- Process connectivity in a naturally prograding river delta
- Authors: Alicia Sendrowski; Paola Passalacqua
Abstract: River deltas are lowland systems that can display high hydrological connectivity. This connectivity can be structural (morphological connections), functional (control of fluxes), and process connectivity (information flow from system drivers to sinks). In this work, we quantify hydrological process connectivity in Wax Lake Delta, coastal Louisiana, by analyzing couplings among external drivers (discharge, tides, and wind) and water levels recorded at five islands and one channel over summer 2014. We quantify process connections with information theory, a branch of mathematics concerned with the communication of information. We represent process connections as a network; variables serve as network nodes and couplings as network links describing the strength, direction, and timescale of information flow. Comparing process connections at long (105-days) and short (10-days) timescales, we show that tides exhibit daily synchronization with water level, with decreasing strength from downstream to upstream, and that tides transfer information as tides transition from spring to neap. Discharge synchronizes with water level and the timescale of its information transfer compares well to physical travel times through the system, computed with a hydrodynamic model. Information transfer and physical transport show similar spatial patterns, although information transfer timescales are larger than physical travel times. Wind events associated with water level set-up lead to increased process connectivity with highly variable information transfer timescales. We discuss the information theory results in the context of the hydrologic behavior of the delta, the role of vegetation as a connector/dis-connector on islands, and the applicability of process networks as tools for delta modeling results. This article is protected by copyright. All rights reserved.
- Using SAS functions and high resolution isotope data to unravel travel
time distributions in headwater catchments
- Authors: Paolo Benettin; Chris Soulsby, Christian Birkel, Doerthe Tetzlaff, Gianluca Botter4, Andrea Rinaldo
Abstract: We use high resolution tracer data from an experimental site to test theoretical approaches that integrate catchment-scale flow and transport processes in a unified framework centered on selective age sampling by streamflow and evapotranspiration fluxes. Transport processes operating at the catchment scale are reflected in the evolving residence time distribution of the catchment water storage and in the age-selection operated by out-fluxes. Such processes are described here through StorAge Selection (SAS) functions parametrized as power laws of the normalized rank storage. Such functions are computed through appropriate solution of the master equation defining formally the evolution of residence and travel times. By representing the way in which catchment storage generates outflows composed by water of different ages, the main mechanism regulating the tracer composition of runoff is clearly identified and detailed comparison with empirical data sets are possible. Properly calibrated numerical tools provide simulations that convincingly reproduce complex measured signals of daily deuterium content in stream waters during wet and dry periods. Results for the catchment under consideration are consistent with other recent studies indicating a tendency for natural catchments to preferentially release younger available water. The study shows that power-law SAS functions prove a powerful tool to explain catchment-scale transport processes that also has potential in less intensively monitored sites. This article is protected by copyright. All rights reserved.
- Experimental Investigation of clogging dynamics in homogeneous porous
- Authors: Jikang Shen; Rui Ni
Abstract: A 3D refractive-index matching Lagrangian particle tracking (3D-RIM-LPT) system was developed to study the filtration and clogging process inside a homogeneous porous medium. A small subset of particles flowing through the porous medium was dyed and tracked. As this subset was randomly chosen, its dynamics is representative of all the rest. The statistics of particle locations, number, and velocity vectors were obtained as functions of different volumetric concentrations. It is found that, in our system, the clogging time decays with particle concentration following a power law relationship. As the concentration increases, there is a transition from depth filtration to cake filtration. At high concentration, more clogged pores lead to frequent flow redirections and more transverse migrations of particles. In addition, the velocity distribution of the transverse direction is symmetrical around zero, and it is slightly more intermittent than the random Gaussian curve due to particle-particle interactions and particle-grain interactions. In contrast, as clogging develops, the longitudinal velocity of particles along the main flow direction has peak near zero because of those trapped particles. But at the same time, the remaining open pores will experience larger pressure and, as a result, particles through those pores will have a larger longitudinal velocity. This article is protected by copyright. All rights reserved.
- The effects of spatial resolution and dimensionality on modeling
regional-scale hydraulics in a multichannel river
- Authors: Elizabeth H. Altenau; Tamlin M. Pavelsky, Paul D. Bates, Jeffrey C. Neal
Abstract: As modeling capabilities at regional and global scales improve, questions remain regarding the appropriate process representation required to accurately simulate multichannel river hydraulics. This study uses the hydrodynamic model LISFLOOD-FP to simulate patterns of water surface elevation (WSE), depth, and inundation extent across a ∼90 km, anabranching reach of the Tanana River, Alaska. To provide boundary conditions, we collected field observations of bathymetry and WSE during a two-week field campaign in summer 2013. For the first time at this scale, we test a simple, raster-based model's capabilities to simulate 2D, in-channel patterns of WSE and inundation extent. Additionally, we compare finer resolution (≤ 25 m) 2D models to four other models of lower dimensionality and coarser resolution (100–500 m) to determine the effects of simplifying process representation. Results indicate that simple, raster-based models can accurately simulate 2D, in-channel hydraulics in the Tanana. Also, the fine-resolution, 2D models produce lower errors in spatiotemporal outputs of WSE and inundation extent compared to coarse-resolution, 1D models: 22.6 cm vs. 56.4 cm RMSE for WSE, and 90% vs. 41% Critical Success Index values for simulating inundation extent. Incorporating the anabranching channel network using subgrid representations for smaller channels is important for simulating accurate hydraulics and lowers RMSE in spatially distributed WSE by at least 16%. As a result, better representation of the converging and diverging multichannel network by using subgrid solvers or downscaling techniques in multichannel rivers is needed to improve errors in regional to global scale models. This article is protected by copyright. All rights reserved.
- Hydro-geomorphic perturbations on the soil-atmosphere CO2 exchange: How
(un)certain are our balances?
- Authors: Yannis G. Dialynas; Rafael L. Bras, Daniel deB. Richter
Abstract: Attempts to estimate the influence of erosion on the carbon (C) cycle are limited by difficulties in accounting for the fate of mobilized organic material and for the uncertainty associated with land management practices. This study proposes a method to quantify the uncertainty introduced by the influence of land management on soil organic C (SOC) generation and decomposition at eroding soils. The framework is implemented in tRIBS-ECO (Triangulated Irregular Network-based Real-time Integrated Basin Simulator-Erosion and Carbon Oxidation). tRIBS-ECO is a spatially- and depth-explicit model of C dynamics coupled with a process-based hydro-geomorphic model. We assess the impact of soil erosion on the net soil-atmosphere CO2 exchange at the Calhoun Critical Zone Observatory, one of the most severely agriculturally eroded regions in the U.S. Measurements of SOC storage are used from different catena positions. We demonstrate that the spatio-temporal variations of land management practices introduce significant uncertainty in estimates of the erosion-induced CO2 exchange with the atmosphere. Observations and simulations suggest that a substantial portion of eroded organic material is buried in alluvial sediments at the study site. According to results, recent reforestation led to a partial decline in soil and SOC erosion rates. It is suggested that the representation of the fine spatio-temporal variability of the dynamics of eroded C is important in the computation of C budgets in regional and global scales. This article is protected by copyright. All rights reserved.
- Permeability, porosity, and mineral surface area changes in basalt cores
induced by reactive transport of CO2-rich brine
- Authors: Andrew J. Luhmann; Benjamin M. Tutolo, Brian C. Bagley, David F.R. Mildner, William E. Seyfried, Martin O. Saar
Abstract: Four reactive flow-through laboratory experiments (two each at 0.1 ml/min and 0.01 ml/min flow rates) at 150°C and 150 bar (15 MPa) are conducted on intact basalt cores to assess changes in porosity, permeability, and surface area caused by CO2-rich fluid-rock interaction. Permeability decreases slightly during the lower flow rate experiments and increases during the higher flow rate experiments. At the higher flow rate, core permeability increases by more than one order of magnitude in one experiment and less than a factor of two in the other due to differences in preexisting flow path structure. X-ray computed tomography (XRCT) scans of pre- and post-experiment cores identify both mineral dissolution and secondary mineralization, with a net decrease in XRCT porosity of ∼0.7% – 0.8% for all four cores. (Ultra) small-angle neutron scattering ((U)SANS) datasets indicate an increase in both (U)SANS porosity and specific surface area (SSA) over the ∼ 1 nm- to 10 µm-scale range in post-experiment basalt samples, with differences due to flow rate and reaction time. Net porosity increases from summing XRCT and (U)SANS analyses are consistent with core mass decreases. (U)SANS data suggest an overall preservation of the pore structure with no change in mineral surface roughness from reaction, and the pore structure is unique in comparison to previously published basalt analyses. Together, these datasets illustrate changes in physical parameters that arise due to fluid-basalt interaction in relatively low pH environments with elevated CO2 concentration, with significant implications for flow, transport, and reaction through geologic formations. This article is protected by copyright. All rights reserved.
- Pore-scale modelling of wettability alteration during primary drainage
- Authors: W. Kallel; M.I.J. van Dijke, K.S. Sorbie, Rachel Wood
Abstract: While carbonate reservoirs are recognized to be weakly-to-moderately oil-wet at the core-scale, pore-scale wettability distributions remain poorly understood. In particular, the wetting state of micropores (pores
- Improving probabilistic prediction of daily streamflow by identifying
Pareto optimal approaches for modeling heteroscedastic residual errors
- Authors: David McInerney; Mark Thyer, Dmitri Kavetski, Julien Lerat, George Kuczera
Abstract: Reliable and precise probabilistic prediction of daily catchment-scale streamflow requires statistical characterization of residual errors of hydrological models. This study focuses on approaches for representing error heteroscedasticity with respect to simulated streamflow, i.e., the pattern of larger errors in higher streamflow predictions. We evaluate 8 common residual error schemes, including standard and weighted least squares, the Box-Cox transformation (with fixed and calibrated power parameter λ) and the log-sinh transformation. Case studies include 17 perennial and 6 ephemeral catchments in Australia and USA, and two lumped hydrological models. Performance is quantified using predictive reliability, precision and volumetric bias metrics. We find the choice of heteroscedastic error modelling approach significantly impacts on predictive performance, though no single scheme simultaneously optimizes all performance metrics. The set of Pareto optimal schemes, reflecting performance trade-offs, comprises Box-Cox schemes with λ of 0.2 and 0.5, and the log scheme (λ=0, perennial catchments only). These schemes significantly outperform even the average-performing remaining schemes (e.g., across ephemeral catchments, median precision tightens from 105% to 40% of observed streamflow, and median biases decrease from 25% to 4%). Theoretical interpretations of empirical results highlight the importance of capturing the skew/kurtosis of raw residuals and reproducing zero flows. Paradoxically, calibration of λ is often counterproductive: in perennial catchments, it tends to overfit low flows at the expense of abysmal precision in high flows. The log-sinh transformation is dominated by the simpler Pareto optimal schemes listed above. Recommendations for researchers and practitioners seeking robust residual error schemes for practical work are provided. This article is protected by copyright. All rights reserved.
- An estimation of the main wetting branch of the soil water retention curve
based on its main drying branch using the machine learning method
- Authors: Krzysztof Lamorski; Jiří Šimůnek, Cezary Sławiński, Joanna Lamorska
Abstract: In this paper, we estimated using the machine learning methodology the main wetting branch of the soil water retention curve based on the knowledge of the main drying branch and other, optional, basic soil characteristics (particle size distribution, bulk density, organic matter content, or soil specific surface). The support vector machine algorithm, was used for the models' development. The data needed by this algorithm for model training and validation consisted of 104 different undisturbed soil core samples collected from the topsoil layer (A horizon) of different soil profiles in Poland. The main wetting and drying branches of SWRC, as well as other basic soil physical characteristics, were determined for all soil samples. Models relying on different sets of input parameters were developed and validated. The analysis showed that taking into account other input parameters (i.e., particle size distribution, bulk density, organic matter content, or soil specific surface) than information about the drying branch of the SWRC has essentially no impact on the models' estimations. Developed models are validated and compared with well-known models that can be used for the same purpose, such as the Mualem  (M77) and Kool and Parker  (KP87) models. The developed models estimate the main wetting SWRC branch with estimation errors (RMSE=0.018 m3/m3) that are significantly lower than those for the M77 (RMSE=0.025 m3/m3) or KP87 (RMSE=0. 047 m3/m3) models. This article is protected by copyright. All rights reserved.
- An Eulerian equation for snow accumulation downstream of an object
- Authors: Noriaki Ohara
Abstract: This study investigated the form of the governing equation for the particle distribution by focusing on the particle motion processes rather than flow regime and particle characteristics. A linear erosion term for a fetch-eddy effect was introduced to the advection dispersion equation. The equation formulated in this paper described most of the particle deposit patterns behind an object including porous and solid snow fences, and a tree. This theory may enable us to estimate particle motion parameters, such as diffusion, drift, and erosion coefficients, from field observed particle distributions. Snow stratigraphy observed by ground penetrating radar (GPR) was used verify to result of the modeled theoretical snow redistribution. These analyses confirmed the effectiveness of the linear erosion term at explaining the particle deposition patterns due to eddys around a porous snow fence. This article is protected by copyright. All rights reserved.
- A parametric approach for simultaneous bias correction and high-resolution
downscaling of climate model rainfall
- Authors: Antonios Mamalakis; Andreas Langousis, Roberto Deidda, Marino Marrocu
Abstract: Distribution mapping has been identified as the most efficient approach to bias correct climate model rainfall, while reproducing its statistics at spatial and temporal resolutions suitable to run hydrologic models. Yet, its implementation based on empirical distributions derived from control samples (referred to as non-parametric distribution mapping), makes the method's performance sensitive to sample length variations, the presence of outliers, the spatial resolution of climate model results, and may lead to biases, especially in extreme rainfall estimation. To address these shortcomings, we propose a methodology for simultaneous bias correction and high resolution downscaling of climate model rainfall products that uses: a) a two component theoretical distribution model (i.e. a generalized Pareto (GP) model for rainfall intensities above a specified threshold u*, and an exponential model for lower rainrates), and b) proper interpolation of the corresponding distribution parameters on a user-defined high-resolution grid, using kriging for uncertain data. We assess the performance of the suggested parametric approach relative to the non-parametric one, using daily raingauge measurements from a dense network in the island of Sardinia (Italy), and rainfall data from 4 GCM/RCM model chains of the ENSEMBLES project. The obtained results shed light on the competitive advantages of the parametric approach, which is proved more accurate and considerably less sensitive to the characteristics of the calibration period, independent of the GCM/RCM combination used. This is especially the case for extreme rainfall estimation, where the GP assumption allows for more accurate and robust estimates, also beyond the range of the available data. This article is protected by copyright. All rights reserved.
- Estimating bed shear stress from remotely measured surface turbulent
dissipation fields in open channel flows
- Authors: E. D. Johnson; E. A. Cowen
Abstract: Synoptic information on bed shear stress is necessary in predicting the transport of sediments and environmental contaminants in rivers and open channels. Existing methods of estimating bed shear stress typically involve measuring vertical profiles of streamwise velocity or Reynolds stress and taking advantage of the logarithmic or the constant stress region, respectively, to determine friction velocity and subsequently, bed shear stress. While effective, these methods yield local measurements of bed shear stress only. Direct measurements of bed shear stress can also be obtained through measurements with a drag plate. However, this method yields average shear stress information over the area of the plate and is impractical for large-scale implementation in the field.Here we present a method capable of providing continuous synoptic measurements of bed shear stress over a large field-of-view. A series of Large-Scale Particle Image Velocimetry (LSPIV) and Acoustic Doppler Velocimetry (ADV) measurements were made in a variety of flows generated in a wide-open channel facility. Turbulent dissipation is calculated on the free-surface from the 2-D LSPIV results and is correlated with near surface ADV measurements of turbulent dissipation in the water column. The ADV results are consistent with the Nezu  established relationship for the vertical variation of turbulent dissipation in the water column. Knowledge of the correlation between free-surface and near-surface dissipation values coupled with Nezu's  relationship allow a robust and accurate estimate of friction velocity to be made and subsequently, shear stress at the bed can be estimated. This article is protected by copyright. All rights reserved.
- Heat and water transport in soils and across the soil-atmosphere
interface: 2. Numerical analysis
- Authors: Thomas Fetzer; Jan Vanderborght, Klaus Mosthaf, Kathleen M. Smits, Rainer Helmig
Abstract: In an accompanying paper, we presented an overview of a wide variety of modeling concepts, varying in complexity, used to describe evaporation from soil. Using theoretical analyses, we explained the simplifications and parameterizations in the different approaches. In this paper, we numerically evaluate the consequences of these simplifications and parameterizations. Two sets of simulations were performed. The first set investigates lateral variations in vertical fluxes, which emerge from both homogeneous and heterogeneous porous media, and their importance to capturing evaporation behavior. When evaporation decreases from parts of the heterogeneous soil surface, lateral flow and transport processes in the free flow and in the porous medium generate feedbacks that enhance evaporation from wet surface areas. In the second set of simulations, we assume that the vertical fluxes do not vary considerably in the simulation domain and represent the system using one-dimensional models which also consider dynamic forcing of the evaporation process, for example, due to diurnal variations in net radiation. Simulated evaporation fluxes subjected to dynamic forcing differed considerably between model concepts depending on how vapor transport in the air phase and the interaction at the interface between the free flow and porous medium were represented or parameterized. However, simulated cumulative evaporation losses from initially wet soil profiles were very similar between model concepts and mainly controlled by the desorptivity, Sevap, of the porous medium, which depends mainly on the liquid flow properties of the porous medium.
- Heat and water transport in soils and across the soil-atmosphere
interface: 1. Theory and different model concepts
- Authors: Jan Vanderborght; Thomas Fetzer, Klaus Mosthaf, Kathleen M. Smits, Rainer Helmig
Abstract: Evaporation is an important component of the soil water balance. It is composed of water flow and transport processes in a porous medium that are coupled with heat fluxes and free air flow. This work provides a comprehensive review of model concepts used in different research fields to describe evaporation. Concepts range from nonisothermal two-phase flow, two-component transport in the porous medium that is coupled with one-phase flow, two-component transport in the free air flow to isothermal liquid water flow in the porous medium with upper boundary conditions defined by a potential evaporation flux when available energy and transfer to the free airflow are limiting or by a critical threshold water pressure when soil water availability is limiting. The latter approach corresponds with the classical Richards equation with mixed boundary conditions. We compare the different approaches on a theoretical level by identifying the underlying simplifications that are made for the different compartments of the system: porous medium, free flow and their interface, and by discussing how processes not explicitly considered are parameterized. Simplifications can be grouped into three sets depending on whether lateral variations in vertical fluxes are considered, whether flow and transport in the air phase in the porous medium are considered, and depending on how the interaction at the interface between the free flow and the porous medium is represented. The consequences of the simplifications are illustrated by numerical simulations in an accompanying paper.
- Improved water balance component estimates through Joint Assimilation of
GRACE water storage and SMOS soil moisture retrievals
- Authors: Siyuan Tian; Paul Tregoning, Luigi J. Renzullo, Albert I.J.M. van Dijk, Jeffrey P. Walker, Valentijn R.N. Pauwels, Sébastien Allgeyer
Abstract: The accuracy of global water balance estimates is limited by the lack of observations at large scale, and the uncertainties of model simulations. Global retrievals of terrestrial water storage (TWS) change and soil moisture (SM) from satellites provide an opportunity to improve model estimates through data assimilation. However, combining these two data sets is challenging due to the disparity in temporal and spatial resolution at both vertical and horizontal scale. For the first time, TWS observations from the Gravity Recovery and Climate Experiment (GRACE) and near-surface SM observations from the Soil Moisture and Ocean Salinity (SMOS) were jointly assimilated into a water balance model using the Ensemble Kalman Smoother from January 2010 to December 2013 for the Australian continent. The performance of joint assimilation was assessed against open-loop model simulations and the assimilation of either GRACE TWS anomalies or SMOS SM alone. The SMOS-only assimilation improved SM estimates but reduced the accuracy of groundwater and TWS estimates. The GRACE-only assimilation improved groundwater estimates but did not always produce accurate estimates of SM. The joint assimilation typically led to more accurate water storage profile estimates with improved surface SM, root-zone SM, and groundwater estimates against in-situ observations. The assimilation successfully downscaled GRACE-derived integrated water storage horizontally and vertically into individual water stores at the same spatial scale as the model and SMOS, and partitioned monthly-averaged TWS into daily estimates. These results demonstrate that satellite TWS and SM measurements can be jointly assimilated to produce improved water balance component estimates. This article is protected by copyright. All rights reserved.
- Assessment of uncertainties in global land cover products for hydroclimate
modeling in India
- Authors: C. G. Madhusoodhanan; K. G. Sreeja, T. I. Eldho
Abstract: Earth's land cover (LC) has significant influence on land-atmospheric processes and affects the climate at multiple scales. There are multiple Global LC (GLC) datasets which are yet to be evaluated for uncertainties and their propagation into the simulation of land surface fluxes (LSFs) in land surface/climate modeling. The present study assesses the uncertainties in seven GLC products with reference to a regional dataset for the simulation of LSFs in India using a macro-scale land surface model. There is considerable overestimation of the extent of croplands in most of the GLCs. The uncertainties in LCs exert significant bias in the simulation of the LSFs of actual evapotranspiration (ETa), latent heat (LE) and sensible heat (H) fluxes. Error propagation in LSFs is proportional to the bias in cropping intensity under rainfed condition. The high under-representation of cropland area in the UMd dataset results in highest bias in LSFs whereas the least cropland bias in Globland30 leads to least bias. Irrigation has higher potential to alter the LSFs than uncertainties related to LC especially in regions with large area under irrigation like India. The changes in LSFs are higher in arid/semi-arid regions with medium irrigation intensity than in sub-humid regions with high irrigation intensity. This has significant implications for the country's future irrigation expansion plans in the arid/semi-arid regions. The study also emphasizes the need for focused efforts to quantify the uncertainties from varying irrigation intensities in the next generation CMIP6 experiments. This article is protected by copyright. All rights reserved.
- An index for drought induced financial risk in the mining industry
- Authors: L. Bonnafous; U. Lall, J. Siegel
Abstract: Water scarcity has emerged as a potential risk for mining operations. High capital spending for desalination and water conflicts leading to asset stranding have recently occurred. Investors in mining companies are interested in the exposure to such risks across portfolios of mining assets (whether the practical at-site consequences are foregone production, higher OPEX and CAPEX and ensuing lost revenues, or asset-stranding). In this paper, an index of the potential financial exposure of a portfolio is developed and its application is illustrated. Since the likely loss at each mine is hard to estimate a priori, one needs a proxy for potential loss. The index considers drought duration, severity and frequency (defined by a return-level in years) at each mining asset, and provides a measure of financial exposure through weighing of production or Net Asset Value. Changes in human needs are not considered, but are relevant, and could be incorporated if global data on mine and other water use were available at the appropriate resolution. Potential for contemporaneous drought incidence across sites in a portfolio is considered specifically. Through an appropriate choice of drought thresholds, an analyst can customize a scenario to assess potential losses in production value or profits, or whether conflicts could emerge that would lead to stranded assets or capital expenditure to secure alternate water supplies. Global climate data sets that allow a customized development of such an index are identified, and selected mining company portfolios are scored as to the risk associated with one publicly available drought index. This article is protected by copyright. All rights reserved.
- The pulse of driftwood export from a very large forested river basin over
multiple timescales, Slave River, Canada
- Authors: Natalie Kramer; Ellen Wohl, Brooke Hess-Homeier, Stephen Leisz
Abstract: This study presents a case study of large wood transport on the great Slave River in northern Canada with the objective to better understand the processes of and variability in pulsed wood fluxes from large forested catchments. We use a varied approach, integrating field characterization of wood, historical anecdotes, repeat aerial imagery of stored wood, and time-lapse imagery of moving wood, for a robust analysis and synthesis of processes behind pulsed wood flux, from yearly uncongested export to rare congested wood floods. Repeat monitoring of known sites of temporary storage with new or historic imagery proved to be a very useful tool for constraining wood flux histories. Pulsed wood export on the Slave River is not an artefact of episodic recruitment from major up-basin disturbances, but rather reflects decadal- to half-century-scale discharge patterns that re-distribute wood recruited from channel migration and bank slumping. We suggest that the multi-year flow history is of paramount importance for estimating wood flux magnitude, followed in declining importance by the yearly sequence of peaks and the magnitude and characteristics of the rising limb of individual floods. This article is protected by copyright. All rights reserved.
- Tracer-based characterization of hyporheic exchange and benthic biolayers
- Authors: Julia L.A. Knapp; Ricardo González-Pinzón, Jennifer D. Drummond, Laurel G. Larsen, Olaf A. Cirpka, Judson W. Harvey
Abstract: Shallow benthic biolayers at the top of the streambed are believed to be places of enhanced biogeochemical turnover within the hyporheic zone. They can be investigated by reactive stream tracer tests with tracer recordings in the streambed and in the stream channel. Common in-stream measurements of such reactive tracers cannot localize where the processing primarily takes place, whereas isolated vertical depth profiles of solutes within the hyporheic zone are usually not representative of the entire stream. We present results of a tracer test where we injected the conservative tracer bromide together with the reactive tracer resazurin into a third-order stream and combined the recording of in-stream breakthrough curves with multi-depth sampling of the hyporheic zone at several locations. The transformation of resazurin was used as an indicator of metabolism, and high-reactivity zones were identified from depth profiles. The results from our subsurface analysis indicate that the potential for tracer transformation (i.e., the reaction rate constant) varied with depth in the hyporheic zone. This highlights the importance of the benthic biolayer, which we found to be on average 2 cm thick in this study, which ranged from one third to one half of the full depth of the hyporheic zone. The reach-scale approach integrated the effects of processes along the reach length, isolating hyporheic processes relevant for whole-stream chemistry and estimating effective reaction rates. This article is protected by copyright. All rights reserved.
- The influence of NaCl concentration on salt precipitation in heterogeneous
- Authors: Mina Bergstad; Dani Or, P.J. Withers, Nima Shokri
Abstract: Evaporation of saline solutions from porous media is governed by the complex interactions between the transport properties of the porous media, the evaporating solution and the external boundary conditions. In the present study, we have investigated the effects of salt concentration on the evaporation process from porous media in the presence of a sharp textural discontinuity; a common heterogeneity in natural porous media formed due to the weathering or formation of soil horizons, wind deposition and erosion. We have conducted a comprehensive series of macro- and micro-scale experiments to delineate how the precipitation pattern is modified as salt concentration varies from relatively low values to a concentration close to the solubility limit. For concentrations much less than the solubility limit, the precipitation begins at the coarse-textured part of the heterogeneous porous media (which is a counter-intuitive result considering the preferential evaporation of water from the fine-textured part of the heterogeneous surface). However, when the concentration is close to the solubility limit, precipitation initiates preferentially at the fine-textured part of the heterogeneous porous surface. This behaviour results from the interaction between the transport properties of the porous media and the properties of the evaporating solution which must be considered. Additionally, using pore-scale images obtained by X-ray micro-computed tomography (CT), we have visualized the dynamics of precipitation in the presence of heterogeneity at high spatial and temporal resolution. The pore-scale results corroborate the mechanisms controlling the precipitation patterns in the presence of textural discontinuities inferred from the macro-scale experiments. This article is protected by copyright. All rights reserved.
- Characterizing the spatial correlation of daily streamflows
- Authors: A. Betterle; M. Schirmer, G. Botter
Abstract: In this study we propose an analytical framework to estimate the spatial correlation of daily flows in two arbitrary locations within a given hydrologic district or river basin. The method builds on the description of the coupled streamflow dynamics at the outlet of two catchments, which are represented as correlated shot noises forced by Poisson rainfall. Novel analytical expressions for the spatial correlation of discharge are derived using a limited number of parameters that encapsulate effective precipitation regime and catchment drainage rates. The method is suited to describe how heterogeneity of climate and landscape features impact the spatial and temporal variability of flow regimes along river systems. The analysis suggests that frequency and intensity of synchronous effective rainfall events in the relevant contributing catchments are the main driver of the spatial correlation of daily discharge, unless the drainage rates of the two catchments differ by almost one order of magnitude. The method also portrays how the topological arrangement of the two outlets along the river network influences the underlying streamflow correlation, and shows how nested catchments tend to maximize the spatial correlation of flow regimes. To demonstrate the potential of the tool, the model is tested on a set of sixteen catchments belonging to a 120,000 km2 region of the United States. The application evidences satisfactory performance (RMSE
- Controls on solute concentration-discharge relationships revealed by
simultaneous hydrochemistry observations of hillslope runoff and stream
flow: The importance of critical zone structure
- Authors: Hyojin Kim; William E. Dietrich, Benjamin M. Thurnhoffer, Jim K. B. Bishop, Inez Y. Fung
Abstract: We investigated controls on concentration-discharge relationships of a catchment underlain by argillite by monitoring both groundwater along a hillslope transect and stream chemistry. Samples were collected at 1-3-day intervals over four years (2009-2013) in Elder Creek in the Eel River Critical Zone Observatory in California. Runoff at our study hillslope is driven by vadose zone flux through deeply weathered argillite (5-25 m thick) to a perched, seasonally dynamic groundwater that then drains to Elder Creek. Low flow derives from the slowly draining deepest perched groundwater that reaches equilibrium between primary and secondary minerals and saturation with calcite under high subsurface pCO2. Arriving winter rains pass through the thick vadose zone, where they rapidly acquire solutes via cation exchange reactions (driven by high pCO2), and then recharge the groundwater that delivers runoff to the stream. These new waters displayed lower solute concentrations than the deep groundwater by less than a factor of 5 (except for Ca). Up to 74% of the total annual solute flux is derived from the vadose zone. The deep groundwater's Ca concentration decreased as it exfiltrates to the stream due to CO2 degassing and this Ca loss is equivalent of 30% of the total chemical weathering flux of Elder Creek. The thick vadose zone in weathered bedrock and the perched groundwater on underlying fresh bedrock result in two distinct processes that lead to the relatively invariant (chemostatic) concentration-discharge behavior. The processes controlling solute chemistry are not evident from stream chemistry and runoff analysis alone. This article is protected by copyright. All rights reserved.
- Impacts of precipitation and potential evapotranspiration patterns on
downscaling soil moisture in regions with large topographic relief
- Authors: Garret S. Cowley; Jeffrey D. Niemann, Timothy R. Green, Mark S. Seyfried, Andrew S. Jones, Peter J. Grazaitis
Abstract: Soil moisture can be estimated at coarse resolutions (>1 km) using satellite remote sensing, but that resolution is poorly suited for many applications. The Equilibrium Moisture from Topography, Vegetation, and Soil (EMT+VS) model downscales coarse-resolution soil moisture using fine-resolution topographic, vegetation, and soil data to produce fine-resolution (10-30 m) estimates of soil moisture. The EMT+VS model performs well at catchments with low topographic relief (≤124 m), but it has not been applied to regions with larger ranges of elevation. Large relief can produce substantial variations in precipitation and potential evapotranspiration (PET), which might affect the fine-resolution patterns of soil moisture. In this research, simple methods to downscale temporal average precipitation and PET are developed and included in the EMT+VS model, and the effects of spatial variations in these variables on the surface soil moisture estimates are investigated. The methods are tested against ground truth data at the 239 km2 Reynolds Creek watershed in southern Idaho, which has 1145 m of relief. The precipitation and PET downscaling methods are able to capture the main features in the spatial patterns of both variables. The space-time Nash-Sutcliffe coefficients of efficiency of the fine-resolution soil moisture estimates improve from 0.33 to 0.36 and 0.41 when the precipitation and PET downscaling methods are included, respectively. PET downscaling provides a larger improvement in the soil moisture estimates than precipitation downscaling likely because the PET pattern is more persistent through time, and thus more predictable, than the precipitation pattern. This article is protected by copyright. All rights reserved.
- Quantifying streambed advection and conduction heat fluxes
- Authors: Daniel Caissie; Charles H. Luce
Abstract: Groundwater and accompanying heat fluxes are particularly relevant for aquatic habitats as they influence living conditions both within the river and streambed. This study focuses on the theory and the development of new equations to estimate conduction and advection heat fluxes into and out of the bed, correcting some earlier misunderstandings and adding parameterizations that extend our understanding of timing of heat fluxes. The new heat flux equations are illustrated using Catamaran Brook (New Brunswick, Canada) stream/streambed temperature data. We show important relationships between fluxes when the surface water temperature 1) follows a sinusoidal function superimposed on a steady-state condition (constant deep streambed temperature) and 2) when sinusoidal variations in stream temperature at two frequencies (annual and diel) are superimposed. When the stream temperature is used as a prescribed boundary condition, the contribution of bed fluid fluxes to stream temperature occurs through the effects of conductive thermal gradients, not through direct contribution/mixing of cold/warm water. Boundary conditions can be modified however to account for direct contribution of cold/warm water (e.g., localised upwelling) and consequences for the conduction heat flux. Equations developed allow for prediction of conductive fluxes to the bed during summer driven by diel and annual temperature fluctuations of the stream water and good agreement was observed between analytic solutions and field data. Results from this study provide a better insight into groundwater and heat fluxes which will ultimately result in better stream temperature models and a better management of fisheries resources. This article is protected by copyright. All rights reserved.
- An interdisciplinary framework for participatory modeling design and
evaluation What makes models effective participatory decision tools?
- Authors: Stefanie M. Falconi; Richard N. Palmer
Abstract: Increased requirements for public involvement in water resources management (WRM) over the past century have stimulated the development of more collaborative decision-making methods. Participatory modeling (PM) uses computer models to inform and engage stakeholders in the planning process in order to influence collaborative decisions in WRM. Past evaluations of participatory models focused on process and final outcomes, yet, were hindered by diversity of purpose and inconsistent documentation. This paper presents a two-stage framework for evaluating PM based on mechanisms for improving model effectiveness as participatory tools. The five dimensions characterize the ‘who, when, how, and why' of each participatory effort (stage 1). Models are evaluated as “boundary objects,” a concept used to describe tools that bridge understanding and translate different bodies of knowledge to improve credibility, salience, and legitimacy (stage 2). This evaluation framework is applied to five existing case studies from the literature. Though the goals of participation can be diverse, the novel contribution of the two-stage proposed framework is the flexibility it has to evaluate a wide range of cases that differ in scope, modeling approach, and participatory context. Also, the evaluation criteria provide a structured vocabulary based on clear mechanisms that extend beyond previous process- and outcome-based evaluations. Effective models are those that take advantage of mechanisms that facilitate dialogue and resolution and improve the accessibility and applicability of technical knowledge. Furthermore, the framework can help build more complete records and systematic documentation of evidence to help standardize the field of PM. This article is protected by copyright. All rights reserved.
- Nonmonotonic and spatial-temporal dynamic slope effects on soil erosion
during rainfall-runoff processes
- Authors: Songbai Wu; Minghui Yu, Li Chen
Abstract: The slope effect on flow erosivity and soil erosion still remains a controversial issue. This theoretical framework explained and quantified the direct slope effect by coupling the modified Green-Ampt equation accounting for slope effect on infiltration, 1D kinematic wave overland flow routing model, and WEPP soil erosion model. The flow velocity, runoff rate, shear stress, interrill, and rill erosion were calculated on 0°-60° isotropic slopes with equal horizontal projective length. The results show that, for short duration rainfall events, the flow erosivity and erosion amounts exhibit a bell-shaped trend which first increase with slope gradient, and then decrease after a critical slope angle. The critical slope angles increase significantly or even vanish with increasing rainfall duration but are nearly independent of the slope projective length. The soil critical shear stress, rainfall intensity and temporal patterns have great influences on the slope effect trend, while the other soil erosion parameters, soil type, hydraulic conductivity, and antecedent soil moisture have minor impacts. Neglecting the slope effect on infiltration would generate smaller erosion and reduce critical slope angles. The relative slope effect on soil erosion in physically-based model WEPP was compared to those in the empirical models USLE and RUSLE. The trends of relative slope effect were found quite different, but the difference may diminish with increasing rainfall duration. Finally, relatively smaller critical slope angles could be obtained with the equal slope length and the range of variation provides a possible explanation for the different critical slope angles reported in previous studies. This article is protected by copyright. All rights reserved.
- Subgrid parameterization for snow depth over mountainous terrain from flat
field snow depth
- Authors: N. Helbig; A. van Herwijnen
Abstract: Snow depth is an important variable for a variety of models including land-surface, meteorological and climate models. Various measurement networks were therefore developed to measure snow depth on the ground. Measurement stations are generally located in gentle terrain (flat field measurements) most often at lower or mid elevation. While these sites have provided a wealth of information, various studies have questioned the representativity of such flat field measurements for the surrounding topography, especially in alpine regions. Using highly-resolved snow depth maps at the peak of winter from two distinct climatic regions in Switzerland and in the Spanish Pyrenees, we developed two parameterizations to estimate domain-averaged snow depth in coarse-scale model applications over complex topography using easy to derive topographic parameters. The first parameterization uses a commonly applied linear lapse rate. Removing the dominant elevation gradient in mean snow depth revealed remaining underlying correlations with other topographic parameters, in particular the sky view factor. The second parameterization combines a power law elevation trend scaled with the subgrid parameterized sky view factor. Using a variety of statistic measures showed that the more complex parameterization performs better when using mean high-resolution flat field snow depth. The performances slightly decreased when formulating the parameterizations for a single flat field snow depth measurement. Nevertheless, the more complex parameterization still outperformed the linear lapse rate model. As the parameterization was developed independently of a specific geographic region we suggest it could be used to assimilate flat field snow depth or snowfall into coarse-scale snow model frameworks. This article is protected by copyright. All rights reserved.
- A new geophone device for understanding environmental impacts caused by
gravel bedload during artificial floods
- Authors: Ryota Tsubaki; Yoshihisa Kawahara, Xin-Hua Zhang, Kentaro Tsuboshita
Abstract: Here, to assess the contribution of gravel bedload on the removal of attached-algae and aquatic plants from a cobble-bed river during small floods, we propose a geophone type method for measuring the local bedload of non-uniform sized gravel. Due to limited peak discharge for focused events during our study, a large fraction of bed material (here cobbles) was immobile and only a small fraction of bed material (sand and gravel) was expected to be transported during the flushing flows we analyzed. The device we developed has a size equivalent to immobile bed material and a shape similar to bed material (rounded cobbles) at the site. The instrument's design allows avoidance of disturbances in river bed micro-topography during installation and local bedload transport during floods. A flume experiment was conducted in order to establish an empirical algorithm for estimating the diameter of impacted gravel and, here, discuss uncertainty related to diameter estimations. The proposed method was utilized to quantify gravel bedload in a cobble-bed river during flushing flows. In the text, we also discuss the contribution of measured gravel bedload during flushing flows on the removal of attached-algae (up to a 37% reduction in chlorophyll-a density) and aquatic plants (a reduction of 38% in dry mass per area). Based on time variation for the measured gravel bedload, we also suggest the propagation of a bed-form composed of the fine sediment fraction migrating on immobile larger sediment and implications for the propagation of the fine sediment wave for attached-algae removal. This article is protected by copyright. All rights reserved.
- Impact of saturation on dispersion and mixing in porous media:
Photo-bleaching pulse injection experiments and shear-enhanced mixing
- Authors: Joaquín Jiménez-Martínez; Tanguy Le Borgne, Hervé Tabuteau, Yves Méheust
Abstract: The dynamics of solute dispersion and mixing in unsaturated flows is analyzed from photo-bleaching experiments in two-dimensional porous micro-models. This technique allows producing pulse line (delta-Dirac) injections of a conservative tracer by bleaching a finite volume of fluorescent without disturbing the flow field. The temporal evolution of the concentration field and the spatial distribution of the air and water phases can be monitored at pore scale. We study the dispersion and mixing of a line of tracer under different water saturations. While dispersion in saturated porous media follows an approximately Fickian scaling, a shift to ballistic scaling is observed as soon as saturation is lowered. Hence, at the time scale of observation, dispersion in our unsaturated flows is dominated by the ballistic separation of tracer blobs within the water phase, between trapped clusters and preferential flow paths. While diffusion plays a minor role in the longitudinal dispersion during the time scale of the experiments, its interplay with fluid deformation is apparent in the dynamics of mixing. The scalar dissipation rates show an initial stretching regime, during which mixing is enhanced by fluid deformation, followed by a dissipation regime, during which diffusion overcomes compression induced by stretching. The transition between these two regimes occurs at the mixing time, when concentration gradients are maximum. We propose a predictive analytical model, based on shear-enhanced diffusion, that captures the dynamics of mixing from basic unsaturated porous media parameters, suggesting that this type of model may be a useful framework at larger scales. This article is protected by copyright. All rights reserved.
- Water quality data for national-scale aquatic research: The Water Quality
- Authors: Emily K. Read; Lindsay Carr, Laura De Cicco, Hilary A. Dugan, Paul C. Hanson, Julia A. Hart, James Kreft, Jordan S. Read, Luke A. Winslow
Abstract: Aquatic systems are critical to food, security, and society. But, water data are collected by hundreds of research groups and organizations, many of which use non-standard or inconsistent data description and dissemination, and disparities across different types of water observation systems represent a major challenge for freshwater research. In response to this, the Water Quality Portal (WQP) was developed by the U.S. Environmental Protection Agency, the U.S. Geological Survey, and the National Water Quality Monitoring Council to be a single point of access for water quality data dating back more than a century. The WQP is the largest standardized water quality data set available at the time of this writing, with more than 290 million records from more than 2.7 million sites in groundwater, inland, and coastal waters. The number of data contributors, data consumers, and third-party application developers making use of the WQP are rapidly growing. Here, we introduce the WQP, including an overview of data, the standardized data model, and data access and services; and we describe challenges and opportunities associated with using WQP data. We also demonstrate the value of the WQP data by characterizing seasonal variation in lake water clarity for regions of the continental U.S. The code used to access, download, analyze, and display this WQP data as shown in the figures is included as Supplemental Materials. This article is protected by copyright. All rights reserved.
- The future role of dams in the United States of America
- Authors: Michelle Ho; Upmanu Lall, Maura Allaire, Naresh Devineni, Hyun Han Kwon, Indrani Pal, David Raff, Dave Wegner
Abstract: Storage and controlled distribution of water have been key elements of a human strategy to overcome the space and time variability of water, which have been marked by catastrophic droughts and floods throughout the course of civilization. In the United States the peak of dam building occurred in the mid-20th century with knowledge limited to the scientific understanding and hydrologic records of the time. Ecological impacts were considered differently than current legislative and regulatory controls would potentially dictate. Additionally, future costs such as maintenance or removal beyond the economic design life were not fully considered. The converging risks associated with aging water storage infrastructure and uncertainty in climate in addition to the continuing need for water storage, flood protection, and hydropower result in a pressing need to address the state of dam infrastructure across the nation. Decisions regarding the future of dams in the United States may, in turn, influence regional water futures through groundwater outcomes, economic productivity, migration, and urban growth. We advocate for a comprehensive national water assessment and a formal analysis of the role dams play in our water future. We emphasize the urgent need for environmentally and economically sound strategies to integrate surface and groundwater storage infrastructure in local, regional, and national water planning considerations. A research agenda is proposed to assess dam failure impacts and the design, operation, and need for dams considering both paleo and future climate, utilization of groundwater resources, and the changing societal values towards the environment. This article is protected by copyright. All rights reserved.
- Examination of the seepage face boundary condition in subsurface and
coupled surface/subsurface hydrological models
- Authors: C. Scudeler; C. Paniconi, D. Pasetto, M. Putti
Abstract: A seepage face is a nonlinear dynamic boundary that strongly affects pressure head distributions, water table fluctuations, and flow patterns. Its handling in hydrological models, especially under complex conditions such as heterogeneity and coupled surface/subsurface flow, has not been extensively studied. In this paper we compare the treatment of the seepage face as a static (Dirichlet) versus dynamic boundary condition, we assess its resolution under conditions of layered heterogeneity, we examine its interaction with a catchment outlet boundary, and we investigate the effects of surface/subsurface exchanges on seepage faces forming at the land surface. The analyses are carried out with an integrated catchment hydrological model. Numerical simulations are performed for a synthetic rectangular sloping aquifer and for an experimental hillslope from the Landscape Evolution Observatory. The results show that the static boundary condition is not always an adequate stand-in for a dynamic seepage face boundary condition, especially under conditions of high rainfall, steep slope, or heterogeneity; that hillslopes with layered heterogeneity give rise to multiple seepage faces that can be highly dynamic; that seepage face and outlet boundaries can coexist in an integrated hydrological model and both play an important role; and that seepage faces at the land surface are not always controlled by subsurface flow. The paper also presents a generalized algorithm for resolving seepage face outflow that handles heterogeneity in a simple way, is applicable to unstructured grids, and is shown experimentally to be equivalent to the treatment of atmospheric boundary conditions in subsurface flow models. This article is protected by copyright. All rights reserved.
- Elemental properties, hydrology, and biology interact to shape
concentration-discharge curves for carbon, nutrients, sediment, and major
- Authors: F. Moatar; B.W. Abbott, M. Minaudo, F. Curie, G. Pinay
Abstract: To investigate the prevalence and cause of concentration-discharge (C-Q) relationships for carbon, nutrients, major ions, and particulates, we analyzed 40 years of water quality data from 293 monitoring stations in France. Catchments drained diverse landscapes and ranged from 50 to 110,000 km2, together covering nearly half of France. To test for differences during low and high flows, we calculated independent C-Q slopes above and below the median discharge. We found that 84% of all catchment-element combinations were chemodynamic for at least half of the hydrograph and 60% of combinations showed non-linear C-Q curves. Only two or three of the nine possible C-Q modalities were manifest for each parameter, and these modalities were stable through time, suggesting that intrinsic and extrinsic elemental properties (e.g. solubility, reactivity, and source dynamics) set basic C-Q templates for each parameter, which are secondarily influenced by biological activity during low flows, and the interaction between hydrology and catchment characteristics at high flows. Several patterns challenged current C-Q views, including low-flow chemostasis for TSS in 66% of catchments, low-flow biological mediation of NO3- in 71% of catchments, and positive C-Q for dissolved organic carbon independent of catchment size in 80% of catchments. Efforts to reduce nutrient loading decreased phosphorus concentration and altered C-Q curves, but NO3- continued to increase. While C-Q segmentation requires more data than a single analysis, the prevalence of non-linear C-Q slopes demonstrates the potential information loss associated with linear or monotonic analysis of C-Q relationships, and conversely, the value of long-term monitoring. This article is protected by copyright. All rights reserved.
- Impact of mountain permafrost on flow path and runoff response in a high
- Authors: M. Rogger; G.B. Chirico, H. Hausmann, K. Krainer, E. Brückl, P. Stadler, G. Blöschl
Abstract: Permafrost in high alpine catchments is expected to disappear in future warmer climates, but the hydrological impact of such changes is poorly understood. This paper investigates the flow paths and the hydrological response in a 5 km2 high alpine catchment in the Ötztal Alps, Austria, and their changes resulting from a loss of permafrost. Spatial permafrost distribution, depth to the permafrost table and depth to the bedrock were mapped by geophysical methods. Catchment runoff and meteorological variables were monitored from June 2008 to December 2011. These data were used along with field experience to infer conceptual schemes of the dominant flow paths in four types of hillslopes that differ in terms of their unconsolidated sediment characteristics and the presence of permafrost. The four types are: talus fans, rock glaciers, Little Ice Age (LIA) till and Pre-LIA till. Permafrost tends to occur in the first three types, but is absent from Pre-LIA till. Based on these flow path concepts, runoff was simulated for present conditions and for future conditions when permafrost has completely disappeared. The simulations indicate that complete disappearance of permafrost will reduce flood peaks by up to 17% and increase runoff during recession by up to 19%. It is argued that change modeling needs to account for flow path types and their changes based on geophysical surveys and field investigations. This article is protected by copyright. All rights reserved.
- Probabilistic inversion with graph cuts: Application to the Boise
hydrogeophysical research site
- Authors: Guillaume Pirot; Niklas Linde, Grégoire Mariethoz, John Bradford
Abstract: Inversion methods that build on multiple-point statistics tools offer the possibility to obtain model realizations that are not only in agreement with field data, but also with conceptual geological models that are represented by training images. A recent inversion approach based on patch-based geostatistical resimulation using graph cuts outperforms state-of-the-art multiple point statistics methods when applied to synthetic inversion examples featuring continuous and discontinuous property fields. Applications of multiple-point statistics tools to field data are challenging due to inevitable discrepancies between actual subsurface structure and the assumptions made in deriving the training image. We introduce several amendments to the original graph cut inversion algorithm and present a first-ever field application by addressing porosity estimation at the Boise Hydrogeophysical Research Site, Boise, Idaho. We consider both a classical multi-Gaussian and an outcrop-based prior model (training image) that are in agreement with available porosity data. When conditioning to available crosshole ground-penetrating radar data using Markov chain Monte Carlo, we find that the posterior realizations honor overall both the characteristics of the prior models and the geophysical data. The porosity field is inverted jointly with the measurement error and the petrophysical parameters that link dielectric permittivity to porosity. Even though the multi-Gaussian prior model leads to posterior realizations with higher likelihoods, the outcrop-based prior model shows better convergence. In addition, it offers geologically more realistic posterior realizations and it better preserves the full porosity range of the prior. This article is protected by copyright. All rights reserved.
- Experimental investigation of the impact of compound-specific dispersion
and electrostatic interactions on transient transport and solute
- Authors: Muhammad Muniruzzaman; Massimo Rolle
Abstract: This study investigates the effects of compound-specific diffusion/dispersion and electrochemical migration on transient solute transport in saturated porous media. We conducted laboratory bench-scale experiments, under advection-dominated regimes (seepage velocity: 0.5, 5, 25 m/day), in a quasi two-dimensional flow-through setup using pulse injection of multiple tracers (both uncharged and ionic species). Extensive sampling and measurement of solutes' concentrations (∼1500 samples; >3000 measurements) were performed at the outlet of the flow-through setup, at high spatial and temporal resolution. The experimental results show that compound-specific effects and charge-induced Coulombic interactions are important not only at low velocities and/or for steady-state plumes but also for transient transport under high flow velocities. Such effects can lead to a remarkably different behavior of measured breakthrough curves also at very high Péclet numbers. To quantitatively interpret the experimental results, we used four modeling approaches: classical advection-dispersion equation (ADE), continuous time random walk (CTRW), dual domain mass transfer model (DDMT), and a multicomponent ionic dispersion model. The latter is based on the multicomponent formulation of coupled diffusive/dispersive fluxes and was used to describe and explain the electrostatic effects of charged species. Furthermore, we determined experimentally the temporal profiles of the flux-related dilution index. This metric of mixing, used in connection with the traditional solute breakthrough curves, proved to be useful to correctly distinguish between plume spreading and mixing, particularly for the cases in which the sole analysis of integrated concentration breakthrough curves may lead to erroneous interpretation of plume dilution. This article is protected by copyright. All rights reserved.
- Modeling multidecadal surface water inundation dynamics and key drivers on
large river basin scale using multiple time series of Earth observation
and river flow data
- Authors: V. Heimhuber; M. G. Tulbure, M. Broich
Abstract: Periodically inundated floodplain areas are hotspots of biodiversity and provide a broad range of ecosystem services but have suffered alarming declines in recent history. Despite their importance, their long-term surface water (SW) dynamics and hydro-climatic drivers remain poorly quantified on continental scales. In this study, we used a 26-year time series of Landsat-derived SW maps in combination with river flow data from 68 gauges and spatial time series of rainfall, evapotranspiration and soil moisture to statistically model SW dynamics as a function of key drivers across Australia's Murray-Darling Basin (∼1 million km2). We fitted generalized additive models for 18,521 individual modeling units made up of 10x10 km grid cells, each split into floodplain, floodplain-lake and non-floodplain area. Average goodness of fit of models was high across floodplains and floodplain-lakes (r2 > 0.65), which were primarily driven by river flow, and was lower for non-floodplain areas (r2 > 0.24), which were primarily driven by rainfall. Local climate conditions were more relevant for SW dynamics in the northern compared to the southern basin and had the highest influence in the least regulated and most extended floodplains. We further applied the models of two contrasting floodplain areas to predict SW extents of cloud-affected time steps in the Landsat series during the large 2010 floods with high validated accuracy (r2 > 0.97). Our framework is applicable to other complex river basins across the world and enables a more detailed quantification of large floods and drivers of SW dynamics compared to existing methods. This article is protected by copyright. All rights reserved.
- Analytical solutions for aquifer thermal energy storage
- Authors: Jan Martin Nordbotten
Abstract: The concept of aquifer thermal energy storage involves injection of water at elevated temperature, and possibly non-ambient salinity, into a host aquifer. We consider axisymmetric injection, wherein both the composition and temperature of the injected fluid differs from the fluid in the target aquifer. In this setting, we derive the governing equations within a vertically integrated framework, and show their self-similar structure. We subsequently derive explicit approximate solutions to the self-similar equations for parameter ranges of relevance to thermal energy storage (small density and viscosity differences), we derive explicit approximate solutions to the self-similar equations.The analysis is supported by numerical validation, covering the relevant parameter regime. The resulting comparisons demonstrate the mathematical qualities of the analytical approximations. A study based on field data from analogue sites, justifies the assertions regarding the magnitude of the dimensionless parameters used in the analysis. This article is protected by copyright. All rights reserved.
- Measurement and simulation of heat exchange in fractured bedrock using
inert and thermally degrading tracers
- Authors: Adam. J. Hawkins; Don. B. Fox, Matthew. W. Becker, Jefferson. W. Tester
Abstract: Multi-component groundwater tracer tests were conducted in a well-characterized field site in Altona, NY using inert carbon-cored nanoparticles and a thermally degrading phenolic compound. Experiments were conducted in a meso-scale reservoir consisting of a single sub-horizontal bedding plane fracture located 7.6 m below ground surface contained between two wells separated by 14.1 m. The reservoir rock, initially at 11.7°C, was heated using 74°C water. During the heating process, a series of tracer tests using thermally degrading tracers were used to characterize the progressive in-situ heating of the fracture. Fiber-Optic Distributed Temperature Sensing (FODTS) was used to measure temperature rise orthogonal to the fracture surface at ten locations. Recovery of the thermally degrading tracer's product was increased as the reservoir was progressively heated indicating that the advancement of the thermal front was proportional to the mass fraction of the thermally degrading tracer recovered. Both GPR imaging and FODTS measurements reveal that flow was reduced to a narrow channel which directly connected the two wells and led to rapid thermal breakthrough. Computational modeling of inert tracer and heat transport in a two-dimensional discrete fracture demonstrate that subsurface characterization using inert tracers alone could not uniquely characterize the Altona field site. However, the inclusion of a thermally degrading tracer may permit accurate subsurface temperature monitoring. At the Altona field site, however, fluid-rock interactions appear to have increased reaction rates relative to lab-based measurements made in the absence of rock surfaces. This article is protected by copyright. All rights reserved.
- Identifying modern and historic recharge events from tracer-derived
groundwater age distributions
- Authors: James L. McCallum; Peter. G. Cook, Shawan Dogramaci, Roland Purtschert, Craig T. Simmons, Lawrence Burk
Abstract: Understanding groundwater ages offers insight into the time scales of recharge, aquifer storage turnover times and contaminant protection time frames. The ability to quantify groundwater age distributions heavily depends on the choice of the interpretive model, and often important features of the age distribution cannot be identified with the subset of available models. In this paper, we implemented a multiple tracer method using a technique that assumes limited details regarding the shape of the age distribution and applied it to dewatering wells at a mine site in the Pilbara region of north-western Australia. Using our method, we were able to identify distinct age components in the groundwater. We calculated the presence of four distinct age groups in the samples. All wells contained water aged between zero and 20 years. However, the rest of the samples were composed of water between 50 and 100 years, 100 and 600 years, or water approximately 1000 years old. These were consistent with local recharge sources (50-100 years) and knowledge of paleoclimate from lake sediment records. We found that although the age components were well constrained, the relative proportions of each component were highly sensitive to errors of environmental tracer data. Our results show that our method can identify distinct age groups in groundwater samples without prior knowledge of the age distribution. The presence of distinct recharge times gives insight into groundwater flow conditions over long periods of time. This article is protected by copyright. All rights reserved.
- Incorporating institutions and collective action into a socio-hydrological
model of flood resilience
- Authors: David J. Yu; Nikhil Sangwan, Kyungmin Sung, Xi Chen, Venkatesh Merwade
Abstract: Stylized socio-hydrological models have mainly used social memory aspects such as community awareness or sensitivity to connect hydrologic change and social response. However, social memory alone does not satisfactorily capture the details of how human behavior is translated into collective action for water resources governance. Nor is it the only social mechanism by which the two-way feedbacks of socio-hydrology can be operationalized. This study contributes towards bridging of this gap by developing a socio-hydrological model of a flood resilience that includes two additional components: (1) institutions for collective action, and (2) connections to an external economic system. Motivated by the case of community-managed flood protection systems (polders) in coastal Bangladesh, we use the model to understand critical general features that affect long-term resilience of human-flood systems. Our findings suggest that occasional adversity can enhance long-term resilience. Allowing some hydrological variability to enter into the polder can increase its adaptive capacity for resilience through the preservation of social norm for collective action. Further, there are potential trade-offs associated with optimization of flood resistance through structural measures. By reducing sensitivity to floods, the system may become more fragile under the double impact of floods and economic change. This article is protected by copyright. All rights reserved.
- Differences in behavior and distribution of permafrost-related lakes in
Central Yakutia and their response to climatic drivers
- Authors: M. Ulrich; H. Matthes, L. Schirrmeister, J. Schütze, H. Park, Y. Iijima, A.N. Fedorov
Abstract: The Central Yakutian permafrost landscape is rapidly being modified by land use and global warming, but small-scale thermokarst process variability and hydrological conditions are poorly understood. We analyze lake-area changes and thaw subsidence of young thermokarst lakes on ice-complex deposits (yedoma lakes) in comparison to residual lakes in alas basins during the last 70 years for a local study site and we record regional lake size and distribution on different ice-rich permafrost terraces using satellite and historical airborne imagery. Statistical analysis of climatic and ground-temperature data identified driving factors of yedoma- and alas-lake changes. Overall, lake area is larger today than in 1944 but alas-lake levels have oscillated greatly over 70 years, with a mean alas-lake-radius change rate of 1.6 ± 3.0 m/yr. Anthropogenic disturbance and forest degradation initiated, and climate forced rapid, continuous yedoma-lake growth. The mean yedoma lake-radius change rate equals 1.2 ± 1.0 m/yr over the whole observation period. Mean thaw subsidence below yedoma lakes is 6.2 ± 1.4 cm/yr. Multiple regression analysis suggests that winter precipitation, winter temperature, and active-layer properties are primary controllers of area changes in both lake types; summer weather and permafrost conditions additionally influence yedoma-lake growth rates. The main controlling factors of alas-lake changes are unclear due to larger catchment areas and subsurface hydrological conditions. Increasing thermokarst activity is currently linked to older terraces with higher ground-ice contents, but thermokarst activity will likely stay high and wet conditions will persist within the near future in Central Yakutian alas basins.
- Insights into hydrologic and hydrochemical processes based on
concentration-discharge and endmember mixing analyses in the mid-Merced
River Basin, Sierra Nevada, California
- Authors: Fengjing Liu; Martha H. Conklin, Glenn D. Shaw
Abstract: Both concentration-discharge relation and endmember mixing analysis were explored to elucidate the connectivity of hydrologic and hydrochemical processes using chemical data collected during 2006-2008 at Happy Isles (468 km2), Pohono Bridge (833 km2), and Briceburg (1,873 km2) in the snowmelt-fed mid Merced River basin, augmented by chemical data collected by the USGS during 1990-2014 at Happy Isles. Concentration-discharge (C-Q) in streamflow was dominated by a well-defined power-law relation, with the magnitude of exponent (0.02-0.6) and R2 values (p
- Transverse and longitudinal mixing in real emergent vegetation at low
- Authors: F. Sonnenwald; J. R. Hart, P. West, V. R. Stovin, I. Guymer
Abstract: Understanding solute mixing within real vegetation is critical to predicting and evaluating the performance of engineered natural systems such as stormwater ponds. For the first time, mixing has been quantified through simultaneous laboratory measurements of transverse and longitudinal dispersion within artificial and real emergent vegetation. Dispersion coefficients derived from a routing solution to the 2D Advection Dispersion Equation (ADE) are presented that compare the effects of vegetation type (artificial, Typha latifolia or Carex acutiformis) and growth season (winter or summer). The new experimental dispersion coefficients are plotted with the experimental values from other studies and used to review existing mixing models for emergent vegetation. The existing mixing models fail to predict the observed mixing within natural vegetation, particularly for transverse dispersion, reflecting the complexity of processes associated with the heterogeneous nature of real vegetation. Observed stem diameter distributions are utilized to highlight the sensitivity of existing models to this key length-scale descriptor, leading to a recommendation that future models intended for application to real vegetation should be based on probabilistic descriptions of both stem diameters and stem spacings. This article is protected by copyright. All rights reserved.
- Approximate solutions for diffusive fracture-matrix transfer: Application
to storage of dissolved CO2 in fractured rocks
- Authors: Quanlin Zhou; Curtis M. Oldenburg, Lee H. Spangler, Jens T. Birkholzer
Abstract: Analytical solutions with infinite exponential series are available to calculate the rate of diffusive transfer between low-permeability blocks and high-permeability zones in the subsurface. Truncation of these series is often employed by neglecting the early-time regime. In this paper, we present unified-form approximate solutions in which the early-time and the late-time solutions are continuous at a switchover time. The early-time solutions are based on three-term polynomial functions in terms of square root of dimensionless time, with the first coefficient dependent only on the dimensionless area-to-volume ratio. The last two coefficients are either determined analytically for isotropic blocks (e.g., spheres and slabs) or obtained by fitting the exact solutions, and they solely depend on the aspect ratios for rectangular columns and parallelepipeds. For the late-time solutions, only the leading exponential term is needed for isotropic blocks, while a few additional exponential terms are needed for highly anisotropic rectangular blocks. The optimal switchover time is between 0.157 and 0.229, with highest relative approximation error less than 0.2%. The solutions are used to demonstrate the storage of dissolved CO2 in fractured reservoirs with low-permeability matrix blocks of single and multiple shapes and sizes. These approximate solutions are building blocks for development of analytical and numerical tools for hydraulic, solute, and thermal diffusion processes in low-permeability matrix blocks. This article is protected by copyright. All rights reserved.
- Labyrinths in large reservoirs: An invisible barrier to fish migration and
the solution through reservoir operation
- Authors: Zhihao Xu; Xinan Yin, Tao Sun, Yanpeng Cai, Yu Ding, Wei Yang, Zhifeng Yang
Abstract: Reservoir construction changes a river's natural flows and temperature, thereby threatening fish migration. Researchers have tried to restore fish migration passages by ensuring environmental flows in downstream river channels. However, reservoir impoundment changes upstream environments from lotic to lentic and thereby hinders fish migration by eliminating migration cues, which has been rarely considered. This study characterized the invisible barriers that large reservoirs create for migratory fish. Water currents are the primary orientation cues for migration due to fish's natural rheotactic tendency. Fish also require suitable temperatures during migration. We built a quasi-3D model to simulate hydrodynamic and temperature conditions in large reservoirs and tested whether these conditions met the velocity and temperature requirements of fish. Due to the strong effects of operation on reservoir conditions, we proposed an eco-friendly technical operating solution to restore migration passages. We added an ecological constraint (i.e., creating a suitable velocity field for fish migration) to reservoir operation model and applied multi-objective optimization to simultaneously protect reservoir benefits. As a case, we applied our approach to China's Danjiangkou Reservoir. We found that velocities in more than half of the zones along the potential fish migration route through the reservoir were lower than the fish requirement and could not offer orientation cues for migration. The eco-friendly operating scheme effectively restored a fish migration passage by managing reservoir releases during key migration periods, slightly reducing the reservoir's socio-economic benefits by 1.67%∼5.03%. This study provides a new perspective on biodiversity and fisheries protection in global regulated rivers. This article is protected by copyright. All rights reserved.
- Generation of 3-D hydrostratigraphic zones from dense airborne
electromagnetic data to assess groundwater model prediction error
- Authors: N.K. Christensen; B.J. Minsley, S. Christensen
Abstract: We present a new methodology to combine spatially dense high-resolution airborne electromagnetic (AEM) data and sparse borehole information to construct multiple plausible geological structures using a stochastic approach. The method developed allows for quantification of the performance of groundwater models built from different geological realizations of structure. Multiple structural realizations are generated using geostatistical Monte Carlo simulations that treat sparse borehole lithological observations as hard data and dense geophysically derived structural probabilities as soft data. Each structural model is used to define 3D hydrostratigraphical zones of a groundwater model, and the hydraulic parameter values of the zones are estimated by using nonlinear regression to fit hydrological data (hydraulic head and river discharge measurements). Use of the methodology is demonstrated for a synthetic domain having structures of categorical deposits consisting of sand, silt, or clay. It is shown that using dense AEM data with the methodology can significantly improve the estimated accuracy of the sediment distribution as compared to when borehole data are used alone. It is also shown that this use of AEM data can improve the predictive capability of a calibrated groundwater model that uses the geological structures as zones. However, such structural models will always contain errors because even with dense AEM data it is not possible to perfectly resolve the structures of a groundwater system. It is shown that when using such erroneous structure in a groundwater model it can lead to biased parameter estimates and biased model predictions, therefore impairing the model's predictive capability. This article is protected by copyright. All rights reserved.
- Comparison of soil wetness from multiple models over Australia with
- Authors: Vinod Kumar; I. Dharssi, J. Bally, P. Steinle, D. McJannet, J. Walker
Abstract: The McArthur Forest Fire Danger Index used in Australia for operational fire warnings has a component representing fuel availability called the Drought Factor (DF). The DF is partly based on soil moisture deficit, calculated as either the Keetch-Byram Drought Index (KBDI) or Mount's Soil Dryness Index (MSDI). The KBDI and MSDI are simplified water balance models driven by observation based daily rainfall and temperature. In this work, gridded KBDI and MSDI analyses are computed at a horizontal resolution of 5 km and are verified against in-situ soil moisture observations. Also verified is another simple model called the Antecedent Precipitation Index (API). Soil moisture analyses from the Australian Community Climate and Earth System Simulator (ACCESS) global Numerical Weather Prediction (NWP) system as well as remotely sensed soil wetness retrievals from the Advanced Scatterometer (ASCAT) are also verified. The verification shows that the NWP soil wetness analyses have greater skill and smaller biases than the KBDI, MSDI and API analyses. This is despite the NWP system having a coarse horizontal resolution and not using observed precipitation. The average temporal correlations (root mean square difference) between cosmic ray soil moisture monitoring facility observations and modelled or remotely sensed soil wetness are 0.82 (0.15±0.02), 0.66 (0.33±0.07), 0.77 (0.20±0.03), 0.74 (0.22±0.03) and 0.83 (0.18±0.04) for NWP, KBDI, MSDI, API and ASCAT. The results from this study suggests that analyses of soil moisture can be greatly improved by using physically based land surface models, remote sensing measurements and data assimilation. This article is protected by copyright. All rights reserved.
- Soil moisture background error covariance and data assimilation in a
coupled land-atmosphere model
- Authors: Liao-Fan Lin; Ardeshir M. Ebtehaj, Jingfeng Wang, Rafael L. Bras
Abstract: This study characterizes the space-time structure of soil moisture background error covariance and paves the way for the development of a soil moisture variational data assimilation system for the Noah land surface model coupled to the Weather Research and Forecasting (WRF) model. The soil moisture background error covariance over the contiguous United States exhibits strong seasonal and regional variability with the largest values occurring in the uppermost soil layer during the summer. Large background error biases were identified, particularly over the Southeastern United States, caused mainly by the discrepancy between the WRF-Noah simulations and the initial conditions derived from the used operational global analysis dataset. The assimilation of the Soil Moisture and Ocean Salinity (SMOS) soil moisture data notably reduces the error of soil moisture simulations. On average, data assimilation with space-time varying background error covariance results in 33% and 35% reduction in the root-mean-square error and the mean absolute error, respectively, in the simulation of hourly top 10-cm soil moisture, mainly due to implicit reductions in soil moisture biases. In terms of correlation, the improvement in soil moisture simulations is also observed but less notable, indicating the limitation of coarse-scale soil moisture data assimilation in capturing fine-scale soil moisture variation. In addition, soil moisture data assimilation improves the simulations of latent heat fluxes but shows a marginal impact on the simulations of sensible latent heat fluxes and precipitation. This article is protected by copyright. All rights reserved.
- Stochastic estimation of hydraulic transmissivity fields using flow
connectivity indicator data
- Authors: G. Freixas; D. Fernàndez-Garcia, X. Sanchez-Vila
Abstract: Most methods for hydraulic test interpretation rely on a number of simplified assumptions regarding the homogeneity and isotropy of the underlying porous media. This way, the actual heterogeneity of any natural parameter, such as transmissivity (), is transferred to the corresponding estimates in a way heavily dependent on the interpretation method used. An example is a long-term pumping test interpreted by means of the Cooper-Jacob method, which implicitly assumes a homogeneous isotropic confined aquifer. The estimates obtained from this method are not local values, but still have a clear physical meaning; the estimated represents a regional-scale effective value, while the log-ratio of the normalized estimated storage coefficient, indicated by , is an indicator of flow connectivity, representative of the scale given by the distance between the pumping and the observation wells. In this work we propose a methodology to use , together with sampled local measurements of transmissivity at selected points, to map the expected value of local values using a technique based on cokriging. Since the interpolation involves two variables measured at different support scales, a critical point is the estimation of the covariance and crosscovariance matrices. The method is applied to a synthetic field displaying statistical anisotropy, showing that the inclusion of connectivity indicators in the estimation method provide maps that effectively display preferential flow pathways, with direct consequences in solute transport. This article is protected by copyright. All rights reserved.
- Evaluating waterpoint sustainability and access implications of revenue
collection approaches in rural Kenya
- Authors: T. Foster; R. Hope
Abstract: Water policies in many sub-Saharan African countries stipulate that rural communities are responsible for self-financing their waterpoint's operation and maintenance. In the absence of policy consensus or evidence on optimal payment models, rural communities adopt a diversity of approaches. This study empirically assesses waterpoint sustainability and access outcomes associated with different revenue collection approaches on the south coast of Kenya. The analysis draws on a unique data set comprising financial records spanning 27 years and 100 communities, operational performance indicators for 200 waterpoints, and water source choices for more than 2,000 households. Results suggest communities collecting pay-as-you-fetch fees on a volumetric basis generate higher levels of income and experience improved operational performance compared with communities charging flat fees. In both cases, financial flows mirror seasonal rainfall peaks and troughs. These outcomes are tempered by evidence that households are more likely to opt for an unimproved drinking water source when a pay-as-you-fetch system is in place. The findings illuminate a possible tension between financial sustainability and universal access. If the Sustainable Development Goal of 'safe water for all' is to become a reality, policymakers and practitioners will need to address this issue and ensure rural water services are both sustainable and inclusive. This article is protected by copyright. All rights reserved.
- Issue Information
- Pages: 9213 - 9214
- A narrative method for analyzing transitions in urban water management:
The case of the Miami-Dade Water and Sewer Department
- Authors: Galen Treuer; Elizabeth Koebele, Aaron Deslatte, Kathleen Ernst, Margaret Garcia, Kim Manago
Abstract: Although the water management sector is often characterized as resistant to risk and change, urban areas across the United States are increasingly interested in creating opportunities to transition toward more sustainable water management practices. These transitions are complex and difficult to predict – the product of water managers acting in response to numerous biophysical, regulatory, and political factors within institutional constraints. Gaining a better understanding of how these transitions occur is crucial for continuing to improve water management. This paper presents a replicable methodology for analyzing how urban water utilities transition toward sustainability. The method combines standardized quantitative measures of variables that influence transitions with contextual qualitative information about a utility's unique decision making context to produce structured, data-driven narratives. Data-narratives document the broader context, the utility's pre-transition history, key events during an accelerated period of change, and the consequences of transition. Eventually, these narratives should be compared across cases to develop empirically-testable hypotheses about the drivers of and barriers to utility-level urban water management transition. The methodology is illustrated through the case of the Miami-Dade Water and Sewer Department (WASD) in Miami-Dade County, Florida and its transition towards more sustainable water management in the 2000s, during which per capita water use declined, conservation measures were enacted, water rates increased, and climate adaptive planning became the new norm. This article is protected by copyright. All rights reserved.
- A long-term perspective of the hydroclimatological impacts of atmospheric
rivers over the central United States
- Authors: Munir Ahmad Nayak; Gabriele Villarini
Abstract: The focus of this study is on the climatology of atmospheric rivers (ARs) over the central United States using six atmospheric reanalysis products. This climatology is used to understand the long-term impacts of ARs on annual precipitation, precipitation extremes, and flooding over the central United States. The relationship between the frequency of ARs and three prominent large-scale atmospheric modes [Pacific-North American (PNA) teleconnection, Artic Oscillation (AO), and North Atlantic Oscillation (NAO)] is investigated, and the results are used to statistically model the frequency of ARs at the seasonal scale.AR characteristics (e.g., frequency, duration) are generally robust across the different reanalysis products. ARs exhibit a marked seasonality, with the largest activity in winter (more than ten ARs per season on average), and the lowest in summer (less than two ARs per season on average). Overall, the duration of most ARs is less than three days, but exceptionally persistent ARs (more than six days) are also observed.The year-to-year variations in the total annual precipitation over the central United States are largely explained by the variations in AR-related precipitation. Moreover, 40% of the top 1% daily precipitation extremes are associated with ARs, and more than 70% of the annual instantaneous peak discharges and peaks-over-threshold floods are associated with these storms, in particular during winter and spring.The seasonal frequency of ARs can be described in terms of large-scale atmospheric modes, with PNA playing a major role in particular in winter and spring. This article is protected by copyright. All rights reserved.
- Experiment on temporal variation of bedload transport in response to
changes in sediment supply in streams
- Authors: Maria A. Elgueta-Astaburuaga; Marwan A. Hassan
Abstract: A flume experiment was conducted to study channel adjustment to episodic sediment supply in mountain streams. The bulk sediment used for the bed and feed included grain sizes 0.5-64 mm with geometric mean Dg (bulk) of 5.7 mm. Water discharge was held constant for 40 h, and 300 kg of sediment was supplied through a range of scenarios. Bed slope, sediment storage, sediment transport and bed surface texture responded to sediment supply. During the first of seven runs, bed slope decreased from 0.022 m/m (flume slope) to 0.018 m/m due to sediment starvation. Bed slope increased beginning in the second run as the bed aggraded due to preferential storage of grains >8 mm. Transport rate and bed-surface particle size were significantly affected by magnitude–frequency of sediment feed. Under constant feed, transport rate increased gradually and Dg (surface) ranged between 12-15 mm. Instead, sediment pulses caused a pronounced increase in sediment transport rate and surface fining, trends that were inverted as sediment evacuated. At the run-scale, sediment transport and storage behaved as with constant feed if pulse relaxation time exceeded time between pulses. The increase in transport rate and surface fining were proportional to pulse size. After the 300 kg pulse, transport rate reached 100 g m−1 s−1 and Dg (surface) was 12 mm. Textural differences on the initial bed surface influenced the patterns of sediment transport. Channel adjustment was controlled by magnitude-frequency of sediment feed and not by total feed. This article is protected by copyright. All rights reserved.
- Estimating snow water equivalent in a Sierra Nevada watershed via
spaceborne radiance data assimilation
- Authors: Dongyue Li; Michael Durand, Steven A. Margulis
Abstract: This paper demonstrates improved retrieval of snow water equivalent (SWE) from spaceborne passive microwave measurements for the sparsely-forested Upper Kern watershed (511 km2) in the southern Sierra Nevada (USA). This is accomplished by assimilating AMSR-E 36.5 GHz measurements into model predictions of SWE at 90-m spatial resolution using the Ensemble Batch Smoother (EnBS) data assimilation framework. For each water year (WY) from 2003 to 2008, SWE was estimated for the accumulation season (October 1st to April 1st) with the assimilation of the measurements in the dry snow season (December 1st to February 28th). The EnBS SWE estimation was validated against snow courses and snow pillows. On average, the EnBS accumulation season SWE RMSE was 77.4 mm (13.1%, relative to peak accumulation), despite deep snow (average peak SWE of 545 mm). The prior model estimate without assimilation had an accumulation season average RMSE of 119.7 mm. After assimilation, the overall bias of the accumulation season SWE estimates was reduced by 84.2%, and the RMSE reduced by 35.4%. The assimilation also reduced the bias and the RMSE of the April 1st SWE estimates by 80.9% and 45.4%, respectively. The EnBS is expected to work well above treeline and for dry snow. This article is protected by copyright. All rights reserved.
- Improving physically based snow simulations by assimilating snow depths
using the particle filter
- Authors: Jan Magnusson; Adam Winstral, Andreas S. Stordal, Richard Essery, Tobias Jonas
Abstract: Data assimilation can help to ensure that model results remain close to observations despite potential errors in the model, parameters and inputs. In this study, we test whether assimilation of snow depth observations using the particle filter, a generic data assimilation method, improves the results of a multi-layer energy-balance snow model, and compare the results against a direct insertion method. At the field site Col de Porte in France, the particle filter reduces errors in SWE, snowpack runoff and soil temperature when forcing the model with coarse resolution reanalysis data, which is a typical input scenario for operational simulations. For those variables, the model performance after assimilation of snow depths is similar to model performance when forcing with high quality, locally observed input data. Using the particle filter, we could also estimate a snowfall correction factor accurately at Col de Porte. The assimilation of snow depths also improves forecasts with lead-times of, at least, seven days. At further forty sites in Switzerland, the assimilation of snow depths in a model forced with numerical weather prediction data reduces the root-mean-squared-error for SWE by 64% compared to the a model without assimilation. The direct-insertion method shows similar performance as the particle filter, but is likely to produce inconsistencies between modelled variables. The particle filter, on the other hand, avoids such limitations without loss of performance. The methods proposed in this study efficiently reduces errors in snow simulations, seems applicable for different climatic and geographic regions and are easy to deploy. This article is protected by copyright. All rights reserved.
- The complementary relationship between actual and potential evaporation
for spatially heterogeneous surfaces
- Authors: Milad Aminzadeh; Dani Or
Abstract: The Complementary Relationship (CR) between actual and potential evaporation offers an attractive framework for estimating actual evaporation of drying land surfaces from simple meteorological measurements. Land surfaces are often heterogeneous with variable soil types, land cover, and local hydrologic conditions that give rise to spatially variable evaporation dynamics. The main aim is to incorporate effects of spatial heterogeneities on estimates of actual evaporation in the CR framework. The study extends the physically based approach of Aminzadeh et al.  and proposes upscaling schemes for land-atmosphere interactions affecting reference evaporation from heterogeneous surfaces comprised of vegetation and bare soil patches. For small-scale surface heterogeneity relative to the extent of convective boundary layer (CBL), area averaged atmospheric boundary conditions were imposed over the domain of interest to integrate contributions from patches with different dynamics. For large-scale heterogeneity (large patches relative to the scale of the mean CBL), fluxes from each patch were weighted by their respective areas. Preliminary results are in reasonable agreement with available field measurements and illustrate various effects of heterogeneous surface evaporative fluxes on the CR response. The results also highlight hidden dynamics not captured by standard CR, such as ability of vegetated patches to support steady evaporative fluxes until the onset of water stress while bare soil has already dried out. The study provides new insights into the roles of different vegetation types, land cover fraction, and atmospheric conditions on regional CR behavior hence advancing predictive capabilities of actual evapotranspiration from spatially heterogeneous land surfaces. This article is protected by copyright. All rights reserved.
- Tidal controls on riverbed denitrification along a tidal freshwater zone
- Authors: Deon Knights; Audrey H. Sawyer, Rebecca Barnes, Cole Musial, Samuel Bray
Abstract: In coastal rivers, tidal pumping enhances the exchange of oxygen-rich river water across the sediment-water interface, controlling nitrogen cycling in riverbed sediment. We developed a one-dimensional, fluid flow and solute transport model that quantifies the influence of tidal pumping on nitrate removal and applied it to the tidal freshwater zone (TFZ) of White Clay Creek (Delaware, USA). In field observations and models, both oxygenated river water and anoxic groundwater deliver nitrate to carbon-rich riverbed sediment. A zone of nitrate removal forms beneath the aerobic interval, which expands and contracts over daily timescales due to tidal pumping. At high tide when oxygen-rich river water infiltrates into the bed, denitrification rates decrease by 25% relative to low tide. In the absence of tidal pumping, our model predicts that the aerobic zone would be thinner, and denitrification rates would increase by 10%. As tidal amplitude increases towards the coast, nitrate removal rates should decrease due to enhanced oxygen exchange across the sediment-water interface, based on sensitivity analysis. Denitrification hot spots in TFZs are more likely to occur in less permeable sediment under lower tidal ranges and higher rates of ambient groundwater discharge. Our models suggest that tidal pumping is not efficient at removing surface water nitrate but can remove up to 81% of nitrate from discharging groundwater in the TFZ of White Clay Creek. Given the high population densities of coastal watersheds, the reactive riverbeds of TFZs play a critical role in mitigating new nitrogen loads to coasts. This article is protected by copyright. All rights reserved.
- Single discharge events increase reactive efficiency of the hyporheic zone
- Authors: Nico Trauth; Jan H. Fleckenstein
Abstract: In this study, we investigate the impact of single stream discharge events on water exchange, solute transport and reactions in the hyporheic zone below a natural in-stream gravel bar. We set up a reactive transport groundwater model with stream flow scenarios that vary by event duration and peak discharge. A steady ambient groundwater flow field is assumed that results in losing, neutral, or gaining stream conditions depending on the stream stage. Across the streambed dissolved oxygen, organic carbon and nitrate are transported into the subsurface. Additional nitrate is received from upwelling groundwater. Aerobic respiration and denitrification are simulated for scenarios with different stream solute concentrations.Results show that hyporheic exchange flux, solute transport and consumption increase during events. However, their intensities depend highly on the interplay between event characteristics and ambient groundwater conditions. During events where reversals in the hydraulic gradient occur stream water and solutes infiltrate deeper into the aquifer where they have more time to react. For those events, the reactive efficiency of the hyporheic zone (solute consumption as fraction of influx) for aerobic respiration and denitrification is up to 2.7 and 10 times higher compared to base flow conditions. The fraction of stream nitrate load consumed in the hyporheic zone increases with stream discharge (up to 150 mg/m2/hour), but remains below the value under base flow conditions for weak events. Events also increase denitrification of groundwater borne nitrate, but groundwater nitrate flux to the stream decreases by up to 33% due to temporary gradient reversals. This article is protected by copyright. All rights reserved.
- Primary weathering rates, water transit times, and concentration-discharge
relations: A theoretical analysis for the critical zone
- Authors: Ali Ameli; Keith Beven, Martin Erlandsson, Irena Creed, Jeffrey J. McDonnell, Kevin Bishop
Abstract: The permeability architecture of the critical zone exerts a major influence on the hydrogeochemistry of the critical zone. Water flowpath dynamics drive the spatio-temporal pattern of geochemical evolution and resulting streamflow concentration-discharge (C-Q) relation, but these flowpaths are complex and difficult to map quantitatively. Here, we couple a new integrated flow and particle tracking transport model with a general reversible Transition-State-Theory style dissolution rate-law to explore theoretically how C-Q relations and concentration in the critical zone respond to decline in saturated hydraulic conductivity (Ks) with soil depth. We do this for a range of flow rates and mineral reaction kinetics.Our results show that for minerals with a high ratio of equilibrium concentration (Ceq) to intrinsic weathering rate (Rmax), vertical heterogeneity in Ks enhances the gradient of weathering-derived solute concentration in the critical zone and strengthens the inverse stream C-Q relation. As CeqRmax decreases, the spatial distribution of concentration in the critical zone becomes more uniform for a wide range of flow rates, and stream C-Q relation approaches chemostatic behaviour, regardless of the degree of vertical heterogeneity in Ks. These findings suggest that the transport-controlled mechanisms in the hillslope can lead to chemostatic C-Q relations in the stream while the hillslope surface reaction-controlled mechanisms are associated with an inverse stream C-Q relation. In addition, as CeqRmax decreases, the concentration in the critical zone and stream become less dependent on groundwater age (or transit time). This article is protected by copyright. All rights reserved.
- The dynamic response of the water retention curve in unsaturated soils
during drainage to acoustic excitations
- Authors: Wei-Cheng Lo; Chi-Chin Yang, Shao-Yiu Hsu, Chu-Hui Chen, Chao- Lung Yeh, Markus Hilpert
Abstract: We examined the effects of acoustic excitations on the water retention curve, i.e., the relationship between capillary pressure (PC) and water saturation (SW) in unsaturated porous media, during drainage. The water retention curves were measured under static and dynamic conditions, where water was withdrawn from a sandbox with three different pumping rates, 12.6, 19.7, and 25.2 mL/s. Excitations with frequencies of 75, 100, 125, and 150 Hz were applied. The acoustic excitations had no effect on the static water retention curve but altered the dynamic water retention curve. The acoustic excitations lowered the dynamic PC, especially under the dynamic condition where the pumping rate was 25.2 mL/s and when SW varied between 0.6 and 0.95. The differences between the capillary pressures measured under static and dynamic conditions decreased when acoustic excitations were applied. We link this finding to the change in contact angle induced by the acoustic excitation. The dynamic coefficients, τ, for the dynamic water retention curves that we fitted to the experimental data were smaller with than without acoustic excitations. We attribute the decrease of the dynamic coefficient to the combination of the increase in the permeability and the decline in the air-entry pressure caused by adding acoustic excitations. This article is protected by copyright. All rights reserved.
- Imaging and quantification of spreading and trapping of carbon dioxide in
saline aquifers using meter-scale laboratory experiments
- Authors: Luca Trevisan; Ronny Pini, Abdullah Cihan, Jens T. Birkholzer, Quanlin Zhou, Ana González-Nicolás, Tissa H. Illangasekare
Abstract: The role of capillary forces during buoyant migration of CO2 is critical towards plume immobilization within the post-injection phase of a geological carbon sequestration operation. However, the inherent heterogeneity of the subsurface makes it very challenging to evaluate the effects of capillary forces on the storage capacity of these formations and to assess in-situ plume evolution. To overcome the lack of accurate and continuous observations at the field scale and to mimic vertical migration and entrapment of realistic CO2 plumes in the presence of a background hydraulic gradient, we conducted two unique long-term experiments in a 2.44 m × 0.5 m tank. X-ray attenuation allowed measuring the evolution of a CO2-surrogate fluid saturation, thus providing direct insight into capillarity- and buoyancy-dominated flow processes occurring under successive drainage and imbibition conditions. The comparison of saturation distributions between two experimental campaigns suggests that layered-type heterogeneity plays an important role on non-wetting phase (NWP) migration and trapping, because it leads to (i) longer displacement times (3.6 months vs. 24 days) to reach stable trapping conditions, (ii) limited vertical migration of the plume (with center of mass at 39% vs. 55% of aquifer thickness), and (iii) immobilization of a larger fraction of injected NWP mass (67.2% vs. 51.5% of injected volume) as compared to the homogenous scenario. While these observations confirm once more the role of geological heterogeneity in controlling buoyant flows in the subsurface, they also highlight the importance of characterizing it at scales that are below seismic resolution (1-10 m). This article is protected by copyright. All rights reserved.
- Effects of initial aquifer conditions on economic benefits from
- Authors: T. Foster; N. Brozović, A. P. Butler
Abstract: Worldwide, there is growing recognition of the need to reduce agricultural groundwater use in response to rapid rates of aquifer depletion. To date, however, few studies have evaluated how benefits of conservation vary along an aquifer's depletion pathway. To address this question, we develop an integrated modeling framework that couples an agro-economic model of farmers' field-level irrigation decision-making with a borehole-scale groundwater flow model. Unique to this framework is the explicit consideration of the dynamic reductions in well yields that occur as an aquifer is depleted, and how these changes in intraseasonal groundwater supply affect farmers' ability to manage production risks caused by climate variability and, in particular, drought. For an illustrative case study in the High Plains region of the United States, we apply our model to analyze the value of groundwater conservation activities for different initial aquifer conditions. Our results demonstrate that there is a range of initial conditions for which reducing pumping will have long-term economic benefits for farmers by slowing reductions in well yields and prolonging the usable lifetime of an aquifer for high-value irrigated agriculture. In contrast, restrictions on pumping that are applied too early or too late will provide limited welfare benefits. We suggest, therefore, that there are ‘windows of opportunity’ to implement groundwater conservation, which will depend on complex feedbacks between local hydrology, climate, crop growth, and economics. This article is protected by copyright. All rights reserved.
- Optimal adaptation to extreme rainfalls in current and future climate
- Authors: Dan Rosbjerg
Abstract: More intense and frequent rainfalls have increased the number of urban flooding events in recent years, prompting adaptation efforts. Economic optimization is considered an efficient tool to decide on the design level for adaptation. The costs associated with a flooding to the T-year level and the annual capital and operational costs of adapting to this level are described with log-linear relations. The total flooding costs are developed as the expected annual damage of flooding above the T-year level plus the annual capital and operational costs for ensuring no flooding below the T-year level. The value of the return period T that corresponds to the minimum of the sum of these costs will then be the optimal adaptation level.The change in climate, however, is expected to continue in the next century, which calls for expansion of the above model. The change can be expressed in terms of a climate factor (the ratio between the future and the current design level) which is assumed to increase in time. This implies increasing costs of flooding in the future for many places in the world. The optimal adaptation level is found for immediate as well as for delayed adaptation. In these cases the optimum is determined by considering the net present value of the incurred costs during a sufficiently long time span. Immediate as well as delayed adaptation is considered. This article is protected by copyright. All rights reserved.
- Numerical study on CO2 leakage detection using electrical streaming
potential (SP) data
- Authors: Henrik Büsing; Christian Vogt, Anozie Ebigbo, Norbert Klitzsch
Abstract: We study the feasibility of detecting carbon dioxide (CO2) movement in the overburden of a storage reservoir due to CO2 leakage through an abandoned well by self-potential measurements at the surface. This is achieved with three-dimensional numerical (SP) modeling of two-phase fluid flow and electrokinetic coupling between flow and streaming potential. We find that, in typical leakage scenarios, for leaky and/or injection wells with conductive metal casing, self-potential signals originating from injection can be identified at the surface. As the injection signal is also observed at the leaky well with metal casing, SP monitoring can be applied for detecting abandoned wells. However, leakage signals are much smaller than the injection signal and thus masked by the latter.We present three alternatives to overcome this problem: (i) simulate the streaming potential of the non-leaky scenario and subtract the result from the measured streaming potential data; (ii) exploit the symmetry of the injection signal by analysing the potential difference of dipoles with the dipole center at the injection well; or (iii) measure SP during periods where the injection is interrupted. In our judgement, the most promising approach for detecting a real-world CO2 leakage is by combining methods (i) and (ii), because this would give the highest signal from the leakage and omit signals originating from the injection well. Consequently, we recommend SP as monitoring method for subsurface CO2 storage, especially because a leakage can be detected shortly after the injection started even before CO2 arrives at the leaky well. This article is protected by copyright. All rights reserved.
- Assessing convection permitting resolutions of WRF for the purpose of
water resource impact assessment and vulnerability work: A southeast
Australian case study
- Authors: Marie Ekström; Eric Gilleland
Abstract: Convective permitting simulations are increasingly pursued for providing physically more credible climate projections of rainfall. Their value is likely to be greater for regions where increased resolution not only resolves physical processes better, but also the topographic features of the target domain. Here we assess the skill of convective permitting simulations to simulate rainfall for water resource assessment work in a climate change context for southeast Australia. Output on 2- and 10-km grid-length resolution from a 5-year regional climate model simulation is assessed for skill in simulating mean seasonal climatologies for days with low or high observed rainfall intensities. Comparison is conducted on spatial grids and for 25 catchments across the study region. No significant difference in skill was found in the loss differential when using absolute error for spatial fields of mean climatologies. Measures focusing on spatial similarity and accuracy in position of high rainfall areas indicate somewhat better skill in the 2-km simulation with regard to positioning (in autumn and winter), and with regard to spatial variability (in autumn and spring). Significant difference in skill was shown when comparing the simulated datasets on a catchment basis; seasonally 5-7 catchments in favour of the 10-km output and somewhat less for the 2-km output (3-6 catchments). When using correlation skill as the test measure, results are overwhelmingly in favour of the 2-km output. We cautiously suggest that results may be overly pessimistic for the 2-km simulation because of inadequate representation of rainfall in high altitude areas in observations. This article is protected by copyright. All rights reserved.
- Improving operating policies of large-scale surface-groundwater systems
through stochastic programming
- Authors: H. Macian-Sorribes; A. Tilmant, M. Pulido-Velazquez
Abstract: The management of large-scale water resource systems with surface and groundwater resources requires considering stream-aquifer interactions. Optimization models applied of large-scale systems have either employed deterministic optimization (with perfect foreknowledge of future inflows, which hinders their applicability to real-life operations) or stochastic programming (in which stream-aquifer interaction is often neglected due to the computational burden associated with these methods). In this paper, stream-aquifer interaction is integrated in a stochastic programming framework by combining the Stochastic Dual Dynamic Programming (SDDP) optimization algorithm with the Embedded Multireservoir Model (EMM). The resulting extension of the SDDP algorithm, named Combined Surface-Groundwater SDDP (CSG-SDDP), is able to properly represent the stream-aquifer interaction within stochastic optimization models of large-scale surface-groundwater resources systems. The algorithm is applied to build a hydroeconomic model for the Jucar River Basin (Spain), in which stream-aquifer interactions are essential to the characterization of water resources in the system. Besides the uncertainties regarding the economic characterization of the demand functions, the results show that the economic efficiency of the operating policies under the current system can be improved by better management of groundwater and surface resources. This article is protected by copyright. All rights reserved.
- A coupled three-dimensional hydrodynamic model for predicting hypolimnetic
oxygenation and epilimnetic mixing in a shallow eutrophic reservoir
- Authors: Shengyang Chen; Chengwang Lei, Cayelan C. Carey, Paul A. Gantzer, John C. Little
Abstract: Artificial mixing and hypolimnetic oxygenation are two common methods for improving water quality in reservoirs. To examine the effects of their operation on the thermal structure of the water column, we used a three-dimensional hydrodynamic model coupled with a newly developed water-jet model and an existing linear bubble-plume model in conjunction with whole-reservoir in-situ mixing experiments in a drinking-water reservoir. This reservoir has a side-stream supersaturation (SSS) hypolimnetic oxygenation system and a bubble-plume epilimnetic mixing (EM) system installed to reduce hypolimnetic hypoxia and algal blooms. The results show that the SSS successfully adds dissolved oxygen to the hypolimnion without destratifying the reservoir, whereas the EM, located at the lower metalimnetic boundary, deepens this boundary and partially mixes the metalimnion and epilimnion. The newly developed water-jet model coupled with the hydrodynamic model can successfully predict the variation of the thermal structure in the reservoir. The extent to which the SSS and EM systems affect the thermal structure of the reservoir is also quantified by further application of the coupled hydrodynamic model. This article is protected by copyright. All rights reserved.
- Quantifying nitrate and oxygen reduction rates in the hyporheic zone using
222Rn to upscale biogeochemical turnover in rivers
- Authors: M. Pittroff; S. Frei, B.S. Gilfedder
Abstract: Quantifying and upscaling chemical turnover in the hyporheic zone (HZ) is difficult due to limited reaction rate data, unknown carbon quality, and few methods for upscaling local measurements to river networks. Here we develop a method for quantifying reaction kinetics in-situ in the HZ and upscaling biogeochemical turnover to catchment scales. Radon-222 was used to quantify water residence times in the HZ of the Roter Main River (RM), Germany. Residence times were then combined with O2, NO3-, CO2, DOC and carbon quality (EEMs, SUVA) data to estimate Monod and first-order reaction rates. Monod parameters µmax and ksat for NO3- reduction were 11 µmol l−1 h−1 and 52 µmol l−1 respectively while the first-order rate was 0.04 h−1. Carbon quality was highly bioavailable in the HZ and is unlikely to be limiting. Reaction kinetics were incorporated into the FINIFLUX model to upscale NO3- mass loss over a 32 km reach of the RM. The aims were to (1) to estimate hyporheic efficiency using Damköhler numbers (Da), and (2) calculate NO3- mass loss in the HZ over the reach. The Da analysis suggests that the hyporheic zone is inefficient for NO3- processing, however this is somewhat misleading as the largest NO3- mass loss occurs at the shortest residence times where Da≪1. This is due to the largest water flux occurring in the uppermost part of the sediment profile. Nitrate processing in the HZ accounted for 24 kg NO3- h−1 over the reach, which was 20% of the NO3- flux from the catchment. This article is protected by copyright. All rights reserved.
- Constraining the annual groundwater contribution to the water balance of
an agricultural floodplain using radon: The importance of floods
- Authors: Jackie R. Webb; Isaac R. Santos, Barbara Robson, Ben Macdonald, Luke Jeffrey, Damien T. Maher
Abstract: The water balance of drained floodplains is highly dynamic with complex groundwater-surface water interactions operating over varying spatial and temporal scales. Here, we hypothesise that the majority of groundwater discharge will follow flood events in a modified wetland. To test this hypothesis, we developed a detailed water balance that quantifies the contribution of groundwater discharge to the annual water budget of an extensively drained agricultural floodplain. A clear relationship between surface water radon measurements and groundwater level indicated alternating connection-disconnection dynamics between the drains and shallow groundwater. This relationship was used to develop a radon mass balance to quantitatively model groundwater discharge continuously throughout the year. Groundwater discharge varied by four orders of magnitude over the study period, with daily average rates ranging from 0 to 27,200 m3 d−1, peaking just a few hours after floods receded. Flood events occurred only 12% of the time yet contributed 72 to 76% of the total groundwater discharge. During flood recession periods, aerial groundwater discharge rates reached up to 325 cm d−1 which were some of the highest rates ever estimated. We proposed that the high drainage density of this site (12.4 km constructed drains km−2 catchment area) enhanced groundwater discharge during wet periods due to increased connectivity with the soil. Overall, groundwater discharge contributed 30-80% to the total surface water discharge. This study offers insight into the dynamic behaviour of groundwater within an extensively drained floodplain, and the importance of capturing flood events to quantify total groundwater contribution to floodplain water balances. This article is protected by copyright. All rights reserved.
- Impact of social preparedness on flood early warning systems
- Authors: M. Girons Lopez; G. Di Baldassarre, J. Seibert
Abstract: Flood early warning systems play a major role in the disaster risk reduction paradigm as cost-effective methods to mitigate flood disaster damage. The connections and feedbacks between the hydrological and social spheres of early warning systems are increasingly being considered as key aspects for successful flood mitigation. The behavior of the public and first responders during flood situations, determined by their preparedness, is heavily influenced by many behavioral traits such as perceived benefits, risk awareness, or even denial.In this study we use the recency of flood experiences as a proxy for social preparedness to assess its impact on the efficiency of flood early warning systems through a simple stylized model and implemented this model using a simple mathematical description. The main findings, which are based on synthetic data, point to the importance of social preparedness for flood loss mitigation, especially in circumstances where the technical forecasting and warning capabilities are limited. Furthermore, we found that efforts to promote and preserve social preparedness may help to reduce disaster-induced losses by almost one half. The findings provide important insights into the role of social preparedness that may help guide decision-making in the field of flood early warning systems. This article is protected by copyright. All rights reserved.
- The influence of mixing on stable isotope ratios in porous media: A
revised Rayleigh model
- Authors: Jennifer L. Druhan; Kate Maher
Abstract: For an irreversible reaction, the Rayleigh or distillation-type relationship between stable isotope enrichment and reactant concentration is compromised if fluid samples are characterized by a range of water ages or different extents of reaction progress. Such mixed samples are rarely avoided in the standard methods of sampling fluid from natural porous media. As a result, application of a Rayleigh model to stable isotope ratios measured in aquifers commonly requires a diminished or effective fractionation factor relative to the intrinsic value obtained in the absence of transport effects. Thus, quantitative application of intrinsic parameter values to a fractionating reaction occurring in porous media flow requires revision to the functional form of the relationship between reactant concentration and isotope fractionation. Here, we derive a series of analytical solutions for the relationship between fractionation and flow subject to non-uniform fluid travel time distributions. These solutions are unique from previous approaches in that they avoid the use of a dispersion coefficient. The results are demonstrated against multi-component reactive transport simulations of stable isotope fractionation in homogeneous and spatially correlated heterogeneous flow fields, and applied to a dataset of stable Cr isotope enrichment obtained from a contaminated aquifer. We show that the flux-weighted isotope ratio of a solute is more sensitive to the effects of physical heterogeneity than solute concentrations. Our results support an updated functional form of the traditional Rayleigh model that describes the relationship between reactant concentration and isotope fractionation and is valid for a mixed-fluid sample. This article is protected by copyright. All rights reserved.
- Statistical distributions for monthly aggregations of precipitation and
streamflow in drought indicator applications
- Authors: Cecilia Svensson; Jamie Hannaford, Ilaria Prosdocimi
Abstract: Drought indicators are used as triggers for action and so are the foundation of drought monitoring and early warning. The computation of drought indicators like the standardized precipitation index (SPI) and standardized streamflow index (SSI) require a statistical probability distribution to be fitted to the observed data. Both precipitation and streamflow have a lower bound at zero, and their empirical distributions tend to have positive skewness. For deriving the SPI, the Gamma distribution has therefore often been a natural choice. The concept of the SSI is newer and there is no consensus regarding distribution. In the present study, twelve different probability distributions are fitted to streamflow and catchment average precipitation for four durations (1, 3, 6, and 12 months), for 121 catchments throughout the United Kingdom. The more flexible three- and four-parameter distributions generally do not have a lower bound at zero, and hence may attach some probability to values below zero. As a result, there is a censoring of the possible values of the calculated SPIs and SSIs. This can be avoided by using one of the bounded distributions, such as the reasonably flexible three-parameter Tweedie distribution, which has a lower bound (and potentially mass) at zero. The Tweedie distribution has only recently been applied to precipitation data, and only for a few sites. We find it fits both precipitation and streamflow data nearly as well as the best of the traditionally used three-parameter distributions, and should improve the accuracy of drought indices used for monitoring and early warning. This article is protected by copyright. All rights reserved.
- Flood type-specific construction of synthetic design hydrographs
- Authors: Manuela I. Brunner; Daniel Viviroli, Anna E. Sikorska, Olivier Vannier, Anne-Catherine Favre, Jan Seibert
Abstract: Accurate estimates of flood peaks, corresponding volumes and hydrographs are required to design safe and cost-effective hydraulic structures. In this paper, we propose a statistical approach for the estimation of the design variables peak and volume by constructing synthetic design hydrographs for different flood types such as flash-floods, short-rain floods, long-rain floods, and rain-on-snow floods. Our approach relies on the fitting of probability density functions to observed flood hydrographs of a certain flood type and accounts for the dependence between peak discharge and flood volume. It makes use of the statistical information contained in the data and retains the process information of the flood type. The method was tested based on data from 39 meso-scale catchments in Switzerland and provides catchment specific and flood type specific synthetic design hydrographs for all of these catchments. We demonstrate that flood type specific synthetic design hydrographs are meaningful in flood risk management when combined with knowledge on the seasonality and the frequency of different flood types. This article is protected by copyright. All rights reserved.
- Development of a discrete-continuum VDFST-CFP numerical model for
simulating seawater intrusion to a coastal karst aquifer with a conduit
- Authors: Zexuan Xu; Bill X. Hu
Abstract: A hybrid discrete-continuum numerical model, Variable-Density Flow and Solute Transport - Conduit Flow Process (VDFST-CFP), is developed to simulate seawater intrusion to a coastal karst aquifer with a conduit network. The Darcy-Weisbach equation is applied to simulate the non-laminar groundwater flow in the conduit system that is conceptualized as pipes, while the Darcy equation is used for laminar groundwater flow in the continuum porous medium. Density-dependent groundwater flow with appropriate additional density terms in the conduit is analytically derived. The flow and transport equations are coupled, and numerically solved by the finite difference method with an implicit iteration procedure. Two synthetic benchmarks are developed to compare the VDFST-CFP model results with other numerical models, such as the variable-density SEAWAT, constant-density continuum MODFLOW/MT3DMS and constant-density discrete-continuum CFPv2/UMT3D models. The VDFST-CFP model compares reasonably well with the other model results in both conduit and porous medium domains, and well describes water and salt exchange between the two systems. Under turbulent flow conditions within the conduit, the Darcy-Weisbach equation calculates the flow rate more accurately without the overestimation by the Darcy equation. Sensitivity analysis indicates that conduit diameter, friction factor, matrix hydraulic conductivity, and effective medium porosity are important parameters in the VDFST-CFP model. The pros and cons of the VDFST-CFP model are discussed, including the model assumptions and simplifications, limitations of the discrete-continuum modeling method, and the convergence criteria. In general, the newly developed VDFST-CFP model provides a new numerical modeling method for simulating seawater intrusion in a coastal karst aquifer with conduits. This article is protected by copyright. All rights reserved.
- A hierarchical Bayesian model for regionalized seasonal forecasts:
Application to low flows in the northeastern United States
- Authors: Kuk-Hyun Ahn; Richard Palmer, Scott Steinschneider
Abstract: This study presents a regional, probabilistic framework for seasonal forecasts of extreme low summer flows in the northeastern United States conditioned on antecedent climate and hydrologic conditions. The model is developed to explore three innovations in hierarchical modeling for seasonal forecasting at ungaged sites: 1) predictive climate teleconnections are inferred directly from ocean fields instead of pre-defined climate indices, 2) a parsimonious modeling structure is introduced to allow climate teleconnections to vary spatially across streamflow gages, and 3) climate teleconnections and antecedent hydrologic conditions are considered jointly for regional forecast development. The proposed model is developed and calibrated in a hierarchical Bayesian framework to pool regional information across sites and enhance regionalization skill. The model is validated in a cross-validation framework along with five simpler nested formulations to test specific hypotheses embedded in the full model structure. Results indicate that each of the three innovations improve out-of-sample summer low-flow forecasts, with the greatest benefits derived from the spatially heterogeneous effect of climate teleconnections. We conclude with a discussion of possible model improvements from a better representation of antecedent hydrologic conditions at ungaged sites. This article is protected by copyright. All rights reserved.
- Water storage in a changing environment: The impact of allocation
institutions on value
- Authors: Alexander Maas; Andre Dozier, Dale T. Manning, Christopher Goemans
Abstract: As populations increase in arid regions of the world, investment in water infrastructure improves resource management by increasing control over the location and timing of water allocation. Many studies have explored freer trade as a substitute for additional infrastructure investment. We instead quantify how water allocation institutions, reservoir management objectives, and storage capacity influence the value derived from a reservoir system. We develop a stochastic dynamic programming model of a reservoir system that faces within-year variation in weather-dependent water demand as well as stochastic semi-annual inflows. We parameterize the model using the Colorado-Big Thompson system, which transports stored water from the West Slope of the Rocky Mountains to the East Slope. We then evaluate the performance of the system under five institutional settings. Our results suggest that rigid allocation mechanisms and inefficient management objectives result in a decrease of up to 13% in the value generated from stored water when compared to a free trade scenario, an impact on par with predicted losses associated with climate-change-induced inflow reductions. We also find that under biased management objectives, increasing storage capacity can decrease the social value obtained from stored water. This article is protected by copyright. All rights reserved.
- Impact of evapotranspiration process representation on runoff projections
from conceptual rainfall-runoff models
- Authors: Danlu Guo; Seth Westra, Holger R. Maier
Abstract: Conceptual rainfall-runoff models are commonly used to estimate potential changes in runoff due to climate change. The development of these models has generally focused on reproducing runoff characteristics, with less scrutiny on other important processes such as the conversion from potential evapotranspiration (PET) to actual evapotranspiration (AET). This study uses three conceptual rainfall-runoff models (GR4J, AWBM and IHACRES_CMD) and five catchments in climatologically different regions of Australia to explore the role of ET process representation on the sensitivity of runoff to plausible future changes in PET. The changes in PET were simulated using the Penman-Monteith model and by perturbing each of the driving variables (temperature, solar radiation, humidity and wind) separately. Surprisingly, the results showed the potential of a more than seven-fold difference in runoff sensitivity per unit change in annual average PET, depending on both the rainfall-runoff model and the climate variable used to perturb PET. These differences were largely due to different ways used to convert PET to AET in the conceptual rainfall-runoff models, with particular dependencies on the daily wet/dry status, as well as the seasonal variations in store levels. By comparing the temporal patterns in simulated AET with eddy-covariance-based observations at two of the study locations, we highlighted some unrealistic behaviour in the simulated AET from AWBM. Such process-based evaluations are useful for scrutinizing the representation of physical processes in alternative conceptual rainfall-runoff models, which can be particularly useful for selecting models for projecting runoff under a changing climate. This article is protected by copyright. All rights reserved.
- An analytical test case for snow models
- Authors: Martyn P. Clark; Bart Nijssen, Charles H. Luce
Abstract: This paper develops general analytical solutions for examples of water movement through snow and compares the derived analytical solutions to numerical simulations from a coupled energy and mass balance model. The intended use of the test cases is to evaluate the impact of different numerical approximations, especially different vertical discretization strategies and different time stepping schemes. The analytical solutions provide both outflow from the snowpack, as well as vertical profiles of temperature and volumetric liquid water content at different times throughout the analysis period. The derived analytical solutions have close correspondence with model simulations in most cases. The most pronounced differences between the numerical simulations and the analytical solutions are for the fresh snow test case, where the numerical simulations predict earlier arrival of snowpack outflow. The analytical solutions provide a useful test case for physically motivated snow models because the solutions can be used to evaluate the coupling of hydrology and thermodynamics as well as the unsaturated flow of water through porous media. This article is protected by copyright. All rights reserved.
- Phase exposure-dependent exchange
- Authors: T. R. Ginn; L. G. Schreyer, K. Zamani
Abstract: Solutes and suspended material often experience delays during exchange between phases one of which may be moving. Consequently transport often exhibits combined effects of advection/dispersion, and delays associated with exchange between phases. Such processes are ubiquitous and include transport in porous/fractured media, watersheds, rivers, forest canopies, urban infrastructure systems and networks. Upscaling approaches often treat the transport and delay mechanisms together, yielding macroscopic “anomalous transport” models. When interaction with the immobile phase is responsible for the delays, it is not the transport that is anomalous, but the lack of it, due to delays. We model such exchanges with a simple generalization of first-order kinetics completely independent of transport. Specifically, we introduce a remobilization rate coefficient that depends on the time in immobile-phase. Memory-function formulations of exchange (with or without transport) can be cast in this framework, and can represent practically all time-nonlocal mass balance models including multirate mass transfer and its equivalent counterparts in the continuous time random walk and time-fractional advection dispersion formalisms, as well as equilibrium exchange. Our model can address delayed single-/multi-event remobilizations as in delay-differential equations and periodic remobilizations that may be useful in sediment transport modeling. It is also possible to link delay mechanisms with transport if so desired, or to superpose an additional source of nonlocality through the transport operator. This approach allows for mechanistic characterization of the mass transfer process with measurable parameters, and the full set of processes representable by these generalized kinetics is a new open question. This article is protected by copyright. All rights reserved.
- A 277 year cool season dam inflow reconstruction for Tasmania,
- Authors: K.J. Allen; S.C. Nichols, R. Evans, S. Allie, G. Carson, F. Ling, E.R. Cook, G. Lee, P.J. Baker
Abstract: Seasonal variability is a significant source of uncertainty in projected changes to precipitation across southeastern Australia (SEA). While existing instrumental records provide seasonal data for recent decades, most proxy records (e.g., tree rings, corals, speleothems) offer only annual reconstructions of hydroclimate. We present the first cool-season (July – August) reconstruction of dam inflow (Lake Burbury) for western Tasmania in SEA based on tree-ring width (Athrotaxis selaginoides) and mean latewood cell wall thickness (Phyllocladus aspleniifolius) chronologies. The reconstruction, produced using principal component regression, verifies back to 1731 and is moderately skillful, explaining around 23% of the variance. According to the reconstruction, relatively low inflow periods occurred around 1860, the early 1900s and 1970, while relatively high inflows occurred in the 1770s and 1810s. Highest reconstructed inflows occurred in 1816, and lowest in 1909. Comparison with available documentary and instrumental records indicates that the reconstruction better captures high rather than low flow events. There is virtually no correlation between our reconstruction and another for December-January inflow for the same catchment, a result consistent with the relationship between seasonal instrumental data. This suggests that conditions in one season have not generally reflected conditions in the other season over the instrumental record, or for the past 277 years. This illustrates the value of obtaining reconstructions of regional hydroclimatic variability for multiple individual seasons in regions where dry and wet seasons are not strongly defined. The results also indicate that the hydroclimate of southeastern Australian region cannot be adequately represented by a single regional reconstruction. This article is protected by copyright. All rights reserved.
- The integrated hydrologic model intercomparison project, IH-MIP2: A second
set of benchmark results to diagnose integrated hydrology and feedbacks
- Authors: Stefan Kollet; Mauro Sulis, Reed Maxwell, Claudio Paniconi, Mario Putti, Giacomo Bertoldi, Ethan T. Coon, Emanuele Cordano, Stefano Endrizzi, Evgeny Kikinzon, Emmanuel Mouche, Claude Mügler, Young-Jin Park, Jens Christian Refsgaard, Simon Stisen, Edward Sudicky
Abstract: Emphasizing the physical intricacies of integrated hydrology and feedbacks in simulating connected, variably-saturated groundwater-surface water systems, the Integrated Hydrologic Model Intercomparison Project initiated a second phase (IH-MIP2), increasing the complexity of the benchmarks of the first phase. The models that took part in the intercomparison were ATS, Cast3M, CATHY, GEOtop, HydroGeoSphere, MIKE-SHE, and ParFlow. IH-MIP2 benchmarks included a tilted v-catchment with 3D subsurface; a superslab case expanding the slab case of the first phase with an additional horizontal subsurface heterogeneity; and the Borden field rainfall-runoff experiment. The analyses encompassed time series of saturated, unsaturated, and ponded storages, as well as discharge. Vertical cross sections and profiles were also inspected in the superslab and Borden benchmarks. An analysis of agreement was performed including systematic and unsystematic deviations between the different models. Results show generally good agreement between the different models, which lends confidence in the fundamental physical and numerical implementation of the governing equations in the different models. Differences can be attributed to the varying level of detail in the mathematical and numerical representation or in the parameterization of physical processes, in particular with regard to ponded storage and friction slope in the calculation of overland flow. These differences may become important for specific applications such as detailed inundation modeling or when strong inhomogeneities are present in the simulation domain. This article is protected by copyright. All rights reserved.
- Unsaturated hydraulic properties of Sphagnum moss and peat reveal trimodal
- Authors: Tobias K. D. Weber; Sascha C. Iden, Wolfgang Durner
Abstract: In ombrotrophic peatlands, the moisture content of the vadose zone (acrotelm) controls oxygen diffusion rates, redox state, and the turnover of organic matter. Whether peatlands act as sinks or sources of atmospheric carbon thus relies on variably-saturated flow processes. The Richards equation is the standard model for water flow in soils, but it is not clear whether it can be applied to simulate water flow in live Sphagnum moss. Transient laboratory evaporation experiments were conducted to observe evaporative water fluxes in the acrotelm, containing living Sphagnum moss, and a deeper layer containing decomposed moss peat. The experimental data were evaluated by inverse modelling using the Richards equation as process model for variably-saturated flow. It was tested whether water fluxes and time series of measured pressure heads during evaporation could be simulated. The results showed that the measurements could be matched very well providing the hydraulic properties are represented by a suitable model. For this, a trimodal parametrization of the underlying pore-size distribution was necessary which reflects three distinct pore systems of the Sphagnum constituted by inter-, intra- and inner plant water. While the traditional van Genuchten-Mualem model led to great discrepancies, the physically more comprehensive Peters-Durner-Iden model which accounts for capillary and non-capillary flow, led to a more consistent description of the observations. We conclude that the Richards equation is a valid process description for variably-saturated moisture fluxes over a wide pressure range in peatlands supporting the conceptualisation of the live moss as part of the vadose zone. This article is protected by copyright. All rights reserved.
- Experimental study of the effect of grain sizes in a bimodal mixture on
bed slope, bed texture, and the transition to washload
- Authors: K. M. Hill; John Gaffney, Sarah Baumgardner, Peter Wilcock, Chris Paola
Abstract: When fine sediment is added to a coarse-grained system, the mobility and composition of the bed can change dramatically. We conducted a series of flume experiments to determine how the size of fine particles introduced to an active gravel bed influences the mobility and composition of the bed. We initiated our experiments using a constant water discharge and feed rate of gravel. After the system reached steady state, we doubled the feed rate by supplying a second sediment of equal or lesser size, creating size ratios from 1:1 to 1:150. As we decreased the relative size of the fine particles, the system transitioned among three regimes: (1) For particle size ratios close to one, the bed slope increased to transport the additional load of similar-sized particles. The bed surface remained planar and unchanged. (2) For intermediate particle size ratios, the bed slope decreased with the additional fines. The bed surface became patchy with regions of fine and coarse grains. (3) For the largest particle size ratios (the smallest fines), the bed slope remained relatively unchanged. The subsurface became clogged with fine sediment, but fine particles were not present in the surface layer. This third regime constitutes washload, defined by those fractions that do not affect bed-material transport conditions. Our results indicate washload should be defined in terms of three conditions: small grain size relative to that of the bed material, full suspension based on the Rouse number, and a small rate of fine sediment supply relative to transport capacity. This article is protected by copyright. All rights reserved.
- Evaporation fractionation in a peatland drainage network affects stream
water isotope composition
- Authors: Matthias Sprenger; Doerthe Tetzlaff, Claire Tunaley, Jonathan Dick, Chris Soulsby
Abstract: There is increasing interest in improving understanding of evaporation within a catchment for an enhanced representation of dominant processes in hydrological models. We used a dual-isotope approach within a nested experimental design in a boreal catchment in the Scottish Highlands (Bruntland Burn) to quantify the spatio-temporal dynamics of evaporation fractionation in a peatland drainage network and its effect on stream water isotopes. We conducted spatially distributed water sampling within the saturated peatland under different wetness conditions. We used the lc-excess – which describes the offset of a water sample from the local meteoric water line in the dual-isotope space - to understand the development of kinetic fractionation during runoff in a peatland network. The evaporation fractionation signal correlated positively with the potential evapotranspiration and negatively with the discharge. The variability of the isotopic enrichment within the peatland drainage network was higher with higher potential evapotranspiration and lower with higher discharge. We found an increased evaporation fractionation towards the center of the peatland, while groundwater seepage from minerogenic soils influenced the isotopic signal at the edge of the peatland. The evaporation signal was imprinted on the stream water, as the discharge from a peatland dominated sub-catchment showed a more intense deviation from the local meteoric water line than the discharge from the Bruntland Burn. The findings underline that evaporation fractionation within peatland drainage networks affects the isotopic signal of headwater catchments, which questions the common assumption in hydrological modelling that the isotopic composition of stream waters did not undergo fractionation processes. This article is protected by copyright. All rights reserved.
- Capillary pressure across a pore throat in the presence of surfactants
- Authors: Junbong Jang; Zhonghao Sun, J. Carlos Santamarina
Abstract: Capillarity controls the distribution and transport of multi-phase and immiscible fluids in soils and fractured rocks; therefore, capillarity affects the migration of non-aqueous contaminants and remediation strategies for both LNAPLs and DNAPLs, constrains gas and oil recovery, and regulates CO2 injection and geological storage. Surfactants alter interfacial tension and modify the invasion of pores by immiscible fluids. Experiments are conducted to explore the propagation of fluid interfaces along cylindrical capillary tubes and across pore constrictions in the presence of surfactants. Measured pressure signatures reflect the interaction between surface tension, contact angle and the pore geometry. Various instabilities occur as the interface traverses the pore constriction, consequently, measured pressure signatures differ from theoretical trends predicted from geometry, lower capillary pressures are generated in advancing wetting fronts, and jumps are prone to under-sampling. Contact angle and instabilities are responsible for pronounced differences between pressure signatures recorded during advancing and receding tests. Pressure signatures gathered with surfactant solutions suggest changes in interfacial tension at the constriction; the transient surface tension is significantly lower than the value measured in quasi-static conditions. Interface stiffening is observed during receding fronts for solutions near the critical micelle concentration. Wetting liquids tend to form plugs at pore constrictions after the invasion of a non-wetting fluid; plugs split the non-wetting fluid into isolated globules and add resistance against fluid flow. This article is protected by copyright. All rights reserved.
- An improved multilevel Monte Carlo method for estimating probability
distribution functions in stochastic oil reservoir simulations
- Authors: Dan Lu; Guannan Zhang, Clayton Webster, Charlotte Barbier
Abstract: In this work we develop an improved multilevel Monte Carlo (MLMC) method for estimating cumulative distribution functions (CDFs) of a quantity of interest (QoI), coming from numerical approximation of large-scale stochastic subsurface simulations. Compared with Monte Carlo (MC) methods, that require a significantly large number of high-fidelity model executions to achieve a prescribed accuracy when computing statistical expectations, MLMC methods were originally proposed to significantly reduce the computational cost with the use of multi-fidelity approximations. The improved performance of the MLMC methods depends strongly on the decay of the variance of the integrand as the level increases. However, the main challenge in estimating CDFs is that the integrand is a discontinuous indicator function whose variance decays slowly. To address this difficult task, we approximate the integrand using a smoothing function that accelerates the decay of the variance. In addition, we design a a-posteriori optimization strategy to calibrate the smoothing function, so as to balance the computational gain and the approximation error. The combined proposed techniques are integrated into a very general and practical algorithm that can be applied to a wide range of subsurface problems for high-dimensional uncertainty quantification, such as a fine-grid oil reservoir model considered in this effort. The numerical results reveal that with the use of the calibrated smoothing function, the improved MLMC technique significantly reduces the computational complexity compared to the standard MC approach. Finally, we discuss several factors that affect the performance of the MLMC method and provide guidance for effective and efficient usage in practice. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice: A
- Authors: Graham E. Fogg; Yong Zhang
Abstract: A geologic perspective on stochastic subsurface hydrology offers insights on representativeness of prominent field experiments and their general relevance to other hydrogeologic settings. Although the gains in understanding afforded by some 30 years of research in stochastic hydrogeology have been important and even essential, adoption of the technologies and insights by practitioners has been limited, due in part to a lack of geologic context in both the field and theoretical studies. In general, unintentional, biased sampling of hydraulic conductivity (K) using mainly hydrologic, well-based methods has resulted in the tacit assumption by many in the community that the subsurface is much less heterogeneous than in reality. Origins of the bias range from perspectives that are limited by scale and the separation of disciplines (geology, soils, aquifer hydrology, groundwater hydraulics, etc.). Consequences include a misfit between stochastic hydrogeology research results and the needs of, for example, practitioners who are dealing with local plume site cleanup that is often severely hampered by very low velocities in the very aquitard facies that are commonly overlooked or missing from low-variance stochastic models or theories. We suggest that answers to many of the problems exposed by stochastic hydrogeology research can be found through greater geologic integration into the analyses, including the recognition of not only the nearly ubiquitously high variances of K, but also the strong tendency for the good connectivity of the high K facies when spatially persistent geologic unconformities are absent. We further suggest that, although such integration may appear to make the contaminant transport problem more complex, expensive and intractable, it may in fact lead to greater simplification and more reliable, less expensive site characterizations and models. This article is protected by copyright. All rights reserved.
- Bed load transport in a very steep mountain stream (Riedbach,
Switzerland): Measurement and prediction
- Authors: Johannes M. Schneider; Dieter Rickenmann, Jens M. Turowski, Bastian Schmid, James W. Kirchner
Abstract: Compared to lower-gradient channels, steep mountain streams typically have rougher beds and shallower flow depths, making macro-scale flow resistance (due to, for example, immobile boulders and irregular bedforms) more important as controls on sediment transport. The marked differences in hydraulics, flow resistance, and grain mobility between steep and lower-gradient streams raise the question of whether the same equations can predict bed load transport rates across wide ranges of channel gradients. We studied a steep, glacier-fed mountain stream (Riedbach, Ct. Valais, Switzerland) that provides a natural experiment for exploring how stream gradients affect bed load transport rates. The streambed gradient increases over a 1-km stream reach by roughly one order of magnitude (from 3% to 38%), while flow discharge and width remain approximately constant. Sediment transport rates were determined in the 3% reach using Bunte bed load traps and in the 38% reach using the Swiss plate geophone system. Despite a ten-fold increase in bed gradient, bed load transport rates did not increase substantially. Observed transport rates for these two very different bed gradients could be predicted reasonably well by using a flow resistance partitioning approach to account for increasing bed roughness (D84 changes from 0.17 m to 0.91 m) within a fractional bed load transport equation. This suggests that sediment transport behavior across this large range of steep slopes agrees with patterns established in previous studies for both lower-gradient and steep reaches, and confirms the applicability of the flow resistance and bed load transport equations at very steep slopes. This article is protected by copyright. All rights reserved.
- Three-dimensional flow structure and bed morphology in large elongate
meander loops with different outer bank roughness characteristics
- Authors: Kory M. Konsoer; Bruce L. Rhoads, James L. Best, Eddy J. Langendoen, Jorge D. Abad, Dan R. Parsons, Marcelo H. Garcia
Abstract: Few studies have examined the three-dimensional flow structure and bed morphology within elongate loops of large meandering channels. The present study focuses on the spatial patterns of three-dimensional flow structure and bed morphology within two elongate meander loops and examines how differences in outer bank roughness influence near-bank flow characteristics. Three-dimensional velocities were measured during two different events – a near-bankfull flow and an overbank event. Detailed data on channel bathymetry and bedform geometry were obtained during a near-bankfull event. Flow structure within the loops is characterized by strong topographic steering by the point bar, by the development of helical motion associated with flow curvature, and by acceleration of flow where bedrock is exposed along the outer bank. Near-bank velocities during the overbank event are less than those for the near-bankfull flow, highlighting the strong influence of the point bar on redistribution of mass and momentum of the flow at sub-bankfull stages. Multiple outer bank pools are evident within the elongate meander loop with low outer bank roughness, but are not present in the loop with high outer bank roughness, which may reflect the influence of abundant large woody debris on near-bank velocity characteristics. The positions of pools within both loops can be linked to spatial variations in planform curvature. The findings indicate that flow structure and bed morphology in these large elongate loops is similar to that in small elongate loops, but differs somewhat from flow structure and bed morphology reported for experimental elongate loops. This article is protected by copyright. All rights reserved.
- Linking hydromorphology with invertebrate ecology in diverse morphological
units of a large river-floodplain system
- Authors: Martín C.M. Blettler; Mario L. Amsler, Eliana G. Eberle, Ricardo Szupiany, Francisco G. Latosinski, Elie Abrial, Paul J. Oberholster, Luis A. Espinola, Aldo Paira, Ailen Poza, Alberto Rodrigues Capítulo
Abstract: Interdisciplinary research in the fields of ecohydrology and ecogeomorphology is becoming increasingly important as a way to understand how biological and physical processes interact with each other in river systems. The objectives of the current study were 1) to determine changes in invertebrate community due to hydrological stages, 2) to link local physical features (flow configuration, sediment composition and morphological feature) with the ecological structure between and within dissimilar morphological units (meander and confluence), and 3) to determine the existence and the origin of bed hydro-geomorphic patches, determining their ecological structure. Results were discussed in the frame of prevailing ecological models and concepts.The study site extends over a floodplain area of the large Paraná River (Argentina), including minor and major secondary channels as well as the main channel.Overall results suggested that hydrodynamics was the driving force determining distribution patterns of benthic assemblages in the floodplain. However, while the invertebrates living in minor secondary channels seem to benefit from flooding, this hydrological phase had the opposite effect on organisms from the main and major secondary channels. We also found a clear linkage between physical features and invertebrate ecology, which caused a dissimilar fauna structure between and within the meander and the confluence. Furthermore, several sandy-patches were recorded in the confluence. These patches were colonized by the particular benthic assemblage recorded in the main channel, supported the view of rivers as patchy discontinua, under uncertain ecological equilibrium. This article is protected by copyright. All rights reserved.
- Implementation and evaluation of a monthly water balance model over the
U.S. on an 800 m grid
- Authors: S.W. Hostetler; J.R. Alder
Abstract: We simulate the 1950-2010 water balance for the conterminous U.S. (CONUS) with a monthly water balance model (MWBM) using the 800-m Parameter-elevation Regression on Independent Slopes Model (PRISM) data set as model input. We employed observed snow and streamflow data sets to guide modification of the snow and potential evapotranspiration components in the default model and to evaluate model performance. Based on various metrics and sensitivity tests, the modified model yields reasonably good simulations of seasonal snowpack in the West (range of bias of ±50 mm at 68% of 713 SNOTEL sites), the gradients and magnitudes of actual evapotranspiration, and runoff (median correlation of 0.83 and median Nash-Sutcliff efficiency of 0.6 between simulated and observed annual time series at 1427 USGS gage sites). The model generally performs well along the Pacific Coast, the high elevations of the Basin and Range and over the Midwest and East, but not as well over the dry areas of the Southwest and upper Plains regions due, in part, to the apportioning of direct versus delayed runoff. Sensitivity testing and application of the MWBM to simulate the future water balance at four National Parks when driven by 30 climate models from the Climate Model Intercomparison Program Phase 5 (CMIP5) demonstrate that the model is useful for evaluating first-order, climate driven hydrologic change on monthly and annual time scales. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice: The
relevance of stochastic subsurface hydrology to practical problems of
contaminant transport and remediation. What is characterization and
stochastic theory good for?
- Authors: A. Fiori; V. Cvetkovic, G. Dagan, S. Attinger, A. Bellin, P. Dietrich, A. Zech, G. Teutsch
Abstract: The emergence of stochastic subsurface hydrology stemmed from the realization that spatial variability of aquifer properties (primarily permeability K) has a profound impact on solute transport. Heterogeneity is characterized by much larger scale than the pore scale and the seemingly erratic variation of K and the uncertainty of its distribution called for its modeling as a random space function, which renders the fluid Darcian velocity and the concentration random as well. This article is protected by copyright. All rights reserved.
- Regional evaporation estimates in the eastern monsoon region of China:
Assessment of a nonlinear formulation of the complementary principle
- Authors: Xiaomang Liu; Changming Liu, Wilfried Brutsaert
Abstract: The performance of a nonlinear formulation of the complementary principle for evaporation estimation was investigated in 241 catchments with different climate conditions in the eastern monsoon region of China. Evaporation (Ea) calculated by the water balance equation was used as the reference. Ea estimated by the calibrated nonlinear formulation was generally in good agreement with the water balance results, especially in relatively dry catchments. The single parameter in the nonlinear formulation, namely αe as a weak analog of the alpha parameter of Priestley and Taylor , tended to exhibit larger values in warmer and humid near-coastal areas, but smaller values in colder, drier environments inland, with a significant dependency on the aridity index (AI). The nonlinear formulation combined with the equation relating the one parameter and AI provides a promising method to estimate regional Ea with standard and routinely measured meteorological data. This article is protected by copyright. All rights reserved.
- Distributed temperature sensing as a down-hole tool in hydrogeology
- Authors: V.F. Bense; T. Read, O. Bour, T. Le Borgne, T. Coleman, S. Krause, A. Chalari, M. Mondanos, F. Ciocca, J.S. Selker
Abstract: Distributed Temperature Sensing (DTS) technology enables down-hole temperature monitoring to study hydrogeological processes at unprecedentedly high frequency and spatial resolution. DTS has been widely applied in passive mode in site investigations of groundwater flow, in-well flow, and subsurface thermal property estimation. However, recent years have seen the further development of the use of DTS in an active mode (A-DTS) for which heat sources are deployed. A suite of recent studies using A-DTS down-hole in hydrogeological investigations illustrate the wide range of different approaches and creativity in designing methodologies. The purpose of this review is to outline and discuss the various applications and limitations of DTS in down-hole investigations for hydrogeological conditions and aquifer geological properties. To this end, we first review examples where passive DTS has been used to study hydrogeology via down-hole applications. Secondly, we discuss and categorize current A-DTS borehole methods into three types. These are thermal advection tests, hybrid cable flow logging, and heat pulse tests. We explore the various options with regards to cable installation, heating approach, duration, and spatial extent in order to improve their applicability in a range of settings. These determine the extent to which each method is sensitive to thermal properties, vertical in well flow, or natural gradient flow. Our review confirms that the application of DTS has significant advantages over discrete point temperature measurements, particularly in deep wells, and highlights the potential for further method developments in conjunction with other emerging fiber optic based sensors such as Distributed Acoustic Sensing. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice:
- Authors: Harihar Rajaram
Abstract: This paper introduces the papers in the “Debates – Stochastic Subsurface Hydrology from Theory to Practice” series. Beginning in the 1970s, the field of stochastic subsurface hydrology has been an active field of research, with over 3500 journal publications, of which over 850 have appeared in Water Resources Research. We are fortunate to have insightful contributions from four groups of distinguished authors who discuss the reasons why the advanced research framework established in stochastic subsurface hydrology has not impacted the practice of groundwater flow and transport modeling and design significantly. There is reasonable consensus that a community effort aimed at developing “toolboxes” for applications of stochastic methods will make them more accessible and encourage practical applications. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice: Does
stochastic subsurface hydrology help solving practical problems of
- Authors: Olaf A. Cirpka; Albert J. Valocchi
Abstract: While stochastic subsurface hydrology has been tremendously successful in understanding how the spatial variability of hydraulic conductivity affects conservative solute transport in idealized settings, it has gained little impact in practice. This is the case because typical assumptions needed for the derivation of analytical expressions are too restrictive for practical applications and often geologically implausible, small-scale variation of hydraulic conductivity is by far not the only cause of uncertainty when considering the fate and remediation of pollutants, and the research community has not developed enough methods that can directly be used by practitioners. To overcome these shortcomings we propose putting more emphasis on providing easy-to-use tools to generate realistic realizations of subsurface properties that are conditioned on all data measured at a site, extending the focus from hydraulic conductivity only to all parameters and processes relevant for reactive transport, making use of self-organizing principles of reactive transport to conceptually simplify the problem, and addressing conceptual uncertainty by stochastic methods. This article is protected by copyright. All rights reserved.
- Debates—Stochastic subsurface hydrology from theory to practice: Why
stochastic modeling has not yet permeated into practitioners?
- Authors: X. Sanchez-Vila; D. Fernàndez-Garcia
Abstract: We address modern topics of Stochastic Hydrogeology from their potential relevance to real modeling efforts at the field scale. While the topics of stochastic hydrogeology and numerical modelling have become routine in hydrogeological studies, non-deterministic models have not yet permeated into practitioners. We point out a number of limitations of stochastic modelling when applied to real applications and comment on the reasons why stochastic models fail to become an attractive alternative for practitioners. We specifically separate issues corresponding to flow, conservative transport and reactive transport. The different topics addressed are: emphasis on process modeling, need for upscaling parameters and governing equations, relevance of properly accounting for detailed geological architecture in hydrogeological modeling, and specific challenges of reactive transport. We end up by concluding that the main responsible for non-deterministic models having not yet permeated in industry can be fully attributed to researchers in stochastic hydrogeology. This article is protected by copyright. All rights reserved.
- Push-pull tracer tests: Their information content and use for
characterizing non-Fickian, mobile-immobile behavior
- Authors: Scott K. Hansen; Brian Berkowitz, Velimir V. Vesselinov, Daniel O'Malley, Satish Karra
Abstract: Path reversibility and radial symmetry are often assumed in push-pull tracer test analysis. In reality, heterogeneous flow fields mean that both assumptions are idealizations. To understand their impact, we perform a parametric study which quantifies the scattering effects of ambient flow, local-scale dispersion and velocity field heterogeneity on push-pull breakthrough curves and compares them to the effects of mobile-immobile mass transfer (MIMT) processes including sorption and diffusion into secondary porosity. We identify specific circumstances in which MIMT overwhelmingly determines the breakthrough curve, which may then be considered uninformative about drift and local-scale dispersion. Assuming path reversibility, we develop a continuous time random walk-based interpretation framework which is flow-field agnostic and well suited to quantifying MIMT. Adopting this perspective, we show that the radial flow assumption is often harmless: to the extent that solute paths are reversible, the breakthrough curve is uninformative about velocity field heterogeneity. Our interpretation method determines a mapping function (i.e. subordinator) from travel time in the absence of MIMT to travel time in its presence. A mathematical theory allowing this function to be directly “plugged into” an existing Laplace-domain transport model to incorporate MIMT is presented and demonstrated. Algorithms implementing the calibration are presented and applied to interpretation of data from a push-pull test performed in a heterogeneous environment. A successful four-parameter fit is obtained, of comparable fidelity to one obtained using a million-node 3D numerical model. Finally, we demonstrate analytically and numerically how push-pull tests quantifying MIMT are sensitive to remobilization, but not immobilization, kinetics. This article is protected by copyright. All rights reserved.
- A generalized non-Darcian radial flow model for constant rate test
- Authors: Ming-Ming Liu; Yi-Feng Chen, Jia-Min Hong, Chuang-Bing Zhou
Abstract: Models used for data interpretation of constant rate tests (CRTs) are commonly derived with the assumption of Darcian flow in an idealized geometry, hence disregarding the non-Darcian nature of fluid flow and the complexity of flow geometry. In this study, an Izbash's law-based analytical model is proposed by means of Laplace transform and linearization approximation for interpretation of non-Darcian flow in a generalized radial formation where the flow dimension may become fractional between 1 and 3. The source storage and skin effects are also considered in the model development. The proposed model immediately reduces to Barker's  model for Darcian flow in the generalized radial formation and to Wen's [2008a] model for non-Darcian flow in a two-dimensional confined aquifer. A comparison with numerical simulations shows that the proposed model behaves well in low non-Darcian flow condition or at late times. The proposed model is finally applied for data interpretation of the constant rate pumping tests performed at Ploemeur [Le Borgne et al., 2004], showing that the estimated hydraulic properties (i.e. hydraulic conductivity, specific storage coefficient, non-Darcy exponent and the dimension of flow geometry) are well representative of the hydrogeologic conditions on the field scale at the test site after the exploitation of groundwater. The proposed model is an extension of the generalized radial flow (GRF) model, which would be of significance in the problem of choosing an appropriate dimension of flow geometry in which non-Darcian flow occurs. This article is protected by copyright. All rights reserved.
- Global evaluation of new GRACE mascon products for hydrologic applications
- Authors: Bridget R. Scanlon; Zizhan Zhang, Himanshu Save, David N. Wiese, Felix W. Landerer, Di Long, Laurent Longuevergne, Jianli Chen
Abstract: Recent developments in mascon (mass concentration) solutions for GRACE (Gravity Recovery and Climate Experiment) satellite data have significantly increased the spatial localization and amplitude of recovered terrestrial Total Water Storage anomalies (TWSA); however, land hydrology applications have been limited. Here we compare TWSA from Apr. 2002 through Mar. 2015 from (1) newly released GRACE mascons from the Center for Space Research (CSR-M) with (2) NASA JPL mascons (JPL-M), and with (3) CSR Tellus gridded spherical harmonics rescaled (sf) (CSRT-GSH.sf) in 176 river basins, ∼60% of the global land area. Time series in TWSA mascons (CSR-M and JPL-M) and spherical harmonics are highly correlated (mostly >0.9). The signal from long-term trends (up to ±20 mm/yr) is much less than that from seasonal amplitudes (up to 250 mm). Net long-term trends, summed over all 176 basins, are similar for CSR and JPL mascons (66–69 km3/yr) but are lower for spherical harmonics (∼14 km3/yr). Long-term TWSA declines are found mostly in irrigated basins (- 41 to -69 km3/yr). Seasonal amplitudes agree among GRACE solutions, increasing confidence in GRACE-based seasonal fluctuations. Rescaling spherical harmonics significantly increases agreement with mascons for seasonal fluctuations, but less for long-term trends. Mascons provide advantages relative to spherical harmonics, including (1) reduced leakage from land to ocean increasing signal amplitude, and (2) application of geophysical data constraints during processing with little empirical post-processing requirements, making it easier for non-geodetic users. Results of this product intercomparison should allow hydrologists to better select suitable GRACE solutions for hydrologic applications. This article is protected by copyright. All rights reserved.
- Discussion of “Estimation of composite hydraulic resistance in
ice-covered alluvial streams”, by Zare et al. (2016) WRR 52(2):
- Authors: Spyros Beltaos
Abstract: Inconsistencies between basic hydraulics and hypotheses advanced by the authors are identified. They are attributed primarily to weaknesses in the analysis of velocity measurements in the ice-controlled flow layer and suggestions for improvement are offered. This article is protected by copyright. All rights reserved.
- Infiltration of fine sediment mixtures through poorly sorted immobile
- Authors: Francisco Núñez-González
Abstract: Percolation of fine sediment is a common process in gravel-bed rivers, which often exhibit extended and overlapping grain size distributions of the bed and the supplied fine sediment. Yet, existing sediment infiltration theory assumes well-sorted fine material with smaller grain size than the bed pores, and as such, is not suitable for many situations encountered in gravel-bed streams. Previous developments for infiltration of uniform material are here generalized to consider poorly-sorted sediment mixtures. Governing equations and a numerical solution to model the vertical distribution of infiltrating sediment are presented. The equations are solved as a function of a trapping coefficient, dependent on the relative size of infiltrating fines in relation to bed material. A method is developed to generate equivalent grain size distributions to calculate the trapping coefficient, when grain sizes of the infiltrating and bed materials overlap. Moreover, a bed cutoff size is defined and computed with a semi-empirical packing-porosity model, to distinguish particles smaller than the bed pores. Published experimental data are used to test the new model and calibrate the trapping coefficient. It is shown that this coefficient is highly sensible to the fine and coarse tails of fine and coarse materials grain size distributions. Accordingly, calibrated values of the coefficient are set as a function of a mean size ratio, computed from the geometric mean of the tails of the size distributions. Incorporating this relation, the model performed well in reproducing indirect observations of sediment infiltration from experiments reported in the literature. This article is protected by copyright. All rights reserved.
- Observed and simulated hydrologic response for a first-order catchment
during extreme rainfall three years after wildfire disturbance
- Authors: Brian A. Ebel; Francis K. Rengers, Gregory E. Tucker
Abstract: Hydrologic response to extreme rainfall in disturbed landscapes is poorly understood because of the paucity of measurements. A unique opportunity presented itself when extreme rainfall in September 2013 fell on a headwater catchment (i.e.
- Response to the discussion of “Estimation of composite hydraulic
resistance in ice-covered alluvial streams”
- Authors: Soheil Ghareh Aghaji Zare; Stephanie A. Moore, Colin D. Rennie, Ousmane Seidou, Habib Ahmari, Jarrod Malenchak
Abstract: Previously utilized techniques for analysis of flow velocity in ice controlled zone are confirmed to be valid. Potential sources of error regarding the estimation of energy grade line slope are corrected considering the comments by the discussant. A modified version of the equation for composite roughness calculation originally introduced in the paper is presented. The revised new method then is tested against the other available methods and its accuracy is evaluated. Other assumptions and analyses presented in the paper are also tested and validated. This article is protected by copyright. All rights reserved.
- A travel-time based approach to model kinetic sorption in highly
heterogeneous porous media via reactive hydrofacies
- Authors: Michael Finkel; Peter Grathwohl, Olaf A. Cirpka
Abstract: We present a semi-analytical model for the transport of solutes being subject to sorption in porous aquifers. We couple a travel-time based model of advective transport with a spherical diffusion model of kinetic sorption in non-uniform material mixtures. The model is formulated in the Laplace domain and transformed to the time domain by numerical inversion. By this, three-dimensional transport of solutes undergoing mass transfer between aqueous and solid phases can be simulated very efficiently. The model addresses both hydraulic and reactive heterogeneity of porous aquifers by means of hydrofacies, which function as homogeneous but non-uniform subunits. The total exposure time to each of these subunits controls the magnitude of sorption effects, whereas the particular sequence of facies through which the solute passes is irrelevant. We apply the model to simulate the transport of phenanthrene in a fluvio-glacial aquifer, for which the hydrofacies distribution is known at high resolution, the lithological composition of each facies has been analyzed, and sorption properties of the lithological components are available. Taking the fully resolved hydrofacies-model as reference, we evaluate different approximations referring to lower information levels, reflecting shortcomings in typical modeling projects. The most important feature for a good description of both the main breakthrough and tailing of phenanthrene is the non-uniformity of the porous medium. While spatial heterogeneity of chemical properties might be neglected without introducing a large error, an approximation of the facies' composition in terms of a uniform substitute material considerably compromises the quality of the modeling result. This article is protected by copyright. All rights reserved.
- Panel regressions to estimate low-flow response to rainfall variability in
- Authors: Maoya Bassiouni; Richard M. Vogel, Stacey A. Archfield
Abstract: Multicollinearity and omitted-variable bias are major limitations to developing multiple linear regression models to estimate streamflow characteristics in ungaged areas and varying rainfall conditions. Panel regression is used to overcome limitations of traditional regression methods, and obtain reliable model coefficients, in particular to understand the elasticity of streamflow to rainfall. Using annual rainfall and selected basin characteristics at 86 gaged streams in the Hawaiian Islands, regional regression models for three stream classes were developed to estimate the annual low-flow duration discharges. Three panel-regression structures (random effects, fixed effects, and pooled) were compared to traditional regression methods, in which space is substituted for time. Results indicated that panel regression generally was able to reproduce the temporal behavior of streamflow and reduce the standard errors of model coefficients compared to traditional regression, even for models in which the unobserved heterogeneity between streams is significant and the variance inflation factor for rainfall is much greater than 10. This is because both spatial and temporal variability were better characterized in panel regression. In a case study, regional rainfall elasticities estimated from panel regressions were applied to ungaged basins on Maui, using available rainfall projections to estimate plausible changes in surface-water availability and usable stream habitat for native species. The presented panel-regression framework is shown to offer benefits over existing traditional hydrologic regression methods for developing robust regional relations to investigate streamflow response in a changing climate. This article is protected by copyright. All rights reserved.
- A generalized threshold model for computing bedload grain-size
- Authors: Alain Recking
Abstract: For morphodynamic studies, it is important to compute not only the transported volumes of bedload, but also the size of the transported material. A few bedload equations compute fractional transport (i.e., both the volume and grain size distribution), but many equations compute only the bulk transport (a volume) with no consideration of the transported grain sizes. To fill this gap, a method is proposed to compute the bedload grain size distribution separately to the bedload flux. The method is called the Generalized Threshold Model (GTM), because it extends the flow competence method for threshold of motion of the largest transported grain size to the full bed surface grain size distribution. This was achieved by replacing dimensional diameters with their size indices in the standard hiding function, which offers a useful framework for computation, carried out for each indices considered in the range [1, 100]. New functions are also proposed to account for partial transport. The method is very simple to implement and is sufficiently flexible to be tested in many environments. In addition to being a good complement to standard bulk bedload equations, it could also serve as a framework to assist in analyzing the physics of bedload transport in future research. This article is protected by copyright. All rights reserved.
- On the correlation of water vapor and CO2: Application to flux
partitioning of evapotranspiration
- Authors: Wen Wang; James A. Smith, Prathap Ramamurthy, Mary Lynn Baeck, Elie Bou-Zeid, Todd M. Scanlon
Abstract: The partitioning of evapotranspiration (ET) between plant transpiration (Et) and direct evaporation (Ed) presents one of the most important and challenging problems for characterizing ecohydrological processes. The exchange of water vapor (q) and CO2 (c) are closely coupled in ecosystem processes and knowledge of their controls can be gained through joint investigation of q and c. In this study we examine the correlation of water vapor and CO2 (Rqc) through analyses of high frequency time series derived from eddy covariance measurements collected over a suburban grass field in Princeton, NJ during a two-year period (2011-2013). Rqc at the study site exhibits pronounced seasonal and diurnal cycles, with maximum anticorrelation in June and maximum decorrelation in January. The diurnal cycle of Rqc varies seasonally and is characterized by a near-symmetric shape with peak anticorrelation around local noon. Wavelet and spectral analyses suggest that q and c are jointly transported for most eddy scales (1-200 m), which is important for flux-variance ET partitioning methods (e.g. Scanlon and Sahu ). The diurnal cycle of the transpiration fraction (ratio of Et to total ET) exhibits an asymmetric diurnal cycle, especially during the warm season, with peak values occurring in the afternoon. These ET partitioning results give similar diurnal and seasonal patterns compared with numerical simulations from the Noah Land Surface Model using the Jarvis canopy resistance formulation. This article is protected by copyright. All rights reserved.
- Predicting shifts in rainfall-runoff partitioning during multiyear
drought: Roles of dry period and catchment characteristics
- Authors: Margarita Saft; Murray C. Peel, Andrew W. Western, Lu Zhang
Abstract: While the majority of hydrological prediction methods assume that observed interannual variability explores the full range of catchment response dynamics, recent cases of prolonged climate drying suggest otherwise. During the ∼decade-long Millennium drought in south-eastern Australia significant shifts in hydrologic behaviour were reported. Catchment rainfall-runoff partitioning changed from what was previously encountered during shorter droughts, with significantly less runoff than expected occurring in many catchments. In this article we investigate the variability in the magnitude of shift in rainfall-runoff partitioning observed during the Millennium drought. We re-evaluate a large range of factors suggested to be responsible for the additional runoff reductions. Our results suggest that the shifts were mostly influenced by catchment characteristics related to pre-drought climate (aridity index and rainfall seasonality) and soil and groundwater storage dynamics (pre-drought interannual variability of groundwater storage and mean solum thickness). The shifts were amplified by seasonal rainfall changes during the drought (spring rainfall deficits). We discuss the physical mechanisms that are likely to be associated with these factors. Our results confirm that shifts in the annual rainfall-runoff relationship represent changes in internal catchment functioning, and emphasise the importance of cumulative multiyear changes in the catchment storage for runoff generation. Prolonged drying in some regions can be expected in the future, and our results provide an indication of which catchments characteristics are associated with catchments more susceptible to a shift in their runoff response behaviour. This article is protected by copyright. All rights reserved.
- Modeling the influence of preferential flow on the spatial variability and
time-dependence of mineral weathering rates
- Authors: Sachin Pandey; Harihar Rajaram
Abstract: Inferences of weathering rates from laboratory and field observations suggest significant scale and time dependence. Preferential flow induced by heterogeneity (manifest as permeability variations or discrete fractures) has been suggested as one potential mechanism causing scale/time dependence. We present a quantitative evaluation of the influence of preferential flow on weathering rates using reactive transport modeling. Simulations were performed in discrete fracture networks (DFNs) and correlated random permeability fields (CRPFs), and compared to simulations in homogeneous permeability fields. The simulations reveal spatial variability in the weathering rate, multi-dimensional distribution of reactions zones, and the formation of rough weathering interfaces and corestones due to preferential flow. In the homogeneous fields and CRPFs, the domain-averaged weathering rate is initially constant as long as the weathering front is contained within the domain, reflecting equilibrium-controlled behavior. The behavior in the CRPFs was influenced by macrodispersion, with more spread-out weathering profiles, an earlier departure from the initial constant rate and longer persistence of weathering. DFN simulations exhibited a sustained time-dependence resulting from the formation of diffusion-controlled weathering fronts in matrix blocks, which is consistent with the shrinking core mechanism. A significant decrease in the domain-averaged weathering rate is evident despite high remaining mineral volume fractions, but the decline does not follow a 1/√t dependence, characteristic of diffusion, due to network scale effects and advection-controlled behavior near the inflow boundary. The DFN simulations also reveal relatively constant horizontally-averaged weathering rates over a significant depth range, challenging the very notion of a weathering front. This article is protected by copyright. All rights reserved.
- Monitoring groundwater storage changes in complex basement aquifers: An
evaluation of the GRACE satellites over East Africa
- Authors: J. Nanteza; C. R. de Linage, B.F. Thomas, J.S. Famiglietti
Abstract: Although the use of the Gravity Recovery and Climate Experiment (GRACE) satellites to monitor groundwater storage changes has become commonplace, our evaluation suggests that careful processing of the GRACE data is necessary to extract a representative signal especially in regions with significant surface water storage (i.e. lakes/reservoirs). In our study, we use cautiously processed datasets, including GRACE, lake altimetry and model soil moisture, to reduce scaling factor bias and compare GRACE-derived groundwater storage changes to in-situ groundwater observations over parts of East Africa. Over the period 2007-2010, a strong correlation between in-situ groundwater storage change and GRACE-groundwater estimates (Spearman's ρ = 0.6) is found. Piecewise trend analyses for the GRACE-groundwater estimates reveal significant negative storage changes that are attributed to groundwater use and climate variability. Further analysis comparing groundwater and satellite precipitation datasets permits identification of regional groundwater characterization. For example, our results identify potentially permeable and/or shallow groundwater systems underlying Tanzania and deep and/or less permeable groundwater systems underlying the Upper-Nile basin. Regional groundwater behaviors in the semi-arid regions of Northern Kenya are attributed to hydraulic connections to recharge zones outside the sub-basin boundary. Our results prove the utility of applying GRACE in monitoring groundwater resources in hydrologically complex regions that are under-sampled and where policies limit data accessibility. This article is protected by copyright. All rights reserved.
- Numerical simulation of transient groundwater age distributions assisting
land and water management in the Middle Wairarapa Valley, New Zealand
- Authors: MichaelW. Toews; Christopher J. Daughney, Fabien J. Cornaton, Uwe Morgenstern, Ryan D. Evison, Bethanna M. Jackson, Karine Petrus, Doug Mzila
Abstract: This study used numerical models to simulate transient groundwater age distributions using a time-marching Laplace transform Galerkin (TMLTG) technique. First, the TMLTG technique was applied to simple box models configured to match idealized lumped parameter models (LPMs). Even for simple box models, time-varying recharge can generate groundwater age distributions with highly irregular shapes that vary over time in response to individual recharge events. Notably, the transient numerical simulations showed that the breakthrough and mean ages are younger than in the steady flow case, and that this difference is greater for sporadic recharge time series than for more regular recharge time series. Second, the TMLTG technique was applied to a transient numerical model of the 270 km2 Middle Wairarapa Valley, New Zealand. To our knowledge this study is the first application of the TMLTG technique to a real-world example, made possible by the dataset of tritium measurements that exists for the Wairarapa Valley. Results from a transient mean age simulation shows variation from a few days to over a decade in either temporal or spatial dimensions. Temporal variations of mean age are dependent on seasonal climate and groundwater abstraction. Results also demonstrated important differences between the transient age distributions derived from the TMLTG technique compared to the much simpler steady-state LPMs that are frequently applied to interpret age tracer data. Finally, results had direct application to land and water management, for example for identification of land areas where age distributions vary seasonally, affecting the security of groundwater supplies used for drinking water. This article is protected by copyright. All rights reserved.