- Characterization of non‐Gaussian conductivities and porosities with
hydraulic heads, solute concentrations, and water temperatures
- Abstract: Reliable characterization of hydraulic parameters is important for the understanding of groundwater flow and solute transport. The normal‐score ensemble Kalman filter (NS‐EnKF) has proven to be an effective inverse method for the characterization of non‐Gaussian hydraulic conductivities by assimilating transient piezometric head data, or solute concentration data. Groundwater temperature, an easily captured state variable, has not drawn much attention as an additional state variable useful for the characterization of aquifer parameters. In this work, we jointly estimate non‐Gaussian aquifer parameters (hydraulic conductivities and porosities) by assimilating three kinds of state variables (piezometric head, solute concentration, and groundwater temperature) using the NS‐EnKF. A synthetic example including seven tests is designed, and used to evaluate the ability to characterize hydraulic conductivity and porosity in a non‐Gaussian setting by assimilating different numbers and types of state variables. The results show that characterization of aquifer parameters can be improved by assimilating groundwater temperature data and that the main patters of the non‐Gaussian reference fields can be retrieved with more accuracy and higher precision if multiple state variables are assimilated. This article is protected by copyright. All rights reserved.
- Conceptualizing sociohydrological drought processes: The case of the Maya
- Abstract: With population growth, increasing water demands and climate change the need to understand the current and future pathways to water security is becoming more pressing. To contribute to addressing this challenge, we examine the link between water stress and society through socio‐hydrological modeling. We conceptualize the interactions between an agricultural society with its environment in a stylized way. We apply the model to the case of the ancient Maya, a population that experienced a peak during the Classic Period (AD 600‐830) and then declined during the ninth century. The hypothesis that modest drought periods played a major role in the society's collapse is explored. Simulating plausible feedbacks between water and society we show that a modest reduction in rainfall may lead to an 80% population collapse.Population density and crop sensitivity to droughts, however, may play an equally important role. The simulations indicate that construction of reservoirs results in less frequent drought impacts, but if the reservoirs run dry, drought impact may be more severe and the population drop may be larger. Index terms: 1812 Drought (4303) 1834 Human impacts (4323) 4330 Vulnerability. Keywords: socio‐hydrology, Ancient Maya, drought, vulnerability. This article is protected by copyright. All rights reserved.
- Interactions among hydraulic conductivity distributions, subsurface
topography, and transport thresholds revealed by a multitracer hillslope
- Authors: C. Rhett Jackson; Enhao Du, Julian Klaus, Natalie A. Griffiths, Menberu Bitew, Jeffrey J. McDonnell
Abstract: Interactions among hydraulic conductivity distributions, subsurface topography, and lateral flow are poorly understood. We applied 407 mm of water and a suite of tracers over 51 hours to a 12 by 16.5 m forested hillslope segment to determine interflow thresholds, preferential pathway pore velocities, large‐scale conductivities, the time series of event water fractions, and the fate of dissolved nutrients. The 12% hillslope featured loamy sand A and E horizons overlying a sandy clay loam Bt at 1.25 m average depth. Interflow measured from two drains within an interception trench commenced after 131 and 208 mm of irrigation. Cumulative interflow equaled 49% of applied water. Conservative tracer differences between the collection drains indicated differences in flow paths and storages within the plot. Event water fractions rose steadily throughout irrigation, peaking at 50% sixteen hours after irrigation ceased. Data implied that tightly held water exchanged with event water throughout the experiment and a substantial portion of pre‐event water was released from the argillic layer. Surface‐applied dye tracers bypassed the matrix, with peak concentrations measured shortly after flow commencement, indicating preferential network conductivities of 864 to 2240 mm/h, yet no macropore flow was observed. Near steady‐state flow conditions indicated average conductivities of 460 mm/h and 2.5 mm/h for topsoils and the Bt horizon, respectively. Low ammonium and phosphorus concentrations in the interflow suggested rapid uptake or sorption, while higher nitrate concentrations suggested more conservative transport. These results reveal how hydraulic conductivity variation and subsurface topographic complexity explain otherwise paradoxical solute and flow behaviors. This article is protected by copyright. All rights reserved.
- Transit time distributions and StorAge Selection functions in a sloping
soil lysimeter with time‐varying flow paths: Direct observation of
internal and external transport variability
- Abstract: Transit times through hydrologic systems vary in time, but the nature of that variability is not well understood. Transit times variability was investigated in a 1 m3 sloping lysimeter, representing a simplified model of a hillslope receiving periodic rainfall events for 28 days. Tracer tests were conducted using an experimental protocol that allows time‐variable transit time distributions (TTDs) to be calculated from data. Observed TTDs varied with the storage state of the system, and the history of inflows and outflows. We propose that the observed time variability of the TTDs can be decomposed into two parts: ‘internal' variability associated with changes in the arrangement of, and partitioning between, flow pathways; and ‘external' variability driven by fluctuations in the flow rate along all flow pathways. These concepts can be defined quantitatively in terms of rank StorAge Selection (rSAS) functions, which is a theory describing lumped transport dynamics. Internal variability is associated with temporal variability in the rSAS function, while external is not. The rSAS function variability was characterized by an ‘inverse storage effect', whereby younger water is released in greater proportion under wetter conditions than drier. We hypothesize that this effect is caused by the rapid mobilization of water in the unsaturated zone by the rising water table. Common approximations used to model transport dynamics that neglect internal variability were unable to reproduce the observed breakthrough curves accurately. This suggests that internal variability can play an important role in hydrologic transport dynamics, with implications for field data interpretation and modeling. This article is protected by copyright. All rights reserved.
- Closed‐flow column experiments: A numerical sensitivity analysis of
reactive transport and parameter uncertainty
- Authors: Thomas Ritschel; Kai Uwe Totsche
Abstract: The identification of transport parameters by inverse modeling often suffers from equifinality or parameter correlation when models are fitted to measurements of the solute breakthrough in column outflow experiments. This parameter uncertainty can be approached by performing multiple experiments with different sets of boundary conditions, each provoking observations that are uniquely attributable to the respective transport processes. A promising approach to further increase the information potential of the experimental outcome is the closed‐flow column design. It is characterized by the recirculation of the column effluent into the solution supply vessel that feeds the inflow, which results in a damped sinusoidal oscillation in the breakthrough curve. In order to reveal the potential application of closed‐flow experiments, we present a comprehensive sensitivity analysis using common models for adsorption and degradation. We show that the sensitivity of inverse parameter determination with respect to the apparent dispersion can be controlled by the experimenter. For optimal settings, a decrease in parameter uncertainty as compared to classical experiments by an order of magnitude is achieved. In addition, we show a reduced equifinality between rate‐limited interactions and apparent dispersion. Furthermore, we illustrate the expected breakthrough curve for equilibrium and nonequilibrium adsorption, the latter showing strong similarities to the behavior found for completely mixed batch reactor experiments. Finally, breakthrough data from a reactive tracer experiment is evaluated using the proposed framework with excellent agreement of model and experimental results. This article is protected by copyright. All rights reserved.
- A proposal of optimal sampling design using a modularity strategy
- Authors: A. Simone; O. Giustolisi, D.B. Laucelli
Abstract: In real water distribution networks (WDNs) are present thousands nodes and optimal placement of pressure and flow observations is a relevant issue for different management tasks. The planning of pressure observations in terms of spatial distribution and number is named sampling design and it was faced considering model calibration. Nowadays, the design of system monitoring is a relevant issue for water utilities e.g. in order to manage background leakages, to detect anomalies and bursts, to guarantee service quality, etc. In recent years, the optimal location of flow observations related to design of optimal district metering areas (DMAs) and leakage management purposes has been faced considering optimal network segmentation and the modularity index using a multi‐objective strategy. Optimal network segmentation is the basis to identify network modules by means of optimal conceptual cuts, which are the candidate locations of closed gates or flow meters creating the DMAs. Starting from the WDN‐oriented modularity index, as a metric for WDN segmentation, this paper proposes a new way to perform the sampling design, i.e. the optimal location of pressure meters, using newly developed sampling‐oriented modularity index. The strategy optimizes the pressure monitoring system mainly based on network topology and weights assigned to pipes according to the specific technical tasks. A multi‐objective optimization minimizes the cost of pressure meters while maximizing the sampling‐oriented modularity index. The methodology is presented and discussed using the Apulian and Exnet networks. This article is protected by copyright. All rights reserved.
- Identification of temporal consistency in rating curve data: Bidirectional
- Authors: Katrien Van Eerdenbrugh; Stijn Van Hoey, Niko E.C. Verhoest
Abstract: In this paper, a methodology is developed to identify consistency of rating curve data based on a quality analysis of model results. This methodology, called Bidirectional Reach (BReach), evaluates results of a rating curve model with randomly sampled parameter sets in each observation. The combination of a parameter set and an observation is classified as non‐acceptable if the deviation between the accompanying model result and the measurement exceeds observational uncertainty. Based on this classification, conditions for satisfactory behavior of a model in a sequence of observations are defined. Subsequently, a parameter set is evaluated in a data point by assessing the span for which it behaves satisfactory in the direction of the previous (or following) chronologically sorted observations. This is repeated for all sampled parameter sets and results are aggregated by indicating the endpoint of the largest span, called the maximum left (right) reach. This temporal reach should not be confused with a spatial reach (indicating a part of a river). The same procedure is followed for each data point and for different definitions of satisfactory behavior. Results of this analysis enable the detection of changes in data consistency. The methodology is validated with observed data and various synthetic stage‐discharge data sets and proves to be a robust technique to investigate temporal consistency of rating curve data. It provides satisfying results despite of low data availability, errors in the estimated observational uncertainty and a rating curve model that is known to cover only a limited part of the observations. This article is protected by copyright. All rights reserved.
- Aquifer Heterogeneity Controls on Adverse Human Health Effects and the
Concept of the Hazard Attenuation Factor
- Authors: F. P. J. de Barros; A. Bellin, V. Cvetkovic, G. Dagan, A. Fiori
Abstract: We analyze the probability distribution of the hazard attenuation factor for a non‐carcinogenic reactive compound captured by a well in heterogeneous porous formations. The hazard attenuation factor is defined as the ratio between the hazard index HI at a detection well and at the source. Heterogeneity of the aquifer is represented through the Multi‐Indicator Model (a collection of blocks of independent permeability) while flow and transport are solved by the means of the Self‐Consistent Approach, that is able to deal with any degree of heterogeneity. Due to formation heterogeneity, HI is a random variable and similar for hazard attenuation index. The latter can be fully characterized by its cumulative distribution function (CDF), which in turn can be related to the statistics of the travel time of solute particles, from the source to the detection well. The approach is applied to the case of a solute which undergoes decay and a well with a screen much smaller than the correlation scale of hydraulic conductivity. The results show that the probability of exceeding a given acceptable threshold of the hazard index is significantly affected by the level of heterogeneity comparable to the one observed for the MADE site, and the distance between the source and the well. This article is protected by copyright. All rights reserved.
- An adaptive Gaussian process‐based method for efficient Bayesian
experimental design in groundwater contaminant source identification
- Authors: Jiangjiang Zhang; Weixuan Li, Lingzao Zeng, Laosheng Wu
Abstract: Surrogate models are commonly used in Bayesian approaches such as Markov Chain Monte Carlo (MCMC) to avoid repetitive CPU‐demanding model evaluations. However, the approximation error of a surrogate may lead to biased estimation of the posterior distribution. This bias can be corrected by constructing a very accurate surrogate or implementing MCMC in a two‐stage manner. Since the two‐stage MCMC requires extra original model evaluations after surrogate evaluations, the computational cost is still high. If the information of measurement is incorporated, a locally accurate surrogate can be adaptively constructed with low computational cost. Based on this idea, we integrate Gaussian process (GP) and MCMC to adaptively construct locally accurate surrogates for Bayesian experimental design in groundwater contaminant source identification problems. Moreover, the uncertainty estimate of GP approximation error is incorporated in the Bayesian formula to avoid over‐confident estimation of the posterior distribution. The proposed approach is tested with a numerical case study. Without sacrificing the estimation accuracy, the new approach achieves about 200 times of speed‐up compared to our previous work which implemented MCMC in a two‐stage manner. This article is protected by copyright. All rights reserved.
- Large earthquakes create vertical permeability by breaching aquitards
- Abstract: Hydrologic responses to earthquakes and their mechanisms have been widely studied. Some responses have been attributed to increases in the vertical permeability. However, basic questions remain: How do increases in the vertical permeability occur? How frequently do they occur? Is there a quantitative measure for detecting the occurrence of aquitard breaching? We try to answer these questions by examining data from a dense network of ∼50 monitoring stations of clustered wells in a sedimentary basin near the epicenter of the 1999 M7.6 Chi‐Chi earthquake in western Taiwan. While most stations show evidence that confined aquifers remained confined after the earthquake, about 10% of the stations show evidence of coseismic breaching of aquitards, creating vertical permeability as high as that of aquifers. The water levels in wells without evidence of coseismic breaching of aquitards show tidal responses similar to that of a confined aquifer before and after the earthquake. Those wells with evidence of coseismic breaching of aquitards, on the other hand, show distinctly different post‐seismic tidal response. Furthermore, the post‐seismic tidal response of different aquifers became strikingly similar, suggesting that the aquifers became hydraulically connected and the connection was maintained many months thereafter. Breaching of aquitards by large earthquakes has significant implications for a number of societal issues such as the safety of water resources, the security of underground waste repositories, and the production of oil and gas. The method demonstrated here may be used for detecting the occurrence of aquitard breaching by large earthquakes in other seismically active areas. This article is protected by copyright. All rights reserved.
- Model simulations of flood and debris flow timing in steep catchments
- Authors: F. K. Rengers; L. A. McGuire, J. W. Kean, D. M. Staley, D. E. J. Hobley
Abstract: Debris flows are a typical hazard on steep slopes after wildfire, but unlike debris flows that mobilize from landslides, most post‐wildfire debris flows are generated from water runoff. The majority of existing debris‐flow modeling has focused on landslide‐triggered debris flows. In this study we explore the potential for using process‐based rainfall‐runoff models to simulate the timing of water flow and runoff‐generated debris flows in recently burned areas. Two different spatially distributed hydrologic models with differing levels of complexity were used: the full shallow water equations and the kinematic wave approximation. Model parameter values were calibrated in two different watersheds, spanning two orders of magnitude in drainage area. These watersheds were affected by the 2009 Station Fire in the San Gabriel Mountains, CA, USA. Input data for the numerical models were constrained by time series of soil moisture, flow stage, and rainfall collected at field sites, as well as high‐resolution lidar‐derived digital elevation models. The calibrated parameters were used to model a third watershed in the burn area, and the results show a good match with observed timing of flow peaks. The calibrated roughness parameter (Manning's $n$) was generally higher when using the kinematic wave approximation relative to the shallow water equations, and decreased with increasing spatial scale. The calibrated effective watershed hydraulic conductivity was low for both models, even for storms occurring several months after the fire, suggesting that wildfire‐induced changes to soil‐water infiltration were retained throughout that time. Overall the two model simulations were quite similar suggesting that a kinematic wave model, which is simpler and more computationally efficient, is a suitable approach for predicting flood and debris flow timing in steep, burned watersheds. This article is protected by copyright. All rights reserved.
- Using practical and social information to influence flood adaptation
- Authors: Maura C. Allaire
Abstract: As the prospect for more frequent and severe extreme weather events gains scientific support, many nations are evaluating mitigation and adaptation options. Insurance and home retrofits could reduce household welfare losses due to flood events. Yet, even after disasters, households often fail to take risk mitigation actions. This paper presents the first randomized field experiment that tests the effect of information provision on household uptake of flood insurance and home retrofits.
A sample of 364 flood‐prone households in Bangkok was randomly split into treatment and control groups. The treatment group received practical details on home retrofits and flood insurance as well as social information regarding the insurance purchase decisions of peers. Results indicate that the information intervention increased insurance purchases by about five percentage points, while no effect was detected for home retrofits. This effect is nearly equal to the increase in uptake that the national insurance program in Thailand has achieved through all other means since its establishment in 2012. If scaled up to include all uninsured, flood‐prone households in Bangkok, nearly 70,000 additional households could be insured. The results suggest that well‐designed information interventions could increase uptake of flood insurance, without additional premium subsidies or mandates. This article is protected by copyright. All rights reserved.
- Integrated surface/subsurface permafrost thermal hydrology: Model
formulation and proof‐of‐concept simulations
- Authors: Scott L. Painter; Ethan T. Coon, Adam Atchley, Markus Berndt, Rao Garimella, David Moulton, Daniil Svyatskiy, Cathy J. Wilson
Abstract: The need to understand potential climate impacts and feedbacks in Arctic regions has prompted recent interest in modeling of permafrost dynamics in a warming climate. A new fine‐scale integrated surface/subsurface thermal hydrology modeling capability is described and demonstrated in proof‐of‐concept simulations. The new modeling capability combines a surface energy balance model with recently developed three‐dimensional subsurface thermal hydrology models and new models for nonisothermal surface water flows and snow distribution in the microtopography. Surface water flows are modeled using the diffusion wave equation extended to include energy transport and phase change of ponded water. Variation of snow depth in the microtopography, physically the result of wind scour, is modeled phenomenologically with a diffusion wave equation. The multiple surface and subsurface processes are implemented by leveraging highly parallel community software. Fully integrated thermal hydrology simulations on the tilted open book catchment, an important test case for integrated surface/subsurface flow modeling, are presented. Fine‐scale 100‐year projections of the integrated permafrost thermal hydrological system on an ice wedge polygon at Barrow Alaska in a warming climate are also presented. These simulations demonstrate the feasibility of microtopography‐resolving, process‐rich simulations as a tool to help understand possible future evolution of the carbon‐rich Arctic tundra in a warming climate. This article is protected by copyright. All rights reserved.
- Spatial statistical network models for stream and river temperature in New
- Authors: Naomi E. Detenbeck; Alisa Morrison, Ralph W. Abele, Darin Kopp
Abstract: Watershed managers are challenged by the need for predictive temperature models with sufficient accuracy and geographic breadth for practical use. We described thermal regimes of New England rivers and streams based on a reduced set of metrics for the May to September growing season (July or August median temperature, diurnal rate of change, and magnitude and timing of growing season maximum) chosen through principal component analysis of 78 candidate metrics. We then developed and assessed spatial statistical models for each of these metrics, incorporating spatial autocorrelation based on both distance along the flow network and Euclidean distance between points. Calculation of spatial autocorrelation based on travel or retention time in place of network distance yielded tighter‐fitting Torgegrams with less scatter but did not improve overall model prediction accuracy. We predicted monthly median July or August stream temperatures as a function of median air temperature, estimated urban heat island effect, shaded solar radiation, main channel slope, watershed storage (percent lake and wetland area), percent coarse‐grained surficial deposits, and presence or maximum depth of a lake immediately upstream, with an overall root‐mean‐square prediction error of 1.4 and 1.5○ C, respectively. Growing season maximum water temperature varied as a function of air temperature, local channel slope, shaded August solar radiation, imperviousness, and watershed storage. Predictive models for July or August daily range, maximum daily rate of change, and timing of growing season maximum were statistically significant but explained a much lower proportion of variance than the above models (5‐14% of total) . This article is protected by copyright. All rights reserved.
- Accurate early and late time modelling of countercurrent spontaneous
- Authors: Rafael March; Florian Doster, Sebastian Geiger
Abstract: Spontaneous counter‐current imbibition into a finite porous medium is an important physical mechanism for many applications, included but not limited to irrigation, CO2 storage and oil recovery. Symmetry considerations that are often valid in fractured porous media allow us to study the process in a one‐dimensional domain. In 1D, the onset of imbibition can be captured by self‐similar solutions and the imbibed volume scales with . At later times, the imbibition rate decreases and the finite size of the medium has to be taken into account. This requires numerical solutions. Here, we present a new approach to approximate the whole imbibition process semi‐analytically. While the onset is captured by a semi‐analytical solution. We also provide an a priori estimate of the time until which the imbibed volume scales with . This time is significantly longer than the time it takes until the imbibition front reaches the model boundary. The remainder of the imbibition process is obtained from a self‐similarity solution. We test our approach against numerical solutions that employ parametrizations relevant for oil recovery and CO2 sequestration. We show that this concept improves common first order approaches that heavily underestimate early‐time behaviour and note that it can be readily included into dual porosity models. This article is protected by copyright. All rights reserved.
- Tap water isotope ratios reflect urban water system structure and dynamics
across a semiarid metropolitan area
- Authors: Yusuf Jameel; Simon Brewer, Stephen P. Good, Brett J. Tipple, James R. Ehleringer, Gabriel J. Bowen
Abstract: Water extraction for anthropogenic use has become a major flux in the hydrological cycle. With increasing demand for water and challenges supplying it in the face of climate change, there is a pressing need to better understand connections between human populations, climate, water extraction, water use, and its impacts. To understand these connections, we collected and analyzed stable isotopic ratios of more than 800 urban tap water samples in a series of semiannual water surveys (spring and fall, 2013 to 2015) across the Salt Lake Valley (SLV) of northern Utah. Consistent with previous work, we found that mean tap water had a lower 2H and 18O concentration than local precipitation, highlighting the importance of nearby montane winter precipitation as source water for the region. However, we observed strong and structured spatiotemporal variation in tap water isotopic compositions across the region which we attribute to complex distribution systems, varying water management practices and multiple sources used across the valley. Water from different sources was not used uniformly throughout the area and we identified significant correlation between water source and demographic parameters including population and income. Isotopic mass balance indicated significant inter‐ and intra‐annual variability in water losses within the distribution network due to evaporation from surface water resources supplying the SLV. Our results demonstrate the effectiveness of isotopes as an indicator of water management strategies and climate impacts within regional urban water systems, with potential utility for monitoring, regulation, forensic and a range of water resource research. This article is protected by copyright. All rights reserved.
- Groundwater depletion in Central Mexico: Use of GRACE and InSAR to support
water resources management
- Authors: Pascal Castellazzi; Richard Martel, Alfonso Rivera, Jianliang Huang, Pavlic Goran, Angus I. Calderhead, Estelle Chaussard, Jaime Garfias, Javier Salas
Abstract: Groundwater deficits occur in several areas of Central Mexico, where water resource assessment is limited by the availability and reliability of field data. In this context, GRACE and InSAR are used to remotely assess groundwater storage loss in one of Mexico's most important watersheds in terms of size and economic activity: the Lerma‐Santiago‐Pacifico (LSP). In situ data and Land Surface Models are used to subtract soil moisture and surface water storage changes from the total water storage change measured by GRACE satellites. As a result, groundwater mass change time‐series are obtained for a 12 years period. ALOS‐PALSAR images acquired from 2007 to 2011 were processed using the SBAS‐InSAR algorithm to reveal areas subject to ground motion related to groundwater over‐exploitation. In the perspective of providing guidance for groundwater management, GRACE and InSAR observations are compared with official water budgets and field observations.
InSAR‐derived subsidence mapping generally agrees well with official water budgets, and shows that deficits occur mainly in cities and irrigated agricultural areas. GRACE does not entirely detect the significant groundwater losses largely reported by official water budgets, literature and InSAR observations. The difference is interpreted as returns of wastewater to the groundwater flow systems, which limits the watershed scale groundwater depletion but suggests major impacts on groundwater quality. This phenomenon is enhanced by ground fracturing as noticed in the field. Studying the fate of the extracted groundwater is essential when comparing GRACE data with higher resolution observations, and particularly in the perspective of further InSAR/GRACE combination in hydrogeology. This article is protected by copyright. All rights reserved.
- Change in streamflow response in unregulated catchments in Sweden over the
- Abstract: A Fourier spectral analysis of daily discharge time series over the last century in 79 unregulated catchments in Sweden reveals a significant gradual steepening of the discharge power spectrum slope over time. Where historical meteorological observations are available (the 41 southernmost catchments), the results of our analyses indicate that local land‐use changes within the catchments have affected discharge power spectra to a greater extent than have changes in precipitation patterns.
1D distributed routing analysis based on current and historical maps and scenario modeling in the Törnestorp Catchment suggests that changes in stream network properties have led to increases in the hydraulic Péclet number (Pe) and subsequent decreases in the discharge power spectrum over short periods. The analysis displays analytically how a change in stream network properties can result in changes in the power spectra, where the relative importance of the geomorphological and hydrodynamic dispersion effects determines the shape of the streamflow response.
The lowering of the discharge power spectrum over short periods observed for many Swedish catchments is likely caused by increasing Pe (a decrease in dispersion) over time, resulting in higher peak values, especially for rapid streamflow responses (i.e., over short periods), demonstrated empirically for the Törnestorp case study.
The finding that the discharge power spectrum can change significantly over time highlights the need for hydrological models to account for the effect of the non‐stationarity of parameters that result from temporal change caused by land use change and/or climate change that is due to anthropogenic or natural causes. This article is protected by copyright. All rights reserved.
- Impact of topography on groundwater salinization due to ocean surge
- Authors: Xuan Yu; Jie Yang, Thomas Graf, Mohammad Koneshloo, Michael A. O'Neal, Holly A. Michael
Abstract: Sea‐level rise and increases in the frequency and intensity of ocean surges caused by climate change are likely to exacerbate adverse effects on low‐lying coastal areas. The landward flow of water during ocean surges introduces salt to surficial coastal aquifers and threatens groundwater resources. Coastal topographic features (e.g., ponds, dunes, barrier islands, and channels) likely have a strong impact on overwash and salinization processes, but are generally highly simplified in modeling studies. To understand topographic impacts on groundwater salinization, we modeled a theoretical overwash event and variable‐density groundwater flow and salt transport in 3D using the fully coupled surface and subsurface numerical simulator, HydroGeoSphere. The model simulates the coastal aquifer as an integrated system considering overland flow, coupled surface and subsurface exchange, variably saturated flow, and variable‐density groundwater flow. To represent various coastal landscape types, we simulated both synthetic fields and real‐world coastal topography from Delaware, USA. The groundwater salinization assessment suggested that the topographic connectivity promoting overland flow controls the volume of aquifer that is salinized. In contrast, the amount of water that can be stored in surface depressions determines the amount seawater that infiltrates the subsurface and the time for seawater to flush from the aquifer. Our study suggests that topography has a significant impact on groundwater salinization due to ocean surge overwash, with important implications for coastal land management and groundwater vulnerability assessment. This article is protected by copyright. All rights reserved.
- Hydrograph variances over different timescales in hydropower production
- Abstract: The operation of water reservoirs involves a spectrum of timescales based on the distribution of stream flow travel times between reservoirs, as well as the technical, environmental and social constraints imposed on the operation. In this research, a hydrodynamically based description of the flow between hydropower stations was implemented to study the relative importance of wave diffusion on the spectrum of hydrograph variance in a regulated watershed. Using spectral decomposition of the effluence hydrograph of a watershed, an exact expression of the variance in the outflow response was derived, as a function of the trends of hydraulic and geomorphologic dispersion and management of production and reservoirs. We show that the power spectra of involved time‐series follow nearly fractal patterns, which facilitates examination of the relative importance of wave diffusion and possible changes in production demand on the outflow spectrum. The exact spectral solution can also identify statistical bounds of future demand patterns due to limitations in storage capacity. The impact of the hydraulic description of the stream flow on the reservoir discharge was examined for a given power demand in River Dalälven, Sweden, as function of a stream flow Peclet number. The regulation of hydropower production on the River Dalälven generally increased the short‐term variance in the effluence hydrograph, whereas wave diffusion decreased the short‐term variance over periods of
- Does resolution of flow field observation influence apparent habitat use
and energy expenditure in juvenile coho salmon?
- Abstract: This study investigated how the resolution of observation influences interpretation of how fish, juvenile Coho Salmon (Oncorhynchus kisutch), exploit the hydraulic environment in streams. Our objectives were to evaluate how spatial resolution of the flow field observation influenced: 1) the velocities considered to be representative of habitat units; 2) patterns of use of the hydraulic environment by fish; and 3) estimates of energy expenditure. We addressed these objectives using observations within a 1:1 scale physical model of a full‐channel log jam in an outdoor experimental stream. Velocities were measured with Acoustic Doppler Velocimetry at a 10 cm grid spacing, whereas fish locations and tailbeat frequencies were documented over time using underwater videogrammetry. Results highlighted that resolution of observation did impact perceived habitat use and energy expenditure, as did the location of measurement within habitat units and the use of averaging to summarize velocities within a habitat unit. In this experiment, the range of velocities and energy expenditure estimates increased with coarsening resolution, reducing the likelihood of measuring the velocities locally experienced by fish. In addition, the coarser resolutions contributed to fish appearing to select velocities that were higher than what was measured at finer resolutions. These findings indicate the need for careful attention to and communication of resolution of observation in investigating the hydraulic environment and in determining the habitat needs and bioenergetics of aquatic biota. This article is protected by copyright. All rights reserved.
- Comment on “Advective transport in heterogeneous aquifers: Are proxy
models predictive?” by A. Fiori, A. Zarlenga, H. Gotovac, I.
Jankovic, E.Volpi, V.Cvetkovic, and G. Dagan
- Authors: Shlomo P. Neuman
Abstract: Fiori et al.  examine the predictive capabilities of (among others) two “proxy” non‐Fickian transport models, MRMT (Multi‐RateMassTransfer) and CTRW (Continuous‐Time Random Walk). In particular, they compare proxy model predictions of mean breakthrough curves (BTCs) at a sequence of control planes with near‐ergodic BTCs generated through two‐ and three‐dimensional simulations of nonreactive, mean‐uniform advective transport in single realizations of stationary, randomly heterogeneous porous media. The authors find fitted proxy model parameters to be nonunique and devoid of clear physical meaning. This notwithstanding, they conclude optimistically that “i. Fitting the proxy models to match the BTC at [one control plane] automatically ensures prediction at downstream control planes [and thus] ii. … the measured BTC can be used directly for prediction, with no need to use models underlain by fitting.” I show that (a) the authors' findings follow directly from (and thus confirm) theoretical considerations discussed earlier by Neuman and Tartakovsky , which (b) additionally demonstrate that proxy models will lack similar predictive capabilities under more realistic, non‐Markovian flow and transport conditions that prevail under flow through nonstationary (e.g. multiscale) media in the presence of boundaries and/or nonuniformly distributed sources, and/or when flow/transport are conditioned on measurements. This article is protected by copyright. All rights reserved.
- Reply to the comment by S.P. Neuman on “Advective transport in
heterogeneous aquifers: Are proxy models predictive?”by A.
Fiori, A. Zarlenga, H. Gotovac, I. Jankovic, E.Volpi, V.Cvetkovic, and G.
- Authors: Aldo Fiori; Antonio Zarlenga, Hrvoje Gotovac, Hrvoje Gotovac, Hrvoje Gotovac, Vladimir Cvetkovic, Gedeon Dagan
- A copula‐based nonstationary frequency analysis for the
2012‐2015 drought in California
- Abstract: Using a multi‐century reconstruction of drought, we investigate how rare the 2012‐2015 California drought is. A Bayesian approach to a nonstationary, bivariate probabilistic model, including the estimation of Copula parameters is used to assess the time varying return period of the current drought. Both the duration and severity of drought exhibit similar multi‐century trends. The period from 800‐1200 AD was perhaps more similar to the recent period than the period from 1200 to 1800 AD. The median return period of the recent drought accounting for both duration and severity, varies from approximately 667 to 2652 years, if the model parameters from the different time periods are considered. However, we find that the recent California drought is of unprecedented severity, especially given the relatively modest duration of the drought. The return period of the severity of the recent drought given its 4‐year duration is estimated to be nearly 21,000 years. This article is protected by copyright. All rights reserved.
- Minimizing the effects of filtering on catchment scale GRACE solutions
- Authors: Bramha Dutt Vishwakarma; Balaji Devaraju, Nico Sneeuw
Abstract: The Gravity Recovery and Climate Experiment (GRACE) satellite mission has provided time variable gravity information since its launch in 2002. Due to short‐wavelength noise, the total water storage variations over a catchment observed from GRACE are usable only after filtering. Filtering smooths both the signal and the noise, inevitably changing the nature of the estimated total water storage change. The filtered estimates suffer from attenuation and leakage, which changes the signal characteristics. Several studies have mainly focused on correcting the changed amplitude with the aid of hydrological models. In this study, it is demonstrated that in addition to the amplitude loss, also significant phase change in the time series of total water storage over a region can occur. The phase change due to leakage from nearby catchments can be around 20° to 30° for catchments with moderate size, which makes it difficult to retrieve signal by only scaling. We propose a strategy to approach the true time series with improved phase and amplitude. The strategy is independent of any hydrological model. It is first demonstrated in a closed‐loop environment over 32 catchments, where we show that the performance of our method is consistent and better than other model dependent approaches. Then we also discuss the limitations of our approach. Finally we apply our method to the GRACE level 2 products for 32 catchments. This article is protected by copyright. All rights reserved.
- A comparison of regional flood frequency analysis approaches in a
- Authors: D. Ganora; F. Laio
Abstract: Regional frequency analysis (RFA) is a well‐established methodology to provide an estimate of the flood frequency curve at ungauged (or scarcely gauged) sites. Different RFA approaches exist, depending on the way the information is transferred to the site of interest, but it is not clear in the literature if a specific method systematically outperforms the others. The aim of this study is to provide a framework wherein carrying out the intercomparison by building up a virtual environment based on synthetically generated data. The considered regional approaches include: (i) a unique regional curve for the whole region; (ii) a multiple‐region model where homogeneous subregions are determined through cluster analysis; (iii) a Region‐of‐Influence model which defines a homogeneous subregion for each site; (iv) a spatially‐smooth estimation procedure where the parameters of the regional model vary continuously along the space. Virtual environments are generated considering different patterns of heterogeneity, including step change and smooth variations. If the region is heterogeneous, with the parent distribution changing continuously within the region, the spatially‐smooth regional approach outperforms the others, with overall errors 10%‐50% lower than the other methods. In the case of a step‐change, the spatially‐smooth and clustering procedures perform similarly if the heterogeneity is moderate, while clustering procedures work better when the step‐change is severe. To extend our findings, an extensive sensitivity analysis has been performed to investigate the effect of sample length, number of virtual stations, return period of the predicted quantile, variability of the scale parameter of the parent distribution, number of predictor variables and different parent distribution. Overall, the spatially‐smooth approach appears as the most robust approach as its performances are more stable across different patterns of heterogeneity, especially when short records are considered. This article is protected by copyright. All rights reserved.
- Mean age distribution of inorganic soil‐nitrogen
- Authors: Dong K. Woo; Praveen Kumar
Abstract: Excess reactive nitrogen in soils of intensively managed landscapes causes adverse environmental impact, and continues to remain a global concern. Many novel strategies have been developed to provide better management practices and, yet, the problem remains unresolved. The objective of this study is to develop a model to characterize the “age” of inorganic soil‐nitrogen (nitrate, and ammonia/ammonium). We use the general theory of age, which provides an assessment of the time elapsed since inorganic nitrogen has been introduced into the soil system. We analyze a corn‐corn‐soybean rotation, common in the Midwest United States, as an example application. We observe two counter‐intuitive results: (1) the mean nitrogen age in the topsoil layer is relatively high; and (2) mean nitrogen age is lower under soybean cultivation compared to corn although no fertilizer is applied for soybean cultivation. The first result can be explained by cation‐exchange of ammonium that retards the leaching of nitrogen, resulting in an increase in the mean nitrogen age near the soil surface. The second result arises because the soybean utilizes the nitrogen fertilizer left from the previous year, thereby removing the older nitrogen and reducing mean nitrogen age. Estimating the mean nitrogen age can thus serve as an important tool to disentangle complex nitrogen dynamics by providing a nuanced characterization of the time scales of soil‐nitrogen transformation and transport processes. This article is protected by copyright. All rights reserved.
- Extending theis' solution: Using transient pumping tests to estimate
parameters of aquifer heterogeneity
- Abstract: A framework for interpreting transient pumping tests in heterogeneous transmissivity fields is developed to infer the overall geostatistical parameters of the medium without reconstructing the specific heterogeneous structure point wise. The methodology of Radial Coarse Graining is applied to deduce an effective radial description of multi‐Gaussian transmissivity. It was used to derive an Effective Well Flow Solution for transient flow conditions including not only the storativity, but also the geometric mean, the variance, and the correlation length of log‐transmissivity. This solution is shown to be appropriate to characterize the pumping test drawdown behavior in heterogeneous transmissivity fields making use of ensembles of simulated pumping tests with multiple combinations of statistical parameters. Based on the Effective Well Flow Solution, a method is developed for inferring heterogeneity parameters from transient pumping test drawdown data by inverse estimation. Thereby, the impact of statistical parameters on the drawdown is analyzed, allowing to determine the dependence of reliability of parameter estimates on location and number of measurements. It is shown, that the number of measurements can be reduced compared to steady state pumping tests. Finally, a sampling strategy for single aquifer analysis is developed, which allows to estimate the statistical parameters, in particular variance and correlation length for individual heterogeneous transmissivity fields making use of transient pumping test measurements at multiple locations. This article is protected by copyright. All rights reserved.
- Assimilation of temperature and hydraulic gradients for quantifying the
spatial variability of streambed hydraulics
- Authors: Xiang Huang; Charles B. Andrews, Jie Liu, Yingying Yao, Chuankun Liu, Scott W. Tyler, John S. Selker, Chunmiao Zheng
Abstract: Understanding the spatial and temporal characteristics of water flux into or out of shallow aquifers is imperative for water resources management and eco‐environmental conservation. In this study, the spatial variability in the vertical specific fluxes and hydraulic conductivities in a streambed were evaluated by integrating distributed temperature sensing (DTS) data and vertical hydraulic gradients into an ensemble Kalman filter (EnKF) and smoother (EnKS) and an empirical thermal‐mixing model. The formulation of the EnKF/EnKS assimilation scheme is based on a discretized 1D advection‐conduction equation of heat transfer in the streambed. We first systematically tested a synthetic case and performed quantitative and statistical analyses to evaluate the performance of the assimilation schemes. Then a real‐world case was evaluated to calculate assimilated specific flux. An initial estimate of the spatial distributions of the vertical hydraulic gradients was obtained from an empirical thermal‐mixing model under steady‐state conditions using a constant vertical hydraulic conductivity. Then, this initial estimate was updated by repeatedly dividing the assimilated specific flux by estimates of the vertical hydraulic gradients to obtain a refined spatial distribution of vertical hydraulic gradients and vertical hydraulic conductivities.
Our results indicate that optimal parameters can be derived with fewer iterations but greater simulation effort using the EnKS compared with the EnKF. For the field application in a stream segment of the Heihe River Basin in northwest China, the average vertical hydraulic conductivities in the streambed varied over three orders of magnitude (5 × 10−1 to 5 × 102 m/d). The specific fluxes ranged from near zero (qz
- The two‐phase flow IPTT method for measurement of
nonwetting‐wetting liquid interfacial areas at higher nonwetting
saturations in natural porous media
- Authors: Hua Zhong; Asma El Ouni, Dan Lin, Bingguo Wang, Mark L Brusseau
Abstract: Interfacial areas between nonwetting‐wetting (NW‐W) liquids in natural porous media were measured using a modified version of the interfacial partitioning tracer test (IPTT) method that employed simultaneous two‐phase flow conditions, which allowed measurement at NW saturations higher than trapped residual saturation. Measurements were conducted over a range of saturations for a well‐sorted quartz sand under three wetting scenarios of primary drainage (PD), secondary imbibition (SI), and secondary drainage (SD). Limited sets of experiments were also conducted for a model glass‐bead medium and for a soil. The measured interfacial areas were compared to interfacial areas measured using the standard IPTT method for liquid‐liquid systems, which employs residual NW saturations. In addition, the theoretical maximum interfacial areas estimated from the measured data are compared to specific solid surface areas measured with the N2/BET method and estimated based on geometrical calculations for smooth spheres. Interfacial areas increase linearly with decreasing water saturation over the range of saturations employed. The maximum interfacial areas determined for the glass beads, which have no surface roughness, are 32±4 and 36±5 cm−1 for PD and SI cycles, respectively. The values are similar to the geometric specific solid surface area (31±2 cm−1) and the N2/BET solid surface area (28±2 cm−1). The maximum interfacial areas are 274±38, 235±27, and 581±160 cm−1 for the sand for PD, SI, and SD cycles, respectively, and ∼7625 cm−1 for the soil for PD and SI. The maximum interfacial areas for the sand and soil are significantly larger than the estimated smooth‐sphere specific solid surface areas (107±8 cm−1 and 152±8 cm−1, respectively), but much smaller than the N2/BET solid surface area (1387±92 cm−1 and 55224 cm−1, respectively). The NW‐W interfacial areas measured with the two‐phase flow method compare well to values measured using the standard IPTT method. This article is protected by copyright. All rights reserved.
- Biofilm effect on soil hydraulic properties: Experimental investigation
using soil‐grown real biofilm
- Authors: Elazar Volk; Sascha C. Iden, Alex Furman, Wolfgang Durner, Ravid Rosenzweig
Abstract: Understanding the influence of attached microbial biomass on water flow in variably saturated soils is crucial for many engineered flow systems. So far, the investigation of the effects of microbial biomass has been mainly limited to water‐saturated systems. We have assessed the influence of biofilms on the soil hydraulic properties under variably‐saturated conditions. A sandy soil was incubated with Pseudomonas Putida and the hydraulic properties of the incubated soil were determined by a combination of methods. Our results show a stronger soil water retention in the inoculated soil as compared to the control. The increase in volumetric water content reaches approximately 0.015 cm3 cm−3 but is only moderately correlated with the carbon deficit, a proxy for biofilm quantity, and less with the cell viable counts. The presence of biofilm reduced the saturated hydraulic conductivity of the soil by up to one order of magnitude. Under unsaturated conditions, the hydraulic conductivity was only reduced by a factor of four. This means that relative water conductance in biofilm‐affected soils is higher compared to the clean soil at low water contents, and that the unsaturated hydraulic conductivity curve of biofilm‐affected soil cannot be predicted by simply scaling the saturated hydraulic conductivity. A flexible parameterization of the soil hydraulic functions accounting for capillary and non‐capillary flow was needed to adequately describe the observed properties over the entire wetness range. More research is needed to address the exact flow mechanisms in biofilm‐affected, unsaturated soil and how they are related to effective system properties. This article is protected by copyright. All rights reserved.
- Structural controls on anomalous transport in fractured porous rock
- Authors: Yaniv Edery; Sebastian Geiger, Brian Berkowitz
Abstract: Anomalous transport is ubiquitous in a wide range of disordered systems, notably in fractured porous formations. We quantitatively identify the structural controls on anomalous tracer transport in a model of a real fractured geological formation that was mapped in an outcrop. The transport, determined by a continuum scale mathematical model, is characterized by breakthrough curves (BTCs) that document anomalous (or “non‐Fickian”) transport, which is accounted for by a power‐law distribution of local transition times ψ(t) within the framework of a continuous time random walk (CTRW). We show that the determination of ψ(t) is related to fractures aligned approximately with the macroscopic direction of flow. We establish the dominant role of fracture alignment, and assess the statistics of these fractures by determining a concentration‐visitation weighted residence time histogram. We then convert the histogram to a probability density function (pdf) that coincides with the CTRW ψ(t) and hence anomalous transport. We show that the permeability of the geological formation hosting the fracture network has a limited effect on the anomalous nature of the transport; rather, it is the fractures transverse to the flow direction that play the major role in forming the long BTC tail associated with anomalous transport. This is a remarkable result, given the complexity of the flow field statistics as captured by concentration transitions. This article is protected by copyright. All rights reserved.
- Multiscale pore‐network representation of heterogeneous carbonate
- Authors: Tannaz Pak; Ian B. Butler, Sebastian Geiger, Marinus I.J. van Dijke, Zeyun Jiang, Rodrigo Surmas
Abstract: A multi‐scale network integration approach introduced by Jiang et al.  is used to generate a representative pore‐network for a carbonate rock with a pore‐size distribution across several orders of magnitude. We predict the macroscopic flow parameters of the rock utilising i) 3D images captured by X‐ray computed micro‐tomography and ii) pore‐network flow simulations. To capture the multi‐scale pore‐size distribution of the rock we imaged four different rock samples at different resolutions and integrated the data to produce a pore‐network model that combines information at several length‐scales that cannot be recovered from a single tomographic image. A workflow for selection of the number and length‐scale of the required input networks for the network integration process, as well as fine tuning the model parameters is presented. Mercury injection capillary‐pressure data were used to evaluate independently the multi‐scale networks. We explore single‐scale, two‐scale, and three‐scale network models and discuss their representativeness by comparing simulated capillary‐pressure versus saturation curves with laboratory measurements. We demonstrate that for carbonate rocks with wide pore‐size distributions, it may be required to integrate networks extracted from two or three discrete tomographic data sets in order to simulate macroscopic flow parameters. This article is protected by copyright. All rights reserved.
- Compositional data analysis as a robust tool to delineate hydrochemical
facies within and between gas‐bearing aquifers
- Abstract: Isometric log ratios of proportions of major ions, derived from intuitive sequential binary partitions, are used to characterise hydrochemical variability within and between coal seam gas (CSG) and surrounding aquifers in a number of sedimentary basins in the USA and Australia. These isometric log ratios are the coordinates corresponding to an orthonormal basis in the sample space (the simplex). The characteristic proportions of ions, as described by linear models of isometric log ratios, can be used for a mathematical‐descriptive classification of water types. This is a more informative and robust method of describing water types than simply classifying a water type based on the dominant ions. The approach allows: a) compositional distinctions between very similar water types to be made; and b) large data sets with a high degree of variability to be rapidly assessed with respect to particular relationships/compositions that are of interest. A major advantage of these techniques is that major and minor ion components can be comprehensively assessed and subtle processes ─ which may be masked by conventional techniques such as Stiff diagrams, Piper plots and classic ion ratios ─ can be highlighted. Results show that while all CSG groundwaters are dominated by Na, HCO3 and Cl ions, the proportions of other ions indicate they can evolve via different means and the particular proportions of ions within total or subcompositions can be unique to particular basins. Using isometric log ratios, subtle differences in the behaviour of Na, K and Cl between CSG water types and very similar Na‐HCO3 water types in adjacent aquifers are also described. A complementary pair of isometric log ratios, derived from a geochemically intuitive sequential binary partition that is designed to reflect compositional variability within and between CSG groundwater, is proposed. These isometric log ratios can be used to model a hydrochemical pathway associated with methanogenesis and/or to delineate groundwater associated with high gas concentrations. This article is protected by copyright. All rights reserved.
- On the deterministic and stochastic use of hydrologic models
- Authors: William H. Farmer; Richard M. Vogel
Abstract: Environmental simulation models, such as precipitation‐runoff watershed models, are increasingly used in a deterministic manner for environmental and water resources design, planning, and management. In operational hydrology, simulated responses are now routinely used to plan, design, and manage a very wide class of water resource systems. However, all such models are calibrated to existing data sets and retain some residual error. This residual, typically unknown in practice, is often ignored, implicitly trusting simulated responses as if they are deterministic quantities. In general, ignoring the residuals will result in simulated responses with distributional properties that do not mimic those of the observed responses. This discrepancy has major implications for the operational use of environmental simulation models as is shown here. Both a simple linear model and a distributed‐parameter precipitation‐runoff model are used to document the expected bias in the distributional properties of simulated responses when the residuals are ignored. The systematic reintroduction of residuals into simulated responses in a manner that produces stochastic output is shown to improve the distributional properties of the simulated responses. Every effort should be made to understand the distributional behavior of simulation residuals and to use environmental simulation models in a stochastic manner. This article is protected by copyright. All rights reserved.
- Simulation of root water uptake under consideration of
non‐equilibrium dynamics in the rhizosphere
- Authors: Eva Kroener; Mohsen Zarebanadkouki, Marco Bittelli, Andrea Carminati
Abstract: The narrow region of soil around roots, the so‐called rhizosphere, defers in its hydraulic properties from the bulk soil. The rhizosphere hydraulic properties primarily depend on the drying and wetting rate of mucilage, a polymeric gel exuded by plant roots. Under equilibrium conditions mucilage increases the water holding capacity. Upon drying mucilage turns hydrophobic and makes the rhizosphere temporarily water repellent.
There are several models of root water uptake, from analytical models of water flow to a single root to complex numerical models that consider the root architecture. Most of these models, however, do not account for the specific hydraulic properties of the rhizosphere.
Here we describe a single‐root model that includes the altered hydraulic properties of the rhizosphere due to mucilage exudation. We use the model to reproduce existing experiments reporting unexpected and puzzling hysteresis in the rhizosphere, which could not be explained under the assumption of homogeneous hydraulic properties. In our model the hydraulic properties depend on the concentration of mucilage. This enables a continuous transition from the bulk soil to the root surface. We assumed that: (a) mucilage increases the water holding capacity in equilibrium conditions, (b) hydrophobicity, swelling and shrinking of mucilage cause a non‐equilibrium relation between water content and water potential and (c) mucilage reduces the mobility of water molecules in the liquid phase resulting in a lower hydraulic conductivity at a given water content.
Our model reproduces well the experiments and suggests that mucilage softens drought stress in plants during severe drying events. This article is protected by copyright. All rights reserved.
- Heat as a tracer for understanding transport processes in fractured media:
theory and field assessment from multi‐scale thermal push‐pull
- Authors: Maria V. Klepikova; Tanguy Le Borgne, Olivier Bour, Marco Dentz, Rebecca Hochreutener, Nicolas Lavenant
Abstract: The characterization and modeling of heat transfer in fractured media is particularly challenging as the existence of fractures at multiple scales induces highly localized flow patterns. From a theoretical and numerical analysis of heat transfer in simple conceptual models of fractured media, we show that flow channeling has a significant effect on the scaling of heat recovery in both space and time. The late time tailing of heat recovery under channeled flow is shown to diverge from the T(t) ∝ t−1.5 behavior expected for the classical parallel plate model and follow the scaling T(t) ∝ 1/t (log t)2 for a simple channel modeled as a tube. This scaling, which differs significantly from known scalings in mobile‐immobile systems, is of purely geometrical origin: late time heat transfer from the matrix to a channel corresponds dimensionally to a radial diffusion process, while heat transfer from the matrix to a plate may be considered as a one‐dimensional process. This phenomenon is also manifested on the spatial scaling of heat recovery as flow channeling affects the decay of the thermal breakthrough peak amplitude and the increase of the peak time with scale. These findings are supported by the results of a field experimental campaign performed on the fractured rock site of Ploemeur. The scaling of heat recovery in time and space, measured from thermal breakthrough c urves measured through a series of push‐pull tests at different scales, shows a clear signature of flow channeling. The whole data‐set can thus be successfully represented by a multi‐channel model parametrized by the mean channel density and aperture. These findings, which bring new insights on the effect of flow channeling on heat transfer in fractured rocks, show how heat recovery in geothermal tests may be controlled by fracture geometry. In addition, this highlights the interest of thermal push‐pull tests as a complement to solute tracers tests to infer fracture aperture and geometry. This article is protected by copyright. All rights reserved.
- Stochastic Simulation of Soil Particle‐Size Curves in Heterogeneous
Aquifer Systems through a Bayes space approach
- Authors: A. Menafoglio; A. Guadagnini, P. Secchi
Abstract: We address the problem of stochastic simulation of soil particle‐size curves (PSCs) in heterogeneous aquifer systems. Unlike traditional approaches that focus solely on a few selected features of PSCs (e.g., selected quantiles), our approach considers the entire particle size curves and can optionally include conditioning on available data. We rely on our prior work [Menafoglio et al, 2014,2015] to model PSCs as cumulative distribution functions, and interpret their density functions as functional compositions. We thus approximate the latter through an expansion over an appropriate basis of functions. This enables us to (a) effectively deal with the data dimensionality and constraints, and (b) to develop a simulation method for PSCs based upon a suitable and well defined projection procedure. The new theoretical framework allows representing and reproducing the complete information content embedded in PSC data. As a first field application, we demonstrate the quality of unconditional and conditional simulations obtained with our methodology by considering a set of particle‐size curves collected within a shallow alluvial aquifer in the Neckar river valley, Germany. This article is protected by copyright. All rights reserved.
- Instability of canopy flows
- Authors: Giuseppe A. Zampogna; Franck Pluvinage, Azeddine Kourta, Alessandro Bottaro
Abstract: Honami and monami waves are caused by large‐scale coherent vortex structures which form in shear layers generated by canopies. In order to reach new insights on the onset of such waves, the instability of these shear layers is studied. Two different approach are used. In the first approach the presence of the canopy is modeled via a drag coefficient, taken to vary along the canopy as by experimental indications. The second approach considers the canopy as a porous medium and different governing equations for the fluid flow are deduced. In this second case the anisotropy of the canopy, composed by rigid cylindrical elements, is accounted for via an apparent permeability tensor. The results obtained with the latter approach approximate better experimental correlations for the synchronous oscillations of the canopy. This article is protected by copyright. All rights reserved.
- A new device for characterizing fracture networks and measuring
groundwater and contaminant fluxes in fractured rock aquifers
- Authors: Harald Klammler; Kirk Hatfield, Mark A. Newman, Jaehyun Cho, Michael D. Annable, Beth L. Parker, John A. Cherry, Irina Perminova
Abstract: This paper presents the fundamental theory and laboratory test results on a new device that is deployed in boreholes in fractured rock aquifers to characterize vertical distributions of water and contaminant fluxes, aquifer hydraulic properties, and fracture network properties (e.g., active fracture density and orientation). The device, a fractured rock passive flux meter (FRPFM), consists of an inflatable core assembled with upper and lower packers that isolate the zone of interest from vertical gradients within the borehole. The outer layer of the core consists of an elastic fabric mesh equilibrated with a visible dye which is used to provide visual indications of active fractures and measures of fracture location, orientation, groundwater flux, and the direction of that flux. Beneath the outer layer is a permeable sorbent that is preloaded with known amounts of water soluble tracers which are eluted at rates proportional to groundwater flow. This sorbent also captures target contaminants present in intercepted groundwater. The mass of contaminant sorbed is used to quantify cumulative contaminant flux; whereas, the mass fractions of resident tracers lost are used to provide measures of water flux. In this paper, the FRPFM is bench tested over a range of fracture velocities (2‐20 m/day) using a single fracture flow apparatus (fracture aperture = 0.5 mm). Test results show a discoloration in visible dye corresponding to the location of the active fracture. The geometry of the discoloration can be used to discern fracture orientation as well as direction and magnitude of flow in the fracture. Average contaminant fluxes were measured within 16% and water fluxes within 25% of known imposed fluxes. This article is protected by copyright. All rights reserved.
- Three‐dimensional numerical simulations of methane gas migration
from decommissioned hydrocarbon production wells into shallow aquifers
- Abstract: Three‐dimensional numerical simulations are used to provide insight into the behavior of methane as it migrates from a leaky decommissioned hydrocarbon well into a shallow aquifer. The conceptual model includes gas‐phase migration from a leaky well, dissolution into groundwater, advective‐dispersive transport and biodegradation of the dissolved methane plume. Gas‐phase migration is simulated using the DuMux multi‐phase simulator, while transport and fate of the dissolved phase is simulated using the BIONAPL/3D reactive transport model. Methane behavior is simulated for two conceptual models: first in a shallow confined aquifer containing a decommissioned leaky well based on a monitored field site near Lindbergh, Alberta, Canada, and secondly on a representative unconfined aquifer based loosely on the Borden, Ontario, field site. The simulations show that the Lindbergh site confined aquifer data are generally consistent with a 2‐year methane leak of 2 to 20 m3/d, assuming anaerobic (sulfate‐reducing) methane oxidation and with maximum oxidation rates of 1 × 10−5 to 1 × 10−3 kg/m3/d. Under the highest oxidation rate, dissolved methane decreased from solubility (110 mg/L) to the threshold concentration of 10 mg/L within 5 years. In the unconfined case with the same leakage rate, including both aerobic and anaerobic methane oxidation, the methane plume was less extensive compared to the confined aquifer scenarios. Unconfined aquifers may therefore be less vulnerable to impacts from methane leaks along decommissioned wells. At other potential leakage sites, site‐specific data on the natural background geochemistry would be necessary to make reliable predictions on the fate of methane in groundwater. This article is protected by copyright. All rights reserved.
- Comparison of fluid‐fluid interfacial areas measured with
X‐ray microtomography and interfacial partitioning tracer tests for
the same samples
- Authors: Kieran McDonald; Kenneth C. Carroll, Mark L. Brusseau
Abstract: Two different methods are currently used for measuring interfacial areas between immiscible fluids within 3‐D porous media, high‐resolution microtomographic imaging and interfacial partitioning tracer tests (IPTT). Both methods were used in this study to measure non‐wetting/wetting interfacial areas for a natural sand. The microtomographic imaging was conducted on the same packed columns that were used for the IPTTs. This is in contrast to prior studies comparing the two methods, for which in all cases different samples were used for the two methods. In addition, the columns were imaged before and after the IPTTs to evaluate the potential impacts of the tracer solution on fluid configuration and attendant interfacial area. The interfacial areas measured using IPTT are ∼5 times larger than the microtomographic‐measured values, which is consistent with previous work. Analysis of the image data revealed no significant impact of the tracer solution on NAPL configuration or interfacial area. Other potential sources of error were evaluated, and all were demonstrated to be insignificant. The disparity in measured interfacial areas between the two methods is attributed to the limitation of the microtomography method to characterize interfacial area associated with microscopic surface roughness due to resolution constraints. This article is protected by copyright. All rights reserved.
- Bayesian nitrate source apportionment to individual groundwater wells in
the Central Valley by use of elemental and isotopic tracers
- Abstract: Groundwater quality is a concern in alluvial aquifers that underlie agricultural areas, such as in the San Joaquin Valley of California. Shallow domestic wells (less than 150 m deep) in agricultural areas are often contaminated by nitrate. Agricultural and rural nitrate sources include dairy manure, synthetic fertilizers, and septic waste. Knowledge of the relative proportion that each of these sources contributes to nitrate concentration in individual wells can aid future regulatory and land management decisions. We show that nitrogen and oxygen isotopes of nitrate, boron isotopes, and iodine concentrations are a useful, novel combination of groundwater tracers to differentiate between manure, fertilizers, septic waste, and natural sources of nitrate. Furthermore, in this work, we develop a new Bayesian mixing model in which these isotopic and elemental tracers were used to estimate the probability distribution of the fractional contributions of manure, fertilizers, septic waste, and natural sources to the nitrate concentration found in an individual well. The approach was applied to 56 nitrate‐impacted private domestic wells located in the San Joaquin Valley. Model analysis found that some domestic wells were clearly dominated by the manure source and suggests evidence for majority contributions from either the septic or fertilizer source for other wells. But, predictions of fractional contributions for septic and fertilizer sources were often of similar magnitude, perhaps because modeled uncertainty about the fraction of each was large. For validation of the Bayesian model, fractional estimates were compared to surrounding landuse and estimated source contributions were broadly consistent with nearby landuse types. This article is protected by copyright. All rights reserved.
- Mechanisms for trapping and mobilization of residual fluids during
capillary‐dominated three‐phase flow in porous rock
- Authors: J. O. Helland; E. Jettestuen
Abstract: We use a multiphase level set approach to simulate capillary‐controlled motions of isolated fluid ganglia surrounded by two other continuous fluids (i.e., double displacements) during three‐phase flow on 3‐D porous rock geometries. Double displacements and three‐phase snap‐off mechanisms are closely related. Water snap‐off on gas/oil interfaces can initiate double displacements that mobilize isolated oil ganglia in water‐wet rock, but it can also terminate ongoing double displacements and trap oil in water. The multiphase level set approach allows for calculating the evolution of disconnected‐phase pressure during the motion. In the events of pore filling by double displacement of oil ganglia, and water snap‐off on gas/oil interfaces, we find that the local gas/oil capillary pressure drops, while local oil/water capillary pressure increases, by a similar magnitude as observed for the capillary pressure drops during single‐pore filling events in dynamic pore‐scale experiments of two‐phase drainage. We also find that oil ganglia decrease their surface area, and achieve a more compact shape, when the gas/oil interfacial area decreases at the expense of increased oil/water interfacial area during double displacement. By comparison with similar two‐phase gas/water simulations, we find that the level of the gas/water capillary pressure curves, including hysteresis loops, are smaller when a mobile, disconnected oil is present, which suggests double displacement of oil is more favorable than direct gas/water displacement. We also present cases in which phase trapping occurred in the three‐phase simulations, but not in the corresponding two‐phase simulations, supporting the view that more trapping is possible in three‐phase flow. This article is protected by copyright. All rights reserved.
- Assessment of a numerical model to reproduce event‐scale erosion and
deposition distributions in a braided river
- Authors: R.D. Williams; R. Measures, M. Hicks, J. Brasington
Abstract: Numerical morphological modelling of braided rivers, using a physics‐based approach, is increasingly used as a technique to explore controls on river pattern and, from an applied perspective, to simulate the impact of channel modifications. This paper assesses a depth averaged non‐uniform sediment model (Delft3D) to predict the morphodynamics of a 2.5 km long reach of the braided Rees River, New Zealand, during a single high‐flow event. Evaluation of model performance primarily focused upon using high‐resolution Digital Elevation Models (DEMs) of Difference, derived from a fusion of terrestrial laser scanning and optical empirical bathymetric mapping, to compare observed and predicted patterns of erosion and deposition, and reach scale sediment budgets. For the calibrated model, this was supplemented with planform metrics (e.g. braiding intensity). Extensive sensitivity analysis of model functions and parameters was executed, including consideration of numerical scheme for bedload component calculations, hydraulics, bed composition, bedload transport and bed slope effects, bank erosion and frequency of calculations. Total predicted volumes of erosion and deposition corresponded well to those observed. The difference between predicted and observed volumes of erosion was less than the factor of two that characterises the accuracy of the Gaeuman et al. bedload transport formula. Grain size distributions were best represented using two‐phi intervals. For unsteady flows, results were sensitive to the morphological time scale factor. The approach of comparing observed and predicted morphological sediment budgets shows the value of using natural experiment datasets for model testing. Sensitivity results are transferable to guide Delft3D applications to other rivers. This article is protected by copyright. All rights reserved.
- Transferring measured discharge time series: Large‐scale comparison
of Top‐kriging to geomorphology‐based inverse modeling
- Abstract: Few methods directly transfer streamflow measurements for continuous prediction of ungauged catchments. Top‐kriging has been used mainly to predict the statistical properties of runoff, but has been shown to outperform traditional regionalization approaches of rainfall‐runoff models. We applied the Top‐kriging approach across the Loire river basin and compared predictions to a geomorphology‐based approach. Whereas Top‐kriging uses spatial correlation, the other approach has the advantage of being more physically‐based by using a well‐known geomorphology‐based hydrological model (WFIUH) and its inversion. Both approaches require an equal degree of calibration and provide similar performances. We also demonstrate that the Ghosh distance, which considers the nested nature of catchments, can be used efficiently to calculate weights and to identify the suitability of gauged catchments for use as donor catchments. This result is particularly relevant for catchments with Strahler orders above five, i.e., where donor catchments are more strongly nested. This article is protected by copyright. All rights reserved.
- Linking high‐frequency DOC dynamics to the age of connected water
- Authors: C. Tunaley; D. Tetzlaff, J. Lessels, C. Soulsby
Abstract: We combined high‐frequency dissolved organic matter fluorescence (FDOM) data with stable isotope observations to identify the sources and ages of runoff that cause temporal variability in dissolved organic carbon (DOC) within a peat‐dominated Scottish catchment. FDOM was strongly correlated (r2 ∼ 0.8) with DOC, allowing inference of a 15 minute time series. We captured 34 events over a range of hydrological conditions. Along with marked seasonality, different event responses were observed during summer depending on dry or wet antecedent conditions. The majority of events exhibited anticlockwise hysteresis as a result of the expansion of the riparian saturation zone, mobilizing previously unconnected DOC sources. Water ages from the main runoff sources were extracted from a tracer–aided hydrological model. Particularly useful were ages of overland flow, which were negatively correlated with DOC concentration. Overland flow age, which ranged between 0.2 and 360 days, reflected antecedent conditions, with younger water generally mobilizing the highest DOC concentrations in summer events. During small events with dry antecedent conditions, DOC response was proportionally higher due to the displacement and mixing of small volumes of previously unconnected highly‐concentrated riparian soil waters by new precipitation. During large events with wet antecedent conditions, the riparian saturation zone expands to organic layers on the hillslopes causing peaks in DOC. However, these peaks were limited by dilution and supply. This study highlights the utility of linking high‐frequency DOC measurements with other tracers, allowing the effects of hydrologic connectivity and antecedent conditions on delivery of DOC to streams to be assessed. This article is protected by copyright. All rights reserved.
- Dual assimilation of satellite soil moisture to improve streamflow
prediction in data‐scarce catchments
- Abstract: This paper explores the use of active and passive microwave satellite soil moisture products for improving streamflow prediction within 4 large (>5,000km2) semi‐arid catchments in Australia. We use the probability distributed model (PDM) under a data‐scarce scenario and aim at correcting two key controlling factors in the streamflow generation: the rainfall forcing data and the catchment wetness condition. The soil moisture analysis rainfall tool (SMART) is used to correct a near‐real time satellite rainfall product (forcing correction scheme) and an ensemble Kalman filter is used to correct the PDM soil moisture state (state correction scheme). These two schemes are combined in a dual correction scheme and we assess the relative improvements of each. Our results demonstrate that the quality of the satellite rainfall product is improved by SMART during moderate‐to‐high daily rainfall events, which in turn leads to improved streamflow prediction during high flows. When employed individually, the soil moisture state correction scheme generally outperforms the rainfall correction scheme, especially for low flows. Overall, the combined dual correction scheme further improves the streamflow predictions (reduction in root mean square error and false alarm ratio, and increase in correlation coefficient and Nash‐Sutcliffe efficiency). Our results provide new evidence of the value of satellite soil moisture observations within data‐scarce regions. We also identify a number of challenges and limitations within the schemes. This article is protected by copyright. All rights reserved.
- Comparative assessment of three‐phase oil relative permeability
- Authors: Ehsan Ranaee; Monica Riva, Giovanni M. Porta, Alberto Guadagnini
Abstract: We assess the ability of eleven models to reproduce three‐phase oil relative permeability (kro) laboratory data obtained in a water‐wet sandstone sample. We do so by considering model performance when (i) solely two‐phase data are employed to render predictions of kro, and (ii) two‐ and three‐phase data are jointly used for model calibration. In the latter case a Maximum Likelihood (ML) approach is used to estimate model parameters. The tested models are selected amongst (i) classical models routinely employed in practical applications and implemented in commercial reservoir softwares and (ii) relatively recent models which are considered to allow overcoming some drawbacks of the classical formulations. Amongst others, the latter set of models includes the formulation recently proposed by Ranaee et al. , which has been shown to embed the critical effects of hysteresis, including the reproduction of oil remobilization induced by gas injection in water‐wet media. We employ formal model discrimination criteria to rank models according to their skill to reproduce the observed data and use ML Bayesian Model Averaging to provide model averaged estimates (and associated uncertainty bounds) of kro by taking advantage of the diverse interpretive abilities of all models analyzed. The occurence of elliptic regions is also analyzed for selected models in the framework of the classical fractional flow theory of displacement. Our study confirms that model outcomes based on channel flow theory and classical saturation‐weighted interpolation models do not generally yield accurate reproduction of kro data, especially in the regime associated with low oil saturations, where water alternating gas injection (WAG) techniques are usually employed for enhanced oil recovery. This negative feature is not observed in the model of Ranaee et al.  due to its ability to embed key effects of pore scale phase distributions, such as hysteresis effects and cycle dependency, for modeling kro observed during WAG. This article is protected by copyright. All rights reserved.
- Attribution of regional flood changes based on scaling fingerprints
- Abstract: Changes in the river flood regime may be due to atmospheric processes (e.g. increasing precipitation), catchment processes (e.g. soil compaction associated with land‐use change), and river system processes (e.g. loss of retention volume in the flood plains). This paper proposes a new framework for attributing flood changes to these drivers based on a regional analysis. We exploit the scaling characteristics (i.e., fingerprints) with catchment area of the effects of the drivers on flood changes. The estimation of their relative contributions is framed in Bayesian terms. Analysis of a synthetic, controlled case suggests that the accuracy of the regional attribution increases with increasing number of sites and record lengths, decreases with increasing regional heterogeneity, increases with increasing difference of the scaling fingerprints, and decreases with an increase of their prior uncertainty. The applicability of the framework is illustrated for a case study set in Austria, where positive flood trends have been observed at many sites in the past decades. The individual scaling fingerprints related to the atmospheric, catchment and river system processes are estimated from rainfall data and simple hydrological modelling. Although the distributions of the contributions are rather wide, the attribution identifies precipitation change as the main driver of flood change in the study region. Overall, it is suggested that the extension from local attribution to a regional framework, including multiple drivers and explicit estimation of uncertainty, could constitute a similar shift in flood change attribution as the extension from local to regional flood frequency analysis. This article is protected by copyright. All rights reserved.
- Response of the hyporheic zone to transient groundwater fluctuations on
the annual and storm event time scales
- Authors: Jonathan M. Malzone; Christopher S. Lowry, Adam S. Ward
Abstract: The volume of the water stored in and exchanged with the hyporheic zone is an important factor in stream metabolism and biogeochemical cycling. Previous studies have identified groundwater direction and magnitude as one key control on the volume of the hyporheic zone, suggesting that fluctuation in the riparian water table could induce large changes under certain seasonal conditions. In this study, we analyze the transient drivers that control the volume of the hyporheic zone by coupling the Brinkman‐Darcy equation to the Navier‐Stokes equations to simulate annual and storm induced groundwater fluctuations. The expansion and contraction of the hyporheic zone was quantified based on temporally dynamic scenarios simulating annual groundwater fluctuations in a humid temperate climate. The amplitude of the groundwater signal was varied between scenarios to represent a range of annual hydrologic forcing. Storm scenarios were then superimposed on the annual scenario to simulate the response to short term storm signals. Simulations used two different groundwater storm responses; one in‐phase with the surface water response and one 14 hours out‐of‐phase with the surface water response to represent our observed site conditions. Results show that annual groundwater fluctuation is a dominant control on the volume of the hyporheic zone, where increasing groundwater fluctuation increases the amount of annual variation. Storm responses depended on the antecedent conditions determined by annual scenarios, where the time of year dictated the duration and magnitude of the storm induced response of the hyporheic zone. This article is protected by copyright. All rights reserved.
- Validating a mass balance accounting approach to using 7Be measurements to
estimate event‐based erosion rates over an extended period at the
- Authors: Paolo Porto; Des E. Walling, Vanessa Cogliandro, Giovanni Callegari
Abstract: Use of the fallout radionuclides cesium‐137 and excess lead‐210 offers important advantages over traditional methods of quantifying erosion and soil redistribution rates. However, both radionuclides provide information on longer‐term (i.e. 50‐100 years) average rates of soil redistribution. Beryllium‐7, with its half‐life of 53 days, can provide a basis for documenting short‐term soil redistribution and it has been successfully employed in several studies. However, the approach commonly used introduces several important constraints related to the timing and duration of the study period. A new approach proposed by the authors that overcomes these constraints has been successfully validated using an erosion plot experiment undertaken in southern Italy. Here, a further validation exercise undertaken in a small (1.38 ha) catchment is reported. The catchment was instrumented to measure event sediment yields and beryllium‐7 measurements were employed to document the net soil loss for a series of 13 events that occurred between November 2013 and June 2015. In the absence of significant sediment storage within the catchment's ephemeral channel system and of a significant contribution from channel erosion to the measured sediment yield, the estimates of net soil loss for the individual events could be directly compared with the measured sediment yields to validate the former. The close agreement of the two sets of values is seen as successfully validating the use of beryllium‐7 measurements and the new approach to obtain estimates of net soil loss for a sequence of individual events occurring over an extended period at the scale of a small catchment. This article is protected by copyright. All rights reserved.
- Combined use of thermal methods and seepage meters to efficiently locate,
quantify, and monitor focused groundwater discharge to a sand‐bed
- Authors: Donald O. Rosenberry; Martin A. Briggs, Geoffrey Delin, Danielle K. Hare
Abstract: Quantifying flow of groundwater through streambeds often is difficult due to the complexity of aquifer‐scale heterogeneity combined with local‐scale hyporheic exchange. We used fiber‐optic distributed temperature sensing (FO‐DTS), seepage meters, and vertical temperature profiling to locate, quantify, and monitor areas of focused groundwater discharge in a geomorphically simple sand‐bed stream. This combined approach allowed us to rapidly focus efforts at locations where prodigious amounts of groundwater discharged to the Quashnet River on Cape Cod, Massachusetts, northeastern USA. FO‐DTS detected numerous anomalously cold reaches one to several m long that persisted over two summers. Seepage meters positioned upstream, within, and downstream of 7 anomalously cold reaches indicated that rapid groundwater discharge occurred precisely where the bed was cold; median upward seepage was nearly 5 times faster than seepage measured in streambed areas not identified as cold. Vertical temperature profilers deployed next to 8 seepage meters provided diurnal‐signal‐based seepage estimates that compared remarkably well with seepage‐meter values. Regression slope and R2 values both were near 1 for seepage ranging from 0.05 to 3.0 m d−1. Temperature‐based seepage model accuracy was improved with thermal diffusivity determined locally from diurnal signals. Similar calculations provided values for streambed sediment scour and deposition at subdaily resolution. Seepage was strongly heterogeneous even along a sand‐bed river that flows over a relatively uniform sand and fine‐gravel aquifer. FO‐DTS was an efficient method for detecting areas of rapid groundwater discharge, even in a strongly gaining river, that can then be quantified over time with inexpensive streambed thermal methods.
- Gaussian and non‐Gaussian inverse modeling of groundwater flow using
copulas and random mixing
- Abstract: This paper presents a new copula‐based methodology for Gaussian and non‐Gaussian inverse modeling of groundwater flow. The presented approach is embedded in a Monte Carlo framework and it is based on the concept of mixing spatial random fields where a spatial copula serves as spatial dependence function. The target conditional spatial distribution of hydraulic transmissivities is obtained as a linear combination of unconditional spatial fields. The corresponding weights of this linear combination are chosen such that the combined field has the prescribed spatial variability, and honors all the observations of hydraulic transmissivities. The constraints related to hydraulic head observations are nonlinear. In order to fulfill these constraints, a connected domain in the weight space, inside which all linear constraints are fulfilled, is identified. This domain is defined analytically and includes an infinite number of conditional fields (i.e., conditioned on the observed hydraulic transmissivities), and the nonlinear constraints can be fulfilled via minimization of the deviation of the modeled and the observed hydraulic heads. This procedure enables the simulation of a great number of solutions for the inverse problem, allowing a reasonable quantification of the associated uncertainties. The methodology can be used for fields with Gaussian copula dependence, and fields with specific non‐Gaussian copula dependence. Further, arbitrary marginal distributions can be considered.
- Methane emission through ebullition from an estuarine mudflat: 1. A
conceptual model to explain tidal forcing based on effective stress
- Authors: Xi Chen; Lee Slater
Abstract: Ebullition is an important pathway for transport of methane (CH4) to the atmosphere in wetlands. Water level changes have been suggested to trigger ebullition, especially in tidally flooded areas, although the controlling mechanisms remain uncertain. Bubble transport in submerged sediment represents a multiphase, dynamic interaction between gaseous and solid phases under the modulation of a liquid phase. An unvegetated sediment monolith was retrieved from an estuarine mudflat area at a tidal marsh site and maintained in a saturated state. Laboratory measurements on the mud monolith confirmed that not only ebbing tides, but also flooding tides could trigger ebullition releases of gas bubbles. We develop a Changing Stress for Simulating Ebullition (CSSE) model to describe mechanisms controlling bubble expansion in response to water level changes to unify these observations. Decreases in water level are assumed to lower the effective stress surrounding isolated trapped gas bubbles, driving upward transport via bubble expansion and deformation, with associated fracturing of overlying sediments. Increases in relative permittivity suggest that additional water invades macropores, with associated pore expansion, during the initial stage of increases in water level. We propose that subsequent matrix expansion under lowered effective stress on rising tides also leads to fracture propagation and bubble release. Our findings demonstrate the importance of effective stress changes in triggering ebullition from mudflat areas in tidal wetlands, modulated by the mechanical properties of shallow soft sediments.
- Can a paleo‐drought record be used to reconstruct streamflow? A
case‐study for the Missouri River Basin
- Authors: Michelle Ho; Upmanu Lall, Edward R. Cook
Abstract: Recent advances in paleoclimatology have revealed dramatic long‐term hydro‐climatic variations that provide a context for limited historical records. A notable dataset derived from a relatively dense network of paleoclimate proxy records in North America is the Living Blended Drought Atlas (LBDA): a gridded tree‐ring based reconstruction of summer Palmer Drought Severity Index. This index has been used to assess North American drought frequency, persistence and spatial extent over the past two millennia. Here, we explore whether the LBDA can be used to reconstruct annual streamflow. Relative to streamflow reconstructions that use tree rings within the river basin of interest, the use of a gridded proxy poses a novel challenge. The gridded series have high spatial correlation, since they rely on tree rings over a common radius of influence. A novel algorithm for reconstructing streamflow using regularized canonical regression and inputs of local and global covariates is developed and applied over the Missouri River Basin, as a test case. Effectiveness in reconstruction is demonstrated with reconstructions showing periods where streamflow deficits may have been more severe than during recent droughts (e.g. the Civil War, Dust Bowl and 1950s droughts). The maximum persistence of droughts and floods over the past 500 years far exceed those observed in the instrumental record and periods of multi‐decadal variability in the 1500s and 1600s are detected. Challenges for an extension to a national streamflow reconstruction or applications using other gridded paleoclimate datasets such as adequate spatial coverage of streamflow and applicability of annual reconstructions are discussed. This article is protected by copyright. All rights reserved.
- Scalable subsurface inverse modeling of huge data sets with an application
- Authors: Jonghyun Lee; Hongkyu Yoon, Peter K. Kitanidis, Charles J. Werth, Albert J. Valocchi
Abstract: Characterizing subsurface properties is crucial for reliable and cost‐effective groundwater supply management and contaminant remediation. With recent advances in sensor technology, large volumes of hydro‐geophysical and geochemical data can be obtained to achieve high‐resolution images of subsurface properties. However, characterization with such a large amount of information requires prohibitive computational costs associated with “big data” processing and numerous large‐scale numerical simulations. To tackle such difficulties, the Principal Component Geostatistical Approach (PCGA) has been proposed as a “Jacobian‐free” inversion method that requires much smaller forward simulation runs for each iteration than the number of unknown parameters and measurements needed in the traditional inversion methods. PCGA can be conveniently linked to any multi‐physics simulation software with independent parallel executions. In this paper, we extend PCGA to handle a large number of measurements (e.g. 106 or more) by constructing a fast preconditioner whose computational cost scales linearly with the data size. For illustration, we characterize the heterogeneous hydraulic conductivity (K) distribution in a laboratory‐scale 3‐D sand box using about 6 million transient tracer concentration measurements obtained using magnetic resonance imaging. Since each individual observation has little information on the K distribution, the data was compressed by the zero‐th temporal moment of breakthrough curves, which is equivalent to the mean travel time under the experimental setting. Only about 2,000 forward simulations in total were required to obtain the best estimate with corresponding estimation uncertainty, and the estimated K field captured key patterns of the original packing design, showing the efficiency and effectiveness of the proposed method. This article is protected by copyright. All rights reserved.
- Actively heated high‐resolution fiber‐optic distributed
temperature sensing to quantify streambed flow dynamics in zones of strong
- Authors: Martin A. Briggs; Sean F. Buckley, Amvrossios C. Bagtzoglou, Dale D. Werkema, John W. Lane
Abstract: Zones of strong groundwater upwelling to streams enhance thermal stability and moderate thermal extremes, which is particularly important to aquatic ecosystems in a warming climate. Passive thermal tracer methods used to quantify vertical upwelling rates rely on downward conduction of surface temperature signals. However, moderate to high groundwater flux rates (> ‐1.5 md−1) restrict downward propagation of diurnal temperature signals, and therefore the applicability of several passive thermal methods. Active streambed heating from within high‐resolution fiber‐optic temperature sensors (A‐HRTS) has the potential to define multidimensional fluid flux patterns below the extinction depth of surface thermal signals, allowing better quantification and separation of local and regional groundwater discharge. To demonstrate this concept, nine A‐HRTS were emplaced vertically into the streambed in a grid with ∼ 0.40 m lateral spacing at a stream with strong upward vertical flux in Mashpee, Massachusetts, USA. Long‐term (8‐9 hr) heating events were performed to confirm the dominance of vertical flow to the 0.6 m depth, well below the extinction of ambient diurnal signals. To quantify vertical flux, short‐term heating events (28 min) were performed at each A‐HRTS, and heat pulse decay over vertical profiles was numerically modeled in radial two dimension (2D) using SUTRA. Modeled flux values are similar to those obtained with seepage meters, Darcy methods, and analytical modeling of shallow diurnal signals. We also observed repeatable differential heating patterns along the length of vertically oriented sensors that may indicate sediment layering and hyporheic exchange superimposed on regional groundwater discharge. This article is protected by copyright. All rights reserved.
- A multiscale approach to determine hydraulic conductivity in thick
claystone aquitards using field, laboratory, and numerical modeling
- Authors: L. A. Smith; S. L. Barbour, M. J. Hendry, K. Novakowski, G. van der Kamp
Abstract: Characterizing the hydraulic conductivity (K) of aquitards is difficult due to technical and logistical difficulties associated with field‐based methods as well as the cost and challenge of collecting representative and competent core samples for laboratory analysis. The objective of this study was to produce a multi‐scale comparison of vertical and horizontal hydraulic conductivity (Kv and Kh, respectively) of a regionally extensive Cretaceous clay‐rich aquitard in southern Saskatchewan. Ten vibrating wire pressure transducers were lowered into place at depths between 25 and 325 m, then the annular was space was filled with a cement‐bentonite grout. The in situ Kh was estimated at the location of each transducer by simulating the early‐time pore pressure measurements following setting of the grout using a 2D axisymmetric, finite element, numerical model. Core samples were collected during drilling for conventional laboratory testing for Kv to compare with the transducer‐determined in situ Kh. Results highlight the importance of scale and consideration of the presence of possible secondary features (e.g. fractures) in the aquitard. The proximity of the transducers to an active potash mine (∼1 km) where depressurization of an underlying aquifer resulted in drawdown through the aquitard provided a unique opportunity to model the current hydraulic head profile using both the Kh and Kv estimates. Results indicate that the transducer‐determined Kh estimates would allow for the development of the current hydraulic head distribution, and that simulating the pore pressure recovery can be used to estimate moderately low in situ Kh (
- Water velocity at water‐air interface is not zero: Comment on
“Three‐dimensional quantification of soil hydraulic properties
using X‐ray computed tomography and image‐based
modeling” By Tracy SR et al.
- Authors: X.X. Zhang; X.Y. Fan, Z.Y. Li
- Land surface states within the flux footprint impact daytime
land‐atmosphere coupling in two semiarid ecosystems of the
- Authors: Cody A. Anderson; Enrique R. Vivoni
Abstract: Land surface states play important roles in the turbulent exchanges between ecosystems and their overlying atmosphere. Field methods to estimate turbulent fluxes have time‐variable source areas, while land surface observations are typically obtained at single plots with a smaller measurement scale. In this study, we characterize land‐atmosphere interactions in two semiarid ecosystems in the southwestern U.S. At each study site, we combine the eddy covariance method with a distributed network of soil moisture and temperature sensors, high‐resolution imagery of the spatial distribution of vegetation and soil patches, and novel spatiotemporal analyses to characterize the turbulent flux footprint analytically and identify the soil moisture, temperature and vegetation conditions underlying the eddy covariance measurements. Four methods for aggregating the land surface observations to the scale of the daily flux footprint are tested. Our results reveal a large degree of spatial variability in the footprint, with stronger variations in soil moisture than in soil temperature. Single plot measurements are less reliable than the distributed network in capturing footprint conditions, particularly for soil moisture. Furthermore, a marked improvement is observed in the relations between turbulent fluxes and land surface states for methods capturing the footprint variability. We also identify that the composition of vegetation and soil patches in the time‐variable source area affects the relative magnitudes of the turbulent fluxes and the partitioning of evapotranspiration. Our study points to the importance of monitoring the spatial distribution of land surface states (e.g., soil moisture and temperature) and vegetation and soil patches when assessing land‐atmosphere interactions. This article is protected by copyright. All rights reserved.
- A hybrid statistical‐dynamical framework for meteorological drought
prediction: Application to the southwestern United States
- Abstract: Improving water management in water stressed‐regions requires reliable seasonal precipitation predication, which remains a grand challenge. Numerous statistical and dynamical model simulations have been developed for predicting precipitation. However, both types of models offer limited seasonal predictability. This study outlines a hybrid statistical‐dynamical modeling framework for predicting seasonal precipitation. The dynamical component relies on the physically based North American Multi‐Model Ensemble (NMME) model simulations (99 ensemble members). The statistical component relies on a multivariate Bayesian‐based model that relates precipitation to atmosphere‐ocean teleconnections (also known as an analog‐year statistical model). Here, the Pacific Decadal Oscillation (PDO), Multivariate ENSO Index (MEI), and Atlantic Multi‐decadal Oscillation (AMO) are used in the statistical component. The dynamical and statistical predictions are linked using the so‐called Expert Advice algorithm, which offers an ensemble response (as an alternative to the ensemble mean). The latter part leads to the best precipitation prediction based on contributing statistical and dynamical ensembles. It combines the strength of physically based dynamical simulations and the capability of an analog‐year model. An application of the framework in the southwestern United States, which has suffered from major droughts over the past decade, improves seasonal precipitation predictions (3‐ to 5‐month lead time) by 5‐60 percent relative to the NMME simulations. Overall, the hybrid framework performs better in predicting negative precipitation anomalies (10‐60% improvement over NMME) than positive precipitation anomalies (5‐25% improvement over NMME). The results indicate that the framework would likely improve our ability to predict droughts such as the 2012‐2014 event in the western United States that resulted in significant socio‐economic impacts. This article is protected by copyright. All rights reserved.
- Impact of river regulation on a Mediterranean delta ‐ assessment of
managed vs unmanaged scenarios
- Abstract: This work addresses the effects of the construction of a reservoir 19 km from the mouth on the dynamics of the Guadalfeo delta (southern Spain), a Mediterranean delta in a semi‐arid and high‐mountain basin. The sediment volume transported as bedload and accumulated in the delta was estimated under two scenarios by means of a calibrated hydrological model: a managed scenario, considering the flows drained by the dam, and an unmanaged scenario, considering the absence of such infrastructure. Bathymetric and topographic measurements were analyzed and correlated with the fluvial and maritime forcing agents. Results indicate that the reservoir has significantly modified the dynamics downstream: the coast has lost almost 0.3 hm3 of sediments since the entry into operation of the dam, generating a 1.4‐km coastline retreat around the mouth, with a maximum retreat of 87 m (92% of the initial). The beach profile decreased by up to 820 m2, whereas the average decrease around the mouth was equal to 214 m2. Under unmanaged conditions, more than 2 hm3 of bedload would have reached the coast. Based on the results, three new management scenarios of flows drained by the dam, in combination with bypassed sediment from the reservoir, were proposed to prevent more severe consequences in the delta and the silting of the reservoir. The proposed methodology for new management scenarios can be extended to other worldwide deltas, especially to those in semi‐arid and Mediterranean basins, and it represents an advanced tool for decision making. This article is protected by copyright. All rights reserved.
- Using expert elicitation to quantify catchment water balances and their
- Authors: E. Sebok; J. C. Refsgaard, J. J. Warmink, S. Stisen, K. H. Jensen
Abstract: Expert elicitation with the participation of 35 experts was used to estimate a water balance for the nested Ahlergaarde and Holtum catchments in Western Denmark. Average annual values of precipitation, evapotranspiration and surface runoff as well as subsurface outflow and recharge and their uncertainty were estimated in a multi‐step elicitation, where experts first gave their opinion on the probability distribution of their water balance component of interest, then the average annual values and uncertainty of water balance components and catchment‐scale water balances were obtained by reaching consensus during group discussions. The obtained water balance errors for the 1055 km2 Ahlergaarde catchment and 120 km2 Holtum catchment were ‐5 mm/year and ‐62 mm/year, respectively, with an uncertainty of 66 mm/year and 86 mm/year, respectively. As an advantage of the expert elicitation, drawing on the intuitive experience and capabilities of experts to assess complex, site‐specific problems, the contribution of independent sources of uncertainties to the total uncertainty was also evaluated similarly to the subsurface outflow component, which traditionally is estimated as the residual of the water balance. This article is protected by copyright. All rights reserved.
- Water yield and sediment export in small, partially glaciated Alpine
watersheds in a warming climate
- Authors: Natan Micheletti; Stuart N. Lane
Abstract: Climate change is expected to modify the hydrological and geomorphological dynamics of mountain watersheds significantly, so impacting on downstream water yield and sediment supply. However, such watersheds are often poorly instrumented, making it difficult to link recent and rapid climate change to landscape response. Here we combine unique records of river flow and sediment export, with historical archival imagery to test the hypothesis that climate warming has substantially increased both water yield and sediment export from small Alpine watersheds (< 3 km2) characterized by small (< 0.5 km2 surface) glaciers. To examine ice and landform response to climate change, we apply archival digital photogrammetry to historical aerial imagery available from 1967 to present. We use the resulting data on ice loss, in combination with reliable records of stream flow from hydroelectric power intakes and climate data to approximate a water budget and to determine the evolution of different contributions to river flow. We use the stream flow records to estimate volumetric sediment transport capacity and compare this with the volumes of sand and gravel exported from the watersheds, quantified from records of intake flushing. The data show clearly that climate forcing since the early 1980s has been accompanied by a net increase in both water yield and sediment transport capacity, and we attribute these as signals of reduced snow accumulation and glacier recession. However, sediment export has not responded in the same way and we attribute this to limits on sediment delivery to streams because of poor rockwall‐hillslope‐channel connectivity. However, we do find that extreme climate conditions can be seen in sediment export data suggesting that these, rather than mean climate warming, may dominate watershed response. This article is protected by copyright. All rights reserved.
- Detection of carbon dioxide leakage during injection in deep saline
formations by pressure tomography
- Authors: Linwei Hu; Peter Bayer, Ralf Brauchler
Abstract: CO2 injected into storage formations may escape to the overlying permeable layers. Mixed‐phase diffusivity, namely the ratio of hydraulic conductivity and specific storage of the phase mixture, declines with increasing CO2 saturation. Thus it can be an indicator of CO2 leakage. In this study, we perform interference brine or CO2 injection tests in a synthetic model, including a storage reservoir, an above aquifer, and a caprock. Pressure transients derived from an observation well are utilized for a travel‐time based inversion technique. Variations of diffusivity are resolved by inverting early travel time diagnostics, providing an insight of plume development. Results demonstrate that the evolution of CO2 leakage in the above aquifer can be inferred by interpreting and comparing the pressure observations, travel times and diffusivity tomograms from different times. The extent of the plume in reservoir and upper aquifer are reconstructed by clustering the time‐lapse data sets of the mixed‐phase diffusivity, as the diffusivity cannot be exactly reproduced by the inversion. Furthermore, this approach can be used to address different leaky cases, especially for leakage occurring during the injection. This article is protected by copyright. All rights reserved.
- Infiltration experiments demonstrate an explicit connection between
heterogeneity and anomalous diffusion behavior
- Authors: N. Filipovitch; K. Hill, A. Longjas, V. R. Voller
Abstract: Transport in systems containing heterogeneity distributed over multiple length scales can exhibit anomalous diffusion behaviors, where the time exponent, determining the spreading length scale of the transported scalar, differs from the expected value of . Here we present experimental measurements of the infiltration of glycerin, under a fixed pressure head, into a Hele‐Shaw cell containing a 3‐D printed distribution of flow obstacles; a system that is an analog for infiltration into a porous medium. In support of previously presented direct simulation results [Voller, 2015], we experimentally demonstrate that, when the obstacles are distributed as a fractal carpet with fractal dimension H
- Modeling relative permeability of water in soil: Application of
effective‐medium approximation and percolation theory
- Authors: Behzad Ghanbarian; Muhammad Sahimi, Hugh Daigle
Abstract: Accurate prediction of the relative permeability to water under partially‐saturated condition has broad applications and has been studied intensively since the 1940s by petroleum, chemical, and civil engineers, as well as hydrologists and soil scientists. Many models have been developed for this purpose, ranging from those that represent the pore space as a bundle of capillary tubes, to those that utilize complex networks of interconnected pore bodies and pore throats with various cross‐section shapes. In this paper we propose an approach based on the effective‐medium approximation (EMA) and percolation theory in order to predict the water relative permeability. The approach is general and applicable to any type of porous media. We use the method to compute the water relative permeability in porous media whose pore‐size distribution follows a power law. The EMA is invoked to predict the relative permeability from the fully‐saturated pore space to some intermediate water saturation that represents a crossover from the EMA to what we refer to as the “critical region.” In the critical region below the crossover water saturation Swx, but still above the critical water saturation Swc (the residual saturation or the percolation threshold of the water phase), the universal power law predicted by percolation theory is used to compute the relative permeability. To evaluate the accuracy of the approach, data for 21 sets of undisturbed laboratory samples were selected from the UNSODA database. For 14 cases, the predicted relative permeabilities are in good agreement with the data. For the remaining 7 samples, however, the theory underestimates the relative permeabilities. Some plausible sources of the discrepancy are discussed. This article is protected by copyright. All rights reserved.
- On the failure of upscaling the single collector efficiency to the
transport of colloids in an array of collectors
- Authors: Francesca Messina; Tiziana Tosco, Rajandrea Sethi
Abstract: Defining the removal efficiency of a filter is a key aspect for colloid transport in porous media. Several efforts were devoted to derive accurate correlations for the single collector removal efficiency, but its up‐scaling to the entire porous medium is still a challenging topic. A common approach involves the assumption of deposition being independent of the history of transport, that is, the collector efficiency is uniform along the porous medium. However, this approach was shown inadequate under unfavorable deposition conditions. In this work, the authors demonstrate that it is not adequate even in the simplest case of favorable deposition. Computational Fluid Dynamics (CFD) simulations were run in a vertical array of 50 identical spherical collectors. Particle transport was numerically solved by analyzing a broad range of parameters. The results evidenced that, when particle deposition is not controlled by Brownian diffusion, non‐exponential concentration profiles are retrieved, in contrast with the assumption of uniform efficiency. If sedimentation and interception dominate, the efficiency of the first sphere is significantly higher compared to the others, and then declines along the array down to an asymptotic value. Finally, a correlation for the up‐scaled removal efficiency of the entire array was derived. This article is protected by copyright. All rights reserved.
- Extraordinary sediment delivery and rapid geomorphic response following
the 2008–2009 eruption of Chaitén volcano, Chile
- Abstract: The 10‐day explosive phase of the 2008–2009 eruption of Chaitén volcano, Chile, draped adjacent watersheds with a few cm to >1 m of tephra. Subsequent lava‐dome collapses generated pyroclastic flows that delivered additional sediment. During the waning phase of explosive activity, modest rainfall triggered an extraordinary sediment flush which swiftly aggraded multiple channels by many meters. Ten km from the volcano, Chaitén River channel aggraded 7 m and the river avulsed through a coastal town. That aggradation and delta growth below the abandoned and avulsed channels allow estimates of post‐disturbance traction‐load transport rate. On the basis of pre‐eruption bathymetry and remotely sensed measurements of delta‐surface growth, we derived a time series of delta volume. The initial flush from 11 to 14 May 2008 deposited 0.5–1.5 million m3 of sediment at the mouth of Chaitén River. By 26 May, after channel avulsion, a second delta amassed about 2 million m3 of sediment; by late 2011 it amassed about 11 million m3. Accumulated sediment consists of low‐density vesicular pumice and lithic rhyolite sand. Rates of channel aggradation and delta growth, channel width, and an assumed deposit bulk density of 1100–1500 kg m−3 indicate mean traction‐load transport rate just before and shortly after avulsion (∼14–15 May) was very high, possibly as great as several tens of kg s−1 m−1. From October 2008 to December 2011, mean traction‐load transport rate declined from about 7 to 0.4 kg −1 m−1. Despite extraordinary sediment delivery, disturbed channels recovered rapidly (a few years). This article is protected by copyright. All rights reserved.
- Response to: “Water velocity at water‐air interface is not
zero: Comment on “Three dimensional quantification of soil hydraulic
properties using X‐ray Computed Tomography and image based
- Authors: Saoirse R. Tracy; Keith R. Daly, Craig J. Sturrock, Neil M. J. Crout, Sacha J. Mooney, Tiina Roose
Abstract: In response to the comment raised by Zhang et al (2016) we explore the differences in average velocity computed using slip and no‐slip boundary conditions at the air water interface. We consider a porous medium in which the air phase acts to impede the movement of water rather than to lubricate it, a case closer to the observed distribution of water in our CT images. We find that, whilst the slip boundary condition may be a more accurate approximation, in cases where the air phase is seen to impede water movement the differences between the two approaches are negligible. This article is protected by copyright. All rights reserved.
- Can we manage groundwater? A method to determine the quantitative
testability of groundwater management plans
- Authors: E. K. White; T. J. Peterson, J. Costelloe, A. W. Western, E. Carrarra
Abstract: Groundwater is the world's largest freshwater resource and due to over‐extraction, levels have declined in many regions causing extensive social and environmental impacts. Groundwater management seeks to balance and mitigate the detrimental impacts of development and plans are commonly used to outline management pathways. Thus, plan efficiency is crucial, but seldom are plans systematically and quantitatively assessed for effectiveness. This study frames groundwater management as a system control problem in order to develop a novel testability assessment rubric to determine if plans meet the requirements of a control loop, and subsequently, whether they can be quantitatively tested. Seven components of a management plan equivalent to basic components of a control loop were determined, and requirements of each component necessary to enable testability were defined. Each component was weighted based upon proposed relative importance, then segmented into rated categories depending on the degree the requirements were met. Component importance varied but, a defined objective or acceptable impact was necessary for plans to be testable. The rubric was developed within the context of the Australian groundwater management industry, and while use of the rubric is not limited to Australia, it was applied to 15 Australian groundwater management plans and approximately 47% were found to be testable. Considering the importance of effective groundwater management, and the central role of plans, our lack of ability to test many plans is concerning. This article is protected by copyright. All rights reserved.
- Role of meteorological controls on interannual variations in
wet‐period characteristics of wetlands
- Authors: Yanlan Liu; Mukesh Kumar
Abstract: Many ecological functions of wetlands are influenced by wet‐periods, i.e., the time interval when groundwater table (GWT) is continuously near the land surface. Hence, there is a crucial need to understand the controls on interannual variations of wet‐periods. Given the scarcity of long term measurements of GWT in wetlands, understanding variations in wet‐periods using a measurement approach alone is challenging. Here we used a physically‐based, fully‐distributed hydrologic model, in synergy with publicly available hydrologic data, to simulate long term wet‐period variations in ten inland forested wetlands in a southeastern US watershed. A Bayesian regression and variable selection framework was then implemented to: (a) evaluate the extent to which the simulated wet‐periods can be estimated and predicted by precipitation (Ppt) and potential evapotranspiration (PET); and (b) infer the relative roles of seasonal Ppt and PET. Our results indicate that wet‐period start date and duration could vary by more than 6 months during the 32 year simulation period. Remarkably, 60\% to 90\% of these variations could be captured using regressions based on seasonal Ppt and PET in most wetlands. Effects of seasonal meteorological conditions on wet‐period variations were found to be non‐uniform, which indicate that the annual variables may not explain interannual variations in wet‐periods. The Bayesian framework was able to predict wet‐period variations with errors smaller than 1 month at a 90\% confidence level. The presented framework provides a minimalistic approach for estimating and predicting wet‐period variations in wetlands, and may be used to understand the future responses of associated ecological functions in wetlands. This article is protected by copyright. All rights reserved.
- Scale dependence of Hortonian rainfall‐runoff processes in a
- Authors: L. Chen; S. Sela, T. Svoray, S. Assouline
Abstract: Scale dependence of Hortonian rainfall‐runoff processes has received much attention in the literature but has not been fully resolved. To further explore this issue, a recently developed model was applied to simulate rainfall‐infiltration‐runoff processes at multiple spatial scales. The model consists of the coupling between a two‐dimensional runoff routing module and a two‐layer infiltration module, thus accounting for spatial variability in soil properties, soil surface sealing, topography and partial vegetation cover. A 76 m2 semiarid experimental plot with sparse cover of vegetation patches and a sealed soil surface in inter‐patch bare areas was used as a representative elementary area (REA). A series of four larger artificial plots of different areas was created based on this REA to examine the scale dependence of rainfall‐runoff relationships in the case of stationary heterogeneity. Results show that runoff depth (or runoff coefficient) decreases with increasing scale. This trend is more prominent at scales less than 10 times the REA length. Power‐law relationships can quantitatively describe the scaling law. The major mechanism of the scale effect is run‐on infiltration. However, rainfall intensity and soil properties can both affect the scaling trend through their interaction with run‐on. Higher intensity and less temporal variability of rainfall can both reduce the scale effect. Temporally intermittent rainfall may produce spatially oscillating infiltration rates at large scales. Vegetation patterns are another factor that may affect the scaling. Random vegetation patterns, compared with regular patterns with similar statistical properties, change the spatial distributions, but do not significantly change either the total amount and statistical properties of infiltration and runoff or the scale dependence of the rainfall‐runoff process. This article is protected by copyright. All rights reserved.
- Evaluating the long‐term hydrology of an
evapotranspiration‐capillary barrier with a 1000 year design life
- Authors: Z. Fred Zhang
Abstract: A surface barrier is a commonly used technology for isolation of subsurface contaminants. Surface barriers for isolating radioactive waste are expected to perform for centuries to millennia, yet there are very few data for field‐scale surface barriers for periods approaching a decade or longer. The Prototype Hanford Barrier (PHB) with a design life of 1000 years was constructed over an existing radioactive waste site in 1994 to demonstrate its long‐term performance. The primary element of the PHB is an evapotranspiration‐capillary (ETC) barrier in which precipitation water is stored in a fine‐textured soil layer and later released to the atmosphere via evapotranspiration. To address the barrier performance under extreme conditions, this study included an enhanced precipitation stress test from 1995 to 1997 to determine barrier response to extreme precipitation events. During this period a 1000‐year 24‐hour return rainstorm was simulated in March every year. The loss of vegetation on barrier hydrology was tested with a controlled fire test in 2008. The 19‐year monitoring record shows that the store‐and‐release mechanism worked as well as or better than the design criterion. Average drainage from the ETC barrier amounted to an average of 0.005 mm yr−1, which is well below the design criterion of 0.5 mm yr−1. After a simulated wildfire, the naturally re‐established vegetation and increased evaporation combined to release the stored water and summer precipitation to the atmosphere such that drainage did not occur in the five years subsequent to the fire. This article is protected by copyright. All rights reserved.
- Dynamic groundwater flows and geochemistry in a sandy nearshore aquifer
over a wave event
- Authors: Spencer Malott; Denis M. O'Carroll, Clare E. Robinson
Abstract: Dynamic coastal forcing influence the transport of pollutants in nearshore aquifers and their ultimate flux to coastal waters. In this study, field data is presented that shows, for the first time, the influence of a period of intensified wave conditions (wave event) on nearshore groundwater flows and geochemistry in a sandy beach. Field measurements at a freshwater beach allow wave effects to be quantified without other complex forcing that are present along marine shorelines (e.g. tides). Pressure transducer data obtained over an isolated wave event reveal the development of transient groundwater flow recirculations. The groundwater flows were simulated in FEFLOW using a phase‐averaged wave setup approach to represent waves acting on the sediment‐water interface. Comparison of measured and simulated data indicate that consideration of wave setup alone is able to adequately capture wave‐induced perturbations in groundwater flows. While prior studies have shown sharp pH and redox spatial zonations in nearshore aquifers, this study reveals rapid temporal variations in conductivity, pH and redox (ORP) in shallow sediments (up to 0.5 m depth) in response to varying wave conditions. Comparison of head gradients with calculated conductivity and pH mixing ratios indicates the controlling effect of the wave‐induced water exchange and flows in driving the observed geochemical dynamics. While we are not able to conclusively determine the extent to which temporal variations are caused by conservative mixing versus reactive processes, the pH and ORP variations observed will have significant implications for the fate of reactive pollutants discharging through sandy nearshore aquifers. This article is protected by copyright. All rights reserved.
- Travel times in the vadose zone: Variability in space and time
- Authors: Matthias Sprenger; Stefan Seeger, Theresa Blume, Markus Weiler
Abstract: Water travel times reflect hydrological processes, yet we know little about how travel times in the unsaturated zone vary with time. Using the soil physical model HYDRUS‐1D we derived time variable travel time distributions for 35 study sites within the Attert catchment in Luxembourg. While all sites experience similar climatic forcing, they differ with regard to soil types (16 Cambisols, 12 Arenosols, and 7 Stagnosols) and the vegetation cover (29 forest, 6 grassland). We estimated site specific water flow and transport parameters by fitting the model simulations to observed soil moisture time series and depth profiles of pore water stable isotopes. With the calibrated model we tracked the water parcels introduced with each rainfall event over a period of several years. Our results show that the median travel time of water from the soil surface to depths down to 200 cm is mainly driven by the subsequent rainfall amounts. The median time until precipitation is taken up by roots is governed by the seasonality of evapotranspiration rates. The ratio between the amount of water that leaves the soil profile by on the one hand and evaporation and transpiration on the other hand also shows an annual cycle. This time variable response due to climatic forcing is furthermore visible in the multi‐modal nature of the site specific master transit time distribution representing the flow averaged probability density for rain water to become recharge. The spatial variability of travel times is mainly driven by soil texture and structure, with significant longer travel times for the clayey Stagnosols than for the loamy to sandy Cambisols and Arenosols. This article is protected by copyright. All rights reserved.
- Dissolved organic matter transport reflects hillslope to stream
connectivity during snowmelt in a montane catchment
- Authors: Margaret A. Burns; Holly R. Barnard, Rachel S. Gabor, Diane M. McKnight, Paul D. Brooks
Abstract: Dissolved organic matter (DOM) transport is a key biogeochemical linkage across the terrestrial‐aquatic interface in headwater catchments, and quantifying the biological and hydrological controls on DOM composition provides insight into DOM cycling at the catchment scale. We evaluated the mobility of DOM components during snowmelt in a montane, semi‐arid catchment. DOM composition was evaluated on a near‐daily basis within the soil and the stream during snowmelt, and was compared to groundwater samples using a site‐specific parallel factor analysis (PARAFAC) model derived from soil extracts. The fluorescent component loadings in the interstitial soil water and in the groundwater were significantly different and did not temporally change during snowmelt. In the stream, a transition occurred during snowmelt from fluorescent DOM with higher contributions of amino acid‐like components indicative of groundwater to higher humic‐like contributions indicative of soil water. Furthermore, we identified a humic‐like fluorescent component in the soil water and the stream that is typically only observed in extracted water soluble organic matter from soil which may suggest hillslope to stream connectivity over very short time scales. Qualitative interpretations of changes in stream fluorescent DOM were supported by two end‐member mixing analyses of conservative tracers. After normalizing fluorescent DOM loadings for dissolved organic carbon (DOC) concentration, we found that the peak in DOC concentration in the stream was driven by the non‐fluorescent fraction of DOM. This study demonstrated how PARAFAC analysis can be used to refine our conceptual models of runoff generation sources, as well as provide a more detailed understanding of stream chemistry dynamics. This article is protected by copyright. All rights reserved.
- How should a rainfall‐runoff model be parameterized in an almost
ungauged catchment? A methodology tested on 609 catchments
- Abstract: This paper examines catchments that are almost ungauged, i.e. catchments for which only a small number of point flow measurements are available. In these catchments, hydrologists may still need to simulate continuous streamflow time series using a rainfall‐runoff model, and the methodology presented here allows using few point measurements for model parameterization. The method combines regional information (parameter sets of neighboring gauged stations) and local information (contributed by the point measurements) within a framework where the relative weight of each source of information is made dependent on the number of point measurements available. This approach is tested with two different hydrological models on a set of 609 catchments in France. The results show that on average a few flow measurements can significantly improve the simulation efficiency, and that ten measurements can reduce the performance gap between the gauged and ungauged situations by more than 50%. The added value of regional information progressively decreases until being almost insignificant when sufficient flow measurements are available. Model parameters tend to come closer to the values obtained by calibration in fully gauged conditions as the number of point flow measurements increases. This article is protected by copyright. All rights reserved.
- Scale dependency of effective hydraulic conductivity on
- Authors: Christoph Langhans; Patrick NJ Lane, Petter Nyman, Philip J Noske, Jane G Cawson, Akiko Oono, Gary J. Sheridan
Abstract: Effective hydraulic conductivity (Ke) for Hortonian overland flow modeling has been defined as a function of rainfall intensity and runon infiltration assuming a distribution of saturated hydraulic conductivities (Ks). But surface boundary condition during infiltration and its interactions with the distribution of Ks are not well represented in models. As a result, the mean value of the Ks distribution ), which is the central parameter for Ke, varies between scales. Here we quantify this discrepancy with a large infiltration data set comprising four different methods and scales from fire‐affected hillslopes in SE Australia using a relatively simple yet widely used conceptual model of Ke. Ponded disk (0.002 m2) and ring infiltrometers (0.07 m2) were used at the small scales and rainfall simulations (3 m2) and small catchments (ca 3000 m2) at the larger scales. We compared between methods measured at the same time and place. Disk and ring infiltrometer measurements had on average 4.8 times higher values of than rainfall simulations and catchment scale estimates. Furthermore, the distribution of Ks was not clearly log‐normal and scale‐independent, as supposed in the conceptual model. In our interpretation water repellency and preferential flow paths increase the variance of the measured distribution of Ks and bias ponding towards areas of very low Ks during rainfall simulations and small catchment runoff events while areas with high preferential flow capacity remain water supply limited more than the conceptual model of Ke predicts. The study highlights problems in the current theory of scaling runoff generation. This article is protected by copyright. All rights reserved.
- Improving Budyko curve‐based estimates of long‐term water
partitioning using hydrologic signatures from GRACE
- Authors: Kuai Fang; Chaopeng Shen, Joshua B. Fisher, Jie Niu
Abstract: The Budyko hypothesis provides a first‐order estimate of water partitioning into runoff (Q) and evapotranspiration (E). Observations, however, often show significant departures from the Budyko curve; moreover, past improvements to Budyko curve tend to lose predictive power when migrated between regions or to small scales. Here, to estimate departures from the Budyko curve, we use hydrologic signatures extracted from Gravity Recovery And Climate Experiment (GRACE) terrestrial water storage anomalies. The signatures include GRACE amplitude as a fraction of precipitation (A/P), inter‐annual variability, and 1‐month‐lag auto‐correlation. We created a group of linear models embodying two alternate hypotheses that departures can be predicted by (a) Taylor series expansion based on deviation of physical characteristics (seasonality, snow fraction and vegetation index) from reference conditions; and (b) surrogate indicators co‐varying with E, e.g., A/P. These models are fitted using a mesoscale USA dataset (HUC4) and then evaluated using world datasets and USA basins 1000 km2 and, according to comparison with other global datasets, is suitable for data fusion purposes, with GRACE error as estimates of uncertainty. This article is protected by copyright. All rights reserved.
- Measurement and modeling of engineered nanoparticle transport and aging
dynamics in a reactive porous medium
- Authors: Aviv Naftaly; Ishai Dror, Brian Berkowitz
Abstract: A continuous time random walk particle tracking (CTRW‐PT) method was employed to model flow cell experiments that measured transport of engineered nanoparticles (ENPs) in a reactive porous medium. The experiments involved a water‐saturated medium containing negatively‐charged, polyacrylamide beads, resembling many natural soils and aquifer materials, and having the same refraction index as water. Negatively‐ and positively‐charged ENPs were injected into a uniform flow field in a 3‐D horizontal flow cell, and the spatial and temporal concentrations of the evolving ENP plumes were obtained via image analysis. As a benchmark, and to calibrate the model, Congo red tracer was employed in 1‐D column and 3‐D flow cell experiments, containing the same beads. Negatively‐charged Au and Ag ENPs demonstrated migration patterns resembling those of the tracer, but were slightly more dispersive; the transport was well represented by the CTRW‐PT model. In contrast, positively‐charged AgNPs displayed an unusual behavior: establishment of an initial plume of essentially immobilized ENPs, followed by development of a secondary, freely‐migrating plume. The mobile plume was found to contain ENPs that, with aging, exhibited aggregation and charge inversion, becoming negatively charged and mobile. In this case, the CTRW‐PT model was modified to include a probabilistic law for particle immobilization, to account for the decreasing tendency (over distance and time) of the positively‐charged AgNPs to attach to the porous medium. The agreement between experimental results and modeling suggests that the CTRW‐PT framework can account for the non‐Fickian and surface charge dependent transport and aging exhibited by ENPs in porous media. This article is protected by copyright. All rights reserved.
- Effects of input discretization, model complexity, and calibration
strategy on model performance in a data‐scarce glacierized catchment
in central Asia
- Authors: L. Tarasova; M. Knoche, J. Dietrich, R. Merz
Abstract: Glacierized high‐mountainous catchments are often the water towers for downstream region and modeling these remote areas are often the only available tool for the assessment of water resources availability. Nevertheless, data scarcity affects different aspects of hydrological modeling in such mountainous glacierized basins. On the example of poorly gauged glacierized catchment in Central Asia we examined the effects of input discretization, model complexity and calibration strategy on model performance. The study was conducted with the GSM‐Socont model driven with climatic input from the corrected High Asia Reanalysis data set of two different discretizations. We analyze the effects of the use of long‐term glacier volume loss, snow cover images and interior runoff as an additional calibration data. In glacierized catchments with winter accumulation type, where the transformation of precipitation into runoff is mainly controlled by snow and glacier melt processes, the spatial discretization of precipitation tends to have less impact on simulated runoff than a correct prediction of the integral precipitation volume. Increasing model complexity by using spatially distributed input or semi‐distributed parameters values does not increase model performance in the Gunt catchment, as the more complex model tends to be more sensitive to errors in the input data set. In our case, better model performance and quantification of the flow components can be achieved by additional calibration data, rather than by using a more distributed model parameters. However, a semi‐distributed model better predicts the spatial patterns of snow accumulation and provides more plausible runoff predictions at the interior sites. This article is protected by copyright. All rights reserved.
- Spatiotemporal processes that contribute to hydrologic exchange between
hillslopes, valley bottoms, and streams
- Authors: Anna Bergstrom; Kelsey Jencso, Brian McGlynn
Abstract: Quantifying how watershed structure influences the exchanges of water among component parts of a watershed, particularly the connection between uplands, valley bottoms, and in‐stream hydrologic exchange remains a challenge. However, this understanding is critical for ascertaining the source areas and temporal contributions of water and associated biogeochemical constituents in streams. We used dilution gauging, mass recovery, and recording discharge stations to characterize streamflow dynamics across 52 reaches, from peak snowmelt to baseflow, in the Tenderfoot Creek Experimental forest, Montana, USA. We found that watershed‐contributing area was only a significant predictor of net changes in streamflow at high moisture states and larger spatial scales. However, at the scale of individual stream reaches, the lateral contributing area in conjunction with underlying lithology and vegetation densities were significant predictors of gross hydrologic gains to the stream. Reach lateral contributing areas underlain by more permeable sandstone yielded less water across flow states relative to those with granite gneiss. Additionally, increases in the frequency of steps across each stream reach contributed to greater hydrologic gross losses. Together, gross gains and losses of water along individual reaches resulted in net changes of discharge that cumulatively scale to the observed outlet discharge dynamics. Our results provide a framework for understanding how hillslope topography, geology, vegetation and valley bottom structure contribute to the exchange of water and cumulative increases of stream flow across watersheds of increasing size. This article is protected by copyright. All rights reserved.
- On the structural limitations of recursive digital filters for baseflow
- Abstract: Recursive digital filters (RDFs) are widely used for estimating baseflow from streamflow hydrographs, and various forms of RDFs have been developed based on different physical models. Numerical experiments have been used to objectively evaluate their performance, but they have not been sufficiently comprehensive to assess a wide range of RDFs. This paper extends these studies to understand the limitations of a generalized RDF method as a pathway for future field calibration. Two formalisms are presented to generalize most existing RDFs, allowing systematic tuning of their complexity. The RDFs with variable complexity are evaluated collectively in a synthetic setting, using modelled daily baseflow produced by Li et al.  from a range of synthetic catchments simulated with HydroGeoSphere. Our evaluation reveals that there are optimal RDF complexities in reproducing baseflow simulations, but shows that there is an inherent physical inconsistency within the RDF construction. Even under the idealized setting where true baseflow data are available to calibrate the RDFs, there is persistent disagreement between true and estimated baseflow over catchments with small baseflow components, low saturated hydraulic conductivity of the soil and larger surface runoff. The simplest explanation is that low baseflow ‘signal' in the streamflow data is hard to distinguish, although more complex RDFs can improve upon the simpler Eckhardt filter at these catchments. This article is protected by copyright. All rights reserved.
- Shot noise modeling of daily streamflows: A hybrid spectral‐domain
and time‐domain calibration approach
- Authors: F. Morlando; L. Cimorelli, L. Cozzolino, G. Mancini, D. Pianese, F. Garofalo
Abstract: The aim of this paper is to describe and evaluate a hybrid spectral‐ and time‐domain approach for the calibration of shot noise models for daily streamflow generation. The calibration approach allows the parameter estimation of a minimum‐phase rainfall/streamflow model using two steps. In the predictor step, the power spectral density of a recorded streamflow series is used to calibrate the parameters connected with the model dynamics. During the corrector step, a classic time‐domain procedure is used to calibrate the parameters connected with the average output of the model and the parameters that characterize the rainfall stochastic process. The procedure is demonstrated through its application to the daily streamflow time series associated with three Italian watersheds, and its results are then compared with those obtained by means of a time‐domain calibration method available in the literature. This article is protected by copyright. All rights reserved.
- The method of distributions for dispersive transport in porous media with
uncertain hydraulic properties
- Authors: Francesca Boso; Daniel M. Tartakovsky
Abstract: Predictions of solute transport in subsurface environments are notoriously unreliable due to aquifer heterogeneity and uncertainty about the values of hydraulic parameters. Probabilistic framework, which treats the relevant parameters and solute concentrations as random fields, allows for quantification of this predictive uncertainty. By providing deterministic equations for either probability density function or cumulative distribution function (CDF) of predicted concentrations, the method of distributions enables one to estimate, e.g., the probability of a contaminant's concentration exceeding a safe dose. We derive a deterministic equation for the CDF of solute concentration, which accounts for uncertainty in flow velocity and initial conditions. The coefficients in this equation are expressed in terms of the mean and variance of concentration. The accuracy and robustness of the CDF equations are analyzed by comparing their predictions with those obtained with Monte Carlo simulations and an assumed beta CDF. This article is protected by copyright. All rights reserved.
- Water balance complexities in ephemeral catchments with different land
uses: Insights from monitoring and distributed hydrologic modeling
- Authors: J. F. Dean; M. Camporese, J. A. Webb, S. P. Grover, P. E. Dresel, E. Daly
Abstract: Although ephemeral catchments are widespread in arid and semi‐arid climates, the relationship of their water balance with climate, geology, topography, and land cover is poorly known. Here we use four years (2011‐2014) of rainfall, streamflow, and groundwater level measurements to estimate the water balance components in two adjacent ephemeral catchments in south‐eastern Australia, with one catchment planted with young eucalypts and the other dedicated to grazing pasture. To corroborate the interpretation of the observations, the physically‐based hydrological model CATHY was calibrated and validated against the data in the two catchments. The estimated water balances showed that despite a significant decline in groundwater level and greater evapotranspiration in the eucalypt catchment (104‐119% of rainfall) compared with the pasture catchment (95‐104% of rainfall), streamflow consistently accounted for 1‐4% of rainfall in both catchments for the entire study period. Streamflow in the two catchments was mostly driven by the rainfall regime, particularly rainfall frequency (i.e. the number of rain days per year), while the downslope orientation of the plantation furrows also promoted runoff. With minimum calibration, the model was able to adequately reproduce the periods of flow in both catchments in all years. Although streamflow and groundwater levels were better reproduced in the pasture than in the plantation, model‐computed water balance terms confirmed the estimates from the observations in both catchments. Overall, the interplay of climate, topography, and geology seems to overshadow the effect of land use in the study catchments, indicating that the management ephemeral catchments remains highly challenging. This article is protected by copyright. All rights reserved.
- Scale dependence of the hydraulic properties of a fractured aquifer
estimated using transfer functions
- Abstract: We present an investigation of the scale dependence of hydraulic parameters in fractured media based on the concept of transfer functions (TF). TF methods provide an inexpensive way to perform aquifer parameter estimation, as they relate the fluctuations of an observation time series (hydraulic head fluctuations) to an input function (aquifer recharge) in frequency domain. Fractured media are specially sensitive to this approach as hydraulic parameters are strongly scale dependent, involving non‐stationary statistical distributions. Our study is based on an extensive data set, involving up to 130 measurement points with periodic head measurements that in some cases extend for more than 30 years. For each point, we use a single‐porosity and dual‐continuum TF formulation to obtain a distribution of transmissivities and storativities in both mobile and immobile domains. Single‐porosity TF estimates are compared with data obtained from the interpretation of over 60 hydraulic tests (slug and pumping tests). Results show that the TF is able to estimate the scale dependence of the hydraulic parameters, and it is consistent with the behavior of estimates from traditional hydraulic tests. In addition, the TF approach seems to provide an estimation of the system variance and the extension of the ergodic behavior of the aquifer (estimated in approximately $500$m in the analyzed aquifer). The scale dependence of transmissivity seems to be independent from the adopted formulation (single or dual‐continuum), while storativity is more sensitive to the presence of multiple continua. This article is protected by copyright. All rights reserved.
- Reply to comments by Keith E. Schilling on “Climate and agricultural
land use change impacts on streamflow in the upper midwestern United
- Authors: Satish C. Gupta; Andrew C. Kessler, Melinda K. Brown, William M. Schuh
Abstract: The reply addresses concerns raised by Schilling  claiming that Gupta et al. [2015a,b] mischaracterized his research on the impact of land use change on streamflow. We disagree with his interpretation. In the reply we show that our interpretation of his work on lack of climate impact on streamflow is consistent with interpretation by others in the literature including some of his co‐authors (Xu, Scanlon, Schilling, and Sun, 2013) who stated that Schilling  and Zhang and Schilling  studies concluded that land surface change played a dominant role compared to climate change. Our writing may have been too explicit but is similar to that of Ryberg et al. . We are convinced that a clarification was needed. Furthermore, we provide additional analysis of the Raccoon River flows in Iowa and show that both annual streamflow and baseflow are mainly controlled by precipitation not only in a given year but also by precipitation in the previous year. This article is protected by copyright. All rights reserved.
- Comment on “Climate and agricultural land use change impacts on
streamflow in the upper midwestern United States” by Gupta et al.
- Authors: Keith E. Schilling
Abstract: Increasing precipitation and land use/land cover (LU/LC) change have contributed to increasing streamflow and baseflow in many Midwestern rivers but the relative importance of causal factors is open to debate. The dominant LULC change in the agricultural Midwest is the emergence of soybean production that occurred in the mid‐ to late‐20th Century that replaced many sod‐based rotations and increased total row crop area devoted to annual maize and soybean crops. Increasing precipitation may be a more important factor for increasing total discharge whereas LULC changes contributed more to baseflow changes. This article is protected by copyright. All rights reserved.
- Trends and sensitivities of low streamflow extremes to discharge timing
and magnitude in Pacific Northwest mountain streams
- Authors: Patrick R. Kormos; Charles H. Luce, Seth J. Wenger, Wouter R. Berghuijs
Abstract: Path analyses of historical streamflow data from the Pacific Northwest indicate that the precipitation amount has been the dominant control on the magnitude of low streamflow extremes compared to the air temperature‐affected timing of snowmelt runoff. The relative sensitivities of low streamflow to precipitation and temperature changes have important implications for adaptation planning because global circulation models produce relatively robust estimates of air temperature changes but have large uncertainties in projected precipitation amounts in the Pacific Northwest. Quantile regression analyses indicate that low streamflow extremes from the majority of catchments in this study have declined from 1948 to 2013, which may significantly affect terrestrial and aquatic ecosystems, and water resource management. Trends in the 25th percentile of mean annual streamflow have declined and the center of timing has occurred earlier. We quantify the relative influences of total precipitation and air temperature on the annual low streamflow extremes from 42 stream gauges using mean annual streamflow as a proxy for precipitation amount effects and streamflow center of timing as a proxy for temperature effects on low flow metrics, including 7q10 summer (the minimum 7‐day flow during summer with a 10‐year return probability), mean August, mean September, mean summer, 7q10 winter, and mean winter flow metrics. These methods have the benefit of using only readily available streamflow data, which makes our results robust against systematic errors in high elevation distributed precipitation data. Winter low flow metrics are weakly tied to both mean annual streamflow and center of timing. This article is protected by copyright. All rights reserved.
- Analysis of reach‐scale elevation distribution in braided rivers:
Definition of a new morphologic indicator and estimation of mean
- Authors: M. Redolfi; M. Tubino, W. Bertoldi, J. Brasington
Abstract: Understanding the role of external controls on the morphology of braided rivers is currently limited by the dearth of robust metrics to quantify and distinguish the diversity of channel form. Most existing measures are strongly dependent on river stage and unable to account for the three‐dimensional complexity that is apparent in digital terrain models of braided rivers. In this paper, we introduce a simple, stage‐independent morphological indicator that enables the analysis of reach‐scale regime morphology as a function of slope, discharge, sediment size and degree of confinement. The index is derived from the bed elevation frequency distribution and characterizes a statistical width‐depth curve averaged longitudinally over multiple channel widths. In this way, we define a “synthetic channel” described by a simple parameter that embeds information about the river morphological complexity. Under the assumption of uniform flow, this approach can be extended to provide estimates of the reach‐averaged shear stress distribution, bed load flux and at‐a‐station‐variability of wetted width. We test this approach using data from a wide range of labile channels including 58 flume experiments and three gravel bed braided rivers. Results demonstrate a strong relationship between the unit discharge and the shape of the elevation distribution, which varies between a U‐shape for typical single‐thread confined channels, to a Y‐shape for multi‐thread reaches. Finally, we discuss the use of the metric as a diagnostic index of river condition that may be used to support inferences about the river morphological trajectory. This article is protected by copyright. All rights reserved.
- Water and sanitation service delivery, pricing, and the poor: An empirical
estimate of subsidy incidence in Nairobi, Kenya
- Abstract: The increasing block tariff (IBT) is among the most widely used tariffs by water utilities, particularly in developing countries. This is due in part to the perception that the IBT can effectively target subsidies to low‐income households. Combining data on households' socioeconomic status and metered water use, this paper examines the distributional incidence of subsidies delivered through the IBT in Nairobi, Kenya. Contrary to conventional wisdom, we find that high‐income residential and non‐residential customers receive a disproportionate share of subsidies and that subsidy targeting is poor even among households with a private metered connection. We also find that stated expenditure on water, a commonly used means of estimating water use, is a poor proxy for metered use and that previous studies on subsidy incidence underestimate the magnitude of the subsidy delivered through water tariffs. These findings have implications for both the design and evaluation of water tariffs in developing countries. This article is protected by copyright. All rights reserved.
- Geomorphological control on variably saturated hillslope hydrology and
- Authors: Formetta Giuseppe; Silvia Simoni, Jonathan W. Godt, Ning Lu, Riccardo Rigon
Abstract: In steep topography, the processes governing variably saturated subsurface hydrologic response and the inter‐particle stresses leading to shallow landslide initiation are physically linked. However, these processes are usually analyzed separately. Here, we take a combined approach, simultaneously analyzing the influence of topography on both hillslope hydrology and the effective stress fields within the hillslope itself. Clearly, runoff and saturated groundwater flow are dominated by gravity and, ultimately, by topography. Less clear is how landscape morphology influences flows in the vadose zone, where transient fluxes are usually taken to be vertical. We aim to assess and quantify the impact of topography on both saturated and unsaturated hillslope hydrology and its effects on shallow slope stability. Three real hillslope morphologies (concave, convex and planar) are analyzed using a 3D, physically‐based, distributed model coupled with a module for computation of the probability of failure, based on the infinite slope assumption. The results of the analyses, which included parameter uncertainty analysis of the results themselves, show that convex and planar slopes are more stable than concave slopes. Specifically, under the same initial, boundary, and infiltration conditions, the percentage of unstable areas ranges from 1.3% for the planar hillslope, 21% for convex, to a maximum value of 33% for the concave morphology. The results are supported by a sensitivity analysis carried out to examine the effect of initial conditions and rainfall intensity. This article is protected by copyright. All rights reserved.
- An enhanced Bayesian fingerprinting framework for studying sediment source
dynamics in intensively managed landscapes
- Authors: B. Abban; A.N. (Thanos) Papanicolaou, M.K. Cowles, C.G. Wilson, O. Abaci, K. Wacha, K. Schilling, D. Schnoebelen
Abstract: An enhanced revision of the Fox and Papanicolaou [F‐P, 2008] Bayesian, Markov Chain Monte Carlo fingerprinting framework for estimating sediment source contributions and their associated uncertainties is presented. The F‐P framework included two key deterministic parameters, α and β, that respectively reflected the spatial origin attributes of sources and the time history of eroded material delivered to and collected at the watershed outlet. However, the deterministic treatment of α and β is limited to cases with well‐defined spatial partitioning of sources, high sediment delivery and relatively short travel times with little variability in transport within the watershed. For event‐based studies in intensively managed landscapes, this may be inadequate since landscape heterogeneity results in variabilities in source contributions, their pathways, delivery times and storage within the watershed. Thus, probabilistic treatments of α and β are implemented in the enhanced framework to account for these variabilities. To evaluate the effects of the treatments of α and β on source partitioning, both frameworks are applied to the South Amana Sub‐Watershed (SASW) in the US Midwest. The enhanced framework is found to estimate mean source contributions that are in good agreement with estimates from other studies in SASW. The enhanced framework is also able to produce expected trends in uncertainty during the study period, unlike the F‐P framework, which does not perform as expected. Overall, the enhanced framework is found to be less sensitive to changes in α and β than the F‐P framework, and, therefore, is more robust and desirable from a management standpoint. This article is protected by copyright. All rights reserved.
- Beyond the SCS‐CN method: A theoretical framework for spatially
lumped rainfall‐runoff response
- Authors: M. S. Bartlett; A. J. Parolari, J. J. McDonnell, A. Porporato
Abstract: Since its introduction in 1954, the Soil Conservation Service curve number (SCS‐CN) method has become the standard tool, in practice, for estimating an event‐based rainfall‐runoff response. However, because of its empirical origins, the SCS‐CN method is restricted to certain geographic regions and land use types. Moreover, it does not describe the spatial variability of runoff. To move beyond these limitations, we present a new theoretical framework for spatially‐lumped, event‐based rainfall‐runoff modeling. In this framework, we describe the spatially‐lumped runoff model as a point description of runoff that is upscaled to a watershed area based on probability distributions that are representative of watershed heterogeneities. The framework accommodates different runoff concepts and distributions of heterogeneities, and in doing so, it provides an implicit spatial description of runoff variability. Heterogeneity in storage capacity and soil moisture are the basis for upscaling a point runoff response and linking ecohydrological processes to runoff modeling. For the framework, we consider two different runoff responses for fractions of the watershed area: “pre‐threshold” and “threshold‐excess” runoff. These occur before and after infiltration exceeds a storage capacity threshold. Our application of the framework results in a new model (called SCS‐CNx) that extends the SCS‐CN method with the pre‐threshold and threshold‐excess runoff mechanisms and an implicit spatial description of runoff. We show proof of concept in four forested watersheds and further that the resulting model may better represent geographic regions and site types that previously have been beyond the scope of the traditional SCS‐CN method. This article is protected by copyright. All rights reserved.
- Sensitivity of emergent sociohydrologic dynamics to internal system
properties and external sociopolitical factors: Implications for water
- Authors: Y. Elshafei; M. Tonts, M. Sivapalan, M. R. Hipsey
Abstract: It is increasingly acknowledged that effective management of water resources requires a holistic understanding of the co‐evolving dynamics inherent in the coupled human‐hydrology system. One of the fundamental information gaps concerns the sensitivity of coupled system feedbacks to various endogenous system properties and exogenous societal contexts. This paper takes a previously calibrated socio‐hydrology model and applies an idealized implementation, in order to: i) explore the sensitivity of emergent dynamics resulting from bi‐directional feedbacks to assumptions regarding (a) internal system properties that control the internal dynamics of the coupled system and (b) the external socio‐political context; and ii) interpret the results within the context of water resource management decision making. The analysis investigates feedback behavior in three ways, (a) via a global sensitivity analysis on key parameters and assessment of relevant model outputs, (b) through a comparative analysis based on hypothetical placement of the catchment along various points on the international socio‐political gradient, and (c) by assessing the effects of various direct management intervention scenarios. Results indicate the presence of optimum windows that might offer the greatest positive impact per unit of management effort. Results further advocate management tools that encourage an adaptive learning, community‐based approach with respect to water management, which are found to enhance centralized policy measures. This paper demonstrates that it is possible to use a place‐based socio‐hydrology model to make abstractions as to the dynamics of bi‐directional feedback behavior, and provide insights as to the efficacy of water management tools under different circumstances. This article is protected by copyright. All rights reserved.
- Flash flooding in small urban watersheds: Storm event hydrologic response
- Authors: Long Yang; James A. Smith, Mary Lynn Baeck, Yan Zhang
Abstract: We analyze flash flooding in small urban watersheds, with special focus on the roles of rainfall variability, antecedent soil moisture and urban stormwater management infrastructure in storm event hydrologic response. Our results are based on empirical analyses of high resolution rainfall and discharge observations over Harry's Brook watershed in Princeton, New Jersey during 2005‐2006, as well as numerical experiments with the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) model. We focus on two subwatersheds of Harry's Brook, a 1.1 km2 subwatershed which was developed prior to modern stormwater management regulations, and a 0.5 km2 subwatershed with an extensive network of stormwater detention ponds. The watershed developed prior to modern stormwater regulations is an “end member” in urban flood response, exhibiting a frequency of flood peaks (with unit discharge exceeding 1 m3 s−1 km−2) that is comparable to the “flashiest” watersheds in the conterminous US. Observational analyses show that variability in storm event water balance is strongly linked to peak rain rates at time intervals of less than 30 minutes and only weakly linked to antecedent soil moisture conditions. Peak discharge for both the 1.1 km2 and 0.5 km2 subwatersheds are strongly correlated with rainfall rate averaged over 1‐30 minutes. Hydrologic modeling analyses indicate that the sensitivity of storm event hydrologic response to spatial rainfall variability decreases with storm intensity. Temporal rainfall variability is relatively more important than spatial rainfall variability in representing urban flood response, especially for extreme storm events. This article is protected by copyright. All rights reserved.
- Imprecise probabilistic estimation of design floods with epistemic
- Authors: Wei Qi; Chi Zhang, Guangtao Fu, Huicheng Zhou
Abstract: An imprecise probabilistic framework for design flood estimation is proposed on the basis of the Dempster‐Shafer theory to handle different epistemic uncertainties from data, probability distribution functions and probability distribution parameters. These uncertainties are incorporated in cost‐benefit analysis to generate the lower and upper bounds of the total cost for flood control, thus presenting improved information for decision making on design floods. Within the total cost bounds, a new robustness criterion is proposed to select a design flood that can tolerate higher levels of uncertainty. A variance decomposition approach is used to quantify individual and interactive impacts of the uncertainty sources on total cost. Results from three case studies, with 127‐, 104‐ and 54‐year flood data sets respectively, show that the imprecise probabilistic approach effectively combines aleatory and epistemic uncertainties from the various sources and provides upper and lower bounds of the total cost. Between the total cost and the robustness of design floods, a clear trade‐off which is beyond the information that can be provided by the conventional minimum cost criterion is identified. The interactions among data, distributions and parameters have a much higher contribution than parameters to the estimate of the total cost. It is found that the contributions of the various uncertainty sources and their interactions vary with different flood magnitude, but remain roughly the same with different return periods. This study demonstrates that the proposed methodology can effectively incorporate epistemic uncertainties in cost‐benefit analysis of design floods. This article is protected by copyright. All rights reserved.
- Photogrammetric discharge monitoring of small tropical mountain rivers: A
case study at Rivière des Pluies, Rèunion island
- Abstract: Reliable discharge measurements are indispensable for an effective management of natural water resources and floods. Limitations of classical current meter profiling and stage‐discharge ratings have stimulated the development of more accurate and efficient gauging techniques such as non‐intrusive photogrammetric techniques. Despite many successful applications of large‐scale particle image velocimetry (LSPIV) for short‐term measurements during flood events there are still very few studies that address its use for long‐term monitoring of small mountain rivers. To fill this gap this study targets the development and testing of largely autonomous photogrammetric discharge measurement system with a special focus on the application to small mountain river with high discharge variability in the tropics. It proposes several enhancements concerning camera calibration, more efficient processing in image geometry, the automatic detection of the water level as well as the statistical calibration and estimation of the discharge from multiple profiles. A case study which comprises the analysis of several thousand videos spanning over two and a half year is carried out to test the robustness and accuracy of different processing steps. Comparisons against classical current meter profiling show a mean absolute percentage error of 9.0% after the statistical calibration of the system. The study suggests that LSPIV can already be considered as a valuable tool for the monitoring of torrential flows, whereas further research is still needed to fully integrate night‐time observation and stereo‐photogrammetric capabilities. This article is protected by copyright. All rights reserved.
- An intercomparison of remote sensing river discharge estimation algorithms
from measurements of river height, width, and slope
- Abstract: The Surface Water and Ocean Topography (SWOT) satellite mission planned for launch in 2020 will map river elevations and inundated area globally for rivers >100 m wide. In advance of this launch, we here evaluated the possibility of estimating discharge in ungauged rivers using synthetic, daily ‘remote sensing' measurements derived from hydraulic models corrupted with minimal observational errors. Five discharge algorithms were evaluated, as well as the median of the five, for nineteen rivers spanning a range of hydraulic and geomorphic conditions. Reliance upon a priori information, and thus applicability to truly ungauged reaches, varied among algorithms: one algorithm employed only global limits on velocity and depth, while the other algorithms relied on globally‐available prior estimates of discharge. We found at least one algorithm able to estimate instantaneous discharge to within 35% relative root mean squared error (RRMSE) on 14/16 non‐braided rivers despite out‐of‐bank flows, multi‐channel planforms, and backwater effects. Moreover, we found RRMSE was often dominated by bias; the median standard deviation of relative residuals across the 16 non‐braided rivers was only 12.5%. SWOT discharge algorithm progress is therefore encouraging, yet future efforts should consider incorporating ancillary data or multi‐algorithm synergy to improve results. This article is protected by copyright. All rights reserved.
- On the choice of analogue fluids in CO2 convective dissolution experiments
- Abstract: Mixtures of ethylene glycol and methanol (EG‐MeOH) have been used as an analogue system (i.e., EG‐MeOH/water) in recent experiments in the context of convective dissolution of CO2 in deep saline aquifers. We have conducted a linear stability analysis of a gravitationally unstable diffusive boundary layer as well as direct numerical simulation of convective mixing involved in dissolution of EG‐MeOH species in water. We provide new evidences that EG‐MeOH does not resemble the dynamics of convective instabilities and subsequent mixing associated with dissolution of CO2 in water. It is found that there are fundamental differences in the evolution of the buoyancy‐driven instability and dynamics of convective mixing between CO2/water and a typical EG‐MeOH/water analogue system. Our results show that for a constant Rayleigh number the onset of convective instabilities for EG‐MeOH/water can be different by an order of magnitude as compared with CO2/water. In addition, EG‐MeOH/water system reveals different dynamics associated with the convective mixing as compared to CO2/water system. This study improves our understanding of the instability behavior of analogue systems, their proper selection, and motivates further experiments. This article is protected by copyright. All rights reserved.
- Reply to comments on “Climate and agricultural land use change
impacts on streamflow in the upper Midwestern United States” by
Schottler et al.
- Authors: Satish C. Gupta; Andrew C. Kessler, Melinda K. Brown, William M. Schuh
- Comment on “Climate and agricultural land use change impacts on
streamflow in the upper Midwestern United States”
- Authors: Shawn Schottler; Jason Ulrich, Daniel Engstrom
- Ensemble forecasting of short‐term system scale irrigation demands
using real time flow data and numerical weather predictions
- Authors: Kushan C. Perera; Andrew W. Western, David E. Robertson, Biju George, Bandara Nawarathna
Abstract: Irrigation demands fluctuate in response to weather variations and a range of irrigation management decisions, which creates challenges for water supply system operators. This paper develops a method for real‐time ensemble forecasting of irrigation demand and applies it to irrigation command areas of various sizes for lead times of 1 to 5 days. The ensemble forecasts are based on a deterministic time series model coupled with ensemble representations of the various inputs to that model. Forecast inputs include past flow, precipitation, and potential evapotranspiration. These inputs are variously derived from flow observations from a modernized irrigation delivery system; short‐term weather forecasts derived from numerical weather prediction models and observed weather data available from automatic weather stations. The predictive performance for the ensemble spread of irrigation demand was quantified using rank histograms, the mean continuous rank probability score (CRPS), the mean CRPS reliability and the temporal mean of the ensemble root mean squared error (MRMSE). The mean forecast was evaluated using root mean squared error (RMSE), Nash–Sutcliffe model efficiency (NSE) and bias. The NSE values for evaluation periods ranged between 0.96 (1 day lead time, whole study area) and 0.42 (5 days lead time, smallest command area). Rank histograms and comparison of MRMSE, mean CRPS, mean CRPS reliability and RMSE indicated that the ensemble spread is generally a reliable representation of the forecast uncertainty for short lead times but underestimates the uncertainty for long lead times. This article is protected by copyright. All rights reserved.
- Modeling nonlinear responses of DOC transport in boreal catchments in
- Authors: Ville Kasurinen; Knut Alfredsen, Anne Ojala, Jukka Pumpanen, Gesa A. Weyhenmeyer, Martyn N. Futter, Hjalmar Laudon, Frank Berninger
Abstract: Stream water dissolved organic carbon (DOC) concentrations display high spatial and temporal variation in boreal catchments. Understanding and predicting these patterns is a challenge with great implications for water quality projections and carbon balance estimates. Although several biogeochemical models have been used to estimate stream water DOC dynamics, model biases common during both rain and snow melt driven events. The parsimonious DOC‐model, K‐DOC, with ten calibrated parameters, uses a non‐linear discharge and catchment water storage relationship including soil temperature dependencies of DOC release and consumption. K‐DOC was used to estimate the stream water DOC concentrations over five years for eighteen nested boreal catchments having total area of 68 km2 (varying from 0.04 to 67.9 km2). The model successfully simulated DOC concentrations during base flow conditions, as well as, hydrological events in catchments dominated by organic and mineral soils reaching NSEs from 0.46 to 0.76. Our semi‐mechanistic model was parsimonious enough to have all parameters estimated using statistical methods. We did not find any clear differences between forest and mire dominated catchments that could be explained by soil type or tree species composition. However, parameters controlling slow release and consumption of DOC from soil water behaved differently for small headwater catchments (less than 2 km2) than for those that integrate larger areas of different ecosystem types (10‐68 km2). Our results emphasize that it is important to account for non‐linear dependencies of both, soil temperature and catchment water storage, when simulating DOC dynamics of boreal catchments This article is protected by copyright. All rights reserved.
- Channel morphology and flow structure of an abandoned channel under
- Authors: Katie H. Costigan; Joseph E. Gerken
Abstract: Abandoned channels are those channels left behind as meandering rivers migrate over their floodplains but remain among the most enigmatic features of the riverscape, especially related to their hydraulics and geomorphology. Abandoned channels are being considered and implemented as restoration and rehabilitation strategies for large rivers but we do not yet have a sound understanding of their hydromorphodynamics. The overall objectives of this work were to assess the bed morphology and flow structure of a large, dynamically connected abandoned channel (e.g., the channel is inundated during annual or decadal floods through bank overflow) under varying stages. Here we document the hydromorphodynamics of an abandoned channel during 3.4, 9.2, and 37.9 return interval discharges using an acoustic Doppler current profiler. Flow separation was observed along the channel entrance during the lowest flow surveying campaign but was not seen during the higher flow campaign. Width to depth ratio and channel width at the exit both progressively decreased from the first surveying campaign, despite the final campaign having the highest measured discharge. Large zones of flow stagnation and recirculation were observed, with depth averaged velocity vectors not aligning in one direction, in the abandoned channel where water from the entrance was meeting water coming up from the exit during moderate discharges. The abandoned channel has been maintained for at least 25 years due to its low diversion angle and it being perched above the Kansas River. Results of this study provide insights of how flow hydraulics and physical characteristics of abandoned channel change under varying stages. This article is protected by copyright. All rights reserved.
- Issue Information
- Pages: 4207 - 4208
- Value of long‐term streamflow forecasts to reservoir operations for
water supply in snow‐dominated river catchments
- Authors: D. Anghileri; N. Voisin, A. Castelletti, F. Pianosi, B. Nijssen, D. P. Lettenmaier
Pages: 4209 - 4225
Abstract: We present a forecast‐based adaptive management framework for water supply reservoirs and evaluate the contribution of long‐term inflow forecasts to reservoir operations. Our framework is developed for snow‐dominated river basins that demonstrate large gaps in forecast skill between seasonal and inter‐annual time horizons. We quantify and bound the contribution of seasonal and inter‐annual forecast components to optimal, adaptive reservoir operation. The framework uses an Ensemble Streamflow Prediction (ESP) approach to generate retrospective, one‐year‐long streamflow forecasts based on the Variable Infiltration Capacity (VIC) hydrology model. We determine the optimal sequence of daily release decisions using the Model Predictive Control (MPC) optimization scheme. We then assess the forecast value by comparing system performance based on the ESP forecasts with the performances based on climatology and perfect forecasts. We distinguish among the relative contributions of the seasonal component of the forecast versus the inter‐annual component by evaluating system performance based on hybrid forecasts, which are designed to isolate the two contributions. As an illustration, we first apply the forecast‐based adaptive management framework to a specific case study, i.e., Oroville Reservoir in California, and we then modify the characteristics of the reservoir and the demand to demonstrate the transferability of the findings to other reservoir systems. Results from numerical experiments show that, on average, the overall ESP value in informing reservoir operation is 35% less than the perfect forecast value and the inter‐annual component of the ESP forecast contributes 20–60% of the total forecast value.
- Spectral‐induced polarization measurements on sieved sands and the
relationship to permeability
- Authors: Sheen Joseph; Malcolm Ingham, Gideon Gouws
Pages: 4226 - 4246
Abstract: Laboratory measurements of the permeability and spectral‐induced polarization (SIP) response of samples consisting of unconsolidated sands typical of those found in New Zealand aquifers have been made. After correction of measured formation factors to allow for the fact that some were measured at only one fluid conductivity, predictions of permeability from the grain size (d) of the samples are found to agree well with measured values of permeability. The Cole‐Cole time constant (derived from the SIP measurements) is found, as expected, to depend upon d2, but can be affected by the inclusion of smaller grains in the sample. Measurements made on samples comprising of mixtures of grain sizes show that inclusion in a sample of even 10% of smaller grains can significantly reduce both the Cole‐Cole time constant (τCC) and the permeability, and support theoretical derivation of how the permeability of a mixture of grain sizes varies with the content of the mixture. Proposed relationships for using τCC as a predictor for permeability are tested and found to be crucially dependent on the assumed relationship between the dynamic pore radius and grain size. The inclusion of a multiplicative constant to take account of numerical approximations results in good predictions for the permeability of the samples in this study. It seems unlikely, however, that there is a single global expression for predicting permeability from SIP data for all samples.
- Temporal and spatial dynamics of large lake hypoxia: Integrating
statistical and three‐dimensional dynamic models to enhance lake
- Authors: Serghei A. Bocaniov; Donald Scavia
Pages: 4247 - 4263
Abstract: Hypoxia or low bottom water dissolved oxygen (DO) is a world‐wide problem of management concern requiring an understanding and ability to monitor and predict its spatial and temporal dynamics. However, this is often made difficult in large lakes and coastal oceans because of limited spatial and temporal coverage of field observations. We used a calibrated and validated three‐dimensional ecological model of Lake Erie to extend a statistical relationship between hypoxic extent and bottom water DO concentrations to explore implications of the broader temporal and spatial development and dissipation of hypoxia. We provide the first numerical demonstration that hypoxia initiates in the nearshore, not the deep portion of the basin, and that the threshold used to define hypoxia matters in both spatial and temporal dynamics and in its sensitivity to climate. We show that existing monitoring programs likely underestimate both maximum hypoxic extent and the importance of low oxygen in the nearshore, discuss implications for ecosystem and drinking water protection, and recommend how these results could be used to efficiently and economically extend monitoring programs.
- Variability in isotopic composition of base flow in two headwater streams
of the southern Appalachians
- Authors: Nitin K. Singh; Ryan E. Emanuel, Brian L. McGlynn
Pages: 4264 - 4279
Abstract: We investigated the influence of hillslope scale topographic characteristics and the relative position of hillslopes along streams (i.e., internal catchment structure) on the isotopic composition of base flow in first‐order, forested headwater streams at Coweeta Hydrologic Laboratory. The study focused on two adjacent forested catchments with different topographic characteristics. We used stable isotopes (18O and 2H) of water together with stream gauging and geospatial analysis to evaluate relationships between internal catchment structure and the spatiotemporal variability of base flow δ18O. Base flow δ18O was variable in space and time along streams, and the temporal variability of base flow δ18O declined with increasing drainage area. Base flow became enriched in 18O moving along streams from channel heads to catchment outlets but the frequency of enrichment varied between catchments. The spatiotemporal variability in base flow δ18O was high adjacent to large hillslopes with short flow paths, and it was positively correlated with the relative arrangement of hillslopes within the catchment. These results point to influence of unique arrangement of hillslopes on the patterns of downstream enrichment. Spatial variability in base flow δ18O within the streams was relatively low during dry and wet conditions, but it was higher during the transition period between dry and wet conditions. These results suggest that the strength of topographic control on the isotopic composition of base flow can vary with catchment wetness. This study highlights that topographic control on base flow generation and isotopic composition is important even at fine spatial scales.
- Determining soil moisture and soil properties in vegetated areas by
assimilating soil temperatures
- Pages: 4280 - 4300
Abstract: This study addresses two critical barriers to the use of Passive Distributed Temperature Sensing (DTS) for large‐scale, high‐resolution monitoring of soil moisture. In recent research, a particle batch smoother (PBS) was developed to assimilate sequences of temperature data at two depths into Hydrus‐1D to estimate soil moisture as well as soil thermal and hydraulic properties. However, this approach was limited to bare soil and assumed that the cable depths were perfectly known. In order for Passive DTS to be more broadly applicable as a soil hydrology research and remote sensing soil moisture product validation tool, it must be applicable in vegetated areas. To address this first limitation, the forward model (Hydrus‐1D) was improved through the inclusion of a canopy energy balance scheme. Synthetic tests were used to demonstrate that without the canopy energy balance scheme, the PBS estimated soil moisture could be even worse than the open loop case (no assimilation). When the improved Hydrus‐1D model was used as the forward model in the PBS, vegetation impacts on the soil heat and water transfer were well accounted for. This led to accurate and robust estimates of soil moisture and soil properties. The second limitation is that, cable depths can be highly uncertain in DTS installations. As Passive DTS uses the downward propagation of heat to extract moisture‐related variations in thermal properties, accurate estimates of cable depths are essential. Here synthetic tests were used to demonstrate that observation depths can be jointly estimated with other model states and parameters. The state and parameter results were only slightly poorer than those obtained when the cable depths were perfectly known. Finally, in situ temperature data from four soil profiles with different, but known, soil textures were used to test the proposed approach. Results show good agreement between the observed and estimated soil moisture, hydraulic properties, thermal properties, and observation depths at all locations. The proposed method resulted in soil moisture estimates in the top 10 cm with RMSE values typically
- Detectability of change in winter precipitation within mountain
landscapes: Spatial patterns and uncertainty
- Authors: N. L. Silverman; M. P. Maneta
Pages: 4301 - 4320
Abstract: Detecting long‐term change in seasonal precipitation using ground observations is dependent on the representativity of the point measurement to the surrounding landscape. In mountainous regions, representativity can be poor and lead to large uncertainties in precipitation estimates at high elevations or in areas where observations are sparse. If the uncertainty in the estimate is large compared to the long‐term shifts in precipitation, then the change will likely go undetected. In this analysis, we examine the minimum detectable change across mountainous terrain in western Montana, USA. We ask the question: What is the minimum amount of change that is necessary to be detected using our best estimates of precipitation in complex terrain? We evaluate the spatial uncertainty in the precipitation estimates by conditioning historic regional climate model simulations to ground observations using Bayesian inference. By using this uncertainty as a null hypothesis, we test for detectability across the study region. To provide context for the detectability calculations, we look at a range of future scenarios from the Coupled Model Intercomparison Project 5 (CMIP5) multimodel ensemble downscaled to 4 km resolution using the MACAv2‐METDATA data set. When using the ensemble averages we find that approximately 65% of the significant increases in winter precipitation go undetected at midelevations. At high elevation, approximately 75% of significant increases in winter precipitation are undetectable. Areas where change can be detected are largely controlled by topographic features. Elevation and aspect are key characteristics that determine whether or not changes in winter precipitation can be detected. Furthermore, we find that undetected increases in winter precipitation at high elevation will likely remain as snow under climate change scenarios. Therefore, there is potential for these areas to offset snowpack loss at lower elevations and confound the effects of climate change on water resources.
- A laboratory study to estimate pore geometric parameters of sandstones
using complex conductivity and nuclear magnetic resonance for permeability
- Authors: Gordon Osterman; Kristina Keating, Andrew Binley, Lee Slater
Pages: 4321 - 4337
Abstract: We estimate parameters from the Katz and Thompson permeability model using laboratory complex electrical conductivity (CC) and nuclear magnetic resonance (NMR) data to build permeability models parameterized with geophysical measurements. We use the Katz and Thompson model based on the characteristic hydraulic length scale, determined from mercury injection capillary pressure estimates of pore throat size, and the intrinsic formation factor, determined from multisalinity conductivity measurements, for this purpose. Two new permeability models are tested, one based on CC data and another that incorporates CC and NMR data. From measurements made on forty‐five sandstone cores collected from fifteen different formations, we evaluate how well the CC relaxation time and the NMR transverse relaxation times compare to the characteristic hydraulic length scale and how well the formation factor estimated from CC parameters compares to the intrinsic formation factor. We find: (1) the NMR transverse relaxation time models the characteristic hydraulic length scale more accurately than the CC relaxation time (
R2 of 0.69 and 0.33 and normalized root mean square errors (NRMSE) of 0.16 and 0.21, respectively); (2) the CC estimated formation factor is well correlated with the intrinsic formation factor (NRMSE=0.23). We demonstrate that that permeability estimates from the joint‐NMR‐CC model (NRMSE=0.13) compare favorably to estimates from the Katz and Thompson model (NRMSE=0.074). This model advances the capability of the Katz and Thompson model by employing parameters measureable in the field giving it the potential to more accurately estimate permeability using geophysical measurements than are currently possible.
- Effects of aridity in controlling the magnitude of runoff and erosion
- Authors: Philip J. Noske; Petter Nyman, Patrick N. J. Lane, Gary J. Sheridan
Pages: 4338 - 4357
Abstract: This study represents a uniquely high‐resolution observation of postwildfire runoff and erosion from dry forested uplands of SE Australia. We monitored runoff and sediment load, and temporal changes in soil surface properties from two (0.2–0.3 ha) dry forested catchments burned during the 2009 Black Saturday wildfire. Event‐based surface runoff to rainfall ratios approached 0.45 during the first year postwildfire, compared to reported values
- Water temperature controls in low arctic rivers
- Authors: Tyler V. King; Bethany T. Neilson, Levi D. Overbeck, Douglas L. Kane
Pages: 4358 - 4376
Abstract: Understanding the dynamics of heat transfer mechanisms is critical for forecasting the effects of climate change on arctic river temperatures. Climate influences on arctic river temperatures can be particularly important due to corresponding effects on nutrient dynamics and ecological responses. It was hypothesized that the same heat and mass fluxes affect arctic and temperate rivers, but that relative importance and variability over time and space differ. Through data collection and application of a river temperature model that accounts for the primary heat fluxes relevant in temperate climates, heat fluxes were estimated for a large arctic basin over wide ranges of hydrologic conditions. Heat flux influences similar to temperate systems included dominant shortwave radiation, shifts from positive to negative sensible heat flux with distance downstream, and greater influences of lateral inflows in the headwater region. Heat fluxes that differed from many temperate systems included consistently negative net longwave radiation and small average latent heat fluxes. Radiative heat fluxes comprised 88% of total absolute heat flux while all other heat fluxes contributed less than 5% on average. Periodic significance was seen for lateral inflows (up to 26%) and latent heat flux (up to 18%) in the lower and higher stream order portions of the watershed, respectively. Evenly distributed lateral inflows from large scale flow differencing and temperatures from representative tributaries provided a data efficient method for estimating the associated heat loads. Poor model performance under low flows demonstrated need for further testing and data collection to support the inclusion of additional heat fluxes.
- Mapping permeability in low‐resolution micro‐CT images: A
multiscale statistical approach
- Authors: Pieter W. S. K. Botha; Adrian P. Sheppard
Pages: 4377 - 4398
Abstract: We investigate the possibility of predicting permeability in low‐resolution X‐ray microcomputed tomography (µCT). Lower‐resolution whole core images give greater sample coverage and are therefore more representative of heterogeneous systems; however, the lower resolution causes connecting pore throats to be represented by intermediate gray scale values and limits information on pore system geometry, rendering such images inadequate for direct permeability simulation. We present an imaging and computation workflow aimed at predicting absolute permeability for sample volumes that are too large to allow direct computation. The workflow involves computing permeability from high‐resolution µCT images, along with a series of rock characteristics (notably open pore fraction, pore size, and formation factor) from spatially registered low‐resolution images. Multiple linear regression models correlating permeability to rock characteristics provide a means of predicting and mapping permeability variations in larger scale low‐resolution images. Results show excellent agreement between permeability predictions made from 16 and 64 µm/voxel images of 25 mm diameter 80 mm tall core samples of heterogeneous sandstone for which 5 µm/voxel resolution is required to compute permeability directly. The statistical model used at the lowest resolution of 64 µm/voxel (similar to typical whole core image resolutions) includes open pore fraction and formation factor as predictor characteristics. Although binarized images at this resolution do not completely capture the pore system, we infer that these characteristics implicitly contain information about the critical fluid flow pathways. Three‐dimensional permeability mapping in larger‐scale lower resolution images by means of statistical predictions provides input data for subsequent permeability upscaling and the computation of effective permeability at the core scale.
- Testing the ability of a semidistributed hydrological model to simulate
- Authors: S. G. Mengistu; C. Spence
Pages: 4399 - 4415
Abstract: A dry climate, the prevalence of small depressions, and the lack of a well‐developed drainage network are characteristics of environments with extremely variable contributing areas to runoff. These types of regions arguably present the greatest challenge to properly understanding catchment streamflow generation processes. Previous studies have shown that contributing area dynamics are important for streamflow response, but the nature of the relationship between the two is not typically understood. Furthermore, it is not often tested how well hydrological models simulate contributing area. In this study, the ability of a semidistributed hydrological model, the PDMROF configuration of Environment Canada's MESH model, was tested to determine if it could simulate contributing area. The study focused on the St. Denis Creek watershed in central Saskatchewan, Canada, which with its considerable topographic depressions, exhibits wide variation in contributing area, making it ideal for this type of investigation. MESH‐PDMROF was able to replicate contributing area derived independently from satellite imagery. Daily model simulations revealed a hysteretic relationship between contributing area and streamflow not apparent from the less frequent remote sensing observations. This exercise revealed that contributing area extent can be simulated by a semi‐distributed hydrological model with a scheme that assumes storage capacity distribution can be represented with a probability function. However, further investigation is needed to determine if it can adequately represent the complex relationship between streamflow and contributing area that is such a key signature of catchment behavior.
- Plant transpiration and groundwater dynamics in water‐limited
climates: Impacts of hydraulic redistribution
- Pages: 4416 - 4437
Abstract: The role of groundwater in sustaining plant transpiration constitutes an important but not well‐understood aspect of the interactions between groundwater, vegetation, the land surface, and the atmosphere. The effect of the hydraulic redistribution (HR) process by plant roots on the interplay between plant transpiration and groundwater dynamics under water‐limited climates is investigated by using the Variable Infiltration Capacity Plus (VIC+) land surface model. Numerical experiments, with or without explicitly considering HR, are conducted on soil columns over a range of groundwater table depths (GWTDs) under different vegetative land covers, soil types, and precipitation conditions. When HR is not included, this study obtains transpiration–GWTD relationships consistent with those from watershed studies that do not include HR. When HR is included, the transpiration–GWTD relationships are modified. The modification introduced by HR is manifested in the soil moisture of the root zone. The mechanism of HR is explained by detailing the roles of the hydraulically redistributed water, the upward diffusion of soil water, and the daytime root uptake. We have found that HR is particularly important in water‐limited climates under which plants have high transpiration demand. At the beginning stage of a dry period, HR modulates the severe impacts that climate has on plant transpiration. Only after a prolonged dry period, impacts of HR are lessened when the groundwater table drops below the depth of water uptake by roots and are diminished when plant transpiration is decoupled from groundwater dynamics.
- Analytical solution to transient Richards' equation with realistic water
profiles for vertical infiltration and parameter estimation
- Authors: Mohamed Hayek
Pages: 4438 - 4457
Abstract: A general analytical model for one‐dimensional transient vertical infiltration is presented. The model is based on a combination of the Brooks and Corey soil water retention function and a generalized hydraulic conductivity function. This leads to power law diffusivity and convective term for which the exponents are functions of the inverse of the pore size distribution index. Accordingly, the proposed analytical solution covers many existing realistic models in the literature. The general form of the analytical solution is simple and it expresses implicitly the depth as function of water content and time. It can be used to model infiltration through semi‐infinite dry soils with prescribed water content or flux boundary conditions. Some mathematical expressions of practical importance are also derived. The general form solution is useful for comparison between models, validation of numerical solutions and for better understanding the effect of some hydraulic parameters. Based on the analytical expression, a complete inverse procedure which allows the estimation of the hydraulic parameters from water content measurements is presented.