude of wet and dry epochs. This article is protected by copyright. All rights reserved.
- A paleoclimate rainfall reconstruction in the Murray‐Darling Basin
(MDB), Australia: 1. Evaluation of different paleoclimate archives,
rainfall networks, and reconstruction techniques
- Abstract: From ∼1997‐2009 the Murray‐Darling Basin (MDB), Australia's largest water catchment and reputed ‘food bowl', experienced a severe drought termed the “Millennium Drought” or “Big Dry” followed by devastating floods in the austral summers of 2010/11, 2011/12 and 2012/13. The magnitude and severity of these extreme events highlight the limitations associated with assessing hydroclimatic risk based on relatively short instrumental records (∼100 years). An option for extending hydroclimatic records is through the use of paleoclimate records. However, there are few in situ proxies of rainfall or streamflow suitable for assessing hydroclimatic risk in Australia and none are available in the MDB. In this paper available paleoclimate records are reviewed and those of suitable quality for hydroclimatic risk assessments are used to develop preinstrumental information for the MDB. Three different paleoclimate reconstruction techniques are assessed using two instrumental rainfall networks: (1) corresponding to rainfall at locations where rainfall‐sensitive Australian paleoclimate archives currently exist and (2) corresponding to rainfall at locations identified as being optimal for explaining MDB rainfall variability. It is shown that the optimised rainfall network results in a more accurate model of MDB rainfall compared to reconstructions based on rainfall at locations where paleoclimate rainfall proxies currently exist. This highlights the importance of first identifying key locations where existing and as yet unrealised paleoclimate records will be most useful in characterising variability. These results give crucial insight as to where future investment and research into developing paleoclimate proxies for Australia could be most beneficial, with respect to better understanding instrumental, pre‐instrumental and potential future variability in the MDB. This article is protected by copyright. All rights reserved.
- Water table salinization due to seawater intrusion
- Authors: Sugiarto Badaruddin; Adrian D. Werner, Leanne K. Morgan
Abstract: Seawater intrusion (SWI) is a significant threat to freshwater resources in coastal aquifers around the world. Previous studies have focused on SWI impacts involving salinization of the lower domain of coastal aquifers. However, under certain conditions, SWI may cause salinization of the entire saturated zone of the aquifer, leading to watertable salinization (WTS) in unconfined aquifers by replacing freshwater within the upper region of the saturated zone with seawater, thereby posing a salinity threat to the overlying soil zone. There is presently limited guidance on the extent to which WTS may occur as a secondary impact of SWI. In this study, physical experiments and numerical modelling were used to explore WTS associated with SWI in various non‐tidal, unconfined coastal aquifer settings. Laboratory experiments and corresponding numerical simulations show that significant WTS can occur under active SWI (i.e., the freshwater hydraulic gradient slopes towards the land) because the cessation of freshwater discharge to the sea and the subsequent landward flow across the entire sea boundary eventually lead to watertable salinities approaching seawater concentration. WTS during active SWI is larger under conditions of high hydraulic conductivity, rapid SWI, high dispersivity and for deeper aquifers. Numerical modelling of four published field cases demonstrates that rates of WTS of up to 60 m/y are plausible. Under passive SWI (i.e., the hydraulic gradient slopes towards the sea), minor WTS may arise as a result of dispersive processes under certain conditions (i.e., high dispersivity and hydraulic conductivity, and low freshwater discharge). Our results show that WTS is probably widespread in coastal aquifers experiencing considerable groundwater decline sustained over several years, although further evidence is needed to identify WTS under field settings. This article is protected by copyright. All rights reserved.
- Linking age, survival, and transit time distributions
- Authors: Salvatore Calabrese; Amilcare Porporato
Abstract: Although the concepts of age, survival and transit time have been widely used in many fields, including population dynamics, chemical engineering, and hydrology, a comprehensive mathematical framework is still missing. Here we discuss several relationships among these quantities by starting from the evolution equation for the joint distribution of age and survival, from which the equations for age and survival time readily follow. It also becomes apparent how the statistical dependence between age and survival is directly related to either the age‐dependence of the loss function or the survival‐time dependence of the input function. The solution of the joint distribution equation also allows us to obtain the relationships between the age at exit (or death) and the survival time at input (or birth), as well as to stress the symmetries of the various distributions under time reversal. The transit time is then obtained as a sum of the age and survival time, and its properties are discussed along with the general relationships between their mean values. The special case of steady state case is analyzed in detail. Some examples, inspired by hydrologic applications, are presented to illustrate the theory with the specific results. This article is protected by copyright. All rights reserved.
- Basin‐scale runoff prediction: An Ensemble Kalman Filter framework
based on global hydrometeorological data sets
- Authors: Christof Lorenz; Mohammad J. Tourian, Balaji Devaraju, Nico Sneeuw, Harald Kunstmann
Abstract: In order to cope with the steady decline of the number of in situ gauges worldwide, there is a growing need for alternative methods to estimate runoff. We present an Ensemble Kalman Filter based approach that allows us to conclude on runoff for poorly or irregularly gauged basins. The approach focuses on the application of publicly available global hydrometeorological datasets for precipitation (gpcc, gpcp, cru, udel), evapotranspiration (modis, fluxnet, gleam, era interim, gldas), and water storage changes (grace, wghm, gldas, merra land). Furthermore, runoff data from the grdc and satellite altimetry derived estimates are used. We follow a least‐squares prediction that exploits the joint temporal and spatial auto‐ and cross‐covariance structures of precipitation, evapotranspiration, water storage changes and runoff. We further consider time‐dependent uncertainty estimates derived from all datasets. Our in‐depth analysis comprises of 29 large river basins of different climate regions, with which runoff is predicted for a subset of 16 basins. Six configurations are analyzed: the Ensemble Kalman Filter (Smoother) and the hard (soft) Constrained Ensemble Kalman Filter (Smoother). Comparing the predictions to observed monthly runoff shows correlations larger than 0.5, percentage biases lower than ± 20%, and nse‐values larger than 0.5. A modified nse‐metric, stressing the difference to the mean annual cycle, shows an improvement of runoff predictions for 14 of the 16 basins. The proposed method is able to provide runoff estimates for nearly 100 poorly gauged basins covering an area of more than 11,500,000 km2 with a freshwater discharge, in volume, of more than 125,000 m3/s. This article is protected by copyright. All rights reserved.
- Modeling the influence of hypsometry, vegetation, and storm energy on
snowmelt contributions to basins during rain‐on‐snow floods
- Authors: Nicholas E. Wayand; Jessica D. Lundquist, Martyn P. Clark
Abstract: Point observations and previous basin modeling efforts have suggested that snowmelt may be a significant input of water for runoff during extreme rain‐on‐snow floods within Western U.S. basins. Quantifying snowmelt input over entire basins is difficult given sparse observations of snowmelt. In order to provide a range of snowmelt contributions for water managers, a physically‐based snow model coupled with an idealized basin representation was evaluated in point simulations and used to quantify the maximum basin‐wide input from snowmelt volume during flood events. Maximum snowmelt basin contributions and uncertainty ranges were estimated as 29% (11‐47%), 29% (8‐37%), and 7% (2‐24%) of total rain plus snowmelt input, within the Snoqualmie, East North Fork Feather, and Upper San Joaquin basins, respectively, during historic flooding events between 1980 and 2008. The idealized basin representation revealed that both hypsometry and forest cover of a basin had similar magnitude of impacts on the basin‐wide snowmelt totals. However, the characteristics of a given storm (antecedent SWE and available energy for melt) controlled how much hypsometry and forest cover impacted basin‐wide snowmelt. These results indicate that for watershed managers, flood forecasting efforts should prioritize rainfall prediction first, but cannot neglect snowmelt contributions in some cases. Efforts to reduce the uncertainty in the above snowmelt simulations should focus on improving the meteorological forcing data (especially air temperature and wind speed) in complex terrain. This article is protected by copyright. All rights reserved.
- Comparing drinking water treatment costs to source water protection costs
using time series analysis
- Authors: Matthew T. Heberling; Christopher T. Nietch, Hale W. Thurston, Michael Elovitz, Kelly H. Birkenhauer, Srinivas Panguluri, Balaji Ramakrishnan, Eric Heiser, Tim Neyer
Abstract: We present a framework to compare water treatment costs to source water protection costs, an important knowledge gap for drinking water treatment plants (DWTPs). This tradeoff helps determine what incentives a DWTP has to invest in natural infrastructure or pollution reduction in the watershed rather than pay for treatment on site. To illustrate, we use daily observations from 2007‐2011 for the Bob McEwen Water Treatment Plant, Clermont County, Ohio, to understand the relationship between treatment costs and water quality and operational variables (e.g., turbidity, total organic carbon [TOC], pool elevation, and production volume). Part of our contribution to understanding drinking water treatment costs is examining both long‐run and short‐run relationships using error correction models (ECMs). Treatment costs per 1000 gallons were based on chemical, pumping, and granular activated carbon costs. Results from the ECM suggest a 1% decrease in turbidity decreases treatment costs by 0.02% immediately and an additional 0.1% over future days. Using mean values for the plant, a 1% decrease in turbidity leads to $1123/year decrease in treatment costs. To compare these costs with source water protection costs, we use a polynomial distributed lag model to link total phosphorus loads, a source water quality parameter affected by land use changes, to turbidity at the plant. We find the costs for source water protection to reduce loads much greater than the reduction in treatment costs during these years. Although we find no incentive to protect source water in our case study, this framework can help DWTPs quantify the tradeoffs. This article is protected by copyright. All rights reserved.
- A reflection on the first 50 years of Water Resources Research
- Abstract: The year 2015 marks the 50th anniversary of Water Resources Research (WRR), which was founded in 1965. More than 15,000 papers have been published in WRR since its inception, and these papers have been cited more than 430,000 times. The history of hydrology and the water sciences are also reflected in WRR, which has served as a premier publication outlet and instigator of scientific growth over the last 50 years. The legacy of WRR provides a strong scientific foundation for the hydrology community to rise to the challenges of sustainable water resources management in a future where dramatic environmental change and increasing human population are expected to stress the world's water resources from local to global scales. This article is protected by copyright. All rights reserved.
- Analysis of discontinuities across thin inhomogeneities,
groundwater/surface water interactions in river networks, and circulation
about slender bodies using slit elements in the Analytic Element Method
- Authors: David R. Steward
Abstract: Groundwater and surface water contain interfaces across which hydrologic functions are discontinuous. Thin elements with high hydraulic conductivity in a porous media focus groundwater, which flows through such inhomogeneities and causes an abrupt change in stream function across their interfaces, and elements with low conductivity retards flow with discontinuous head. Baseflow interactions at the interface between groundwater and surface water transport water between these stores and generate a discontinuous normal component of flow. Thin objects in surface water with Kutta condition generates circulation by the discontinuous tangential component of flow across their interface. These discontinuities across hydrologic interfaces are quantified and visualized using the Analytic Element Method, where slit elements are formulated using the Joukowsky transformation with Laurent series and new influence functions to represent sinks and circulation, and methods are developed for these applications expressing discontinuities as Fourier series. The specific geometries illustrate solutions for a randomly generated heterogeneous porous media with non‐intersecting inhomogeneities, for groundwater/surface water interaction in a synthetic river network, and for a slender body with geometry similar to the wings of the Wright Brothers. The mathematical details are reduced to series solutions and matrix multiplications, which are easily extensible to other geometries and applications. This article is protected by copyright. All rights reserved.
- Water storage changes as a marker for base flow generation processes in a
tropical humid basement catchment (Benin): Insights from hybrid gravimetry
- Abstract: In basement catchments of sub‐humid West Africa, baseflow is the main component of annual streamflow. However, the important heterogeneity of lithology hinders the understanding of baseflow generation processes. Since these processes are linked with water storage changes (WSCs) across the catchment, we propose the use of hybrid gravity data in addition to neutron probe‐derived water content and water levels to monitor spatiotemporal WSC of a typical crystalline basement headwater catchment (16 ha) in Benin. These behaviors are shown to provide insights into hydrological processes in terms of water redistribution toward the catchment outlet. Hybrid gravimetry produces gravity change observations from time‐lapse microgravity surveys coupled with gravity changes monitored at a base station using a superconducting gravimeter and/or an absolute gravimeter. A dense microgravity campaign (70 surveys of 14 stations) covering three contrasted years was set up with a rigorous protocol, leading to low uncertainties (< 2.5 µGal) on station gravity determinations (with respect to the network reference station). Empirical orthogonal function analyses of both gravity changes and WSCs from neutron probe data show similar spatial patterns in the seasonal signal. Areas where storage and water table show a capping behavior (when data reach a plateau during the wet season), suggesting threshold‐governed fast subsurface redistribution, are identified. This observed storage dynamics, together with geological structures investigated by electrical resistivity tomography and drill log analysis make it possible to derive a conceptual model for the catchment hydrology. This article is protected by copyright. All rights reserved.
- Hydrologic control of dissolved organic matter concentration and quality
in a semiarid artificially drained agricultural catchment
- Authors: Rebecca A. Bellmore; John A. Harrison, Joseph A. Needoba, Erin Brooks, C. Kent Keller
Abstract: Agricultural practices have altered watershed‐scale dissolved organic matter (DOM) dynamics, including in‐stream concentration, biodegradability, and total catchment export. However, mechanisms responsible for these changes are not clear, and field‐scale processes are rarely directly linked to the magnitude and quality of DOM that is transported to surface water. In a small (12 ha) agricultural catchment in eastern Washington State, we tested the hypothesis that hydrologic connectivity in a catchment is the dominant control over the concentration and quality of DOM exported to surface water via artificial subsurface drainage. Concentrations of dissolved organic carbon (DOC) and humic‐like components of DOM decreased while the Fluorescence Index and Freshness Index increased with depth through the soil profile. In drain discharge, these characteristics were significantly correlated with drain flow across seasons and years, with drain DOM resembling deep sources during low flow and shallow sources during high flow, suggesting that DOM from shallow sources bypasses removal processes when hydrologic connectivity in the catchment is greatest. Assuming changes in streamflow projected for the Palouse River (which contains the study catchment) under the A1B climate scenario (rapid growth, dependence on fossil fuel and renewable energy sources) apply to the study catchment, we project greater interannual variability in annual DOC export in the future, with significant increases in the driest years. This study highlights the variability in DOM inputs from agricultural soil to surface water on daily to interannual timescales, pointing to the need for a more nuanced understanding of agricultural impacts on DOM dynamics in surface water. This article is protected by copyright. All rights reserved.
- Dual control of flow field heterogeneity and immobile porosity on
non‐Fickian transport in Berea sandstone
- Authors: Filip Gjetvaj; Anna Russian, Philippe Gouze, Marco Dentz
Abstract: Both flow field heterogeneity and mass transfer between mobile and immobile domains have been studied separately for explaining observed anomalous transport. Here, we investigate non‐Fickian transport using high‐resolution 3D X‐ray micro‐tomographic images of Berea sandstone containing microporous cement with pore size below the setup resolution. Transport is computed for a set of representative elementary volumes and results from advection and diffusion in the resolved macroporosity (mobile domain) and diffusion in the microporous phase (immobile domain) where the effective diffusion coefficient is calculated from the measured local porosity using a phenomenological model that includes a porosity threshold (ɸϴ) below which diffusion is null and the exponent n that characterizes tortuosity‐porosity power‐law relationship. We show that both flow field heterogeneity and microporosity trigger anomalous transport. Breakthrough curve (BTC) tailing is positively correlated to microporosity volume and mobile‐immobile interface area. The sensitivity analysis showed that the BTC tailing increases with the value of ɸϴ, due to the increase of the diffusion path tortuosity until the volume of the microporosity becomes negligible. Furthermore, increasing the value of n leads to an increase in the standard deviation of the distribution of effective diffusion coefficients, which in turn results in an increase of the BTC tailing. Finally, we propose a continuous time random walk upscaled model where the transition time is the sum of independently distributed random variables characterized by specific distributions. It allows modeling a 1D equivalent macroscopic transport honoring both the control of the flow field heterogeneity and the multi‐rate mass transfer between mobile and immobile domains. This article is protected by copyright. All rights reserved.
- Global sensitivity of high‐resolution estimates of crop water
- Authors: Marta Tuninetti; Stefania Tamea, Paolo D'Odorico, Francesco Laio, Luca Ridolfi
Abstract: Most of the human appropriation of freshwater resources is for agriculture. Water availability is a major constraint to mankind's ability to produce food. The notion of virtual water content (VWC), also known as crop water footprint, provides an effective tool to investigate the linkage between food and water resources as a function of climate, soil and agricultural practices. The spatial variability in the virtual water content of crops is here explored, disentagling its dependency on climate and crop yields, and assessing the sensitivity of VWC estimates to parameter variability and uncertainty. Here we calculate the virtual water content of four staple crops (i.e., wheat, rice, maize, and soybean) for the entire world developing a high‐resolution (5 by 5 arc minute) model, and we evaluate the VWC sensitivity to input‐parameters. We find that food production almost entirely depends on green water (>90%), but, when applied, irrigation makes crop production more water efficient, thus requiring less water. The spatial variability of the VWC is mostly controlled by the spatial patterns of crop yields with an average correlation coefficient of 0.83. The results of the sensitivity analysis show that wheat is most sensitive to the length of the growing period, rice to reference evapotranspiration, maize and soybean to the crop planting date. The VWC sensitivity varies not only among crops, but also across the harvested areas of the world, even at the sub‐national scale. This article is protected by copyright. All rights reserved.
- Modeling NAPL dissolution from pendular rings in idealized porous media
- Authors: Junqi Huang; John A. Christ, Mark N. Goltz, Avery H. Demond
Abstract: The dissolution rate of non‐aqueous phase liquid (NAPL) often governs the remediation time frame at subsurface hazardous waste sites. Most formulations for estimating this rate are empirical and assume that the NAPL is the non‐wetting fluid. However, field evidence suggests that some waste sites might be organic‐wet. Thus, formulations that assume the NAPL is non‐wetting may be inappropriate for estimating the rates of NAPL dissolution. An exact solution to the Young‐Laplace equation, assuming NAPL resides as pendular rings around the contact points of porous media idealized as spherical particles in a hexagonal close packing arrangement, is presented in this work to provide a theoretical prediction for NAPL‐water interfacial area. This analytic expression for interfacial area is then coupled with an exact solution to the advection‐diffusion equation in a capillary tube assuming Hagen‐Poiseuille flow to provide a theoretical means of calculating the mass transfer rate coefficient for dissolution at the NAPL‐water interface in an organic‐wet system. A comparison of the predictions from this theoretical model with predictions from empirically‐derived formulations from the literature for water‐wet systems showed a consistent range of values for the mass transfer rate coefficient, despite the significant differences in model foundations (water‐wetting vs NAPL‐wetting, theoretical vs. empirical). This finding implies that, under these system conditions, the important parameter is interfacial area, with a lesser role played by NAPL configuration. This article is protected by copyright. All rights reserved.
- Influence of small‐scale fluvial architecture on CO2 trapping
processes in deep brine reservoirs
- Authors: Naum I. Gershenzon; Robert W. Ritzi, David F. Dominic, Mohamadreza Soltanian, Edward Mehnert, Roland T. Okwen
Abstract: A number of important candidate CO2 reservoirs exhibit sedimentary architecture reflecting fluvial deposition. Recent studies have led to new conceptual and quantitative models for sedimentary architecture in fluvial deposits over a range of scales that are relevant to CO2 injection and storage. We used a geocellular modelling approach to represent this multi‐scaled and hierarchical sedimentary architecture. With this model, we investigated the dynamics of CO2 plumes, during and after injection, in such reservoirs.
The physical mechanism of CO2 trapping by capillary trapping incorporates a number of related processes, i.e. residual trapping, trapping due to hysteresis of the relative permeability, and trapping due to hysteresis of the capillary pressure. Additionally CO2 may be trapped due to differences in capillary entry pressure for different textural sedimentary facies (e.g. coarser‐ vs. finer‐grained cross‐sets). The amount of CO2 trapped by these processes depends upon a complex system of non‐linear and hysteretic characteristic relationships including how relative permeability and capillary pressure vary with brine and CO2 saturation. The results strongly suggest that representing small‐scale features (decimeter to meter), including their organization within a hierarchy of larger‐scale features, and representing their differences in characteristic relationships, can all be critical to understanding trapping processes in some important candidate CO2 reservoirs. This article is protected by copyright. All rights reserved.
- A framework of change‐point detection for multivariate hydrological
- Abstract: Under changing environments, not only univariate but also multivariate hydrological series might become nonstationary. Nonstationarity, in forms of change‐point or trend, has been widely studied for univariate hydrological series, while it attracts attention only recently for multivariate hydrological series. For multivariate series, two types of change‐point need to be distinguished, i.e. change‐point in marginal distributions and change‐point in the dependence structure among individual variables. In this paper, a three‐step framework is proposed to separately detect two types of change‐point in multivariate hydrological series, i.e. change‐point detection for individual univariate series, estimation of marginal distributions, and change‐point detection for dependence structure. The last step is implemented using both the Cramér‐von Mises statistic (CvM) method and the copula‐based likelihood‐ratio test (CLR) method. For CLR, three kinds of copula model (symmetric, asymmetric, and pair‐copula) are employed to construct the dependence structure of multivariate series. Monte Carlo experiments indicate that CLR is far more powerful than CvM in detecting the change‐point of dependence structure. This framework is applied to the trivariate flood series composed of annual maxima daily discharge (AMDD), annual maxima 3‐day flood volume and annual maxima 15‐day flood volume of the Upper Hanjiang River, China. It is found that each individual univariate flood series has a significant change‐point; and the trivariate series presents a significant change‐point in dependence structure due to the abrupt change in the dependence structure between AMDD and annual maxima 3‐day flood volume. All these changes are caused by the construction of the Ankang Reservoir. This article is protected by copyright. All rights reserved.
- Output‐feedback control of combined sewer networks through receding
horizon control with moving horizon estimation
- Abstract: An output‐feedback control strategy for pollution mitigation in combined sewer networks is presented. The proposed strategy provides means to apply model‐based predictive control to large‐scale sewer networks, in‐spite of the lack of measurements at most of the network sewers. In previous works, the authors presented a hybrid linear control‐oriented model for sewer networks together with the formulation of Optimal Control Problems (OCP) and State Estimation Problems (SEP). By iteratively solving these problems, preliminary Receding Horizon Control with Moving Horizon Estimation (RHC/MHE) results, based on flow measurements, were also obtained. In this work, the RHC/MHE algorithm has been extended to take into account both flow and water level measurements and the resulting control loop has been extensively simulated to assess the system performance according different measurement availability scenarios and rain events. All simulations have been carried out using a detailed physically‐based model of a real case‐study network as virtual reality. This article is protected by copyright. All rights reserved.
- Control of tree water networks: A geometric programming approach
- Authors: L. Sela Perelman; S. Amin
Abstract: This paper presents a modeling and operation approach for tree water supply systems. The network control problem is approximated as a geometric programming (GP) problem. The original nonlinear nonconvex network control problem is transformed into a convex optimization problem. The optimization model can be efficiently solved to optimality using state‐of‐the‐art solvers. Two control schemes are presented: (1) operation of network actuators (pumps and valves) and (2) controlled demand shedding allocation between network consumers with limited resources. The dual of the network control problem is formulated and is used to perform sensitivity analysis with respect to hydraulic constraints. The approach is demonstrated on a small branched‐topology network and later extended to a medium‐size irrigation network. The results demonstrate an intrinsic trade‐off between energy costs and demand shedding policy, providing an efficient decision support tool for active management of water systems. This article is protected by copyright. All rights reserved.
- Nonpayment of water bills in Guatemala: Dissatisfaction or inability to
- Abstract: This paper investigates nonpayment behavior in Guatemala. Determinants of nonpayment behavior are identified through zero‐inflated negative binomial regression models in order to take into account particular distributional characteristics of the amount of outstanding payments. Findings indicate that nonpayment behavior is a demonstration of consumer dissatisfaction with current water services. The amount of outstanding bill payments also responds to system unreliability. Results also suggest that nonpayment behaviors are more prominent in community‐managed systems than in municipal systems. No evidence was found on a potential relationship between nonpayment behavior and household income. Policy implications are discussed. This article is protected by copyright. All rights reserved.
- An analytical study on artesian flow conditions in
unconfined‐aquifer drainage basins
- Abstract: Although it has been reported that flowing artesian wells could be topographically‐controlled, there is no quantitative research on artesian flow conditions in unconfined aquifers. In this study, the water table, which has a lower amplitude than the land surface, is damped from the topography and used as the boundary condition to obtain the analytical solution of hydraulic head of a unit basin with a single flow system. The term artesian head is defined to characterize the condition of flowing artesian wells. The zone with positive artesian head is called artesian zone while with negative artesian head is non‐artesian zone. The maximum artesian head and the size of artesian zones are found to increase with the damping factor and the anisotropy ratio, and decrease with the ratio of basin width to depth and the depth‐decay exponent of hydraulic conductivity. Moreover, the artesian head increases with depth nearby the valley and decreases with depth near by the divide, and the variation rates are influenced by the decay exponent and the anisotropy ratio. Finally, the distribution of flowing artesian wells and the artesian head measurements in different depths of a borehole in a small catchment in the Ordos Plateau, Northwestern China is used to illustrate the theoretical findings. The change in artesian head with depth was used to estimate the anisotropy ratio and the decay exponent. This study opens up a new door to analyze basin‐scale groundwater flow. This article is protected by copyright. All rights reserved.
- The stationarity paradigm revisited: Hypothesis testing using diagnostics,
summary metrics, and DREAM(ABC)
- Authors: Mojtaba Sadegh; Jasper A. Vrugt, Chonggang Xu, Elena Volpi
Abstract: Many watershed models used within the hydrologic research community assume (by default) stationary conditions ‐ that is ‐ the key watershed properties that control water flow are considered to be time‐invariant. This assumption is rather convenient and pragmatic and opens up the wide arsenal of (multivariate) statistical and nonlinear optimization methods for inference of the (temporally‐fixed) model parameters. Several contributions to the hydrologic literature have brought into question the continued usefulness of this stationary paradigm for hydrologic modeling. This paper builds on the likelihood‐free diagnostics approach of Vrugt and Sadegh  and uses a diverse set of hydrologic summary metrics to test the stationary hypothesis and detect changes in the watersheds response to hydro‐climatic forcing. Models with fixed parameter values cannot simulate adequately temporal variations in the summary statistics of the observed catchment data, and consequently the DREAM(ABC) algorithm cannot find solutions that sufficiently honor the observed metrics. We demonstrate that the presented methodology is able to differentiate successfully between watersheds that are classified as stationary and those that have undergone significant changes in land‐use, urbanization and/or hydro‐climatic conditions, and thus are deemed nonstationary. This article is protected by copyright. All rights reserved.
- Issue Information
- PubDate: 2015-09-18T13:06:52.353017-05:
- Evidence of an emerging levee failure mechanism causing disastrous floods
- Authors: Stefano Orlandini; Giovanni Moretti, John D. Albertson
Abstract: A levee failure occurred along the Secchia River, Northern Italy, on January 19, 2014, resulting in flood damage in excess of $500 Million. In response to this failure, immediate surveillance of other levees in the region led to the identification of a second breach developing on the neighboring Panaro River, where rapid mitigation efforts were successful in averting a full levee failure. The paired breach events that occurred along the Secchia and Panaro Rivers provided an excellent window on an emerging levee failure mechanism. In the Secchia River, by combining the information content of photographs taken from helicopters in the early stage of breach development and 10‐cm resolution aerial photographs taken in 2010 and 2012, animal burrows were found to exist in the precise levee location where the breach originated. In the Panaro River, internal erosion was observed to occur at a location where a crested porcupine den was known to exist and this erosion led to the collapse of the levee top. This paper uses detailed numerical modeling of rainfall, river flow, and variably saturated flow in the levee to explore the hydraulic and geotechnical mechanisms that were triggered along the Secchia and Panaro Rivers by activities of burrowing animals leading to levee failures. As habitats become more fragmented and constrained along river corridors it is possible that this failure mechanism could become more prevalent and, therefore, will demand greater attention in both the design and maintenance of earthen hydraulic structures as well as in wildlife management. This article is protected by copyright. All rights reserved.
- Water consumption patterns as a basis for water demand modeling
- Authors: Noa Avni; Barak Fishbain, Uri Shamir
Abstract: Future water demand is a main consideration in water system management. Consequently, water demand models (WDMs) have evolved in past decades, identifying principal demand‐generating factors and modeling their influence on water demand. Regional water systems serve consumers of various types (e.g., municipalities, farmers, industrial regions) and consumption patterns. Thus, one of the challenges in regional water demand modeling is the heterogeneity of the consumers served by the water system. When a high‐resolution, regional WDM is desired, accounting for this heterogeneity becomes all the more important. This paper presents a novel approach to regional water demand modeling. The two‐step approach includes aggregating the dataset into groups of consumers having similar consumption characteristics, and developing a WDM for each homogeneous group. The development of WDMs is widely applied in the literature and thus, the focus of this paper is to discuss the first step of data aggregation. The research hypothesis is that water consumption records in their original or transformed form can provide a basis for aggregating the dataset into groups of consumers with similar consumption characteristics. This paper presents a methodology for water consumption data clustering by comparing several data representation methods (termed Feature Vectors): monthly normalized average, monthly consumption coefficient of variation, a combination of the monthly average and monthly variation, and the autocorrelation coefficients of the consumption time‐series. Clustering using solely normalized monthly average provided homogeneous and distinct clusters with respect to monthly consumption, which succeed in capturing different consumer characteristics (water use, geographical location) that were not specified a‐priori. Clustering using the monthly coefficient of variation provided different, yet homogeneous clusters, clustering consumers characterized by similar variation trends that were closely related to consumer water use type. The concatenation of these two Feature Vectors provided further insight into the relationship between consumption patterns and variability of consumers. An autocorrelation Feature Vector provided results that can form a basis for constructing a time‐series model that is based on a group of resembling time‐series. The approaches presented here are steps towards utilizing the increasing amount of available water consumption data and data analysis techniques to facilitate the modeling of water demands in larger and heterogeneous regions with sufficient resolution. This article is protected by copyright. All rights reserved.
- Potential of hydraulically induced fractures to communicate with existing
- Authors: James A. Montague; George F. Pinder
Abstract: The probability that new hydraulically fractured wells drilled within the area of New York underlain by the Marcellus Shale will intersect existing an wellbore is calculated using a statistical model, which incorporates: the depth of a new fracturing well, the vertical growth of induced fractures, and the depths and locations of existing nearby wells. The model first calculates the probability of encountering an existing well in plan view and combines this with the probability of an existing well being at sufficient depth to intersect the fractured region. Average probability estimates for the entire region of New York underlain by the Marcellus Shale range from 0.00% to 3.45% based upon the input parameters used. The largest contributing parameter on the probability value calculated is the nearby density of wells meaning that due diligence by oil and gas companies during construction in identifying all nearby wells will have the greatest effect in reducing the probability of interwellbore communication. This article is protected by copyright. All rights reserved.
- A generalized complementary principle with physical constraints for
- Authors: Wilfried Brutsaert
Abstract: The idea of complementary evaporative fluxes, first advanced by Bouchet in 1963 is reformulated as a general polynomial, satisfying boundary conditions based on strictly physical considerations. Experimental evidence supports the validity of the imposed constraints. Earlier complementary relationships are shown to be special cases which satisfy only one of the necessary conditions. The new formulation provides a more rigorous base for the complementary principle. This article is protected by copyright. All rights reserved.
- Capillarity and wetting of carbon dioxide and brine during drainage in
Berea sandstone at reservoir conditions
- Abstract: The wettability of CO2‐brine‐rock systems will have a major impact on the management of carbon sequestration in subsurface geological formations. Recent contact angle measurement studies have reported sensitivity in wetting behaviour of this system to pressure, temperature and brine salinity. We report observations of the impact of reservoir conditions on the capillary pressure characteristic curve and and relative permeability of a single Berea sandstone during drainage ‐ CO2 displacing brine ‐ through effects on the wetting state. Eight reservoir condition drainage capillary pressure characteristic curves were measured using CO2 and brine in a single fired Berea sandstone at pressures (5 to 20 MPa), temperatures (25 to 50°C) and ionic strengths (0 to 5 mol kg−1 NaCl). A ninth measurement using a N2‐water system provided a benchmark for capillarity with a strongly water wet system. The capillary pressure curves from each of the tests were found to be similar to the N2‐water curve when scaled by the interfacial tension. Reservoir conditions were not found to have a significant impact on the capillary strength of the CO2‐brine system during drainage through a variation in the wetting state. Two steady‐state relative permeability measurements with CO2 and brine and one with N2 and brine similarly show little variation between conditions, consistent with the observation that the CO2‐brine‐sandstone system is water wetting and multiphase flow properties invariant across a wide range of reservoir conditions. This article is protected by copyright. All rights reserved.
- Interactive design of experiments: A priori global versus sequential
optimization, revised under changing states of knowledge
- Authors: A. Geiges; Y. Rubin, W. Nowak
Abstract: Predicting hydro(geo)logical or environmental systems is subject to high levels of uncertainties, especially if appropriate data for model calibration are lacking. For subsurface systems, where data acquisition is cost intensive and time demanding, it is especially important to collect only those data that provide the largest amount of relevant information. The high expenses call for optimal experimental design, which is widely recognized for maximizing the efficiency of experiments. In model‐based design of experiments, the analysis of the design efficiency and the resulting optimal design are based on the initial state of knowledge about the modeled system. Joint optimization of multi‐measurement designs is a well known challenge and the usefulness of global optimization approaches is widely recognized in this context. However, we will show that the benefit for such global optimization becomes questionable when measurement data become available sequentially. Instead, the optimization effort should be invested within an interactive design approach. Today's fast telecommunication, global connectivity and high‐performance computing allow to consider such interactive coupling. This study will use a synthetic case study to compare the standard en‐bloc global optimization approach to two interactive design approaches. The approaches are implemented in a Bayesian framework and are compared based on their complexity and overall performance. The key conclusion confirms a previously untested presumption: for models that trigger nonlinear parameter inference problems, interaction (which may come at a loss of global optimization) is more beneficial than global optimization based on the initial state of knowledge (which typically implies the impossibility of interactivity). This article is protected by copyright. All rights reserved.
- Using noble gas tracers to constrain a groundwater flow model with
recharge elevations: A novel approach for mountainous terrain
- Authors: Jessica. M. Doyle; Tom Gleeson, Andrew H. Manning, Ulrich Mayer
Abstract: Environmental tracers provide information on groundwater age, recharge conditions, and flow processes which can be helpful for evaluating groundwater sustainability and vulnerability. Dissolved noble gas data have proven particularly useful in mountainous terrain because they can be used to determine recharge elevation. However, tracer‐derived recharge elevations have not been utilized as calibration targets for numerical groundwater flow models. Herein we constrain and calibrate a regional groundwater flow model with noble‐gas‐derived recharge elevations for the first time. Tritium and noble gas tracer results improved the site conceptual model by identifying a previously uncertain contribution of mountain‐block recharge from the Coast Mountains to an alluvial coastal aquifer in humid southwestern British Columbia. The revised conceptual model was integrated into a three‐dimensional numerical groundwater flow model and calibrated to hydraulic head data in addition to recharge elevations estimated from noble gas recharge temperatures. Recharge elevations proved to be imperative for constraining hydraulic conductivity, recharge location and bedrock geometry, and thus minimizing model non‐uniqueness. Results indicate that 45% of recharge to the aquifer is mountain‐block recharge. A similar match between measured and modeled heads was achieved in a second numerical model that excludes the mountain block (no mountain block recharge), demonstrating that hydraulic head data alone are incapable of quantifying mountain block recharge. This result has significant implications for understanding and managing source water protection in recharge areas, potential effects of climate change, the overall water budget, and ultimately ensuring groundwater sustainability. This article is protected by copyright. All rights reserved.
- Flushing of distal hillslopes as an alternative source of stream dissolved
organic carbon in a headwater catchment
- Authors: John P. Gannon; Scott W. Bailey, Kevin J. McGuire, James B. Shanley
Abstract: We investigated potential source areas of dissolved organic carbon (DOC) in headwater streams by examining DOC concentrations in lysimeter, shallow well, and streamwater samples from a reference catchment at the Hubbard Brook Experimental Forest. These observations were then compared to high frequency temporal variations in fluorescent dissolved organic matter (FDOM) at the catchment outlet and the predicted spatial extent of shallow groundwater in soils throughout the catchment. While near‐stream soils are generally considered a DOC source in forested catchments, DOC concentrations in near‐stream groundwater were low (mean = 2.4 mg/L, standard error = 0.6 mg/L), less than hillslope groundwater farther from the channel (mean = 5.7 mg/L, standard error = 0.4 mg/L). Furthermore, water tables in near‐stream soils did not rise into the carbon rich upper B or O horizons even during events. In contrast, soils below bedrock outcrops near channel heads where lateral soil formation processes dominate had much higher DOC concentrations. Soils immediately downslope of bedrock areas had thick eluvial horizons indicative of leaching of organic materials, Fe, and Al and had similarly high DOC concentrations in groundwater (mean = 14.5 mg/L, standard error = 0.8 mg/L). Flow from bedrock outcrops partially covered by organic soil horizons produced the highest groundwater DOC concentrations (mean = 20.0 mg/L, standard error = 4.6 mg/L) measured in the catchment. Correspondingly, streamwater in channel heads sourced in part by shallow soils and bedrock outcrops had the highest stream DOC concentrations measured in the catchment. Variation in FDOM concentrations at the catchment outlet followed water table fluctuations in shallow to bedrock soils near channel heads. We show that shallow hillslope soils receiving runoff from organic matter‐covered bedrock outcrops may be a major source of DOC in headwater catchments in forested mountainous regions where catchments have exposed or shallow bedrock near channel heads. This article is protected by copyright. All rights reserved.
- A hybrid‐3‐D hillslope hydrological model for use in Earth
- Abstract: Hillslope‐scale rainfall‐runoff processes leading to a fast catchment response are not explicitly included in land surface models (LSMs) for use in earth system models (ESMs) due to computational constraints.
This study presents a hybrid‐3D hillslope hydrological model (h3D) that couples a 1D vertical soil column model with a lateral pseudo‐2D saturated zone and overland flow model for use in ESMs. By representing vertical and lateral responses separately at different spatial resolutions, h3D is computationally efficient.
The h3D model was first tested for three different hillslope planforms (uniform, convergent and divergent). We then compared h3D (with single and multiple soil columns) with a complex physically‐based 3D model and a simple 1D soil moisture model coupled with an unconfined aquifer (as typically used in LSMs).
It is found that simulations obtained by the simple 1D model vary considerably from the complex 3D model and are not able to represent hillslope‐scale variations in the lateral flow response. In contrast, the single soil column h3D model shows a much better performance and saves computational time by 2‐3 orders of magnitude compared with the complex 3D model. When multiple vertical soil columns are implemented, the resulting hydrological responses (soil moisture, water table depth, and baseflow along the hillslope) from h3D are nearly identical to those predicted by the complex 3D model, but still saves computational time. As such, the computational efficiency of the h3D model provides a valuable and promising approach to incorporating hillslope‐scale hydrological processes into continental and global‐scale ESMs. This article is protected by copyright. All rights reserved.
- Exploring the water storage changes in the largest lake (Selin Co) over
the Tibetan Plateau during 2003–2012 from a basin‐wide
- Authors: Jing Zhou; Lei Wang, Yinsheng Zhang, Yanhong Guo, Xiuping Li, Wenbin Liu
Abstract: Lake water storage change (ΔSw) is an important indicator of the hydrologic cycle and greatly influences lake expansion/shrinkage over the Tibetan Plateau (TP). Accurate estimation of ΔSw will contribute to improved understanding of lake variations in the TP. Based on a water balance, this study explored the variations of ΔSw for the Lake Selin Co (the largest closed lake on the TP) during 2003‐2012 using the Water and Energy Budget‐based Distributed Hydrological Model (WEB‐DHM) together with two different evapotranspiration (ET) algorithms (the Penman‐Monteith method and a simple sublimation estimation approach for water area in unfrozen and frozen period). The contributions of basin discharge and climate causes to the ΔSw are also quantitatively analyzed. The results showed that WEB‐DHM could well reproduce daily discharge, the spatial pattern and basin‐averaged values of MODIS land surface temperature (LST) during nighttime and daytime. Compared with the ET reference values estimated from the basin‐wide water balance, our ET estimates showed better performance than three global ET products in reproducing basin‐averaged ET. The modeled ET at point scale matches well with short‐term in situ daily measurements (RMSE = 0.82 mm/day). Lake inflows and precipitation over the water area had stronger relationships with ΔSw in the warm season and monthly scale, whereas evaporation from the water area had remarkable effects on ΔSw in the cold season. The total contribution of the three factors to ΔSw was about 90%, and accounting for 49.5%, 22.1% and 18.3%, respectively. This article is protected by copyright. All rights reserved.
- Migration behavior of supercritical and liquid CO2 in a stratified system:
Experiments and numerical simulations
- Abstract: Multiple scenarios of upward CO2 migration driven by both injection‐induced pressure and buoyancy force were investigated in a horizontally and vertically stratified core utilizing a core‐flooding system with a 2D X‐ray scanner. Two reservoir type scenarios were considered: (1) the terrestrial reservoir scenario (10 MPa and 50°C), where CO2 exists in a supercritical state and (2) the deep‐sea sediment reservoir scenario (28 MPa and 25°C), where CO2 is stored in the liquid phase. The core‐flooding experiments showed a 36% increase in migration rate in the vertical core setting compared with the horizontal setting, indicating the significance of the buoyancy force under the terrestrial reservoir scenario. Under both reservoir conditions, the injected CO2 tended to find a preferential flow path (low capillary entry pressure and high‐permeability (high‐k) path) and bypass the unfavorable pathways, leaving low CO2 saturation in the low‐permeability (low‐k) layers. No distinctive fingering was observed as the CO2 moved upward, and the CO2 movement was primarily controlled by media heterogeneity. The CO2 saturation in the low‐k layers exhibited a more sensitive response to injection rates, implying that the increase in CO2 injection rates could be more effective in terms of storage capacity in the low‐k layers in a stratified reservoir. Under the deep‐sea sediment condition, the storage potential of liquid CO2 was more than twice as high as that of supercritical CO2 under the terrestrial reservoir scenario. In the end, multiphase transport simulations were conducted to assess the effects of heterogeneity on the spatial variation of pressure build‐up, CO2 saturation and CO2 flux. Finally, we showed that a high gravity number () tended to be more influenced by the heterogeneity of the porous media. This article is protected by copyright. All rights reserved.
- Effect of advective flow in fractures and matrix diffusion on natural gas
- Authors: Satish Karra; Nataliia Makedonska, Hari S. Viswanathan, Scott L. Painter, Jeffrey D. Hyman
Abstract: Although hydraulic fracturing has been used for natural gas production for the past couple of decades, there are significant uncertainties about the underlying mechanisms behind the production curves that are seen in the field. A discrete fracture network based reservoir‐scale work flow is used to identify the relative effect of flow of gas in fractures and matrix diffusion on the production curve. With realistic three dimensional representations of fracture network geometry and aperture variability, simulated production decline curves qualitatively resemble observed production decline curves. The high initial peak of the production curve is controlled by advective fracture flow of free gas within the network and is sensitive to the fracture aperture variability. Matrix diffusion does not significantly affect the production decline curve in the first few years, but contributes to production after approximately 10 years. These results suggest that the initial flushing of gas‐filled background fractures combined with highly heterogeneous flow paths to the production well are sufficient to explain observed initial production decline. These results also suggest that matrix diffusion may support reduced production over longer time frames. This article is protected by copyright. All rights reserved.
- Steady state analytical solutions for pumping in a fully bounded
- Authors: Chunhui Lu; Pei Xin, Ling Li, Jian Luo
Abstract: Using the Schwartz‐Christoffel conformal mapping method together with the complex variable techniques, we derive steady‐state analytical solutions for pumping in a rectangular aquifer with four different combinations of impermeable and constant‐head boundaries. These four scenarios include: (1) one constant‐head boundary and three impermeable boundaries, (2) two pairs of orthogonal impermeable and constant‐head boundaries, (3) three constant‐head boundaries and one impermeable boundary, and (4) four constant‐head boundaries. For these scenarios, the impermeable and constant‐head boundaries can be combined after applying the mapping functions, and hence only three image wells exist in the transformed plane, despite an infinite number of image wells in the real plane. The closed‐form solutions reflect the advantage of the conformal mapping method, though the method is applicable for the aspect ratio of the rectangle between 1/10.9 and 10.9/1 due to the limitation in the numerical computation of the conformal transformation from a half plane onto an elongated region (i.e., so‐called “crowding” phenomenon). By contrast, for an additional scenario with two parallel constant‐head boundaries and two parallel impermeable boundaries, an infinite series of image wells is necessary to express the solution, since it is impossible to combine these two kinds of boundaries through the conformal transformation. The usefulness of the results derived is demonstrated by an application to pumping in a finite coastal aquifer. This article is protected by copyright. All rights reserved.
- A simple algorithm for identifying periods of snow accumulation on a
- Authors: Karl E. Lapo; Laura M. Hinkelman, Christopher C. Landry, Adam K. Massmann, Jessica D. Lundquist
Abstract: Downwelling solar, Qsi, and longwave, Qli, irradiances at the earth's surface are the primary energy inputs for many hydrologic processes, and uncertainties in measurements of these two terms confound evaluations of estimated irradiances and negatively impact hydrologic modeling. Observations of Qsi and Qli in cold environments are subject to conditions that create additional uncertainties not encountered in other climates, specifically the accumulation of snow on uplooking radiometers. To address this issue, we present an automated method for estimating these periods of snow accumulation. Our method is based on forest interception of snow and uses common meteorological observations. In this algorithm, snow accumulation must exceed a threshold to obscure the sensor and is only removed through scouring by wind or melting. The algorithm is evaluated at two sites representing different mountain climates: 1) Snoqualmie Pass, Washington (maritime) and 2) the Senator Beck Basin Study Area, Colorado (continental). The algorithm agrees well with time‐lapse camera observations at the Washington site and with multiple measurements at the Colorado site, with 70 to 80% of observed snow accumulation events correctly identified. We suggest using the method for quality controlling irradiance observations in snow‐dominated climates where regular, daily maintenance is not possible. This article is protected by copyright. All rights reserved.
- Stream water age distributions controlled by storage dynamics and
nonlinear hydrologic connectivity: Modeling with high‐resolution
- Authors: C. Soulsby; C. Birkel, J. Geris, J. Dick, C. Tunaley, D. Tetzlaff
Abstract: To assess the influence of storage dynamics and non‐linearities in hydrological connectivity on time‐variant stream water ages, we used a new long‐term record of daily isotope measurements in precipitation and stream flow to calibrate and test a parsimonious tracer‐aided runoff model. This can track tracers and the ages of water fluxes through and between conceptual stores in steeper hillslopes, dynamically saturated riparian peatlands and deeper groundwater; these represent the main landscape units involved in runoff generation. Storage volumes are largest in groundwater and on the hillslopes, though most dynamic mixing occurs in the smaller stores in riparian peat. Both stream flow and isotope variations are generally well‐captured by the model, and the simulated storage and tracer dynamics in the main landscape units are consistent with independent measurements. The model predicts that the average age of stream water is ∼1.8 years. On a daily basis, this varies between ∼1 month in storm events, when younger waters draining the hillslope and riparian peatland dominates, to around 4 years in dry periods when groundwater sustains flow. This variability reflects the integration of differently aged water fluxes from the main landscape units and their mixing in riparian wetlands. The connectivity between these spatial units varies in a non‐linear way with storage that depends upon precipitation characteristics and antecedent conditions. This, in turn, determines the spatial distribution of flow paths and the integration of their contrasting non‐stationary ages. This approach is well‐suited for constraining process‐based modelling in a range of northern temperate and boreal environments. This article is protected by copyright. All rights reserved.
- The role of stratification on lakes' thermal response: The case of Lake
- Authors: Sebastiano Piccolroaz; Marco Toffolon, Bruno Majone
Abstract: During the last several decades, the Great Lakes region has been experiencing a significant rise in temperatures, with the extraordinary summer warming that affected Lake Superior in 1998 as an example of the marked response of the lake to increasingly warmer atmospheric conditions. In this work we combine the analysis of this exceptional event with some synthetic scenarios, to achieve a deeper understanding of the main processes driving the thermal dynamics of surface water temperature in Lake Superior. The analysis is performed by means of the lumped model air2water, which simulates lake surface temperature as a function of air temperature alone. The model provides information about the seasonal stratification dynamics, suggesting that unusual warming events can result from two factors: anomalously high summer air temperatures, and increased strength of stratification resulting from a warm spring. The relative contribution of the two factors is quantified using the model by means of synthetic scenarios, which provide a simple but effective description of the positive feedback between the thermal behavior and the stratification dynamics of the lake. This article is protected by copyright. All rights reserved.
- Understanding handpump sustainability: Determinants of rural water source
functionality in the Greater Afram Plains region of Ghana
- Authors: Michael B. Fisher; Katherine F. Shields, Terence U. Chan, Elizabeth Christenson, Ryan D. Cronk, Hannah Leker, Destina Samani, Patrick Apoya, Alexandra Lutz, Jamie Bartram
Abstract: Safe drinking water is critical to human health and development. In rural sub‐Saharan Africa, most improved water sources are boreholes with handpumps; studies suggest that up to one third of these handpumps are non‐functional at any given time. This work presents findings from a secondary analysis of cross‐sectional data from 1509 water sources in 570 communities in the rural Greater Afram Plains (GAP) region of Ghana; one of the largest studies of its kind.
79.4% of enumerated water sources were functional when visited; in multivariable regressions, functionality depended on source age, management, the number of other sources in the community, and the district. A Bayesian network (BN) model developed using the same dataset found strong dependencies of functionality on implementer, pump type, management, and the availability of tools, with synergistic effects from management determinants on functionality, increasing the likelihood of a source being functional from a baseline of 72% to more than 97% with optimal management and available tools.
We suggest that functionality may be a dynamic equilibrium between regular breakdowns and repairs, with management a key determinant of repair rate. Management variables may interact synergistically in ways better captured by BN analysis than by logistic regressions. These qualitative findings may prove generalizable beyond the study area, and may offer new approaches to understanding and increasing handpump functionality and safe water access. This article is protected by copyright. All rights reserved.
- Correlation equations for average deposition rate coefficients of
nanoparticles in a cylindrical pore
- Authors: N. Seetha; S. Majid Hassanizadeh, M. S. Mohan Kumar, Amir Raoof
Abstract: Nanoparticle deposition behavior observed at the Darcy scale represents an average of the processes occurring at the pore scale. Hence, the effect of various pore‐scale parameters on nanoparticle deposition can be understood by studying nanoparticle transport at pore scale and upscaling the results to the Darcy scale. In this work, correlation equations for the deposition rate coefficients of nanoparticles in a cylindrical pore are developed as a function of nine pore‐scale parameters: the pore radius, nanoparticle radius, mean flow velocity, solution ionic strength, viscosity, temperature, solution dielectric constant, and nanoparticle and collector surface potentials. Based on dominant processes, the pore space is divided into three different regions, namely, bulk, diffusion, and potential regions. Advection‐diffusion equations for nanoparticle transport are prescribed for the bulk and diffusion regions, while the interaction between the diffusion and potential regions is included as a boundary condition. This interaction is modeled as a first‐order reversible kinetic adsorption. The expressions for the mass transfer rate coefficients between the diffusion and the potential regions are derived in terms of the interaction energy profile. Among other effects, we account for nanoparticle‐collector interaction forces on nanoparticle deposition. The resulting equations are solved numerically for a range of values of pore‐scale parameters. The nanoparticle concentration profile obtained for the cylindrical pore is averaged over a moving averaging volume within the pore in order to get the 1D concentration field. The latter is fitted to the 1D advection‐dispersion equation with an equilibrium or kinetic adsorption model to determine the values of the average deposition rate coefficients. In this study, pore‐scale simulations are performed for three values of Péclet number, Pe = 0.05, 5 and 50. We find that under unfavorable conditions, the nanoparticle deposition at pore scale is best described by an equilibrium model at low Péclet numbers (Pe = 0.05), and by a kinetic model at high Péclet numbers (Pe = 50). But, at an intermediate Pe (e.g., near Pe = 5), both equilibrium and kinetic models fit the 1D concentration field. Correlation equations for the pore‐averaged nanoparticle deposition rate coefficients under unfavorable conditions are derived by performing a multiple‐linear regression analysis between the estimated deposition rate coefficients for a single pore and various pore‐scale parameters. The correlation equations, which follow a power law relation with nine pore‐scale parameters, are found to be consistent with the column‐scale and pore‐scale experimental results, and qualitatively agree with the colloid filtration theory. These equations can be incorporated into pore network models to study the effect of pore‐scale parameters on nanoparticle deposition at larger length scales such as Darcy scale. This article is protected by copyright. All rights reserved.
- Dry‐season length and runoff control annual variability in stream
DOC dynamics in a small, shallow‐groundwater‐dominated
- Authors: G. Humbert; A. Jaffrezic, O. Fovet, G. Gruau, P. Durand
Abstract: As a phenomenon integrating climate conditions and hydrological control of the connection between streams and terrestrial dissolved organic carbon (DOC) sources, groundwater dynamics control patterns of stream DOC characteristics (concentrations and fluxes). Influence of intra‐annual variations in groundwater level, discharge and climatic factors on DOC concentrations and fluxes were assessed over 13 years at the headwater watershed of Kervidy‐Naizin (5 km2) in western France. Four seasonal periods were delineated within each year according to groundwater fluctuations (A: rewetting, B: high flow, C: recession, and D: drought). Annual and seasonal base flow vs stormflow DOC concentrations were defined based on daily hydrograph readings. High inter‐annual variability of annual DOC fluxes (5.4‐39.5 kg.ha−1.yr−1) indicates that several years of data are required to encompass variations in water flux to evaluate the actual DOC export capacity of a watershed. Inter‐annual variability of mean annual DOC concentrations was much lower (4.9‐7.5 mg C.l−1), with concentrations decreasing within each year from ca. 9.2 mg C.l−1 in A to ca. 3.0 mg C.l−1 in C. This indicates an intra‐annual pattern of stream DOC concentrations controlled by DOC source characteristics and groundwater dynamics very similar across years. Partial least square regressions combined with multiple linear regressions showed that the dry season characteristics (length and drawdown) determine the mean annual DOC concentration while annual runoff determines the annual flux. Antagonistic mechanisms of production‐accumulation and dilution‐depletion combined with an unlimited DOC supply from riparian wetland soils can mitigate the response of stream concentrations to global changes and climatic variations. This article is protected by copyright. All rights reserved.
- Annual water, sediment, nutrient, and organic carbon fluxes in river
basins: A global meta‐analysis as a function of scale
- Abstract: Process controls on water, sediment, nutrient and organic carbon exports from the landscape through runoff are not fully understood. This paper provides analyses from 446 sites worldwide to evaluate the impact of environmental factors (MAP and MAT: mean annual precipitation and temperature; CLAY and BD: soil clay content and bulk density; S: slope gradient and LU: land use) on annual exports (RC: runoff coefficients; SL: sediment loads; TOCL: organic carbon losses; TNL: nitrogen losses and TPL: phosphorus losses) from different spatial scales. RC was found to increase, on average, from 18% at local scale (in headwaters), 25% at micro and subcatchment scale (mid‐reaches) to 41% at catchment scale (lower reaches of river basins) in response to multiple factors. SL increased from microplots (468 g m−2 yr−1) to plots (901 g m−2 yr−1), accompanied by decreasing TOCL and TNL. Climate was a major control masking the effects of other factors. For example, RC, SL, TOCL, TNL and TPL tended to increase with MAP at all spatial scales. These variables, however, decreased with MAT. The impact of CLAY, BD, LU and S on erosion variables was largely confined to the hillslope scale, where RC, SL and TOCL decreased with CLAY, while TNL and TPL increased. The results contribute to better understanding of water, nutrient and carbon cycles in terrestrial ecosystems, and should inform river basin modelling and ecosystem management. The important role of spatial climate variability points to a need for comparative research in specific environments at nested spatio‐temporal scales. This article is protected by copyright. All rights reserved.
- Estimating mountain basin‐mean precipitation from streamflow using
- Authors: Brian Henn; Martyn P. Clark, Dmitri Kavetski, Jessica D. Lundquist
Abstract: Estimating basin‐mean precipitation in complex terrain is difficult due to uncertainty in the topographical representativeness of precipitation gauges relative to the basin. To address this issue, we use Bayesian methodology coupled with a multi‐model framework to infer basin‐mean precipitation from streamflow observations, and we apply this approach to snow‐dominated basins in the Sierra Nevada of California. Using streamflow observations, forcing data from lower‐elevations stations, the Bayesian Total Error Analysis (BATEA) methodology and the Framework for Understanding Structural Errors (FUSE), we infer basin‐mean precipitation, and compare it to basin‐mean precipitation estimated using topographically‐informed interpolation from gauges (PRISM, the Parameter‐elevation Regression on Independent Slopes Model). The BATEA‐inferred spatial patterns of precipitation show agreement with PRISM in terms of the rank of basins from wet to dry, but differ in absolute values. In some of the basins, these differences may reflect biases in PRISM, because some implied PRISM runoff ratios may be inconsistent with the regional climate. We also infer annual time series of basin precipitation using a two‐step calibration approach. Assessment of the precision and robustness of the BATEA approach suggests that uncertainty in the BATEA‐inferred precipitation is primarily related to uncertainties in hydrologic model structure. Despite these limitations, time series of inferred annual precipitation under different model and parameter assumptions are strongly correlated with one another, suggesting that this approach is capable of resolving year‐to‐year variability in basin‐mean precipitation. This article is protected by copyright. All rights reserved.
- Assessing the performance of the independence method in modeling spatial
- Authors: Feifei Zheng; Emeric Thibaud, Michael Leonard, Seth Westra
Abstract: Spatial statistical methods are often employed to improve precision when estimating marginal distributions of extreme rainfall. Methods such as max‐stable and copula models parameterize the spatial dependence and provide a continuous spatial representation. Alternatively, the independence method can be used to estimate marginal parameters without the need for parameterizing the spatial dependence, and this method has been under‐utilized in hydrologic applications. This paper investigates the effectiveness of the independence method for marginal parameter estimation of spatially dependent extremes. Its performance is compared with three spatial dependence models (max‐stable Brown‐Resnick, max‐stable Schlather, and Gaussian copula) by means of a simulation study. The independence method is statistically robust in estimating parameters and their associated confidence intervals for spatial extremes with various underlying dependence structures. The spatial dependence models perform comparably with the independence method when the spatial dependence structure is correctly specified; otherwise they exhibit considerably worse performance. We conclude that the independence method is more appealing for modelling the marginal distributions of spatial extremes (e.g., regional estimation of trends in rainfall extremes) due to its greater robustness and simplicity. The four statistical methods are illustrated using a spatial dataset comprising 69 subdaily rainfall series from the Greater Sydney region, Australia. This article is protected by copyright. All rights reserved.
- Numerical simulations of hydraulic redistribution across climates: The
role of the root hydraulic conductivities
- Authors: Juan C. Quijano; Praveen Kumar
Abstract: Hydraulic redistribution, a process by which vegetation roots redistribute soil moisture, has been recognized as an important mechanism impacting several processes that regulate plant water uptake, energy and water partitioning, and biogeochemical cycling. We analyze how the magnitude of hydraulic redistribution varies across ecosystems that are exposed to different climates and seasonal patterns of incoming shortwave radiation and precipitation. Numerical simulation studies are performed over ten Ameriflux sites, which show that hydraulic redistribution predictions are significantly influenced by the specified root hydraulic conductivities. We performed sensitivity analyses by considering expected ranges of root conductivities based on previous experimental studies, and found contrasting patterns in energy‐limited and water‐limited ecosystems. In energy‐limited ecosystems, there is a threshold above which high root conductivities enhance hydraulic redistribution with no increase in transpiration, while in water‐limited ecosystems increase in root conductivities was always associated with enhancements in both transpiration and hydraulic redistribution. Further we found differences in the magnitude and seasonality of hydraulic redistribution and transpiration across different climates, regulated by interplay between precipitation and transpiration. The annual hydraulic redistribution to transpiration flux ratio (HR/Tr) was significant in Mediterranean climates (HR/Tr ≈ 30%), and in the tropical humid climates (HR/Tr ≈ 15%). However, in the continental climates hydraulic redistribution occurs only during sporadic precipitation events throughout the summer resulting in lower annual magnitudes (HR/Tr
- Temporal variability of the optimal monitoring setup assessed using
- Authors: Marcus Fahle; Tobias L. Hohenbrink, Ottfried Dietrich, Gunnar Lischeid
Abstract: Hydrology is rich in methods that use information theory to evaluate monitoring networks. Yet, in most existing studies only the available data set as a whole is used, which neglects the intra‐annual variability of the hydrological system. In this paper, we demonstrate how this variability can be considered by extending monitoring evaluation to subsets of the available data. Therefore, we separately evaluated time windows of fixed length, which were shifted through the data set, and successively extended time windows. We used basic information theory measures and a greedy ranking algorithm based on the criterion of maximum information/minimum redundancy. The network investigated monitored surface and groundwater levels at quarter‐hourly intervals and was located at an artificially drained lowland site in the Spreewald region in north‐east Germany. The results revealed that some of the monitoring stations were of value permanently while others were needed only temporally. The prevailing meteorological conditions, particularly the amount of precipitation, affected the degree of similarity between the water levels measured. The hydrological system tended to act more individually during periods of no or little rainfall. The optimal monitoring setup, its stability and the monitoring effort necessary were influenced by the meteorological forcing. Altogether, the methodology presented can help achieve a monitoring network design that has a more even performance or covers the conditions of interest (e.g., floods or droughts) best. This article is protected by copyright. All rights reserved.
- A transition in the spatially integrated reaction rate of bimolecular
- Authors: Masoud Arshadi; Harihar Rajaram
Abstract: Numerical simulations of diffusion with bimolecular reaction demonstrate a transition in the spatially integrated reaction rate ‐ increasing with time initially, and transitioning to a decrease with time. In previous work, this reaction‐diffusion problem has been analyzed as a Stefan problem involving a distinct moving boundary (reaction front), leading to predictions that front motion scales as t, and correspondingly the spatially integrated reaction rate decreases as the square root of time 1/t. We present a general non‐dimensionalization of the problem and a perturbation analysis to show that there is an early time regime where the spatially integrated reaction rate scales as t rather than 1/t. The duration of this early time regime (where the spatially integrated reaction rate is kinetically rather than diffusion controlled) is shown to depend on the kinetic rate parameters, diffusion coefficients and initial concentrations of the two species. Numerical simulation results confirm the theoretical estimates of the transition time. We present illustrative calculations in the context of in‐situ chemical oxidation for remediation of fractured rock systems where contaminants are largely dissolved in the rock matrix. We consider different contaminants of concern (COCs), including TCE, PCE, MTBE and RDX. While the early‐time regime is very short‐lived for TCE, it can persist over months to years for MTBE and RDX, due to slow oxidation kinetics. This article is protected by copyright. All rights reserved.
- Nickel migration and retention dynamics in natural soil columns
- Abstract: Nickel migration measured in laboratory‐scale, natural soil column experiments is shown to display anomalous (non‐Fickian) transport, non‐equilibrium adsorption and desorption patterns, and precipitation/dissolution. Similar experiments using a conservative tracer also exhibit anomalous behavior. The occurrence of ion exchange of nickel, mainly with calcium (but also with other soil components), is measured in both batch and flow‐through column experiments; adsorption and desorption isotherms demonstrate hysteresis. Strong retention of nickel during transport in soil columns leads to delayed initial breakthrough (∼40 pore volumes), slow increase in concentration, and extended concentration tailing at long times. We describe the mechanisms of transport and retention in terms of a continuous time random walk (CTRW) model, and use a particle tracking formulation to simulate nickel migration in the column. This approach allows us to capture the non‐Fickian transport and the subtle local effects of adsorption/desorption and precipitation/dissolution. Consideration also of preferential pathways accounts for the evolution of the measured breakthrough curve and measured spatial concentration profiles. The model uses non Fickian transport parameters estimated from the conservative tracer and, as a starting point, adsorption/desorption parameters based on batch experiments and a precipitation parameter based on Ksp values. The batch parameters are found to underestimate the actual amount of adsorption. We suggest that the sorption and precipitation/dissolution dynamics, and resulting breakthrough curves, are influenced strongly by preferential pathways; such pathways significantly alter the availability of sorption sites and ion availability for precipitation. Analysis of these results provides further understanding of the interaction and dynamics among transport, precipitation and sorption mechanisms in natural soil. This article is protected by copyright. All rights reserved.
- Continuum‐scale investigation of evaporation from bare soil under
different boundary and initial conditions: An evaluation of nonequilibrium
- Authors: Andrew C. Trautz; Kathleen M. Smits, Abdullah Cihan
Abstract: Evaporation and condensation in bare soils govern water and energy fluxes between the land and atmosphere. Phase change between liquid water and water vapor is commonly evaluated in soil hydrology using an assumption of instantaneous phase change (i.e. chemical equilibrium). Past experimental studies have shown that finite volatilization and condensation times can be observed under certain environmental conditions, thereby questioning the validity of this assumption. A comparison between equilibrium and non‐equilibrium phase change modeling approaches showed that the latter is able to provide better estimates of evaporation, justifying the need for more research on this topic. Several formulations based on irreversible thermodynamics, first order reaction kinetics, or the kinetic theory of gases have been employed to describe non‐equilibrium phase change at the continuum scale. In this study, results from a fully coupled non‐isothermal heat and mass transfer model applying four different non‐equilibrium phase change formulations were compared with experimental data generated under different initial and boundary conditions. Results from a modified Hertz‐Knudsen formulation based on kinetic theory of gases, proposed herein, were consistently in best agreement in terms of preserving both magnitude and trends of experimental data under all environmental conditions analyzed. Simulation results showed that temperature dependent formulations generally better predict evaporation than formulations independent of temperature. Analysis of vapor concentrations within the porous media showed that conditions were not at equilibrium under the experimental conditions tested. This article is protected by copyright. All rights reserved.
- Resolving two‐dimensional flow structure in rivers using
large‐scale particle image velocimetry: An example from a stream
- Authors: Quinn W. Lewis; Bruce L. Rhoads
Abstract: Large scale particle image velocimetry (LSPIV) has emerged as a valuable tool for measuring surface velocity in a variety of fluvial systems. LSPIV has typically been used in the field to obtain velocity or discharge measurements in relatively simple one‐dimensional flow. Detailed two‐dimensional or three‐dimensional characterization of flow structure has been relegated to laboratory settings because of the difficulty in controlling PIV limiting factors such as poor particle seeding, the need for camera rectification, and challenging field conditions. In this study we implement a low‐cost LSPIV setup using a high‐resolution action camera mounted above a stream confluence and water seeded with recycled landscape mulch. Time‐averaged 2D velocities derived from LSPIV are compared with those measured with an acoustic Doppler velocimeter (ADV) in the camera's field of view. We also assess the capabilities of this setup to resolve both turbulent and time‐averaged flow structures at a stream confluence. Our results reveal that even in challenging field conditions a basic LSPIV setup can yield accurate data on velocity and resolve in detail the temporal evolution of flow structures on the surface of rivers. The resulting dataset contains velocity information at high spatial and temporal resolution, a significant advance in understanding flow processes at stream confluences. Our LSPIV analysis provides support for previous numerical modeling studies that have distinguished between Kelvin‐Helmholtz and wake modes of turbulent behavior within the mixing interface at confluences. This study shows that LSPIV should be considered as both an alternative for traditional methods and a tool that can provide unprecedented levels of resolution of surface velocity patterns on rivers that can be used to evaluate numerical predictions of flow structure in complex fluvial environments. This article is protected by copyright. All rights reserved.
- Combined positron emission tomography and computed tomography to visualize
and quantify fluid flow in sedimentary rocks
- Abstract: Here we show for the first time combined positron emission tomography (PET) and computed tomography (CT) imaging of flow processes within porous rocks to quantify the development in local fluid saturations. The coupling between local rock structure and displacement fronts is demonstrated in exploratory experiments using this novel approach. We also compare quantification of 3D temporal and spatial water saturations in two similar CO2 storage tests in sandstone imaged separately with PET and CT. The applicability of each visualization technique is evaluated for a range of displacement processes, and the favorable implementation of combining PET/CT for laboratory core analysis is discussed. We learn that the signal‐to‐noise ratio (SNR) is over an order of magnitude higher for PET compared with CT for the studied processes. This article is protected by copyright. All rights reserved.
- Impact of degrading permafrost on subsurface solute transport pathways and
- Authors: Andrew Frampton; Georgia Destouni
Abstract: Subsurface solute transport under surface warming and degrading permafrost conditions is studied using a physically‐based model of coupled cryotic and hydrogeological flow processes combined with a particle tracking method. Changes in the subsurface water and inert solute pathways and travel times are analysed for different modelled geological configurations. For all simulated cases, the minimum and mean travel times increase non‐linearly with warming irrespective of geological configuration and heterogeneity structure. The timing of the start of increase in travel time depends on heterogeneity structure, combined with the rate of permafrost degradation that also depends on material thermal and hydrogeological properties. The travel time changes depend on combined warming effects of: i) increase in pathway length due to deepening of the active layer, ii) reduced transport velocities due to a shift from horizontal saturated groundwater flow near the surface to vertical water percolation deeper into the subsurface, and iii) pathway length increase and temporary immobilization caused by cryosuction‐induced seasonal freeze cycles. This article is protected by copyright. All rights reserved.
- Comment on critiques of “Stationarity is dead: Whither water
- Authors: P. C. D. Milly; Julio Betancourt, Malin Falkenmark, Robert M. Hirsch, Zbigniew W. Kundzewicz, Dennis P. Lettenmaier, Ronald J. Stouffer, Michael D. Dettinger, Valentina Krysanova
Abstract: We review and comment upon some themes in the recent stream of critical commentary on the assertion that “stationarity is dead,” attempting to clear up some misunderstandings; to note points of agreement; to elaborate on matters in dispute; and to share further relevant thoughts. This article is protected by copyright. All rights reserved.
- A Statistical Concept to Assess the Uncertainty in Bayesian Model Weights
and its Impact on Model Ranking
- Abstract: Bayesian model averaging (BMA) ranks the plausibility of alternative conceptual models according to Bayes' theorem. A prior belief about each model's adequacy is updated to a posterior model probability based on the skill to reproduce observed data and on the principle of parsimony. The posterior model probabilities are then used as model weights for model ranking, selection, or averaging. Despite the statistically rigorous BMA procedure, model weights can become uncertain quantities due to measurement noise in the calibration data set or due to uncertainty in model input. Uncertain weights may in turn compromise the reliability of BMA results. We present a new statistical concept to investigate this weighting uncertainty, and thus to assess the significance of model weights and the confidence in model ranking. Our concept is to resample the uncertain input or output data, and then to analyze the induced variability in model weights. In the special case of weighting uncertainty due to measurement noise in the calibration data set, we interpret statistics of Bayesian model evidence to assess the distance of a model's performance from the theoretical upper limit. To illustrate our suggested approach, we investigate the reliability of soil‐plant model selection following up on a study by Wöhling et al. . Results show that the BMA routine should be equipped with our suggested upgrade to (1) reveal the significant but otherwise undetected impact of measurement noise on model ranking results, and (2) to decide whether the considered set of models should be extended with better performing alternatives. This article is protected by copyright. All rights reserved.
- Parameter estimation and prediction for groundwater contamination based on
- Authors: S. A. Mattis; T. D. Butler, C. N. Dawson, D. Estep, V. V. Vesselinov
Abstract: The problem of groundwater contamination in an aquifer is one with many uncertainties. Properly quantifying these uncertainties is essential in order to make reliable probabilistic based predictions and decisions regarding remediation strategies. In this work, a measure‐theoretic framework is employed to quantify uncertainties in a simplified groundwater contamination transport model. Given uncertain data from observation wells, the stochastic inverse problem is solved numerically to obtain a probability measure on the space of unknown model parameters characterizing groundwater flow and contaminant transport in an aquifer, as well as unknown model boundary or source terms such as the contaminant source release into the environment. This probability measure is used to make predictions of future contaminant concentrations and to analyze possible remediation techniques. The ability to identify regions of small but nonzero probability using this method is illustrated. This article is protected by copyright. All rights reserved.
- 50 years of water resources research legacy and perspectives for the
science of hydrology introduction
- Abstract: We present an overview of the contributions collected to celebrate the 50th anniversary of Water Resources Research along with a critical discussion of the legacy and perspectives for the science of hydrology in the 21st century. This collection of papers highlights exciting pathways to the future of water sciences. New monitoring and modeling techniques and increasing opportunities for data and knowledge sharing from hydrological research will provide innovative means to improve water management and to ensure a sustainable development to society. We believe that this set of papers will provide valuable inspiration for future hydrologists, and will support the intensification of international cooperation among scientists. This article is protected by copyright. All rights reserved.
- An advanced process‐based distributed model for the investigation of
rainfall‐induced landslides: The effect of process representation
and boundary conditions
- Authors: Grigorios G. Anagnostopoulos; Simone Fatichi, Paolo Burlando
Abstract: Extreme rainfall events are the major driver of shallow landslide occurrences in mountainous and steep terrain regions around the world. Subsurface hydrology has a dominant role on the initiation of rainfall‐induced shallow landslides, since changes in the soil water content affect significantly the soil shear strength. Rainfall infiltration produces an increase of soil water potential, which is followed by a rapid drop in apparent cohesion. Especially on steep slopes of shallow soils, this loss of shear strength can lead to failure even in unsaturated conditions before positive water pressures are developed. We present HYDROlisthisis, a process‐based model, fully distributed in space with fine time resolution, in order to investigate the interactions between surface and subsurface hydrology and shallow landslides initiation. Fundamental elements of the approach are the dependence of shear strength on the three dimensional (3D) field of soil water potential, as well as the temporal evolution of soil water potential during the wetting and drying phases. Specifically, 3D variably saturated flow conditions, including soil hydraulic hysteresis and preferential flow phenomena, are simulated for the subsurface flow, coupled with a surface runoff routine based on the kinematic wave approximation. The geotechnical component of the model is based on a multidimensional limit equilibrium analysis, which takes into account the basic principles of unsaturated soil mechanics. A series of numerical simulations were carried out with various boundary conditions and using different hydrological and geotechnical components. Boundary conditions in terms of distributed soil depth were generated using both empirical and process based models. The effect of including preferential flow and soil hydraulic hysteresis was tested together with the replacement of the infinite slope assumption with the multidimensional limit equilibrium analysis. The results show that boundary conditions play a crucial role in the model performance and that the introduced hydrological (preferential flow and soil hydraulic hysteresis) and geotechnical components (multidimensional limit equilibrium analysis) significantly improve predictive capabilities in the presented case study. This article is protected by copyright. All rights reserved.
- Internal connectivity of meandering rivers: Statistical generalization of
channel hydraulic geometry
- Authors: M.J. Czapiga; V.B. Smith, J.A. Nittrouer, D. Mohrig, G. Parker
Abstract: The geometry of rivers has been characterized in terms of downstream and at‐a‐station hydraulic geometry, based on individual cross‐sections. Such analyses do not, however, provide insight as to how these cross‐sections are connected. We generalize the concept of hydraulic geometry, using data on bathymetry from four reaches of meandering rivers that include at least five bends. We quantify connectivity in terms of the probability that a connected path exists such that a given attribute remains within specified bounds along it. While the concept is general, here we apply it to vessel navigability. We develop a predictor for navigability in meandering rivers, which requires only the following, relatively easily obtained input: vessel draft, vessel width, bankfull depth, bankfull width, relative difference between current and bankfull water surface elevation, and length of desired navigation path. The predictor is applicable to both bankfull and below‐bankfull stage. A key input parameter is the standard deviation of the probability distribution of depth. This parameter, in and of itself, yields no information on connectivity as it does not capture the spatial orientation of depth variation. We find, however, that a) the probability function for connectivity does depend on this parameter, and b) its use allows for an approximate similarity collapse of the probability function, so providing a quasi‐universal predictive relation applying to all four reaches. The results also suggest potential application to more complex forms for connectivity that involve other or multiple in‐stream physical variables. This article is protected by copyright. All rights reserved.
- Is unique scaling of aquifer macrodispersivity supported by field
- Authors: A. Zech; S. Attinger, V. Cvetkovic, G. Dagan, P. Dietrich, A. Fiori, Y. Rubin, G. Teutsch
Abstract: Spreading of conservative solutes in groundwater due to aquifer heterogeneity is quantified by the macrodispersivity, which was found to be scale dependent. It increases with travel distance, stabilizing eventually at a constant value. However, the question of its asymptotic behaviour at very large scale is still a matter of debate. It was surmised in the literature that macrodispersivity scales up following a unique scaling law. Attempts to define such a law were made by fitting a regression line in the log‐log representation of an ensemble of macrodispersivities from multiple experiments. The functional relationships differ among the authors, based on the choice of data. Our study revisits the data basis, used for inferring unique scaling, through a detailed analysis of literature marcodispersivities. In addition, values were collected from the most recent tracer tests reported in the literature. We specified a system of criteria for reliability and re‐evaluated the reliability of the reported values. The final collection of reliable estimates of macrodispersivity does not support a unique scaling law relationship. On the contrary, our results indicate, that the field data can be explained as a collection of macrodispersivities of aquifers with varying degree of heterogeneity where each exhibits its own constant asymptotic value. Our investigation concludes that transport, and particularly the macrodispersivity, is formation‐specific, and that modeling of transport cannot be relegated to a unique scaling law. Instead, transport requires characterization of aquifer properties, e.g. spatial distribution of hydraulic conductivity, and the use of adequate models. This article is protected by copyright. All rights reserved.
- Self‐adjustment of stream bed roughness and flow velocity in a steep
- Authors: Johannes M. Schneider; Dieter Rickenmann, Jens M. Turowski, James W. Kirchner
Abstract: Understanding how channel bed morphology affects flow conditions (and vice versa) is important for a wide range of fluvial processes and practical applications. We investigated interactions between bed roughness and flow velocity in a steep, glacier‐fed mountain stream (Riedbach, Ct. Valais, Switzerland) with almost flume‐like boundary conditions. Bed gradient increases along the 1‐km study reach by roughly one order of magnitude (S=3‐41%), with a corresponding increase in streambed roughness, while flow discharge and width remain approximately constant due to the glacial runoff regime. Streambed roughness was characterized by semi‐variograms and standard deviations of point clouds derived from terrestrial laser scanning. Reach‐averaged flow velocity was derived from dye tracer breakthrough curves measured by 10 fluorometers installed along the channel. Commonly used flow resistance approaches (Darcy‐Weisbach equation and dimensionless hydraulic geometry) were used to relate the measured bulk velocity to bed characteristics. As a roughness measure, D84 yielded comparable results to more laborious measures derived from point clouds. Flow resistance behavior across this large range of steep slopes agreed with patterns established in previous studies for both lower‐gradient and steep reaches, regardless of which roughness measures were used. We linked empirical critical shear stress approaches to the variable power equation for flow resistance to investigate the change of bed roughness with channel slope. The predicted increase in D84 with increasing channel slope was in good agreement with field observations. This article is protected by copyright. All rights reserved.
- Does small‐bodied salmon spawning activity enhance streambed
- Authors: Marwan A. Hassan; Daniele Tonina, Todd H. Buxton
Abstract: Female salmonids bury and lay their eggs in streambeds by digging a pit, which is then covered with sediment from a second pit. The spawning process alters streambed topography, winnows fine sediment, and mixes sediment in the active layer. The resulting egg nests (redds) contain coarser and looser sediments than those of unspawned streambed areas, and display a dune‐like shape with an amplitude and length that vary with fish size, substrate conditions, and flow conditions. Redds increase local bed surface roughness (
- Climate index weighting of ensemble streamflow forecasts using a simple
- Authors: A. Allen Bradley; Mohamed Habib, Stuart S. Schwartz
Abstract: Climate state can be an important predictor of future hydrologic conditions. In ensemble streamflow forecasting, where historical weather inputs or streamflow observations are used to generate the ensemble, climate index weighting is one way to represent the influence of climate state. Using a climate index, each forecast variable member of the ensemble is selectively weighted to reflect the climate state at the time of the forecast. A new approach to climate index weighting of ensemble forecasts is presented. The method is based on a sampling‐resampling approach for Bayesian updating. The original hydrologic ensemble members define a sample drawn from the prior distribution; the relationship between the climate index and the ensemble member forecast variable is used to estimate a likelihood function. Given an observation of the climate index at the time of the forecast, the estimated likelihood function is then used to assign weights to each ensemble member. The weights define the probability of each ensemble member outcome given the observed climate index. The weighted ensemble forecast is then used to estimate the posterior distribution of the forecast variable conditioned on the climate index. The Bayesian climate index weighting approach is easy to apply to hydrologic ensemble forecasts; its parameters do not require calibration with hindcasts, and it adapts to the strength of the relation between climate and the forecast variable, defaulting to equal weighting of ensemble members when no relationship exists. A hydrologic forecasting application illustrates the approach and contrasts it with traditional climate index weighting approaches. This article is protected by copyright. All rights reserved.
- On the limits of heat as a tracer to estimate reach‐scale
river‐aquifer exchange flux
- Authors: Yueqing Xie; Peter G. Cook, Craig T. Simmons, Chunmiao Zheng
Abstract: For the past few decades, heat has been used to estimate river‐aquifer exchange flux at discrete locations by comparison of river and groundwater temperature. In recent years, heat has also been employed to estimate reach‐scale river‐aquifer exchange flux based only on river temperature. However, there are many more parameters that govern heat exchange and transport in surface water than in groundwater. In this study, we analyzed the sensitivities of surface water temperature to various parameters and assessed the accuracy of temperature‐based estimates of exchange flux in two synthetic rivers and in a field setting. For the large synthetic river with a flow rate of 63 m3 s−1 (i.e., 5.44 × 106 m3 d−1), the upper and lower bounds of the groundwater inflow rate can be determined when the actual groundwater inflow is around 100 m2 d−1. For higher and lower fluxes, only minimum and maximum bounds respectively can be determined. For the small synthetic river with the flow rate of 0.63 m3 s−1 (i.e., 5.44 × 104 m3 d−1), the bounds of the groundwater inflow rate can only be estimated when the actual groundwater inflow rate is near 10 m2 d−1. In the field setting, results show that the inflow rate must be less than 100 m2 d−1, but a lower bound for groundwater inflow cannot be determined. The large ranges of estimated groundwater inflow rates in both theoretical and field settings indicate the need to reduce parameter errors and combine heat measurements with other isotopic and/or chemical methods. This article is protected by copyright. All rights reserved.
- Simulating riparian disturbance: Reach‐scale impacts on aquatic
habitat in gravel bed streams
- Authors: S. L. Davidson; B.C. Eaton
Abstract: Large wood governs channel morphology, as well as the availability of in‐stream habitat, in many forested streams. In this paper we use a stochastic, physically based model to simulate wood recruitment and in‐stream geomorphic processes, in order to explore the influence of disturbance history on the availability of aquatic habitat. Specifically, we consider the effects of fire on a range of stream sizes by varying the rate of tree toppling over time in a simulated forest characterized by a tree height of 30 m. We also consider the effects of forest harvesting with various riparian buffer sizes, by limiting the lateral extent of the riparian stand. Our results show that pulsed inputs of wood increase the availability and variability of physical habitat in the post‐fire period; reach‐averaged pool area and deposit area double in small streams, while side‐channels increase by over 50% in intermediate‐sized channels. By contrast, forest harvesting reduces the availability of habitat within the reach, though the effects diminish with increasing buffer size or stream width; in laterally stable streams the effects are minimal so long as buffer width is large enough for key pieces to be recruited to the reach. This research emphasizes the importance of natural disturbance in creating and maintaining habitat heterogeneity and shows that scenario‐based numerical modeling provides a useful tool for assessing the historical range of variability associated with natural disturbance, as well as changes in habitat relevant to fish. It can be also used to inform forest harvesting and management. This article is protected by copyright. All rights reserved.
- Precipitation‐snowmelt timing and snowmelt augmentation of large
peak flow events, Western Cascades, Oregon
- Authors: Keith Jennings; Julia A. Jones
Abstract: This study tested multiple hydrologic mechanisms to explain snowpack dynamics in extreme rain‐on‐snow floods, which occur widely in the temperate and polar regions. We examined 26, 10‐day large storm events over the period 1992 to 2012 in the H.J. Andrews Experimental Forest in western Oregon, using statistical analyses (regression, ANOVA, and wavelet coherence) of hourly snowmelt lysimeter, air and dewpoint temperature, wind speed, precipitation, and discharge data. All events involved snowpack outflow, but only seven events had continuous net snowpack outflow, including three of the five top‐ranked peak discharge events. Peak discharge was not related to precipitation rate, but it was related to the 10‐day sum of precipitation and net snowpack outflow, indicating an increased flood response to continuously melting snowpacks. The two largest peak discharge events in the study had significant wavelet coherence at multiple time scales over several days; a distribution of phase differences between precipitation and net snowpack outflow at the 12 to 32‐hour time scale with a sharp peak at π/2 radians; and strongly correlated snowpack outflow among lysimeters representing 42% of basin area. The recipe for an extreme rain‐on‐snow event includes persistent, slow melt within the snowpack, which appears to produce a near‐saturated zone within the snowpack throughout the landscape, such that the snowpack may transmit pressure waves of precipitation directly to streams, and this process is synchronized across the landscape. Further work is needed to understand the internal dynamics of a melting snowpack throughout a snow‐covered landscape and its contribution to extreme rain‐on‐snow floods. This article is protected by copyright. All rights reserved.
- The effect of soil surface sealing on vegetation water uptake along a dry
- Authors: Shai Sela; Tal Svoray, Shmuel Assouline
Abstract: Soil surface sealing is a widespread natural process occurring frequently in bare soil areas between vegetation patches. The low hydraulic conductivity that characterizes the seal layer reduces both infiltration and evaporation fluxes from the soil, and thus has the potential to affect local vegetation water uptake (VWU). This effect is investigated here using experimental data, 2D physically based modelling and a long‐term climatic dataset from three dry sites presenting a climatic gradient in the Negev Desert, Israel. The Feddes VWU parameters for the dominant shrub at the study site (Sarcopoterium spinosum) were acquired using lysimeter experiments. The results indicate that during the season surface sealing could either increase or decrease VWU depending on initial soil water content, rainfall intensity, and the duration of the subsequent drying intervals. These factors have a marked effect on inter‐annual variability of the seal layer effect on VWU, which on average was found to be 26% higher under sealed conditions than in the case of unsealed soil surfaces. The seal layer was found to reduce the period where the vegetation was under water stress by 31% compared with unsealed conditions. This effect was more pronounced for seasons with total rainfall depth higher than 10 cm/y, and was affected by interseasonal climatic variability. These results shed light on the importance of surface sealing in dry environments and its contribution to the resilience of woody vegetation. This article is protected by copyright. All rights reserved.
- A high‐resolution global flood hazard model
- Authors: Christopher C. Sampson; Andrew M. Smith, Paul B. Bates, Jeffrey C. Neal, Lorenzo Alfieri, Jim E. Freer
Abstract: Floods are a natural hazard that affect communities worldwide, but to date the vast majority of flood hazard research and mapping has been undertaken by wealthy developed nations. As populations and economies have grown across the developing world, so too has demand from governments, businesses and NGOs for modelled flood hazard data in these data‐scarce regions. We identify six key challenges faced when developing a flood hazard model that can be applied globally, and present a framework methodology that leverages recent cross‐disciplinary advances to tackle each challenge. The model produces return period flood hazard maps at ∼90 m resolution for the whole terrestrial land surface between 56˚S and 60˚N, and results are validated against high resolution government flood hazard datasets from the UK and Canada. The global model is shown to capture between two thirds and three quarters of the area determined to be at risk in the benchmark data without generating excessive false positive predictions. When aggregated to ∼1 km, mean absolute error in flooded fraction falls to ∼5%. The full complexity global model contains an automatically parameterised subgrid channel network, and comparison to both a simplified 2D only variant and an independently developed pan‐European model shows the explicit inclusion of channels to be a critical contributor to improved model performance. Whilst careful processing of existing global terrain datasets enables reasonable model performance in urban areas, adoption of forthcoming next‐generation global terrain datasets will offer the best prospect for a step‐change improvement in model performance. This article is protected by copyright. All rights reserved.
- Inroads of remote sensing into hydrologic science during the WRR era
- Authors: Dennis P. Lettenmaier; Doug Alsdorf, Jeff Dozier, George J. Huffman, Ming Pan, Eric F. Wood
Abstract: The first issue of WRR appeared eight years after the launch of Sputnik, but by WRR's 25th anniversary, only seven papers that used remote sensing had appeared. Over the journal's second 25 years, that changed remarkably, and remote sensing is now widely used in hydrology and other geophysical sciences. We attribute this evolution to production of datasets that scientists not well versed in remote sensing can use, and to educational initiatives like NASA's Earth System Science Fellowship program that has supported over a thousand scientists, many in hydrology. We review progress in remote sensing in hydrology from a water balance perspective. We argue that progress is primarily attributable to a creative use of existing and past satellite sensors to estimate such variables as evapotranspiration rates or water storage in lakes and reservoirs and to new and planned missions. Recent transforming technologies include the Gravity Recovery and Climate Experiment (GRACE), the European Soil Moisture and Ocean Salinity (SMOS) and U.S. Soil Moisture Active Passive (SMAP) missions, and the Global Precipitation Measurement (GPM) mission. Future missions include Surface Water and Ocean Topography (SWOT) to measure river discharge and lake, reservoir, and wetland storage. Measurement of some important hydrologic variables remains problematic: retrieval of snow water equivalent (SWE) from space remains elusive especially in mountain areas, even though snow cover extent is well observed, and was the topic of 4 of the first 5 remote sensing papers published in WRR. We argue that this area deserves more strategic thinking from the hydrology community. This article is protected by copyright. All rights reserved.
- Do we need a Community Hydrological Model?
- Authors: Markus Weiler; Keith Beven
Abstract: We believe that there are too many models in hydrology and we should ask ourselves the question, if we are currently wasting time and effort in developing another model again instead of focusing on the development of a community hydrological model. In other fields this kind of models have been quite successful, but due to several reasons, no single community model has been developed in the field of hydrology yet. The concept, strength and weakness of a community model was discussed at the Chapman Conference on Catchment Spatial Behaviour and Complex Organisation held in Luxembourg in September 2014. This discussion as well as out own opinions about the potential of a community models, or at least the necessary discussion to establish one are debated in this commentary. This article is protected by copyright. All rights reserved.
- A new temperature profiling probe for investigating
groundwater‐surface water interaction
- Authors: Ramon C Naranjo; Robert Turcotte
Abstract: Measuring vertically nested temperatures at the streambed interface poses practical challenges that are addressed here with a new discrete subsurface temperature profiling probe. We describe a new temperature probe and its application for heat as a tracer investigations to demonstrate the probe's utility. Accuracy and response time of temperature measurements made at 6 discrete depths in the probe were analyzed in the laboratory using temperature bath experiments. We find the temperature probe to be an accurate and robust instrument that allows for easily installation and long‐term monitoring in highly variable environments. Because the probe is inexpensive and versatile, it is useful for many environmental applications that require temperature data collection for periods of several months in environments that are difficult to access or require minimal disturbance. This article is protected by copyright. All rights reserved.
- Whither field hydrology? The need for discovery science and outrageous
- Authors: T.P. Burt; J.J. McDonnell
Abstract: Field hydrology is on the decline. Meanwhile, the need for new field‐derived insight into the age, origin and pathway of water in the headwaters, where most runoff is generated, is more needed than ever. Water Resources Research (WRR) has included some of the most influential papers in field‐based runoff process understanding, particularly in the formative years when the knowledge base was developing rapidly. Here, we take advantage of this 50th anniversary of the journal to highlight a few of these important field‐based papers and show how field scientists have posed strong and sometimes outrageous hypotheses—approaches so needed in an era of largely model‐only research. We chronicle the decline in field work and note that it is not only the quantity of field work that is diminishing but its character is changing too: from discovery science to data collection for model parameterisation. While the latter is a necessary activity, the loss of the former is a major concern if we are to advance the science of watershed hydrology. We outline a vision for field research to seek new fundamental understanding, new mechanistic explanations of how watershed systems work, particularly outside the regions of traditional focus. This article is protected by copyright. All rights reserved.
- Using RFID and accelerometer‐embedded tracers to measure
probabilities of transport, step lengths, and rest times in a mountain
- Authors: Lindsay Olinde; Joel P. L. Johnson
Abstract: We present new measurements of bedload tracer transport in a mountain stream over several snowmelt seasons. Cumulative displacements were measured using passive tracers, which consisted of gravel and cobbles embedded with radio frequency identification tags. The timing of bedload motion during eleven transporting events was quantified with active tracers, i.e., accelerometer‐embedded cobbles. Probabilities of cobble transport increased with discharge above a threshold, and exhibited slight to moderate hysteresis during snowmelt hydrographs. Dividing cumulative displacements by the number of movements recorded by each active tracer constrained average step lengths. Average step lengths increased with discharge, and distributions of average step lengths and cumulative displacements were thin‐tailed. Distributions of rest times followed heavy‐tailed power law scaling. Rest time scaling varied somewhat with discharge and with the degree to which tracers were incorporated into the stream bed. The combination of thin‐tailed displacement distributions and heavy‐tailed rest time distributions predict superdiffusive dispersion. This article is protected by copyright. All rights reserved.
- Dissolved gas dynamics in wetland soils: Root‐mediated gas transfer
kinetics determined via push‐pull tracer tests
- Abstract: Gas transfer processes are fundamental to the biogeochemical and water quality functions of wetlands, yet there is limited knowledge of the rates and pathways of soil ‐ atmosphere exchange for gases other than oxygen and methane (CH4). In this study we use a novel push‐pull technique with sulfur hexafluoride (SF6) and helium (He) as dissolved gas tracers to quantify the kinetics of root‐mediated gas transfer, which is a critical efflux pathway for gases from wetland soils. This tracer approach disentangles the effects of physical transport from simultaneous reaction in saturated, vegetated wetland soils. We measured significant seasonal variation in first‐order gas exchange rate constants, with smaller spatial variations between different soil depths and vegetation zones in a New Jersey tidal marsh. Gas transfer rates for most biogeochemical trace gases are expected to be bracketed by the rate constants for SF6 and He, which ranged from ∼10−2 to 2x10−1 h−1 at our site. A modified Damköhler number analysis is used to evaluate the balance between biochemical reaction and root‐driven gas exchange in governing the fate of environmental trace gases in rooted, anaerobic soils. This approach confirmed the importance of plant gas transport for CH4, and showed that root‐driven transport may affect nitrous oxide (N2O) balances in settings where N2O reduction rates are slow This article is protected by copyright. All rights reserved.
- On the use of spatially distributed, time‐lapse microgravity surveys
to inform hydrological modeling
- Authors: Sebastiano Piccolroaz; Bruno Majone, Francesco Palmieri, Giorgio Cassiani, Alberto Bellin
Abstract: In the last decades significant technological advances together with improved modeling capabilities fostered a rapid development of geophysical monitoring techniques in support of hydrological modeling. Geophysical monitoring offers the attractive possibility to acquire spatially distributed information on state variables. These provide complementary information about the functioning of the hydrological system to that provided by standard hydrological measurements, which are either intrinsically local or the result of a complex spatial averaging process. Soil water content is an example of state variable, which is relatively simple to measure pointwise (locally) but with a vanishing constraining effect on catchment‐scale modeling, while streamflow data, the typical hydrological measurement, offer limited possibility to disentangle the controlling processes. The objective of this work is to analyze the advantages offered by coupling traditional hydrological data with unconventional geophysical information in inverse modeling of hydrological systems. In particular, we explored how the use of time‐lapse, spatially distributed microgravity measurements may improve the conceptual model identification of a topographically complex Alpine catchment (the Vermigliana catchment, South‐Eastern Alps, Italy). The inclusion of microgravity data resulted in a better constraint of the inversion procedure and an improved capability to identify limitations of concurring conceptual models to a level that would be impossible relying only on streamflow data. This allowed for a better identification of model parameters and a more reliable description of the controlling hydrological processes, with a significant reduction of uncertainty in water storage dynamics with respect to the case when only streamflow data are used. This article is protected by copyright. All rights reserved.
- Influence of injection mode on transport properties in
kilometer‐scale three‐dimensional discrete fracture networks
- Authors: J. D. Hyman; S. L. Painter, H. Viswanathan, N. Makedonska, S. Karra
Abstract: We investigate how the choice of injection mode impacts transport properties in kilometer‐scale three‐dimensional discrete fracture networks (DFN). The choice of injection mode, resident or flux‐weighted, is designed to mimic different physical phenomena. It has been hypothesized that solute plumes injected under resident conditions evolve to behave similarly to solutes injected under flux‐weighted conditions. Previously, computational limitations have prohibited the large scale simulations required to investigate this hypothesis. We investigate this hypothesis by using a high performance DFN suite, dfnWorks, to simulate flow in kilometer‐scale three‐dimensional DFNs based on fractured granite at the Forsmark site in Sweden, and adopt a Lagrangian approach to simulate transport therein. Results show that after traveling through a pre‐equilibrium region both injection methods exhibit linear scaling of the first moment of travel time and power law scaling of the breakthrough curve with similar exponents, slightly larger than two. The physical mechanisms behind this evolution appear to be the combination of in‐network channeling of mass into larger fractures, which offer reduced resistance to flow, and in‐fracture channeling, which results from the topology of the DFN. This article is protected by copyright. All rights reserved.
- Multivariate postprocessing techniques for probabilistic hydrological
- Authors: S. Hemri; D. Lisniak, B. Klein
Abstract: Hydrologic ensemble forecasts driven by atmospheric ensemble prediction systems need statistical post‐processing in order to account for systematic errors in terms of both location and spread. Runoff is an inherently multivariate process with typical events lasting from hours in case of floods to weeks or even months in case of droughts. This calls for multivariate post‐processing techniques that yield well calibrated forecasts in univariate terms and ensure a realistic temporal dependence structure at the same time. To this end, the univariate ensemble model output statistics (EMOS) post‐processing method is combined with two different copula approaches that ensure multivariate calibration throughout the entire forecast horizon. The domain of this study covers three sub‐catchments of the river Rhine that represent different sizes and hydrological regimes: the Upper Rhine up to the gauge Maxau, the river Moselle up to the gauge Trier, and the river Lahn up to the gauge Kalkofen. In this study the two approaches to model the temporal dependence structure are ensemble copula coupling (ECC), which preserves the dependence structure of the raw ensemble, and a Gaussian copula approach (GCA), which estimates the temporal correlations from training observations. The results indicate that both methods are suitable for modelling the temporal dependencies of probabilistic hydrologic forecasts. This article is protected by copyright. All rights reserved.
- Semianalytical solutions for transport in aquifer and fractured clay
- Authors: Junqi Huang; Mark N. Goltz
Abstract: A three‐dimensional mathematical model that describes transport of contaminant in a horizontal aquifer with simultaneous diffusion into a fractured clay formation is proposed. A group of semi‐analytical solutions is derived based on specific initial and boundary conditions as well as various source functions. The analytical model solutions are evaluated by numerical Laplace inverse transformation and analytical Fourier inverse transformation. The model solutions can be used to study the fate and transport in a three‐dimensional spatial domain in which a non‐aqueous phase liquid exists as a pool atop a fractured low permeability clay layer. The non‐aqueous phase liquid gradually dissolves into the groundwater flowing past the pool, while simultaneously diffusing into the fractured clay formation below the aquifer. Mass transfer of the contaminant into the clay formation is demonstrated to be significantly enhanced by the existence of the fractures, even though the volume of fractures is relatively small compared to the volume of the clay matrix. The model solution is a useful tool in assessing contaminant attenuation processes in a confined aquifer underlain by a fractured clay formation. This article is protected by copyright. All rights reserved.
- Hydrological partitioning in the critical zone: Recent advances and
opportunities for developing transferrable understanding of water cycle
- Authors: Paul D. Brooks; Jon Chorover, Ying Fan, Sarah E. Godsey, Reed M. Maxwell, James P. McNamara, Christina Tague
Abstract: Hydrology is an integrative discipline linking the broad array of water‐related research with physical, ecological, and social sciences. The increasing breadth of hydrological research, often where subdisciplines of hydrology partner with related sciences, reflects the central importance of water to environmental science, while highlighting the fractured nature of the discipline itself. This lack of coordination among hydrologic subdisciplines has hindered the development of hydrologic theory and integrated models capable of predicting hydrologic partitioning across time and space. The recent development of the concept of the critical zone (CZ), an open system extending from the top of the canopy to the base of groundwater, brings together multiple hydrological subdisciplines with related physical and ecological sciences. Observations obtained by CZ researchers provide a diverse range of complementary process and structural data to evaluate both conceptual and numerical models. Consequently, a cross‐site focus on “critical zone hydrology” has potential to advance the discipline of hydrology and to facilitate the transition of CZ observatories into a research network with immediate societal relevance.
Here we review recent work in catchment hydrology and hydrochemistry, hydrogeology, and ecohydrology that highlights a common knowledge gap in how precipitation is partitioned in the critical zone: “how is the amount, routing, and residence time of water in the subsurface related to the biogeophysical structure of the CZ?” Addressing this question will require coordination among hydrologic subdisciplines and interfacing sciences, and catalyze rapid progress in understanding current CZ structure and predicting how climate and land cover changes will affect hydrologic partitioning. This article is protected by copyright. All rights reserved.
- Simulation of yearly rainfall time series at microscale resolution with
actual properties: Intermittency, scale invariance, and rainfall
- Abstract: Rainfall is a physical phenomenon resulting from the combination of numerous physical processes involving a wide range of scales, from microphysical processes to the general circulation of the atmosphere. Moreover unlike other geophysical variables such as water vapour concentration, rainfall is characterized by a relaxation behavior that leads to an alternation of wet and dry periods. It follows that rainfall is a complex process which is highly variable both in time and space. Precipitation is thus characterized by the following features: rain/no‐rain intermittency, multiple scaling regimes and extreme events. All these properties are difficult to model simultaneously, especially when a large time and/or space scale domain is required. The aim of this paper is to develop a simulator capable of generating high resolution rain‐rate time series (15 seconds), the main statistical properties of which are close to an observed rain‐rate time series. We also attempt to develop a model having consistent properties even when the fine resolution simulated time series are aggregated to a coarser resolution. In order to break the simulation problem down into sub‐components, the authors have focused their attention on several key properties of rainfall. The simulator is based on a sequential approach in which, firstly, the simulation of rain/no rain durations permits the retrieval of fractal properties of the rain support. Then, the generation of rain‐rates through the use of a multifractal, Fractionally Integrated Flux (FIF), model enables the restitution of the rainfall's multifractal properties. This second step includes a de‐normalization process that was added in order to generate realistic rain‐rate distributions. This article is protected by copyright. All rights reserved.
- Mixing effects on nitrogen and oxygen concentrations and the relationship
to mean residence time in a hyporheic zone of a riffle‐pool sequence
- Authors: Ramon C. Naranjo; Richard G. Niswonger, Clinton Davis
Abstract: Flow paths and residence times in the hyporheic zone are known to influence biogeochemical processes such as nitrification and denitrification. The exchange across the sediment‐water interface may involve mixing of surface water and groundwater through complex hyporheic flow paths that contribute to highly variable biogeochemically active zones. Despite the recognition of these patterns in the literature, conceptualization and analysis of flow paths and nitrogen transformations beneath riffle‐pool sequences often neglect to consider bed form driven exchange along the entire reach. In this study, the spatial and temporal distribution of dissolved oxygen (DO), nitrate (NO3‐) and ammonium (NH4+) were monitored in the hyporheic zone beneath a riffle‐pool sequence on a losing section of the Truckee River, NV. Spatially‐varying hyporheic exchange and the occurrence of multi‐scale hyporheic mixing cells are shown to influence concentrations of DO and NO3‐ and the mean residence time (MRT) of riffle and pool areas. Distinct patterns observed in piezometers are shown to be influenced by the first large flow event following a steady 8 month period of low flow conditions. Increases in surface water discharge resulted in reversed hydraulic gradients and production of nitrate through nitrification at small vertical spatial scales (0.10 to 0.25 m) beneath the sediment‐water interface. In areas with high downward flow rates and low MRT, denitrification may be limited. The use of a longitudinal two‐dimensional flow model helped identify important mechanisms such as multi‐scale hyporheic mixing cells and spatially varying MRT, an important driver for nitrogen transformation in the riverbed. Our observations of DO and NO3‐ concentrations and model simulations highlight the role of multi‐scale hyporheic mixing cells on MRT and nitrogen transformations in the hyporheic zone of riffle‐pool sequences. This article is protected by copyright. All rights reserved.
- The effect of lateral confinement on gravel bed river morphology
- Authors: G.A. Garcia Lugo; W. Bertoldi, A.J. Henshaw, A.M. Gurnell
Abstract: In this paper we use a physical modelling approach to explore the effect of lateral confinement on gravel bed river planform style, bed morphology, and sediment transport processes. A set of 27 runs was performed in a large flume (25 m long, 2.9 m wide), with constant longitudinal slope (0.01) and uniform grain size (1 mm), changing the water discharge (1.5 to 2.5 l/s) and the channel width (0.15 m to 1.5 m) to model a wide range of channel configurations, from narrow, straight, embanked channels to wide braided networks. The outcomes of each run were characterized by a detailed digital elevation model describing channel morphology, a map of dry areas and areas actively transporting sediment within the channel, and continuous monitoring of the amount of sediment transported through the flume outlet. Analysis reveals strong relationships between unit stream power and parameters describing the channel morphology. In particular, a smooth transition is observed between narrow channels with an almost rectangular cross section profile (with sediment transport occurring across the entire channel width) and complex braided networks where only a limited proportion (30%) of the bed is active. This transition is captured by descriptors of the bed elevation frequency distribution, e.g. standard deviation, skewness and kurtosis. These summary statistics represent potentially useful indicators of bed morphology that are compared with other commonly used summary indicators such as the braiding index and the type and number of bars. This article is protected by copyright. All rights reserved.
- Time scale interactions and the coevolution of humans and water
- Abstract: We present a co‐evolutionary view of hydrologic systems, revolving around feedbacks between environmental and social processes operating across different time scales. This brings to the fore an emphasis on emergent phenomena in changing water systems, such as the levee effect, adaptation to change, system lock‐in, and system collapse due to resource depletion. Changing human values play a key role in the emergence of these phenomena and should therefore be considered as internal to the system. Guidance is provided for the framing and modeling of these phenomena to test alternative hypotheses about how they arose. A plurality of co‐evolutionary models, from stylized to comprehensive system‐of‐system models, may assist strategic water management for long time scales through facilitating stakeholder participation, exploring the possibility space of alternative futures, and helping to synthesize the observed dynamics in a wide range of case studies. Future research opportunities lie in exploring emergent phenomena arising from time scale interactions through historical, comparative and process studies of human‐water feedbacks. This article is protected by copyright. All rights reserved.
- The impact of transitions between two and three‐fluid phases on
fluid configuration and fluid‐fluid interfacial area in porous media
- Authors: Kenneth C. Carroll; Kieran McDonald, Justin Marble, Ann E. Russo, Mark L. Brusseau
Abstract: Multiphase‐fluid distribution and flow is inherent in numerous areas of hydrology. Yet, pore‐scale characterization of transitions between two and three immiscible‐fluids is limited. The objective of this study was to examine the impact of such transitions on the pore‐scale configuration of organic liquid in a multi‐fluid system comprising natural porous media. Three‐dimensional images of an organic liquid (trichloroethene) in two‐phase (organic‐liquid/water) and three‐phase (air/organic‐liquid/water) systems were obtained using X‐ray microtomography before and after drainage and imbibition. Upon transition from a two‐phase to a three‐phase system, a significant portion of the organic liquid (intermediate wetting fluid) was observed to exist as lenses and films in contact with air (nonwetting fluid). In these cases, the air was either encased by or contiguous to the organic liquid. The presence of air resulted in an increase in the surface‐area‐to‐volume ratios for the organic‐liquid blobs. Upon imbibition, the air was displaced downgradient, and concomitantly, the morphology of the organic‐liquid blobs no longer in contact with air reverted to that characteristic of a two‐phase distribution (i.e., more spherical blobs and ganglia). This change in morphology resulted in a reduction in the surface‐area‐to‐volume ratio. These results illustrate the impact of transitions between two‐phase and three‐phase conditions on fluid configuration, and they demonstrate the malleable nature of fluid configuration under dynamic, multiphase‐flow conditions. The results have implications for characterizing and modeling pore‐scale flow and mass‐transfer processes. This article is protected by copyright. All rights reserved.
- Self‐affinity and surface‐area‐dependent fluctuations of
lake‐level time series
- Authors: Zachary C. Williams; Jon D. Pelletier
Abstract: We performed power‐spectral analyses on 133 globally distributed lake‐level time series after removing annual variability. Lake‐level power spectra are found to be power‐law functions of frequency over the range of 20 days‐1 to 27 years‐1, suggesting that lake levels are globally a f‐β‐type noise. The spectral exponent (β), i.e. the best‐fit slope of the logarithm of the power spectrum to the logarithm of frequency, is a nonlinear function of lake surface area, indicating that lake size is an important control on the magnitude of water‐level variability over the range of time scales we considered. A simple cellular model for lake‐level fluctuations that reproduces the observed spectral‐scaling properties is presented. The model (an adaptation of a surface‐growth model with random deposition and relaxation) is based on the equations governing flow in an unconfined aquifer with stochastic inputs and outputs of water (e.g. random storms). The agreement between observation and simulation suggests that lake surface area, spatio‐temporal stochastic forcing, and diffusion of the groundwater table are the primary factors controlling lake water‐level variability in natural (unmanaged) lakes. Water‐level variability is generally considered to be a manifestation of climate trends or climate change, yet our work shows that an input with short or no memory (i.e. weather) gives rise to a long‐memory non‐stationary output (lake water‐level). This work forms the basis for a null hypothesis of lake water‐level variability that should be disproven before water‐level trends are to be attributed to climate. This article is protected by copyright. All rights reserved.
- Natural length scales define the range of applicability of the Richards
equation for capillary flows
- Authors: Dani Or; Peter Lehmann, Shmuel Assouline
Abstract: The rapid expansion of remotely‐sensed spatial information and enhanced computational capabilities fuel increasing scientific and public expectations for reliable hydrologic predictions across time and spatial scales. Process‐based hydrologic models often rely on the Richards equation (RE) formalism to represent unsaturated flow processes at different scales which raise the much debated question: does the underlying physics in the RE formulation apply at large scales of practical interest? The study analyses recent findings from various unsaturated flow processes (soil evaporation, internal redistribution, and capillary flow from point sources) revealing inherent characteristic length scales that delineate the range of applicability of the RE. These length scales reflect the role of intrinsic porous medium properties that shape liquid phase continuity and interplay of forces that drive and resist unsaturated flow. The study revisits some of the key assumptions in the RE and their ramifications for numerical discretization. An intrinsic length scale for hydraulic continuity deduced from pore size distribution has been shown to control soil evaporation dynamics (i.e., stage 1 to stage 2 transition), to provide upper bounds for regional evaporative losses, and governs the dynamics of internal redistribution towards field capacity. For large scale hydrologic applications, we show that the extent of lateral flow interactions under most natural capillary gradients rarely exceed a few meters. The study provides a framework for guiding numerical and mathematical models for capillary flows across different scales considering the conditions for coexistence of stationarity, hydraulic continuity and capillary gradients ‐ essential ingredients for physically‐consistent application of the RE. This article is protected by copyright. All rights reserved.
- Human‐impacted waters: New perspectives from global high resolution
- Authors: Serena Ceola; Francesco Laio, Alberto Montanari
Abstract: The human presence close to streams and rivers is known to have consistently increased worldwide, therefore introducing dramatic anthropogenic and environmental changes. However, a spatio‐temporal detailed analysis is missing to date. In this paper, we propose a novel method to quantify the temporal evolution and the spatial distribution of the anthropogenic presence along streams and rivers and in their immediate proximity at the global scale and at a high spatial resolution (i.e., nearly 1 km at the equator). We use satellite images of nocturnal lights, available as yearly snapshots from 1992 to 2013, and identify five distinct distance classes from the river network position. Our results show a temporal enhancement of human presence across the considered distance classes. In particular, we observed a higher human concentration in the vicinity of the river network, even though the frequency distribution of human beings in space has not significantly changed in the last two decades. Our results prove that fine scale remotely sensed data, as nightlights, may provide new perspectives in water science, improving our understanding of the human impact on water resources and water‐related environments. This article is protected by copyright. All rights reserved.
- Impact of multicomponent ionic transport on pH fronts propagation in
saturated porous media
- Authors: Muhammad Muniruzzaman; Massimo Rolle
Abstract: We investigate the propagation of pH fronts during multicomponent ionic transport in saturated porous media under flow‐through conditions. By performing laboratory bench‐scale experiments combined with numerical modeling we show the important influence of Coulombic effects on proton transport in the presence of ionic admixtures. The experiments were performed in a quasi two‐dimensional flow‐through setup under steady‐state flow and transport conditions. Dilute solutions of hydrochloric acid with MgCl2 (1:2 strong electrolyte) were used as tracer solutions to experimentally test the effect of electrochemical cross‐coupling on the migration of diffusive/dispersive pH fronts. We focus on two experimental scenarios, with different composition of tracer solutions, causing remarkably different effects on the propagation of the acidic fronts with relative differences in the penetration depth of pH fronts of 36% between the two scenarios and of 25% and 15% for each scenario with respect to the transport of ions at liberated state (i.e., without considering the charge effects). Also differences in the dilution of the distinct ions plumes up to 28% and 45% in experiment 1 and 2, respectively, were measured at the outflow of the flow‐through system. The dilution of the pH plumes also changed considerably (26% relative difference) in the two flow‐through experiments only due to the different composition of the pore water solution and to the electrostatic coupling of the ions in the flow‐through setups. Numerical transport simulations were performed to interpret the laboratory experiments. The simulations were based on a multicomponent ionic formulation accurately capturing the Coulombic interactions between the transported ions in the flow‐through system. The results of purely forward simulations show a very good agreement with the high‐resolution measurements performed at the outlet of the flow‐through setup and confirms the importance of charge effects on pH transport in porous media. This article is protected by copyright. All rights reserved.
- Supraglacial channel inception: Modeling and processes
- Authors: E. Mantelli; C. Camporeale, L. Ridolfi
Abstract: Supraglacial drainage systems play a key role in glacial hydrology. Nevertheless, physical processes leading to spatial organization in supraglacial networks are still an open issue. In the present work we address from a quantitative point of view the question of what is the physics leading to widely observed patterns made up of evenly spaced channels. To this aim, we set up a novel mathematical model describing a condition antecedent channel formation, i.e. the down–glacier flow of a distributed meltwater film, and perform a linear stability analysis to assess whether the ice–water interface undergoes a morphological instability compatible with observed patterns. The instability is detected, its features depending on glacier surface slope, friction factor and water as well as ice thermal conditions. By contrast, in our model channel spacing is solely hydrodynamically driven and relies on the interplay between pressure perturbations, flow‐depth response and Reynolds stresses. Geometrical features of the predicted pattern are quantitatively consistent with available field data. The hydrodynamic origin of supraglacial channel morphogenesis suggests that alluvial patterns might share the same physical controls. This article is protected by copyright. All rights reserved.
- A formulation for vertically integrated groundwater flow in a stratified
- Authors: O.D.L. Strack; B.K. Ausk
Abstract: We present the comprehensive discharge potential for steady three‐dimensional flow in horizontally stratified coastal aquifers with a horizontal base and a vertical coastline. The gradient of this comprehensive potential gives the vertically integrated discharge throughout the aquifer, i.e., the specific discharge vector as a function of three‐dimensional space integrated over the saturated portion of the aquifer. The boundary values of the comprehensive potential along the coast can be computed precisely, given the geometry of the aquifer: the hydraulic conductivities of the strata, the elevations of the horizontal planes that separate the strata, and the elevation of the impermeable base of the aquifer relative to sea level. Boundary conditions of the comprehensive potential may either be given in terms of its gradient, or computed from given heads along the boundaries. The governing equation of the comprehensive potential is the Poisson equation in areas of infiltration and the Laplace equation elsewhere. The computation of interface elevations, piezometric heads, and the vertical distribution of flow, requires that an assumption be made regarding the relation between the comprehensive potential and piezometric heads. We adopt the Dupuit‐Forchheimer approximation for this purpose and make use of the Ghyben‐Herzberg equation. We present several applications of the approach, and find that the stratification may have a significant effect on the boundary value of the comprehensive potential, and thus on the flow rates in the aquifer. This article is protected by copyright. All rights reserved.
- Hydrologic issues associated with nuclear waste repositories
- Abstract: Significant progress in hydrology, especially in subsurface flow and solute transport, has been made over the last 35 years because of sustained interest in underground nuclear waste repositories. The present paper provides an overview of the key hydrologic issues involved, and to highlight advances in their understanding and treatment because of these efforts. The focus is not on the development of radioactive waste repositories and their safety assessment, but instead on the advances in hydrologic science that have emerged from such studies. Work and results associated with three rock types which are being considered to host the repositories, are reviewed, with a different emphasis for each rock type. The first rock type is fractured crystalline rock, for which the discussion will be mainly on flow and transport in saturated fractured rock. The second rock type is unsaturated tuff, for which the emphasis will be on flow from the shallow subsurface through the unsaturated zone to the repository. The third rock type is clay‐rich formations, whose permeability is very low in an undisturbed state. In this case, the emphasis will be on hydrologic issues that arise from mechanical and thermal disturbances; i.e., on the relevant coupled thermo‐hydro‐mechanical processes. The extensive research results, especially those from multi‐year large‐scale underground research laboratory investigations, represent a rich body of information and data that can form the basis for further development in the related areas of hydrologic research. This article is protected by copyright. All rights reserved.
- Status of CO2 storage in deep saline aquifers with emphasis on modeling
approaches and practical simulations
- Authors: M. A. Celia; S. Bachu, J. M. Nordbotten, K. W. Bandilla
Abstract: Carbon capture and storage (CCS) is the only viable technology to mitigate carbon emissions while allowing continued large‐scale use of fossil fuels. The storage part of CCS involves injection of carbon dioxide, captured from large stationary sources, into deep geological formations. Deep saline aquifers have the largest identified storage potential, with estimated storage capacity sufficient to store emissions from large stationary sources for at least a century. This makes CCS a potentially important bridging technology in the transition to carbon‐free energy sources. Injection of CO2 into deep saline aquifers leads to a multi‐component, multi‐phase flow system, in which geomechanics, geochemistry, and non‐isothermal effects may be important. While the general system can be highly complex and involve many coupled, nonlinear partial differential equations, the underlying physics can sometimes lead to important simplifications. For example, the large density difference between injected CO2 and brine may lead to relatively fast buoyant segregation, making an assumption of vertical equilibrium reasonable. Such simplifying assumptions lead to a range of simplified governing equations whose solutions have provided significant practical insights into system behavior, including improved estimates of storage capacity, easy‐to‐compute estimates of CO2 spatial migration and pressure response, and quantitative estimates of leakage risk. When these modeling studies are coupled with observations from well‐characterized injection operations, understanding of the overall system behavior is enhanced significantly. This improved understanding shows that, while economic and policy challenges remain, CO2 storage in deep saline aquifers appears to be a viable technology and can contribute substantially to climate change solutions. This article is protected by copyright. All rights reserved.
- Group‐sparsity regularization for ill‐posed subsurface flow
- Abstract: Sparse representations provide a flexible and parsimonious description of high‐dimensional model parameters for reconstructing subsurface flow property distributions from limited data. To further constrain ill‐posed inverse problems, group‐sparsity regularization can take advantage of possible relations among the entries of unknown sparse parameters when: (i) groups of sparse elements are either collectively active or inactive; and (ii) only a small subset of the groups is needed to approximate the parameters of interest. Since subsurface properties exhibit strong spatial connectivity patterns, they may lead to sparse descriptions that satisfy the above conditions. When these conditions are established a group‐sparsity regularization can be invoked to facilitate the solution of the resulting inverse problem by promoting sparsity across the groups (and not within each group). The proposed regularization penalizes the number of groups that are active without promoting sparsity within each group. Two implementations are presented in this paper: one based on the multi‐resolution tree structure of Wavelet decomposition, without a need for explicit prior models, and another learned from explicit prior model realizations using sparse principal component analysis (SPCA). In each case, the approach first classifies the parameters of the inverse problem into groups with specific connectivity features, and then takes advantage of the grouped structure to recover the relevant patterns in the solution from the flow data. Several numerical experiments are presented to demonstrate the advantages of additional constraining due to group‐sparsity in solving ill‐posed subsurface model calibration problems. This article is protected by copyright. All rights reserved.
- Physically based modeling in catchment hydrology at 50: Survey and outlook
- Authors: Claudio Paniconi; Mario Putti
Abstract: Integrated, process‐based numerical models in hydrology are rapidly evolving, spurred by novel theories in mathematical physics, advances in computational methods, insights from laboratory and field experiments, and the need to better understand and predict the potential impacts of population, land use, and climate change on our water resources. At the catchment scale, these simulation models are commonly based on conservation principles for surface and subsurface water flow and solute transport (e.g., the Richards, shallow water, and advection‐dispersion equations), and they require robust numerical techniques for their resolution. Traditional (and still open) challenges in developing reliable and efficient models are associated with heterogeneity and variability in parameters and state variables; nonlinearities and scale effects in process dynamics; and complex or poorly known boundary conditions and initial system states. As catchment modeling enters a highly interdisciplinary era, new challenges arise from the need to maintain physical and numerical consistency in the description of multiple processes that interact over a range of scales and across different compartments of an overall system. This paper first gives an historical overview (past 50 years) of some of the key developments in physically‐based hydrological modeling, emphasizing how the interplay between theory, experiments, and modeling has contributed to advancing the state of the art. The second part of the paper examines some outstanding problems in integrated catchment modeling from the perspective of recent developments in mathematical and computational science. This article is protected by copyright. All rights reserved.
- Bayesian‐information‐gap decision theory with an application
to CO2 sequestration
- Authors: D. O'Malley; V.V. Vesselinov
Abstract: Decisions related to subsurface engineering problems such as groundwater management, fossil fuel production, and geologic carbon sequestration are frequently challenging because of an overabundance of uncertainties (related to conceptualizations, parameters, observations, etc.). Because of the importance of these problems to agriculture, energy, and the climate (respectively), good decisions that are scientifically defensible must be made despite the uncertainties.We describe a general approach to making decisions for challenging problems such as these in the presence of severe uncertainties that combines probabilistic and non‐probabilistic methods. The approach uses Bayesian sampling to assess parametric uncertainty and Information‐Gap Decision Theory (IGDT) to address model inadequacy. The combined approach also resolves an issue that frequently arises when applying Bayesian methods to real‐world engineering problems related to the enumeration of possible outcomes. In the case of zero non‐probabilistic uncertainty, the method reduces to a Bayesian method. To illustrate the approach, we apply it to a site‐selection decision for geologic CO2 sequestration. This article is protected by copyright. All rights reserved.
- Stochastic modeling of solute transport in aquifers: From heterogeneity
characterization to risk analysis
- Authors: A. Fiori; A. Bellin, V. Cvetkovic, F.P.J. de Barros, G. Dagan
Abstract: The article presents a few recent developments advanced by the authors in a few key areas of stochastic modeling of solute transport in heterogeneous aquifers. First, a brief review of the Lagrangean approach to modeling plumes longitudinal mass distribution and temporal (the breakthrough curve) mass arrival, is presented. Subsequently, transport in highly heterogeneous aquifers is analyzed by using a recently developed predictive model. It relates the non‐Gaussian BTC to the permeability univariate pdf and integral scale, with application to the MADE field observations. Next, the approach is extended to transport of reactive solute, combinnig the effects of the random velocity field and multirate mass transfer on the BTC, with application to mass attenuation. The following topic is modeling of the local concentration field as affected by mixing and dilution due to pore scale dispersion. The results are applied to the analysis of concentration measurements at the Cape Cod field experiment. The last section incorporates the results of the preceding ones in health risk assesment by analyzing the impact of concentration prediction on risk uncertainty. It is illustrated by assesing the effect of identification of macrodispersivity from field characterization and transport modeling, upon the probability of health risk. This article is protected by copyright. All rights reserved.
- Hydraulic Fracturing Fluid Migration in the Subsurface: A Review and
- Authors: Daniel T. Birdsell; Harihar Rajaram, David Dempsey, Hari Viswanathan
Abstract: Understanding the transport of hydraulic fracturing (HF) fluid that is injected into the deep subsurface for shale gas extraction is important to ensure that shallow drinking water aquifers are not contaminated. Topographically driven flow, overpressured shale reservoirs, permeable pathways such as faults or leaky wellbores, the increased formation pressure due to HF fluid injection, and the density contrast of the HF fluid to the surrounding brine can encourage upward HF fluid migration. In contrast, the very low shale permeability and capillary imbibition of water into partially saturated shale may sequester much of the HF fluid, and well production will remove HF fluid from the subsurface. We review the literature on important aspects of HF fluid migration. Single‐phase flow and transport simulations are performed to quantify how much HF fluid is removed via the wellbore with flowback and produced water, how much reaches overlying aquifers, and how much is permanently sequestered by capillary imbibition, which is treated as a sink term based on a semi‐analytical, one‐dimensional solution for two‐phase flow. These simulations include all of the important aspects of HF fluid migration identified in the literature review and are performed in 5 stages to faithfully represent the typical operation of a hydraulically fractured well. No fracturing fluid reaches the aquifer without a permeable pathway. In the presence of a permeable pathway, ten times more fracturing fluid reaches the aquifer if well production and capillary imbibition are not included in the model. This article is protected by copyright. All rights reserved.
- Potential for real‐time understanding of coupled hydrologic and
biogeochemical processes in stream ecosystems: Future integration of
telemetered data with process models for glacial meltwater streams
- Authors: Diane M McKnight; Karen Cozzetto, James D. S. Cullis, Michael N Gooseff, Christopher Jaros, Joshua C Koch, William B Lyons, Roseanna Neupauer, Adam Wlostowski
Abstract: While continuous monitoring of stream flow and temperature has been common for some time, there is great potential to expand continuous monitoring to include water quality parameters such as nutrients, turbidity, oxygen and dissolved organic material. In many systems distinguishing between watershed and stream ecosystem controls can be challenging. The usefulness of such monitoring can be enhanced by application of quantitative models to interpret observed patterns in real time. Examples are discussed primarily from the glacial meltwater streams of the McMurdo Dry Valleys, Antarctica. Although the dry valley landscape is barren of plants, many streams harbor thriving cyanobacterial mats. Whereas a daily cycle of stream flow is controlled by the surface energy balance on the glaciers and the temporal pattern of solar exposure, the daily signal for biogeochemical processes controlling water quality is generated along the stream. These features result in an excellent outdoor laboratory for investigating fundamental ecosystem process and the development and validation of process based models. As part of the McMurdo Dry Valleys Long Term Ecological Research project, we have conducted field experiments and developed coupled biogeochemical transport models for the role of hyporheic exchange in controlling weathering reactions, microbial nitrogen cycling, and stream temperature regulation. We have adapted modeling approaches from sediment transport to understand mobilization of stream biomass with increasing flows. These models help to elucidate the role of in‐stream processes in systems where watershed processes also contribute to observed patterns, and may serve as a test case for applying real‐time stream ecosystem models. This article is protected by copyright. All rights reserved.
- Effective parameterizations of three nonwetting phase relative
- Authors: Zhenlei Yang; Binayak P. Mohanty
Abstract: Describing convective nonwetting phase flow in unsaturated porous media requires knowledge of the nonwetting phase relative permeability. This study was conducted to formulate and derive a generalized expression for the nonwetting phase relative permeability via combining with the Kosugi water retention function. This generalized formulation is then used to flexibly investigate the Burdine, Mualem and Alexander and Skaggs models' prediction accuracy for relative nonwetting phase permeability. The model and data comparison results show that these three permeability models, if used in their original form, but applied to the nonwetting phase, could not predict the experimental data well. The optimum pore tortuosity and connectivity value is thus obtained for the improved prediction of relative nonwetting phase permeability. As a result, the effective parametrization of (α,β,η) parameters in the modified Burdine, modified Mualem and modified Alexander and Skaggs permeability models were found to be (2.5, 2, 1), (2, 1, 2) and (2.5, 1, 1), respectively. These three suggested models display the highest accuracy among the nine relative permeability models investigated in this study. However, the corresponding discontinuous nonwetting phase and the liquid film flow should be accounted for in future for the improved prediction of nonwetting phase relative permeability at very high and very low water saturation range, respectively. This article is protected by copyright. All rights reserved.
- A review of surrogate models and their application to groundwater modeling
- Authors: M. J. Asher; B. F. W. Croke, A. J. Jakeman, L. J. M. Peeters
Abstract: The spatially and temporally variable parameters and inputs to complex groundwater models typically result in long runtimes which hinder comprehensive calibration, sensitivity and uncertainty analysis. Surrogate modeling aims to provide a simpler, and hence faster, model which emulates the specified output of a more complex model in function of its inputs and parameters.
In this review paper, we summarize surrogate modeling techniques in three categories: data‐driven, projection, and hierarchical‐based approaches. Data‐driven surrogates approximate a groundwater model through an empirical model that captures the input‐output mapping of the original model. Projection based models reduce the dimensionality of the parameter space by projecting the governing equations onto a basis of orthonormal vectors. In hierarchical or multi‐fidelity methods the surrogate is created by simplifying the representation of the physical system, such as by ignoring certain processes, or reducing the numerical resolution.
In discussing the application to groundwater modeling of these methods, we note several imbalances in the existing literature: a large body of work on data‐driven approaches seemingly ignores major drawbacks to the methods; only a fraction of the literature focuses on creating surrogates to reproduce outputs of fully distributed groundwater models, despite these being ubiquitous in practice; and a number of the more advanced surrogate modeling methods are yet to be fully applied in a groundwater modeling context. This article is protected by copyright. All rights reserved.
- Video observations of bed form morphodynamics in a meander bend
- Authors: Margaret L. Palmsten; Jessica L. Kozarek, Joseph Calantoni
Abstract: A new optical remote sensing technique for estimating water depth from an oblique camera view is described. The water surface and the bed were imaged simultaneously to create time‐dependent maps of the water surface velocities and the bed elevations that can be used to validate numerical models at high spatial and temporal resolution. The technique was applied in a sandy meander bend at the University of Minnesota Saint Anthony Falls Laboratory Outdoor StreamLab. The root mean square differences between optical estimates of the bed and in situ observations ranged between 0.01 and 0.03 m. Mean bedform wavelength was 0.73 m and mean crest height was 0.07 m, but both varied with distance around the meander bend. Bedform classification varied with distance downstream, and sinuosity of bedforms varied with local radius of curvature. Bedform roughness scaled similarly to other natural riverine environments although wavelength and height magnitude and variability were larger than predicted by empirical formulations for straight reaches. Bedform translation rate varied between 1 and 5 mm s−1. Estimates of velocity from particle image velocimetry (PIV) on the water surface were ∼10% higher than in situ observations collected ∼0.05 m below the water surface. Using the PIV observations to drive simple equations for bedload sediment flux, we explained up to 72% of the observed variance in downstream sediment flux. The new methodology described here provides non‐intrusive, high spatial and temporal resolution measurements of both the bed and the flow. This article is protected by copyright. All rights reserved.
- Hierarchical Bayesian clustering for nonstationary flood frequency
analysis: Application to trends of annual maximum flow in Germany
- Authors: Xun Sun; Upmanu Lall, Bruno Merz, Nguyen Viet Dung
Abstract: Especially for extreme precipitation or floods, there is considerable spatial and temporal variability in long term trends or in the response of station time series to large scale climate indices. Consequently, identifying trends or sensitivity of these extremes to climate parameters can be marked by high uncertainty. When one develops a nonstationary frequency analysis model, a key step is the identification of potential trends or effects of climate indices on the station series. An automatic clustering procedure that effectively pools stations where there are similar responses is desirable to reduce the estimation variance, thus improving the identification of trends or responses, and accounting for spatial dependence. This paper presents a new hierarchical Bayesian approach for exploring homogeneity of response in large area data sets, through a multi‐component mixture model. The approach allows the reduction of uncertainties through both full pooling and partial pooling of stations across automatically chosen subsets of the data. We apply the model to study the trends in annual maximum daily stream flow at 68 gauges over Germany. The effects of changing the number of clusters and the parameters used for clustering are demonstrated. The results show that there are large, mainly upward trends in the gauges of the River Rhine Basin in Western Germany and along the main stream of the Danube River in the south, while there are also some small upward trends at gauges in Central and Northern Germany. This article is protected by copyright. All rights reserved.
- Evaluating the relationship between topography and groundwater using
outputs from a continental‐scale integrated hydrology model
- Authors: Laura E. Condon; Reed M. Maxwell
Abstract: We study the influence of topography on groundwater fluxes and water table depths across the Contiguous United States (CONUS). Groundwater tables are often conceptualized as subdued replicas of topography. While it is well known that groundwater configuration is also controlled by geology and climate, nonlinear interactions between these drivers within large real world systems are not well understood and are difficult to characterize given sparse groundwater observations. We address this limitation using the fully integrated physical hydrology model ParFlow to directly simulate groundwater fluxes and water table depths within a complex heterogeneous domain that incorporates all three primary groundwater drivers. Analysis is based on a first of its kind, continental scale, high‐resolution (1km), groundwater‐surface water simulation spanning more than 6.3 million km2. Results show that groundwater fluxes are most strongly driven by topographic gradients (as opposed to gradients in pressure head) in humid regions with small topographic gradients or low conductivity. These regions are generally consistent with the topographically controlled groundwater regions identified in previous studies. However, we also show that areas where topographic slopes drive groundwater flux do not generally have strong correlations between water table depth and elevation. Nonlinear relationships between topography and water table depth are consistent with groundwater flow systems that are dominated by local convergence and could also be influenced by local variability in geology and climate. One of the strengths of the numerical modeling approach is its ability to evaluate continental scale groundwater behavior at a high resolution not possible with other techniques. This article is protected by copyright. All rights reserved.
- A well‐balanced FV scheme for compound channels with complex
geometry and movable bed
- Authors: L. Minatti
Abstract: This work focuses on the implementation of a Shallow Water‐Exner model for compound natural channels with complex geometry and movable bed within the finite volume framework.
The model is devised for compound channels modeling: cross‐section overbanks are treated with fixed bed conditions, while the main channel is left free to modify its morphology. A capacitive approach is used for bedload transport modeling, in which the solid flow rates are estimated with bedload transport formulas.
The model equations pose some numerical issues in the case of natural channels, where bedload transport may occur for both subcritical and supercritical flows and geometry varies in space. An explicit path‐conservative scheme, designed to overcome all these issues, is presented in the paper. The scheme solves liquid and solid phases dynamics in a coupled manner, in order to correctly model near critical currents/channel interactions and is well‐balanced, that is able to properly reproduce steady states. The Roe and Osher Riemann solvers are implemented, so as to take into account the spatial geometry variations of natural channels. The scheme reaches up to 2nd order accuracy.
Validation is performed with fixed and movable bed test cases whose analytical solution is known, and with flume experimental data. An application of the model to a real case study is also shown. This article is protected by copyright. All rights reserved.
- River corridor science: Hydrologic exchange and ecological consequences
from bed forms to basins
- Authors: Jud Harvey; Michael Gooseff
Abstract: Previously regarded as the passive drains of watersheds, over the past 50 years, rivers have progressively been recognized as being actively connected with off channel environments. These connections prolong physical storage and enhance reactive processing to alter water chemistry and downstream transport of materials and energy. Here we propose river corridor science as a concept that integrates downstream transport with lateral and vertical exchange across interfaces. Thus the river corridor, rather than the wetted river channel itself, is an increasingly common unit of study. Main channel exchange with recirculating marginal waters, hyporheic exchange, bank storage, and overbank flow onto floodplains are all included under a broad continuum of interactions known as “hydrologic exchange flows”. Hydrologists, geomorphologists, geochemists, and aquatic and terrestrial ecologists are cooperating in studies that reveal the dynamic interactions among hydrologic exchange flows and consequences for water quality improvement, modulation of river metabolism, habitat provision for vegetation, fish, and wildlife, and other valued ecosystem services. The need for better integration of science and management is keenly felt, from testing effectiveness of stream restoration and riparian buffers all the way to reevaluating the definition of the waters of the United States to clarify the regulatory authority under the Clean Water Act. A major challenge for scientists is linking the small‐scale physical drivers with their larger scale fluvial and geomorphic context and ecological consequences. Although the fine scales of field and laboratory studies are best suited to identifying the fundamental physical and biological processes, that understanding must be successfully linked to cumulative effects at watershed to regional and continental scales. This article is protected by copyright. All rights reserved.
- In situ determination of surface relaxivities for unconsolidated sediments
- Authors: Markus Duschl; Petrik Galvosas, Timothy I. Brox, Andreas Pohlmeier, Harry Vereecken
Abstract: NMR relaxometry has developed into a method for rapid pore size determination of natural porous media. Nevertheless, it is prone to uncertainties because of unknown surface relaxivities which depend mainly on the chemical composition of the pore walls as well as on the interfacial dynamics of the pore fluid. The classical approach for the determination of surface relaxivities is the scaling of NMR relaxation times by surface to volume ratios measured by gas adsorption or mercury intrusion. However, it is preferable that a method for the determination of average pore sizes uses the same substance, water, as probe molecule for both relaxometry and surface to volume measurements. One should also ensure that in both experiments the dynamics of the probe molecule takes place on similar length scales, which are in the order of some microns. Therefore, we employed NMR diffusion measurements with different observation times using bipolar pulsed field gradients and applied them to unconsolidated sediments (two purified sands, two natural sands, and one soil). The evaluation by Mitra's short time model for diffusion in restricted environments yielded information about the surface to volume ratios which is independent of relaxation mechanisms. We point out that methods based on NMR diffusometry yield pore dimensions and surface relaxivities consistent with a pore space as sampled by native pore fluids via the diffusion process. This opens a way to calibrate NMR relaxation measurements with other NMR techniques, providing information about the pore size distribution of natural porous media directly from relaxometry. This article is protected by copyright. All rights reserved.
- Predicting permeability from the characteristic relaxation time and
intrinsic formation factor of complex conductivity spectra
- Authors: A. Revil; A. Binley, L. Mejus, P. Kessouri
Abstract: Low‐frequency quadrature conductivity spectra of siliclastic materials exhibit typically a characteristic relaxation time, which either corresponds to the peak frequency of the phase or the quadrature conductivity or a typical corner frequency, at which the quadrature conductivity starts to decrease rapidly towards lower frequencies. This characteristic relaxation time can be combined with the (intrinsic) formation factor and a diffusion coefficient to predict the permeability to flow of porous materials at saturation. The intrinsic formation factor can either be determined at several salinities using an electrical conductivity model or at a single salinity using a relationship between the surface and quadrature conductivities. The diffusion coefficient entering into the relationship between the permeability, the characteristic relaxation time and the formation factor, takes only two distinct values for isothermal conditions. For pure silica, the diffusion coefficient of cations, like sodium or potassium, in the Stern layer is equal to the diffusion coefficient of these ions in the bulk pore water, indicating weak sorption of these couterions. For clayey materials and clean sands and sandstones whose surface have been exposed to alumina (possibly iron), the diffusion coefficient of the cations in the Stern layer appears to be 350 times smaller than the diffusion coefficient of the same cations in the pore water. These values are consistent with the values of the ionic mobilities used to determine the amplitude of the low and high‐frequency quadrature conductivities and surface conductivity. The database used to test the model comprises a total of 202 samples. Our analysis reveals that permeability prediction with the proposed model is usually within an order of magnitude from the measured value above 0.1 mD. We also discuss the relationship between the different time constants that have been considered in previous works as characteristic relaxation time, including the mean relaxation time obtained from a Debye decomposition of the spectra and the Cole‐Cole time constant. This article is protected by copyright. All rights reserved.
- The science and practice of river restoration
- Authors: Ellen Wohl; Stuart N. Lane, Andrew C. Wilcox
Abstract: River restoration is one of the most prominent areas of applied water‐resources science. From an initial focus on enhancing fish habitat or river appearance, primarily through structural modification of channel form, restoration has expanded to incorporate a wide variety of management activities designed to enhance river process and form. Restoration is conducted on headwater streams, large lowland rivers, and entire river networks in urban, agricultural, and less intensively human‐altered environments. We critically examine how contemporary practitioners approach river restoration and challenges for implementing restoration, which include clearly identified objectives, holistic understanding of rivers as ecosystems, and the role of restoration as a social process. We also examine challenges for scientific understanding in river restoration. These include: how physical complexity supports biogeochemical function, stream metabolism, and stream ecosystem productivity; characterizing response curves of different river components; understanding sediment dynamics; and increasing appreciation of the importance of incorporating climate change considerations and resiliency into restoration planning. Finally, we examine changes in river restoration within the past decade, such as increasing use of stream mitigation banking; development of new tools and technologies; different types of process‐based restoration; growing recognition of the importance of biological‐physical feedbacks in rivers; increasing expectations of water quality improvements from restoration; and more effective communication between practitioners and river scientists. This article is protected by copyright. All rights reserved.
- Controls on the breach geometry and flood hydrograph during overtopping of
noncohesive earthen dams
- Authors: Joseph S. Walder; Richard M. Iverson, Jonathan W. Godt, Matthew Logan, Stephen A. Solovitz
Abstract: Overtopping failure of non‐cohesive earthen dams was investigated in 13 large‐scale experiments with dams built of compacted, damp, fine‐grained sand. Breaching was initiated by cutting a notch across the dam crest and allowing water escaping from a finite upstream reservoir to form its own channel. The channel developed a stepped profile, and upstream migration of the steps, which coalesced into a headcut, led to the establishment of hydraulic control (critical flow) at the channel head, or breach crest, an arcuate erosional feature that functions hydraulically as a weir. Novel photogrammetric methods, along with underwater videography, revealed that the retreating headcut maintained a slope near the angle of friction of the sand, while the cross section at the breach crest maintained a geometrically similar shape through time. That cross‐sectional shape was nearly unaffected by slope failures, contrary to the assumption in many models of dam breaching. Flood hydrographs were quite reproducible–for sets of dams ranging in height from 0.55 m to 0.98 m–when the time datum was chosen as the time that the migrating headcut intersected the breach crest. Peak discharge increased almost linearly as a function of initial dam height. Early‐time variability between flood hydrographs for nominally identical dams is probably a reflection of subtle experiment‐to‐experiment differences in groundwater hydrology and the interaction between surface water and groundwater. This article is protected by copyright. All rights reserved.
- Quantifying the impacts of climate change and ecological restoration on
streamflow changes based on a Budyko hydrological model in China's Loess
- Authors: Wei Liang; Dan Bai, Feiyu Wang, Bojie Fu, Junping Yan, Shuai Wang, Yuting Yang, Di Long, Minquan Feng
Abstract: Understanding hydrological effects of ecological restoration (ER) is fundamental to develop effective measures guiding future ER and to adapt climate change in China's Loess Plateau (LP). Streamflow (Q) is an important indicator of hydrological processes that represents the combined effects of climatic and land surface conditions. Here, fourteen catchments located in the LP were chosen to explore the Q response to different driving factors during the period 1961‐2009 by using elasticity and decomposition methods based on the Budyko framework. Our results show that: (1) annual Q exhibited a decreasing trend in all catchments (‐0.30 ∼ ‐1.71 mm y−2), with an average reduction of ‐0.87 mm y−2. The runoff coefficient in flood season and non‐flood season were both decreasing between two periods divided by the changing point in annual Q series; (2) the precipitation (P) and potential evapotranspiration (E0) elasticity of Q are 2.75 and ‐1.75, respectively, indicating that Q is more sensitive to changes in P than that in E0; (3) the two methods consistently demonstrated that, on average, ER (62%) contributing to Q reduction was much larger than that of climate change (38%). In addition, parameter n that entails catchment characteristics in the Budyko framework showed positive correlation with the relative area of ER measures in all catchments (eight of them are statistically significant with p
- Water balance‐based actual evapotranspiration reconstruction from
ground and satellite observations over the conterminous United States
- Authors: Zhanming Wan; Ke Zhang, Xianwu Xue, Zhen Hong, Yang Hong, Jonathan J. Gourley
Abstract: The objective of this study is to produce an observationally based monthly evapotranspiration (ET) product using the simple water balance equation across the conterminous United States (CONUS). We adopted the best quality ground‐ and satellite‐based observations of the water budget components, i.e., precipitation, runoff, and water storage change, while ET is computed as the residual. Precipitation data is provided by the bias‐corrected PRISM observation‐based precipitation dataset, while runoff comes from observed monthly streamflow values at 592 USGS stream gauging stations that have been screened by strict quality controls. We developed a land surface model‐based downscaling approach to disaggregate the monthly GRACE equivalent water thickness data to daily, 0.125º values. The derived ET computed as the residual from the water balance equation is evaluated against three sets of existing ET products. The similar spatial patterns and small differences between the reconstructed ET in this study and the other three products show the reliability of the observationally based approach. The new ET product and the disaggregated GRACE data provide a unique, important hydro‐meteorological data set that can be used to evaluate the other ET products as a benchmark dataset, assess recent hydrological and climatological changes, and terrestrial water and energy cycle dynamics across the CONUS. These products will also be valuable for studies and applications in drought assessment, water resources management, and climate change evaluation. This article is protected by copyright. All rights reserved.
- Improving the representation of hydrologic processes in Earth System
- Authors: Martyn P. Clark; Ying Fan, David M. Lawrence, Jennifer C. Adam, Diogo Bolster, David J. Gochis, Richard P. Hooper, Mukesh Kumar, L. Ruby Leung, D. Scott Mackay, Reed M. Maxwell, Chaopeng Shen, Sean C. Swenson, Xubin Zeng
Abstract: Many of the scientific and societal challenges in understanding and preparing for global environmental change rest upon our ability to understand and predict the water cycle change at large river basin, continent, and global scales. However, current large‐scale land models (as a component of Earth System Models, or ESMs) do not yet reflect the best hydrologic process understanding or utilize the large amount of hydrologic observations for model testing. This paper discusses the opportunities and key challenges to improve hydrologic process representations and benchmarking in ESM land models, suggesting that (1) land model development can benefit from recent advances in hydrology, both through incorporating key processes (e.g., groundwater‐surface interactions) and new approaches to describe multi‐scale spatial variability and hydrologic connectivity; (2) accelerating model advances requires comprehensive hydrologic benchmarking in order to systematically evaluate competing alternatives, understand model weaknesses, and prioritize model development needs, and (3) stronger collaboration is needed between the hydrology and ESM modeling communities, both through greater engagement of hydrologists in ESM land model development, and through rigorous evaluation of ESM hydrology performance in research watersheds or Critical Zone Observatories. Such coordinated efforts in advancing hydrology in ESMs have the potential to substantially impact energy, carbon and nutrient cycle prediction capabilities through the fundamental role hydrologic processes play in regulating these cycles. This article is protected by copyright. All rights reserved.
- Computationally inexpensive identification of noninformative model
parameters by sequential screening
- Abstract: Environmental models tend to require increasing computational time and resources as physical process descriptions are improved or new descriptions are incorporated. Many‐query applications such as sensitivity analysis or model calibration usually require a large number of model evaluations leading to high computational demand. This often limits the feasibility of rigorous analyses. Here we present a fully automated sequential screening method that selects only informative parameters for a given model output. The method requires a number of model evaluations that is approximately ten times the number of model parameters. It was tested using the mesoscale hydrologic model mHM in three hydrologically unique European river catchments. It identified around 20 informative parameters out of 52, with different informative parameters in each catchment. The screening method was evaluated with subsequent analyses using all 52 as well as only the informative parameters. Subsequent Sobol's global sensitivity analysis led to almost identical results yet required 40\% fewer model evaluations after screening. mHM was calibrated with all and with only informative parameters in the three catchments. Model performances for daily discharge were equally high in both cases with Nash‐Sutcliffe efficiencies above 0.82. Calibration using only the informative parameters needed just one third of the number of model evaluations. The universality of the sequential screening method was demonstrated using several general test functions from the literature. We therefore recommend the use of the computationally inexpensive sequential screening method prior to rigorous analyses on complex environmental models. This article is protected by copyright. All rights reserved.
- Charting unknown waters—On the role of surprise in flood risk
assessment and management
- Abstract: Unexpected incidents, failures and disasters are abundant in the history of flooding events. In this paper we introduce the metaphors of terra incognita and terra maligna to illustrate unknown and wicked flood situations, respectively. We argue that surprise is a neglected element in flood risk assessment and management. Two sources of surprise are identified: (1) the complexity of flood risk systems, represented by non‐linearities, interdependencies and non‐stationarities, and (2) cognitive biases in human perception and decision making. Flood risk assessment and management are particularly prone to cognitive biases due to the rarity and uniqueness of extremes, and the nature of human risk perception. We reflect on possible approaches to better understanding and reducing the potential for surprise and its adverse consequences which may be supported by conceptually charting maps that separate terra incognita from terra cognita, and terra maligna from terra benigna. We conclude that flood risk assessment and management should account for the potential for surprise and devastating consequences which will require a shift in thinking. This article is protected by copyright. All rights reserved.
- Integrating social and physical sciences in water management
- Authors: Jay R. Lund
Abstract: Water management has always required more than physical science. This paper reviews the accomplishments of integrating social with physical sciences for water management in the last 50 years. Particular successes are highlighted to illustrate how fundamentals from both physical science and social science have been brought together to improve the performance of water management systems. Some forward‐looking lessons for managing practical and academic interdisciplinary research for water management also are provided. This article is protected by copyright. All rights reserved.
- Impact of prescribed burning on blanket peat hydrology
- Authors: Joseph Holden; Sheila M. Palmer, Kerrylyn Johnston, Catherine Wearing, Brian Irvine, Lee E. Brown
Abstract: Fire is known to impact soil properties and hydrological flowpaths. However, the impact of prescribed vegetation burning on blanket peatland hydrology is poorly understood. We studied ten blanket peat headwater catchments. Five were subject to prescribed burning, while five were unburnt controls. Within the burnt catchments we studied plots where the last burn occurred ∼2 (B2), 4 (B4), 7 (B7) or greater than 10 years (B10+) prior to the start of measurements. These were compared with plots at similar topographic wetness index locations in the control catchments. Plots subject to prescribed vegetation burning had significantly deeper water tables (difference in means = 5.3 cm) and greater water‐table variability than unburnt plots. Water‐table depths were significantly different between burn age classes (B2>B4>B7>B10+) while B10+ water tables were not significantly different to the unburnt controls. Overland flow was less common on burnt peat than on unburnt peat, recorded in 9% and 17% of all runoff trap visits, respectively. Storm lag times and hydrograph recession limb periods were significantly greater (by ∼ 1 hr and 13 hr on average, respectively) in the burnt catchments overall, but for the largest 20% of storms sampled, there was no significant difference in storm lag times between burnt and unburnt catchments. For the largest 20% of storms the hydrograph intensity of burnt catchments was significantly greater than those of unburnt catchments (means of 4.2 x10−5 s−1 and 3.4 × 10−5 s−1, respectively), thereby indicating a non‐linear streamflow response to prescribed burning. Together, these results from plots to whole river catchments indicate that prescribed vegetation burning has important effects on blanket peatland hydrology at a range of spatial scales. This article is protected by copyright. All rights reserved.
- Complexity and organization in hydrology: A personal view
- Authors: Rafael L. Bras
Abstract: The hydrologic cycle is an exquisitely coordinated and, balanced interaction between the atmosphere, the ocean and the land that controls, among other things, the planet's temperature by moving large quantities of matter and energy. The system is incredibly complex with a myriad of positive and negative feedbacks acting at a variety of scales. Much of what we experience in our natural and altered environments results from these complex interactions. Surprisingly (or maybe not) this complexity many times results in beautifully organized expressions of the hydrologic state that are commonly amenable to fairly simple explanations.
This paper illustrates hydrologic complexity and organization in the context of the author's and collaborator's work during the past decades, a lot published in Water Resources Research. Topics include the impact of soil moisture on the atmosphere and vice‐versa, the impact of deforestation on the Amazon cloud climate and precipitation, the estimation of surface energy and mass fluxes, the self‐organization of landscapes and river basins over very long time periods and the roles of vegetation on landscape evolution. This article is protected by copyright. All rights reserved.
- Predicting the resilience and recovery of aquatic systems: A framework for
model evolution within environmental observatories
- Authors: Matthew R. Hipsey; David P. Hamilton, Paul C. Hanson, Cayelan C. Carey, Janaine Z. Coletti, Jordan S. Read, Bas W. Ibelings, Fiona Valesini, Justin D. Brookes
Abstract: Maintaining the health of aquatic systems is an essential component of sustainable catchment management, however, degradation of water quality and aquatic habitat continues to challenge scientists and policy‐makers. To support management and restoration efforts aquatic system models are required that are able to capture the often complex trajectories that these systems display in response to multiple stressors. This paper explores the abilities and limitations of current model approaches in meeting this challenge, and outlines a strategy based on integration of flexible model libraries and data from observation networks, within a learning framework, as a means to improve the accuracy and scope of model predictions. The framework is comprised of a data assimilation component that utilizes diverse data streams from sensor networks, and a second component whereby model structural evolution can occur once the model is assessed against theoretically relevant metrics of system function. Given the scale and trans‐disciplinary nature of the prediction challenge, network science initiatives are identified as a means to develop and integrate diverse model libraries and workflows, and to obtain consensus on diagnostic approaches to model assessment that can guide model adaptation. We outline how such a framework can help us explore the theory of how aquatic systems respond to change by bridging bottom‐up and top‐down lines of enquiry, and, in doing so, also advance the role of prediction in aquatic ecosystem management. This article is protected by copyright. All rights reserved.
- The change of nature and the nature of change in agricultural landscapes:
Hydrologic regime shifts modulate ecological transitions
- Abstract: Hydrology in many agricultural landscapes around the world is changing in unprecedented ways due to the development of extensive surface and subsurface drainage systems that optimize productivity. This plumbing of the landscape alters water pathways, timings, and storage, creating new regimes of hydrologic response and driving a chain of environmental changes in sediment dynamics, nutrient cycling, and river ecology. In this work we non‐parametrically quantify the nature of hydrologic change in the Minnesota River Basin, an intensively managed agricultural landscape, and study how this change might modulate ecological transitions. During the growing season when climate effects are shown to be minimal, daily streamflow hydrographs exhibit sharper rising limbs and stronger dependence on the previous‐day precipitation. We also find a changed storage‐discharge relationship and show that the artificial landscape connectivity has most drastically affected the rainfall‐runoff relationship at intermediate quantiles. Considering the whole year, we show that the combined climate and land‐use change effects reduce the inherent nonlinearity in the dynamics of daily streamflow, perhaps reflecting a more linearized engineered hydrologic system. Using a simplified dynamic interaction model that couples hydrology to river ecology, we demonstrate how the observed hydrologic change and/or the discharge‐driven sediment generation dynamics may have modulated a regime shift in river ecology, namely extirpation of native mussel populations. We posit that such non‐parametric analyses and reduced complexity modeling can provide more insight than highly parameterized models and can guide development of vulnerability assessments and integrated watershed management frameworks. This article is protected by copyright. All rights reserved.
- A model of the sociohydrologic dynamics in a semiarid catchment: Isolating
feedbacks in the coupled human‐hydrology system
- Authors: Y. Elshafei; M. Sivapalan, J.Z. Coletti, M. R. Hipsey
Abstract: The challenge of sustainable freshwater management requires identification and characterization of the underlying components and dynamic interactions within the coupled human‐hydrology system. This paper builds a model that captures the dynamic water balance evolution and coupled human response within the Lake Toolibin catchment in West Australia's wheatbelt region. Two sub‐catchments in different parts of the landscape were selected to examine the key emergent properties of the coupled socio‐hydrology system over a 100 year period, by analyzing the two‐way feedbacks of land‐use management (human system feedback) and land degradation (natural system feedback). Using a relatively simple parameterization of community sensitivity to land degradation within the model, we identified positive and negative feedbacks, the presence of threshold behavior, timescale differences between fast and slow moving variables, differences in time lags resulting from disparate resistance levels of the natural system, and the degree of adaptive learning inherent in the human system. Specifically, the valley floor sub‐catchment transitioned through four phases ‐ Expansion, Contraction, Recession and Recovery ‐ demonstrating a threshold shift in the human feedback after 60 years, whilst the upslope sub‐catchment appears to still be in the Contraction phase, with no sign of reaching a threshold shift in 100 years. These results demonstrate that the model is capable of isolating the two‐way feedbacks of the coupled system, and has implications for resilience theory, suggesting that greater resistance in the underlying natural system counteracts the onset of a negative feedback loop and instigation of adaptive behaviors in the human system. This article is protected by copyright. All rights reserved.
- Predicting colloid transport through saturated porous media: A critical
- Authors: Ian L. Molnar; William P. Johnson, Jason I. Gerhard, Clinton S. Willson, Denis M. O'Carroll
Abstract: Understanding and predicting colloid transport and retention in water saturated porous media is important for the protection of human and ecological health. Early applications of colloid transport research before the 1990's included the removal of pathogens in granular drinking water filters. Since then, interest has expanded significantly to include such areas as source zone protection of drinking water systems and injection of nanometals for contaminated site remediation. This review summarizes predictive tools for colloid transport from the pore to field scales. First, we review experimental breakthrough and retention of colloids under favorable and unfavorable colloid/collector interactions (i.e., no significant and significant colloid‐surface repulsion, respectively). Second, we review the continuum‐scale modeling strategies used to describe observed transport behavior. Third, we review two components of colloid filtration theory: (i) mechanistic force/torque balance models of pore‐scale colloid trajectories, and (ii) approximating correlation equations used to predict colloid retention. The successes and limitations of these approaches for favorable conditions are summarized, as are recent developments to predict colloid retention under the unfavorable conditions particularly relevant to environmental applications. Fourth, we summarize the influences of physical and chemical heterogeneities on colloid transport and avenues for their prediction. Fifth, we review the upscaling of mechanistic model results to rate constants for use in continuum models of colloid behavior at the column and field scales. Overall, this paper clarifies the foundation for existing knowledge of colloid transport and retention, features recent advances in the field, critically assesses where existing approaches are successful and the limits of their application, and highlights outstanding challenges and future research opportunities. These challenges and opportunities include: improving mechanistic descriptions, and subsequent correlation equations, for nanoparticle (i.e., Brownian particle) transport through soil, developing mechanistic descriptions of colloid retention in so‐called ‘unfavorable' conditions via methods such as the ‘discrete heterogeneity' approach, and employing imaging techniques such as x‐ray tomography to develop realistic expressions for grain topology and mineral distribution that can aid the development of these mechanistic approaches. This article is protected by copyright. All rights reserved.
- Enhanced fixed‐size parallel speedup with the Muskingum method using
a trans‐boundary approach and a large subbasins approximation
- Abstract: This study presents a new algorithm for parallel computation of river flow that builds on recent work demonstrating the relative independence of distant river reaches in the update step of the Muskingum method. The algorithm is designed to achieve enhanced fixed‐size parallel speedup and uses a mathematical approximation applied at the boundaries of large sub‐basins. In order to use such an algorithm, a balanced domain decomposition method that differs from the traditional classifications of river reaches and sub‐basins and based on network topology is developed. An application of the algorithm and domain decomposition method to the Mississippi River Basin results in an 8‐fold decrease in computing time with 16 computing cores which is unprecedented for Muskingum‐type algorithms applied in classic parallel‐computing paradigms having a one‐to‐one relationship between cores and sub‐basins. An estimated 300 km between upstream and downstream reaches of sub‐basins guarantees the applicability of the algorithm in our study and motivates further investigation of domain decomposition methods. This article is protected by copyright. All rights reserved.
- Appreciation of peer reviewers for 2014
- Pages: 5869 - 5887
Abstract: During 2014 Water Resources Research benefited from the voluntary effort of 2103 reviewers. Their constructive and professional effort was instrumental for publishing high‐quality contributions thereby supporting the development of our knowledge of water resources. The contribution of the reviewers is instrumental to science for reaching the target of benefiting humanity. Editors and Associate Editors of Water Resources Research are grateful to the reviewers for their talented, unselfish, and continuous support to the journal.
- Persistent questions of heterogeneity, uncertainty, and scale in
subsurface flow and transport
- Authors: Peter K. Kitanidis
Pages: 5888 - 5904
Abstract: When Water Resources Research was launched in 1965, heterogeneity, uncertainty, and scale issues in subsurface hydrology were in the backburner. Only about 10 years later, under the stimulus of dealing with solute transport problems, these problems received attention. The stochastic approach brought tools to deal both with problems of upscaling, also known as homogenization and coarse‐graining, and uncertainty quantification. Effective conductivity and effective dispersion, also known as macrodispersion, coefficients in statistically homogeneous formations were extensively studied. Mixing, in its role of affecting reaction rates, started receiving attention. While in the dispersion problem emphasis was on Fickian representations, more sophisticated models have also been studied. Uncertainty quantification in the inverse problem has also made progress and geostatistical ideas, as well as ideas originating in signal processing, influenced how we approach problems of inference like interpolation and inverse modeling. My view is that we should emphasize information aspects, i.e., the collection of more and better data, their correct assimilation, the quantification of uncertainty associated with predictions, and the selection of designs or policies that accurately reflect what we actually know and thus manage risk. Progress in this department has been hampered by ingrained ideas, inadequate training, and inadequate resources. Research in problems of upscaling will continue to shed new light and provide better tools to deal with onerous problems. At the same time, no cure is more universally potent than using a more refined grid. Finally, although research is active, the diffusion of research results to education and practice has been slow.
- Using in situ vertical displacements to characterize changes in moisture
- Authors: Lawrence C. Murdoch; Clay E. Freeman, Leonid N. Germanovich, Colby Thrash, Scott DeWolf
Pages: 5998 - 6016
Abstract: Changes in soil moisture content alter the load on underlying material, and we have developed a technique for characterizing this effect by using an extensometer to measure the displacement caused by the load change. The extensometer is pushed into soil at depths of 5 m or more, and displacement between two anchors separated by ∼1.5 m is measured with a resolution of better than 0.01 μm (10−8 m). The instrument is sensitive to load changes at the ground surface within a radial distance that is roughly twice its depth, potentially providing a method for averaging changes in water content over hundreds of m2 or more. During a field trial at a site in South Carolina, compressive displacements in unsaturated saprolite were strongly correlated to rainfall with a calibration factor of 0.16 μm displacement per mm of rainfall ±0.002 μm/mm (R2 = 0.95). Estimates of the net change in water volume per unit area made using the calibration factor from rainfall were similar to independent estimates of evapotranspiration. The technique was affected by barometric pressure variations, but the sensitivity was less than expected and does not hinder meaningful application. A companion instrument demonstrated the displacement signal was repeatable.
- Discharge and water‐depth estimates for ungauged rivers: Combining
hydrologic, hydraulic, and inverse modeling with stage and
water‐area measurements from satellites
- Pages: 6017 - 6035
Abstract: Anticipating future global freshwater scarcity and providing mitigation require timely knowledge of spatiotemporal dynamics of discharge for gauged and, more challengingly, ungauged rivers. This study describes a coupled hydrologic (SWAT) and hydraulic (XSECT) modeling approach set in a genetic algorithm framework for estimating discharge and water depth for ungauged rivers from space. The method was tested in the Red River of the North basin by comparing simulated discharges and depths from 2006 to 2010 to in situ observations from across the basin. Results showed that calibration using only remotely sensed data (i.e., water levels from ENVISAT altimetry and water extents from LANDSAT) along the main stem of the Red River yielded daily and monthly estimates of river discharge, which correlated to measured discharges at three gaging stations on the main stem with R2 values averaging 0.822 and 0.924, respectively. The comparisons of modeled and measured discharges were also extended to smaller tributaries, yielding a mean R2 of 0.809 over seven gaging stations. The modeling approach also provided estimates of water depth that correlated to observations at four stations with an average R2 of 0.831. We conclude that the integrated modeling approach is able to estimate discharge and water depth from space for larger ungauged rivers. This study also implies that in situ discharge data may not be necessary for successful hydrologic model calibration.
- Climatic and landscape controls on water transit times and silicate
mineral weathering in the critical zone
- Pages: 6036 - 6051
Abstract: The critical zone (CZ) can be conceptualized as an open system reactor that is continually transforming energy and water fluxes into an internal structural organization and dissipative products. In this study, we test a controlling factor on water transit times (WTT) and mineral weathering called Effective Energy and Mass Transfer (EEMT). We hypothesize that EEMT, quantified based on local climatic variables, can effectively predict WTT within—and mineral weathering products from—the CZ. This study tests whether EEMT or static landscape characteristics are good predictors of WTT, aqueous phase solutes, and silicate weathering products. Our study site is located around Redondo Peak, a rhyolitic volcanic resurgent dome, in northern New Mexico. At Redondo Peak, springs drain slopes along an energy gradient created by differences in terrain aspect. This investigation uses major solute concentrations, the calculated mineral mass undergoing dissolution, and the age tracer tritium and relates them quantitatively to EEMT and landscape characteristics. We found significant correlations between EEMT, WTT, and mineral weathering products. Significant correlations were observed between dissolved weathering products (Na+ and DIC), 3H concentrations, and maximum EEMT. In contrast, landscape characteristics such as contributing area of spring, slope gradient, elevation, and flow path length were not as effective predictive variables of WTT, solute concentrations, and mineral weathering products. These results highlight the interrelationship between landscape, hydrological, and biogeochemical processes and suggest that basic climatic data embodied in EEMT can be used to scale hydrological and hydrochemical responses in other sites.
- Multimodel analysis of anisotropic diffusive tracer‐gas transport in
a deep arid unsaturated zone
- Authors: Christopher T. Green; Michelle A. Walvoord, Brian J. Andraski, Robert G. Striegl, David A. Stonestrom
Pages: 6052 - 6073
Abstract: Gas transport in the unsaturated zone affects contaminant flux and remediation, interpretation of groundwater travel times from atmospheric tracers, and mass budgets of environmentally important gases. Although unsaturated zone transport of gases is commonly treated as dominated by diffusion, the characteristics of transport in deep layered sediments remain uncertain. In this study, we use a multimodel approach to analyze results of a gas‐tracer (SF6) test to clarify characteristics of gas transport in deep unsaturated alluvium. Thirty‐five separate models with distinct diffusivity structures were calibrated to the tracer‐test data and were compared on the basis of Akaike Information Criteria estimates of posterior model probability. Models included analytical and numerical solutions. Analytical models provided estimates of bulk‐scale apparent diffusivities at the scale of tens of meters. Numerical models provided information on local‐scale diffusivities and feasible lithological features producing the observed tracer breakthrough curves. The combined approaches indicate significant anisotropy of bulk‐scale diffusivity, likely associated with high‐diffusivity layers. Both approaches indicated that diffusivities in some intervals were greater than expected from standard models relating porosity to diffusivity. High apparent diffusivities and anisotropic diffusivity structures were consistent with previous observations at the study site of rapid lateral transport and limited vertical spreading of gas‐phase contaminants. Additional processes such as advective oscillations may be involved. These results indicate that gases in deep, layered unsaturated zone sediments can spread laterally more quickly, and produce higher peak concentrations, than predicted by homogeneous, isotropic diffusion models.
- The relative stability of salmon redds and unspawned streambeds
- Authors: Todd H. Buxton; John M. Buffington, Elowyn M. Yager, Marwan A. Hassan, Alexander K. Fremier
Pages: 6074 - 6092
Abstract: Where female salmon build nests (“redds”), streambed material is mixed, fine sediment is winnowed, and bed material is moved into a tailspill mound resembling the shape of a dune. Completed redd surfaces are coarser and better sorted than unspawned beds, which is thought to increase redd stability because larger grains are heavier and harder to move, and sorting increases friction angles for mobility. However, spawning also loosens sediment and creates topography that accelerates flow, which can increase particle mobility. We address these factors controlling the relative stability of redds and unspawned beds in flume experiments where redds were constructed with a dynamic technique that mimics the nesting behavior of female salmon. Although redds exhibited relatively coarse surfaces, measured entrainment forces indicate particle loosening by spawning lowered grain resistance to motion by 12–37% on average compared to unspawned beds. In addition, for the same discharges, boundary shear stress was 13–41% higher on a redd due to flow convergence on the tailspill. Visual measurements of particle entrainment further indicated redd instability, as bed‐average shear stress was 22% lower at incipient motion and 29% lower at the discharge that mobilized all grain sizes on a redd. Overall, results demonstrate that redds are unstable compared to unspawned beds, which increases the risk of scour for buried eggs but may facilitate fine sediment flushing and improve the quality of spawning gravels for future generations of spawners. Therefore, managing salmon returns to increase streambed disturbance may be an effective tool for reducing sedimentation impacts on salmon reproduction.
- Well integrity assessment under temperature and pressure stresses by a 1:1
scale wellbore experiment
- Authors: J. C. Manceau; J. Tremosa, P. Audigane, C. Lerouge, F. Claret, Y. Lettry, T. Fierz, C. Nussbaum
Pages: 6093 - 6109
Abstract: A new in situ experiment is proposed for observing and understanding well integrity evolution, potentially due to changes that could occur during a well lifetime. The focus is put on temperature and pressure stresses. A small section of a well is reproduced at scale 1:1 in the Opalinus Clay formation, representative of a low permeable caprock formation (in Mont Terri Underground Rock Laboratory, Switzerland). The well‐system behavior is characterized over time both by performing hydro‐tests to quantify the hydraulic properties of the well and their evolution, and sampling the fluids to monitor the chemical composition and its changes. This paper presents the well integrity assessment under different imposed temperature (17–52°C) and pressure (10–28 bar) conditions. The results obtained in this study confirm the ability of the chosen design and observation scale to estimate the evolution of the well integrity over time, the characteristics of the flow along the well‐system and the reasons of the observed evolution. In particular, the estimated effective well permeability is higher than cement or caprock intrinsic permeability, which suggest preferential flow pathways at interfaces especially at the very beginning of the experiment; the significant variations of the effective well permeability observed after setting pressure and temperature stresses indicate that operations could influence well integrity in similar proportions than the cementing process.
- The future of water resources systems analysis: Toward a scientific
framework for sustainable water management
- Authors: Casey M. Brown; Jay R. Lund, Ximing Cai, Patrick M. Reed, Edith A. Zagona, Avi Ostfeld, Jim Hall, Gregory W. Characklis, Winston Yu, Levi Brekke
Pages: 6110 - 6124
Abstract: This paper presents a short history of water resources systems analysis from its beginnings in the Harvard Water Program, through its continuing evolution toward a general field of water resources systems science. Current systems analysis practice is widespread and addresses the most challenging water issues of our times, including water scarcity and drought, climate change, providing water for food and energy production, decision making amid competing objectives, and bringing economic incentives to bear on water use. The emergence of public recognition and concern for the state of water resources provides an opportune moment for the field to reorient to meet the complex, interdependent, interdisciplinary, and global nature of today's water challenges. At present, water resources systems analysis is limited by low scientific and academic visibility relative to its influence in practice and bridled by localized findings that are difficult to generalize. The evident success of water resource systems analysis in practice (which is set out in this paper) needs in future to be strengthened by substantiating the field as the science of water resources that seeks to predict the water resources variables and outcomes that are important to governments, industries, and the public the world over. Doing so promotes the scientific credibility of the field, provides understanding of the state of water resources and furnishes the basis for predicting the impacts of our water choices.
- On the use of rhodamine WT for the characterization of stream
hydrodynamics and transient storage
- Authors: Robert L. Runkel
Pages: 6125 - 6142
Abstract: Recent advances in fluorometry have led to increased use of rhodamine WT as a tracer in streams and rivers. In light of this increased use, a review of the dye's behavior in freshwater systems is presented. Studies in the groundwater literature indicate that rhodamine WT is transported nonconservatively, with sorption removing substantial amounts of tracer mass. Column studies document a two‐step breakthrough curve in which two structural isomers are chromatographically separated. Although the potential for nonconservative transport is acknowledged in the surface water literature, many studies assume that sorptive losses will not affect the characterization of physical transport processes. A literature review and modeling analysis indicates that this assumption is valid for quantification of physical properties that are based on the bulk of the tracer mass (traveltime), and invalid for the characterization of processes represented by the tracer tail (transient storage attributable to hyporheic exchange). Rhodamine WT should be considered nonconservative in the hyporheic zone due to nonconservative behavior demonstrated for similar conditions in groundwater. As such, rhodamine WT should not be used as a quantitative tracer in hyporheic zone investigations, including the study of long flow paths and the development of models describing hyporheic zone processes. Rhodamine WT may be used to qualitatively characterize storage in large systems, where there are few practical alternatives. Qualitative investigations should rely on early portions of the tracer profile, making use of the temporal resolution afforded by in situ fluorometry, while discarding later parts of the tracer profile that are adversely affected by sorption.
- A novel equation for determining the suction stress of unsaturated soils
from the water retention curve based on wetted surface area in pores
- Authors: Roberto Greco; Rudy Gargano
Pages: 6143 - 6155
Abstract: A novel equation is proposed for the evaluation of the suction stress of an unsaturated soil. The equation, based on the assumption that suction is transmitted to soil solid particles only through their wet external surface, allows to easily derive the soil suction characteristic curve from the water retention curve. The proposed equation has been verified against published experimental data of suction stress smaller than 1 MPa for soils of various characteristics. In all cases, an excellent agreement between predicted and observed values of suction stress is achieved, showing that the proposed equation performs better than other currently adopted expressions for the evaluation of soil suction stress.
- Interdependence of chronic hydraulic dysfunction and canopy processes can
improve integrated models of tree response to drought
- Authors: D. Scott Mackay; David E. Roberts, Brent E. Ewers, John S. Sperry, Nathan G. McDowell, William T. Pockman
Pages: 6156 - 6176
Abstract: Hydraulic systems of plants have evolved in the context of carbon allocation and fitness trade‐offs of maximizing carbon gain and water transport in the face of short and long‐term fluctuations in environmental conditions. The resulting diversity of traits include a continuum of isohydry‐anisohydry or high to low relative stomatal closure during drought, shedding of canopy foliage or disconnecting roots from soil to survive drought, and adjusting root areas to efficiently manage canopy water costs associated with photosynthesis. These traits are examined within TREES, an integrated model that explicitly couples photosynthesis and carbon allocation to soil‐plant hydraulics and canopy processes. Key advances of the model are its ability to account for differences in soil and xylem cavitation, transience of hydraulic impairment associated with delayed or no refilling of xylem, and carbon allocation to plant structures based on photosynthetic uptake of carbon and hydraulic limitations to water transport. The model was used to examine hydraulic traits of cooccurring isohydric (piñon pine) and anisohydric (one‐seed juniper) trees from a field‐based experimental drought. Model predictions of both transpiration and leaf water potential were improved when there was no refilling of xylem over simulations where xylem was able refill in response to soil water recharge. Model experiments with alternative root‐to‐leaf area ratios (RR/L) showed the RR/L that supports maximum cumulative water use is not beneficial for supporting maximum carbon gain during extended drought, illustrating how a process model reveals trade‐offs in plant traits.
- Four‐dimensional electrical conductivity monitoring of
stage‐driven river water intrusion: Accounting for water table
effects using a transient mesh boundary and conditional inversion
- Authors: Tim Johnson; Roelof Versteeg, Jon Thomle, Glenn Hammond, Xingyuan Chen, John Zachara
Pages: 6177 - 6196
Abstract: This paper describes and demonstrates two methods of providing a priori information to the surface‐based time‐lapse three‐dimensional electrical resistivity tomography (ERT) problem for monitoring stage‐driven or tide‐driven surface water intrusion into aquifers. First, a mesh boundary is implemented that conforms to the known location of the water table through time, thereby enabling the inversion to place a sharp bulk conductivity contrast at that boundary without penalty. Second, a nonlinear inequality constraint is used to allow only positive or negative transient changes in EC to occur within the saturated zone, dependent on the relative contrast in fluid electrical conductivity between surface water and groundwater. A 3‐D field experiment demonstrates that time‐lapse imaging results using traditional smoothness constraints are unable to delineate river water intrusion. The water table and inequality constraints provide the inversion with the additional information necessary to resolve the spatial extent of river water intrusion through time.
- Surface water types and sediment distribution patterns at the confluence
of mega rivers: The Solimões‐Amazon and Negro Rivers junction
- Authors: Edward Park; Edgardo M. Latrubesse
Pages: 6197 - 6213
Abstract: Large river channel confluences are recognized as critical fluvial features because both intensive and extensive hydrophysical and geoecological processes take place at this interface. However, identifications of suspended sediment routing patterns through channel junctions and the roles of tributaries on downstream sediment transport in large rivers are still poorly explored. In this paper, we propose a remote sensing‐based approach to characterize the spatiotemporal patterns of the postconfluence suspended sediment transport by mapping the surface water distribution in the ultimate example of large river confluence on Earth where distinct water types meet: The Solimões‐Amazon (white water) and Negro (black water) rivers. The surface water types distribution was modeled for three different years: average hydrological condition (2007) and 2 years when extreme events occurred (drought‐2005 and flood‐2009). Amazonian surface water domination along the main channel is highest during the water discharge rising season. Surface water mixing along the main channel depends on the hydrological seasons with the highest mixed‐homogenized area observed during water discharge peak season and the lowest during discharge rising season. Water mixture also depends on the yearly hydrological regime with the highest rates of water mixing in 2009, followed by 2005 and 2007. We conclude that the dominant mixing patterns observed in this study have been persistent over a decadal scale and the anabranching patterns contribute to avoid a faster mixing in a shorter distance. Our proposed approach can be applied to a variety of morphodynamic and environmental analyses in confluences of large rivers around the world.
- Reintroducing radiometric surface temperature into the
- Authors: Kaniska Mallick; Eva Boegh, Ivonne Trebs, Joseph G. Alfieri, William P. Kustas, John H. Prueger, Dev Niyogi, Narendra Das, Darren T. Drewry, Lucien Hoffmann, Andrew J. Jarvis
Pages: 6214 - 6243
Abstract: Here we demonstrate a novel method to physically integrate radiometric surface temperature (TR) into the Penman‐Monteith (PM) formulation for estimating the terrestrial sensible and latent heat fluxes (H and λE) in the framework of a modified Surface Temperature Initiated Closure (STIC). It combines TR data with standard energy balance closure models for deriving a hybrid scheme that does not require parameterization of the surface (or stomatal) and aerodynamic conductances (gS and gB). STIC is formed by the simultaneous solution of four state equations and it uses TR as an additional data source for retrieving the “near surface” moisture availability (M) and the Priestley‐Taylor coefficient (α). The performance of STIC is tested using high‐temporal resolution TR observations collected from different international surface energy flux experiments in conjunction with corresponding net radiation (RN), ground heat flux (G), air temperature (TA), and relative humidity (RH) measurements. A comparison of the STIC outputs with the eddy covariance measurements of λE and H revealed RMSDs of 7–16% and 40–74% in half‐hourly λE and H estimates. These statistics were 5–13% and 10–44% in daily λE and H. The errors and uncertainties in both surface fluxes are comparable to the models that typically use land surface parameterizations for determining the unobserved components (gS and gB) of the surface energy balance models. However, the scheme is simpler, has the capabilities for generating spatially explicit surface energy fluxes and independent of submodels for boundary layer developments.
- A space and time scale‐dependent nonlinear geostatistical approach
for downscaling daily precipitation and temperature
- Authors: Sanjeev Kumar Jha; Gregoire Mariethoz, Jason Evans, Matthew F. McCabe, Ashish Sharma
Pages: 6244 - 6261
Abstract: A geostatistical framework is proposed to downscale daily precipitation and temperature. The methodology is based on multiple‐point geostatistics (MPS), where a multivariate training image is used to represent the spatial relationship between daily precipitation and daily temperature over several years. Here the training image consists of daily rainfall and temperature outputs from the Weather Research and Forecasting (WRF) model at 50 and 10 km resolution for a 20 year period ranging from 1985 to 2004. The data are used to predict downscaled climate variables for the year 2005. The result, for each downscaled pixel, is daily time series of precipitation and temperature that are spatially dependent. Comparison of predicted precipitation and temperature against a reference data set indicates that both the seasonal average climate response together with the temporal variability are well reproduced. The explicit inclusion of time dependence is explored by considering the climate properties of the previous day as an additional variable. Comparison of simulations with and without inclusion of time dependence shows that the temporal dependence only slightly improves the daily prediction because the temporal variability is already well represented in the conditioning data. Overall, the study shows that the multiple‐point geostatistics approach is an efficient tool to be used for statistical downscaling to obtain local‐scale estimates of precipitation and temperature from General Circulation Models.
- Continental U.S. streamflow trends from 1940 to 2009 and their
relationships with watershed spatial characteristics
- Authors: Joshua S. Rice; Ryan E. Emanuel, James M. Vose, Stacy A. C. Nelson
Pages: 6262 - 6275
Abstract: Changes in streamflow are an important area of ongoing research in the hydrologic sciences. To better understand spatial patterns in past changes in streamflow, we examined relationships between watershed‐scale spatial characteristics and trends in streamflow. Trends in streamflow were identified by analyzing mean daily flow observations between 1940 and 2009 from 967 U.S. Geological Survey stream gages. Results indicated that streamflow across the continental U.S., as a whole, increased while becoming less extreme between 1940 and 2009. However, substantial departures from the continental U.S. (CONUS) scale pattern occurred at the regional scale, including increased annual maxima, decreased annual minima, overall drying trends, and changes in streamflow variability. A subset of watersheds belonging to a reference data set exhibited significantly smaller trend magnitudes than those observed in nonreference watersheds. Boosted regression tree models were applied to examine the influence of watershed characteristics on streamflow trend magnitudes at both the CONUS and regional scale. Geographic location was found to be of particular importance at the CONUS scale while local variability in hydroclimate and topography tended to have a strong influence on regional‐scale patterns in streamflow trends. This methodology facilitates detailed, data‐driven analyses of how the characteristics of individual watersheds interact with large‐scale hydroclimate forces to influence how changes in streamflow manifest.
- Johnson SB as general functional form for raindrop size distribution
- Authors: Katia Cugerone; Carlo De Michele
Pages: 6276 - 6289
Abstract: Drop size distribution represents the statistical synthesis of rainfall dynamics at particle size scale. Gamma and Lognormal distributions have been widely used in the literature to approximate the drop diameter variability, contrarily to the natural upper boundary of the variable, with almost always site‐specific studies and without the support of statistical goodness‐of‐fit tests. In this work, we present an extensive statistical investigation of raindrop size distribution based on eight data sets, well distributed on the Earth's surface, which have been analyzed by using skewness‐kurtosis plane, AIC and BIC indices and Kolmogorov‐Smirnov test. Here for the first time, the Johnson SB is proposed as general functional form to describe the drop diameter variability specifically at 1 min time scale. Additional analyses demonstrate that the model is well suitable even for larger time intervals (≥1 min).
- Abiotic control of underwater light in a drinking water reservoir: Photon
budget analysis and implications for water quality monitoring
- Authors: Shohei Watanabe; Isabelle Laurion, Stiig Markager, Warwick F. Vincent
Pages: 6290 - 6310
Abstract: In optically complex inland waters, the underwater attenuation of photosynthetically active radiation (PAR) is controlled by a variable combination of absorption and scattering components of the lake or river water. Here we applied a photon budget approach to identify the main optical components affecting PAR attenuation in Lake St. Charles, a drinking water reservoir for Québec City, Canada. This analysis showed the dominant role of colored dissolved organic matter (CDOM) absorption (average of 44% of total absorption during the sampling period), but with large changes over depth in the absolute and relative contribution of the individual absorption components (water, nonalgal particulates, phytoplankton and CDOM) to PAR attenuation. This pronounced vertical variation occurred because of the large spectral changes in the light field with depth, and it strongly affected the average in situ diffuse absorption coefficients in the water column. For example, the diffuse absorption coefficient for pure‐water in the ambient light field was 10‐fold higher than the value previously measured in the blue open ocean and erroneously applied to lakes and coastal waters. Photon absorption budget calculations for a range of limnological conditions confirmed that phytoplankton had little direct influence on underwater light, even at chlorophyll a values above those observed during harmful algal blooms in the lake. These results imply that traditional measures of water quality such as Secchi depth and radiometric transparency do not provide a meaningful estimate of the biological state of the water column in CDOM‐colored lakes and reservoirs.
- Modeling mixed retention and early arrivals in multidimensional
heterogeneous media using an explicit Lagrangian scheme
- Authors: Yong Zhang; Mark M. Meerschaert, Boris Baeumer, Eric M. LaBolle
Pages: 6311 - 6337
Abstract: This study develops an explicit two‐step Lagrangian scheme based on the renewal‐reward process to capture transient anomalous diffusion with mixed retention and early arrivals in multidimensional media. The resulting 3‐D anomalous transport simulator provides a flexible platform for modeling transport. The first step explicitly models retention due to mass exchange between one mobile zone and any number of parallel immobile zones. The mobile component of the renewal process can be calculated as either an exponential random variable or a preassigned time step, and the subsequent random immobile time follows a Hyper‐exponential distribution for finite immobile zones or a tempered stable distribution for infinite immobile zones with an exponentially tempered power‐law memory function. The second step describes well‐documented early arrivals which can follow streamlines due to mechanical dispersion using the method of subordination to regional flow. Applicability and implementation of the Lagrangian solver are further checked against transport observed in various media. Results show that, although the time‐nonlocal model parameters are predictable for transport with retention in alluvial settings, the standard time‐nonlocal model cannot capture early arrivals. Retention and early arrivals observed in porous and fractured media can be efficiently modeled by our Lagrangian solver, allowing anomalous transport to be incorporated into 2‐D/3‐D models with irregular flow fields. Extensions of the particle‐tracking approach are also discussed for transport with parameters conditioned on local aquifer properties, as required by transient flow and nonstationary media.
- Untangling the effects of shallow groundwater and soil texture as drivers
of subfield‐scale yield variability
- Authors: Samuel C. Zipper; Mehmet Evren Soylu, Eric G. Booth, Steven P. Loheide
Pages: 6338 - 6358
Abstract: Water table depth (WTD), soil texture, and growing season weather conditions all play critical roles in determining agricultural yield; however, the interactions among these three variables have never been explored in a systematic way. Using a combination of field observations and biophysical modeling, we answer two questions: (1) under what conditions can a shallow water table provide a groundwater yield subsidy and/or penalty to corn production?; and (2) how do soil texture and growing season weather conditions influence the relationship between WTD and corn yield?. Subfield‐scale yield patterns during a dry (2012) and wet (2013) growing season are used to identify sensitivity to weather. Areas of the field that are negatively impacted by wet growing seasons have the shallowest observed WTD (3 m) and coarse soil textures. Modeling results find that beneficial impacts of shallow groundwater are more common than negative impacts under the conditions studied, and that the optimum WTD is shallower in coarser soils. While groundwater yield subsidies have a higher frequency and magnitude in coarse‐grained soils, the optimum WTD responds to growing season weather at a relatively constant rate across soil types. We conclude that soil texture defines a baseline upon which WTD and weather interact to determine overall yield. Our work has implications for water resource management, climate/land use change impacts on agricultural production, and precision agriculture.
- Scenario tree reduction in stochastic programming with recourse for
- Pages: 6359 - 6380
Abstract: A stochastic programming with recourse model requires the consequences of recourse actions be modeled for all possible realizations of the stochastic variables. Continuous stochastic variables are approximated by scenario trees. This paper evaluates the impact of scenario tree reduction on model performance for hydropower operations and suggests procedures to determine the optimal level of scenario tree reduction. We first establish a stochastic programming model for the optimal operation of a cascaded system of reservoirs for hydropower production. We then use the neural gas method to generate scenario trees and employ a Monte Carlo method to systematically reduce the scenario trees. We conduct in‐sample and out‐of‐sample tests to evaluate the impact of scenario tree reduction on the objective function of the hydropower optimization model. We then apply a statistical hypothesis test to determine the significance of the impact due to scenario tree reduction. We develop a stochastic programming with recourse model and apply it to real‐time operation for hydropower production to determine the loss in solution accuracy due to scenario tree reduction. We apply the proposed methodology to the Qingjiang cascade system of reservoirs in China. The results show: (1) the neural gas method preserves the mean value of the original streamflow series but introduces bias to variance, cross variance, and lag‐one covariance due to information loss when the original tree is systematically reduced; (2) reducing the scenario number by as much as 40% results in insignificant change in the objective function and solution quality, but significantly reduces computational demand.
- Reliability, return periods, and risk under nonstationarity
- Authors: Laura K. Read; Richard M. Vogel
Pages: 6381 - 6398
Abstract: Water resources design has widely used the average return period as a concept to inform management and communication of the risk of experiencing an exceedance event within a planning horizon. Even though nonstationarity is often apparent, in practice hydrologic design often mistakenly assumes that the probability of exceedance, p, is constant from year to year which leads to an average return period To equal to 1/p; this expression is far more complex under nonstationarity. Even for stationary processes, the common application of an average return period is problematic: it does not account for planning horizon, is an average value that may not be representative of the time to the next flood, and is generally not applied in other areas of water planning. We combine existing theoretical and empirical results from the literature to provide the first general, comprehensive description of the probabilistic behavior of the return period and reliability under nonstationarity. We show that under nonstationarity, the underlying distribution of the return period exhibits a more complex shape than the exponential distribution under stationary conditions. Using a nonstationary lognormal model, we document the increased complexity and challenges associated with planning for future flood events over a planning horizon. We compare application of the average return period with the more common concept of reliability and recommend replacing the average return period with reliability as a more practical way to communicate event likelihood in both stationary and nonstationary contexts.
- FINIFLUX: An implicit finite element model for quantification of
groundwater fluxes and hyporheic exchange in streams and rivers using
- Authors: S. Frei; B. S. Gilfedder
Pages: 6776 - 6786
Abstract: A quantitative understanding of groundwater‐surface water interactions is vital for sustainable management of water quantity and quality. The noble gas radon‐222 (Rn) is becoming increasingly used as a sensitive tracer to quantify groundwater discharge to wetlands, lakes, and rivers: a development driven by technical and methodological advances in Rn measurement. However, quantitative interpretation of these data is not trivial, and the methods used to date are based on the simplest solutions to the mass balance equation (e.g., first‐order finite difference and inversion). Here we present a new implicit numerical model (FINIFLUX) based on finite elements for quantifying groundwater discharge to streams and rivers using Rn surveys at the reach scale (1–50 km). The model is coupled to a state‐of‐the‐art parameter optimization code Parallel‐PEST to iteratively solve the mass balance equation for groundwater discharge and hyporheic exchange. The major benefit of this model is that it is programed to be very simple to use, reduces nonuniqueness, and provides numerically stable estimates of groundwater fluxes and hyporheic residence times from field data. FINIFLUX was tested against an analytical solution and then implemented on two German rivers of differing magnitude, the Salzach (∼112 m3 s−1) and the Rote Main (∼4 m3 s−1). We show that using previous inversion techniques numerical instability can lead to physically impossible negative values, whereas the new model provides stable positive values for all scenarios. We hope that by making FINIFLUX freely available to the community that Rn might find wider application in quantifying groundwater discharge to streams and rivers and thus assist in a combined management of surface and groundwater systems.
- Imbibition of hydraulic fracturing fluids into partially saturated shale
- Authors: Daniel T. Birdsell; Harihar Rajaram, Greg Lackey
Pages: 6787 - 6796
Abstract: Recent studies suggest that imbibition of hydraulic fracturing fluids into partially saturated shale is an important mechanism that restricts their migration, thus reducing the risk of groundwater contamination. We present computations of imbibition based on an exact semianalytical solution for spontaneous imbibition. These computations lead to quantitative estimates of an imbibition rate parameter (A) with units of
LT−1/2 for shale, which is related to porous medium and fluid properties, and the initial water saturation. Our calculations suggest that significant fractions of injected fluid volumes (15–95%) can be imbibed in shale gas systems, whereas imbibition volumes in shale oil systems is much lower (3–27%). We present a nondimensionalization of A, which provides insights into the critical factors controlling imbibition, and facilitates the estimation of A based on readily measured porous medium and fluid properties. For a given set of medium and fluid properties, A varies by less than factors of ∼1.8 (gas nonwetting phase) and ∼3.4 (oil nonwetting phase) over the range of initial water saturations reported for the Marcellus shale (0.05–0.6). However, for higher initial water saturations, A decreases significantly. The intrinsic permeability of the shale and the viscosity of the fluids are the most important properties controlling the imbibition rate.