- 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.
- Analytical solution to transient Richards' equation with realistic water
profiles for vertical infiltration and parameter estimation
- Authors: Mohamed Hayek
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.
- 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.
- Determining soil moisture and soil properties in vegetated areas by
assimilating soil temperatures
- 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
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.
- 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
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.
- 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.
- Effects of aridity in controlling the magnitude of runoff and erosion
- Authors: Philip J Noske; Patrick NJ Lane, Petter Nyman, Gary J Sheridan
Abstract: This study represents a uniquely high resolution observation of post‐wildfire 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 post‐wildfire, compared to reported values
- Spectral‐induced polarization measurements on sieved sands and the
relationship to permeability
- Authors: Sheen Joseph; Malcolm Ingham, Gideon Gouws
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. This article is protected by copyright. All rights reserved.
- Plant transpiration and groundwater dynamics in water‐limited
climates: Impacts of hydraulic redistribution
- Abstract: The role of groundwater in sustaining plant transpiration constitutes an important but not well understood aspect of the interactions between groundwater, the land surface, vegetation 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. This article is protected by copyright. All rights reserved.
- Assimilation of gridded terrestrial water storage observations from GRACE
into a Land Surface Model
- Abstract: Observations of terrestrial water storage (TWS) from the Gravity Recovery and Climate Experiment (GRACE) satellite mission have a coarse resolution in time (monthly) and space (roughly 150,000 km2 at mid‐latitudes) and vertically integrate all water storage components over land, including soil moisture and groundwater. Data assimilation can be used to horizontally downscale and vertically partition GRACE‐TWS observations. This work proposes a variant of existing ensemble‐based GRACE‐TWS data assimilation schemes. The new algorithm differs in how the analysis increments are computed and applied. Existing schemes correlate the uncertainty in the modeled monthly TWS estimates with errors in the soil moisture profile state variables at a single instant in the month and then apply the increment either at the end of the month or gradually throughout the month. The proposed new scheme first computes increments for each day of the month and then applies the average of those increments at the beginning of the month. The new scheme therefore better reflects sub‐monthly variations in TWS errors. The new and existing schemes are investigated here using gridded GRACE‐TWS observations. The assimilation results are validated at the monthly time‐scale, using in situ measurements of groundwater depth and soil moisture across the US. The new assimilation scheme yields improved (although not in a statistically significant sense) skill metrics for groundwater compared to the open‐loop (no assimilation) simulations and compared to the existing assimilation schemes. A smaller impact is seen for surface and root‐zone soil moisture, which have a shorter memory and receive smaller increments from TWS assimilation than groundwater. These results motivate future efforts to combine GRACE‐TWS observations with observations that are more sensitive to surface soil moisture, such as L‐band brightness temperature observations from Soil Moisture Ocean Salinity (SMOS) or Soil Moisture Active Passive (SMAP). Finally, we demonstrate that the scaling parameters that are applied to the GRACE observations prior to assimilation should be consistent with the land surface model that is used within the assimilation system. 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
- Water temperature controls in low arctic rivers
- Authors: Tyler V. King; Bethany T. Neilson, Levi D. Overbeck, Douglas L. Kane
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 (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 inclusion of additional heat fluxes. 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”
- Authors: Shawn Schottler; Jason Ulrich, Daniel Engstrom
- Wavelet‐based time series bootstrap model for multidecadal
streamflow simulation using climate indicators
- Authors: Solomon Tassew Erkyihun; Balaji Rajagopalan, Edith Zagona, Upmanu Lall, Kenneth Nowak
Abstract: A model to generate stochastic streamflow projections conditioned on quasi‐oscillatory climate indices such as Pacific Decadal Oscillation (PDO) and Atlantic Multi‐decadal Oscillation (AMO) is presented. Recognizing that each climate index has underlying band‐limited components that contribute most of the energy of the signals, we first pursue a wavelet decomposition of the signals to identify and reconstruct these features from annually resolved historical data and proxy based paleo‐reconstructions of each climate index covering the period from 1650 to 2012. A K‐Nearest Neighbor block bootstrap approach is then developed to simulate the total signal of each of these climate index series while preserving its time‐frequency structure and marginal distributions. Finally, given the simulated climate signal time series, a K‐Nearest Neighbor bootstrap is used to simulate annual streamflow series conditional on the joint state space defined by the simulated climate index for each year. We demonstrate this method by applying it to simulation of streamflow at Lees Ferry gauge on the Colorado River using indices of two large scale climate forcings: Pacific Decadal Oscillation (PDO) and Atlantic Multi‐decadal Oscillation (AMO), which are known to modulate the Colorado River Basin (CRB) hydrology at multi‐decadal time scales. Skill in stochastic simulation of multi‐decadal projections of flow using this approach is demonstrated. This article is protected by copyright. All rights reserved.
- Tracking tracer motion in a 4‐D electrical resistivity tomography
- Authors: W. O. C. Ward; P. B. Wilkinson, J. E. Chambers, H. Nilsson, O. Kuras, L. Bai
Abstract: A new framework for automatically tracking subsurface tracers in electrical resistivity tomography (ERT) monitoring images is presented. Using computer vision and Bayesian inference techniques, in the form of a Kalman filter, the trajectory of a subsurface tracer is monitored by predicting and updating a state model representing its movements. Observations for the Kalman filter are gathered using the maximally stable volumes algorithm, which is used to dynamically threshold local regions of an ERT image sequence to detect the tracer at each time‐step. The application of the framework to the results of 2‐D and 3‐D tracer monitoring experiments show that the proposed method is effective for detecting and tracking tracer plumes in ERT images in the presence of noise, without intermediate manual intervention. This article is protected by copyright. All rights reserved.
- Using stochastic dual dynamic programming in problems with multiple
- Abstract: Stochastic dual dynamic programming (SDDP) is one of the few algorithmic solutions available to optimize large‐scale water resources systems while explicitly considering uncertainty. This paper explores the consequences of, and proposes a solution to, the existence of multiple near‐optimal solutions (MNOS) when using SDDP for mid‐ or long‐term river basin management. These issues arise when the optimization problem cannot be properly parametrized due to poorly defined and/or unavailable data sets. This work shows that when MNOS exists, 1) SDDP explores more than one solution trajectory in the same run, suggesting different decisions in distinct simulation years even for the same point in the state‐space, and 2) SDDP is shown to be very sensitive to even minimal variations of the problem setting, e.g. initial conditions ‐ we call this “algorithmic chaos”. Results that exhibit such sensitivity are difficult to interpret. This work proposes a re‐optimization method, which simulates system decisions by periodically applying cuts from one given year from the SDDP run. Simulation results obtained through this re‐optimization approach are steady‐state solutions, meaning that their probability distributions are stable from year to year. This article is protected by copyright. All rights reserved.
- Acoustic mapping velocimetry
- Authors: M. Muste; S. Baranya, R. Tsubaki, D. Kim, H. Ho, H. Tsai, D. Law
Abstract: Knowledge of sediment dynamics in rivers is of great importance for various practical purposes. Despite its high relevance in riverine environment processes, the monitoring of sediment rates remains a major and challenging task for both suspended and bedload estimation. While the measurement of suspended load is currently an active area of testing with non‐intrusive technologies (optical and acoustic), bedload measurement does not mark a similar progress. This paper describes an innovative combination of measurement techniques and analysis protocols that establishes the proof‐of‐concept for a promising technique, labeled herein Acoustic Mapping Velocimetry (AMV). The technique estimates bedload rates in rivers developing bedforms using a non‐intrusive measurements approach.
The raw information for AMV is collected with acoustic multi‐beam technology that in turn provides maps of the bathymetry over longitudinal swaths. As long as the acoustic maps can be acquired relatively quickly and the repetition rate for the mapping is commensurate with the movement of the bedforms, successive acoustic maps capture the progression of the bedform movement. Two‐dimensional velocity maps associated with the bedform migration are obtained by implementing algorithms typically used in particle image velocimetry to acoustic maps converted in gray‐level images. Furthermore, use of the obtained acoustic and velocity maps in conjunction with analytical formulations (e.g., Exner equation) enables estimation of multi‐directional bedload rates over the whole imaged area. This paper presents a validation study of the AMV technique using a set of laboratory experiments. This article is protected by copyright. All rights reserved.
- Mapping permeability in low‐resolution micro‐CT images: A
multiscale statistical approach
- Authors: Pieter W.S.K. Botha; Adrian P. Sheppard
Abstract: We investigate the possibility of predicting permeability in low‐resolution x‐ray micro‐computed 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. 3D permeability mapping in larger scale lower resolution images by means of statistical predictions lay the groundwork for permeability upscaling and the computation of effective permeability at the core scale. This article is protected by copyright. All rights reserved.
- Temporal and spatial dynamics of large lake hypoxia: Integrating
statistical and three‐dimensional dynamic models to enhance lake
- Authors: Serghei A. Bocaniov; Donald Scavia
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. This article is protected by copyright. All rights reserved.
- Testing the ability of a semidistributed hydrological model to simulate
- Authors: SG. Mengistu; C. Spence
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 semi‐distributed 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 behaviour. This article is protected by copyright. All rights reserved.
- Capture zone delineation methodology based on the maximum
concentration—Preventative groundwater well protection areas for
heat exchange fluid mixtures
- Authors: Jarkko Okkonen; Roseanna M. Neupauer
Abstract: Capture zones of water supply wells are most often delineated based on travel times of water or solute to the well, with the assumption that if the travel time is sufficiently large, the concentration of chemical at the well will not exceed the drinking water standards. In many situations, the likely source concentrations or release masses of contamination from the potential sources are unknown; therefore, the exact concentration at the well cannot be determined. In situations in which the source mass can be estimated with some accuracy, the delineation of the capture zone should be based on the maximum chemical concentration that can be expected at the well, rather than on an arbitrary travel time. We present a new capture zone delineation methodology that is based on this maximum chemical concentration. The method delineates capture zones by solving the adjoint of the advection‐dispersion‐reaction equation and relating the adjoint state and the known release mass to the expected chemical concentration at the well. We demonstrate the use of this method through a case study in which soil heat exchange systems are potential sources of contamination. The heat exchange fluid mixtures contain known fluid volumes and chemical concentrations; thus, in the event of a release, the release mass of the chemical is known. We also demonstrate the use of a concentration basis in quantifying other measures of well vulnerability including exposure time and time to exceed a predefined threshold concentration at the well. This article is protected by copyright. All rights reserved.
- Sources and interpretation of channel complexity in forested subalpine
streams of the Southern Rocky Mountains
- Authors: Bridget Livers; Ellen Wohl
Abstract: We evaluate correlations between stream geomorphic complexity and characteristics of the adjacent riparian forest, valley geometry, and land use history in forested subalpine streams of the Colorado Front Range. Measures of geomorphic complexity focus on cross‐sectional, planform, and instream wood piece and logjam variables. We categorize adjacent riparian forests as old‐growth unmanaged forest (OU), younger unmanaged forest (YU), and younger managed forest (YM), and valley geometry as laterally confined, partly confined, or unconfined. Significant differences in geomorphic stream complexity between OU, YU, and YM result primarily from differences in wood pieces and logjams, and these differences correlate strongly with pool volume and organic matter storage. Significant differences in planform and cross‐sectional complexity correlate more strongly with valley geometry, but do not explain as much of the observed variability in complexity between streams as do the wood variables. Unconfined OU streams have the largest wood loads and the greatest complexity, whereas legacy effects of logging, tie‐drives, and channel simplification create lower complexity in YM streams, even relative to YU streams flowing through similarly‐aged forest. We find that management history of riparian forests exerts the strongest control on reduced functional stream channel complexity, regardless of riparian forest stand age. This article is protected by copyright. All rights reserved.
- Scale invariance of subsurface flow patterns and its limitation
- Authors: S. Hergarten; G. Winkler, S. Birk
Abstract: Preferential flow patterns in the subsurface are of great importance for the availability and the quality of water resources. However, knowledge of their spatial structure is still behind their importance, so that understanding the nature of preferential flow patterns is a major issue in subsurface hydrology. Comparing the statistics of river catchment sizes and spring discharges we found that the morphology of preferential subsurface flow patterns is probably scale‐invariant and similar to that of dendritic river networks. This result is not limited to karstic aquifers where the occurrence of dendritic structures has been known at least qualitatively for a long time. The scale invariance even seems to be independent of the lithology of the aquifer. However, scale invariance of river patterns seems to be only limited by the continental scale, while scale invariance of subsurface flow patterns breaks down at much smaller scales. The upper limit of scale invariance in subsurface flow patterns is highly variable. We found a range from thousands of square kilometers for limestone aquifers down to less than one square kilometer in the weathered zone and debris accumulations of crystalline rocks. This article is protected by copyright. All rights reserved.
- Hazard function analysis for flood planning under nonstationarity
- Authors: Laura K. Read; Richard M. Vogel
Abstract: The field of hazard function analysis (HFA) involves a probabilistic assessment of the ‘time to failure' or ‘return period', T, of an event of interest. HFA is used in epidemiology, manufacturing, medicine, actuarial statistics, reliability engineering, economics and elsewhere. For a stationary process, the probability distribution function (pdf) of the return period always follows an exponential distribution, the same is not true for nonstationary processes. When the process of interest, X, exhibits nonstationary behavior, HFA can provide a complementary approach to risk analysis with analytical tools particularly useful for hydrological applications. After a general introduction to HFA we describe a new mathematical linkage between the magnitude of the flood event, X, and its return period, T, for nonstationary processes. We derive the probabilistic properties of T for a nonstationary 1‐parameter exponential model of X, and then use both Monte‐Carlo simulation and HFA to generalize the behavior of T when X arises from a nonstationary 2‐parameter lognormal distribution. For this case, our findings suggest that a 2‐parameter Weibull distribution provides a reasonable approximation for the pdf of T. We document how HFA can provide an alternative approach to characterizing the probabilistic properties of both nonstationary flood series and the resulting pdf of T. This article is protected by copyright. All rights reserved.
- Comment on “Is unique scaling of aquifer macrodispersivity supported
by field data?” by A. Zech et al.
- Authors: Shlomo P. Neuman
- Reply to comment by S. Neuman on “Is unique scaling of aquifer
macrodispersivity supported by field data?”
- Authors: A. Zech; S. Attinger, V. Cvetkovic, G. Dagan, P. Dietrich, A. Fiori, Y. Rubin, G. Teutsch
- Revisiting hydraulic hysteresis based on long term monitoring of hydraulic
states in lysimeters
- Abstract: Hysteretic processes have been recognized for decades as an important characteristic of soil hydraulic behaviour. Several studies confirmed that wetting and drying periods cannot be described by a simple functional relationship, and that some non‐equilibrium of the water retention characteristics has to be taken into account. A large number of models describing the hysteresis of the soil water retention characteristic were successfully tested on soil cores under controlled laboratory conditions. However, its relevance under field conditions under natural forcings has rarely been investigated. In practice, the modeling of field soils usually neglects the hysteretic nature of soil hydraulic properties. In this study, long‐term observations of water content and matric potential in lysimeters of the lysimeter network TERENO‐SoilCan are presented, clearly demonstrating the hysteretic behavior of field soils. We propose a classification into three categories related to different time scales. Based on synthetic and long term monitoring data, three different models of hysteresis (Mualem , Parker and Lenhard , Poulovassilis and Kargas ) were applied to data sets showing different degrees of hysteresis. We found no single model to be superior to the others. The model ranking depended on the degree of hysteresis. All models were able to reflect the general structure of hysteresis in most cases but failed to reproduce the detailed trajectories of state variables especially under highly transient conditions. As an important result we found that the temporal dynamics of wetting and drying significantly affects these trajectories which should be accounted for in future model concepts. This article is protected by copyright. All rights reserved.
- High‐resolution modeling of coastal freshwater discharge and glacier
mass balance in the Gulf of Alaska watershed
- Authors: J.P. Beamer; D.F. Hill, A. Arendt, G.E. Liston
Abstract: A comprehensive study of the Gulf of Alaska (GOA) drainage basin was carried out to improve understanding of the coastal freshwater discharge (FWD) and glacier volume loss (GVL). Hydrologic processes during the period 1980‐2014 were modeled using a suite of physically based, spatially distributed weather, energy‐balance snow/ice melt, soil water balance, and runoff routing models at a high resolution (1 km horizontal grid; daily time step). Meteorological forcing was provided by the North American Regional Reanalysis (NARR), Modern Era Retrospective Analysis for Research and Applications (MERRA), and Climate Forecast System Reanalysis (CFSR) datasets. Streamflow and glacier mass balance modeled using MERRA and CFSR compared well with observations in four watersheds used for calibration in the study domain. However, only CFSR produced regional seasonal and long term trends in water balance that compared favorably with independent Gravity Recovery and Climate Experiment (GRACE) and airborne altimetry data. Mean annual runoff using CFSR was 760 km3 yr−1, 8% of which was derived from the long‐term removal of stored water from glaciers (glacier volume loss). The annual runoff from CFSR was partitioned into 63% snowmelt, 17% glacier ice melt, and 20% rainfall. Glacier runoff, taken as the sum of rainfall, snow and ice melt occurring each season on glacier surfaces, was 38% of the total seasonal runoff, with the remaining runoff sourced from non‐glacier surfaces. Our simulations suggests that existing GRACE solutions, previously reported to represent glacier mass balance alone, are actually measuring the full water budget of land and ice surfaces. This article is protected by copyright. All rights reserved.
- 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.
- Acceleration of groundwater remediation by deep sweeps and vortex
ejections induced by rapidly pulsed pumping
- Authors: David M. Kahler; Zbigniew J. Kabala
Abstract: One key limiting factor to groundwater remediation is contaminant sequestered in pores whose contents do not mix well with the bulk flow. Mixing between well‐connected (pores whose volume is flushed as water flows through the aquifer) and poorly‐connected pores (pores whose volume does not exchange readily when water flows through the aquifer) is of primary concern. Under steady flow, contaminants are effectively trapped in the poorly‐connected pores and are transferred only by molecular diffusion. This slow mixing process between pore types is a bottleneck to remediation. We present a novel rapidly pulsed pumping method that increases the mixing between these pore types. We do it in the context of pump‐and‐treat remediation because it is the most common remediation practice. In rapidly pulsed pumping, the increase in flow causes a deep sweep, which pushes the flow into poorly‐connected pores and sweeps out sequestered contaminants. The decrease in flow causes a vortex ejection, which causes the vortex within the poorly‐connected pore to emerge with contaminant. These actions are modeled with computational fluid mechanics to elucidate the individual mechanisms and determine how they function and interact. Cleanup of single and multiple poorly‐connected pore systems were simulated and show the acceleration possible. This technique can decrease the time and cost needed to remediate contaminated aquifers, which in the United States has been estimated to exceed $1 trillion. Since our rapidly pulsed pumping method enhances mixing between well‐ and poorly‐connected pores, it can be applied to other remediation schemes such as in situ methods. This article is protected by copyright. All rights reserved.
- Inferring river bathymetry via Image‐to‐Depth Quantile
- Authors: Carl J. Legleiter
Abstract: Conventional, regression‐based methods of inferring depth from passive optical image data undermine the advantages of remote sensing for characterizing river systems. This study introduces and evaluates a more flexible framework, Image‐to‐Depth Quantile Transformation (IDQT), that involves linking the frequency distribution of pixel values to that of depth. In addition, a new image processing workflow involving deep water correction and Minimum Noise Fraction (MNF) transformation can reduce a hyperspectral data set to a single variable related to depth and thus suitable for input to IDQT. Applied to a gravel‐bed river, IDQT avoided negative depth estimates along channel margins and under‐predictions of pool depth. Depth retrieval accuracy (R2 = 0.79) and precision (0.27 m) were comparable to an established band ratio‐based method, although a small shallow bias (0.04 m) was observed. Several ways of specifying distributions of pixel values and depths were evaluated but had negligible impact on the resulting depth estimates, implying that IDQT was robust to these implementation details. In essence, IDQT uses frequency distributions of pixel values and depths to achieve an aspatial calibration; the image itself provides information on the spatial distribution of depths. The approach thus reduces sensitivity to misalignment between field and image data sets and allows greater flexibility in the timing of field data collection relative to image acquisition, a significant advantage in dynamic channels. IDQT also creates new possibilities for depth retrieval in the absence of field data if a model could be used to predict the distribution of depths within a reach. This article is protected by copyright. All rights reserved.
- Hydrological drivers of record‐setting water level rise on Earth's
largest lake system
- Authors: A.D. Gronewold; J. Bruxer, D. Durnford, J.P. Smith, A.H. Clites, F. Seglenieks, S.S. Qian, T.S. Hunter, V. Fortin
Abstract: Between January 2013 and December 2014, water levels on Lake Superior and Lake Michigan‐Huron, the two largest lakes on Earth by surface area, rose at the highest rate ever recorded for a two‐year period beginning in January and ending in December of the following year. This historic event coincided with below‐average air temperatures and extensive winter ice cover across the Great Lakes. It also brought an end to a 15‐year period of persistently below‐average water levels on Lakes Superior and Michigan‐Huron that included several months of record‐low water levels. To differentiate hydrological drivers behind the recent water level rise, we developed a Bayesian Markov chain Monte Carlo (MCMC) routine for inferring historical estimates of the major components of each lake's water budget. Our results indicate that, in 2013, the water level rise on Lake Superior was driven by increased spring runoff and over‐lake precipitation. In 2014, reduced over‐lake evaporation played a more significant role in Lake Superior's water level rise. The water level rise on Lake Michigan‐Huron in 2013 was also due to above‐average spring runoff and persistent over‐lake precipitation, while in 2014, it was due to a rare combination of below‐average evaporation, above‐average runoff and precipitation, and very high inflow rates from Lake Superior through the St. Marys River. We expect, in future research, to apply our new framework across the other Laurentian1 Great Lakes, and to Earth's other large freshwater basins as well. This article is protected by copyright. All rights reserved.
- Can PDSI inform extreme precipitation?: An exploration with a 500 year
long paleoclimate reconstruction over the United States
- Authors: Scott Steinschneider; Michelle Ho, Edward R. Cook, Upmanu Lall
Abstract: This study explores whether it is possible to reconstruct the frequency of extreme precipitation occurrence across the contiguous United States (CONUS) using the Living Blended Drought Atlas (LBDA), a 500‐year paleoclimate reconstruction of the summer (June‐August) Palmer Drought Severity Index (PDSI). We first identify regions of the country where the LBDA may reflect the occurrence of extremes based on their seasonality and contribution to total annual moisture delivery. Correlation measures are used to assess the relationship between the frequencies of extreme precipitation occurrence and both the instrumental monthly PDSI and the annual LBDA estimated PDSI. Extreme precipitation is found to account for a large portion of total precipitation west of the Mississippi River and clusters in particular seasons (winter and summer), supporting a strong relationship with the LBDA without much information loss from the instrumental PDSI data. Dimension reduction techniques are used to explore the joint spatiotemporal structure of extreme precipitation occurrence and LBDA across the country. The primary modes of variability of the LBDA and extreme precipitation occurrence relate remarkably well for a region centered over the Southwest that exhibits an ENSO‐like time‐frequency structure. Generalized linear models (GLMs) are used to demonstrate the feasibility of reconstructing the annual extreme precipitation frequency over the 500‐year pre‐historic record at two sites in the Southwest and Southern Plains. GLM‐based reconstructions show a high degree of structured variability in the likelihood of extreme precipitation occurrences over the pre‐historic record. This article is protected by copyright. All rights reserved.
- Impact of kinetic mass transfer on free convection in a porous medium
- Authors: Chunhui Lu; Liangsheng Shi, Yiming Chen, Yueqing Xie, Craig T. Simmons
Abstract: We investigate kinetic mass transfer effects on unstable density‐driven flow and transport processes by numerical simulations of a modified Elder problem. The first‐order dual‐domain mass transfer model coupled with a variable‐density‐flow model is employed to describe transport behaviour in porous media. Results show that in comparison to the no‐mass‐transfer case, a higher degree of instability and more unstable system is developed in the mass transfer case due to the reduced effective porosity and correspondingly a larger Rayleigh number (assuming permeability is independent on the mobile porosity). Given a constant total porosity, the magnitude of capacity ratio (i.e. immobile porosity/mobile porosity) controls the macroscopic plume profile in the mobile domain, while the magnitude of mass transfer timescale (i.e., the reciprocal of the mass transfer rate coefficient) dominates its evolution rate. The magnitude of capacity ratio plays an important role on the mechanism driving the mass flux into the aquifer system. Specifically, for a small capacity ratio, solute loading is dominated by the density‐driven transport, while with increasing capacity ratio local mass transfer dominated solute loading may occur at later times. At significantly large times, however, both mechanisms contribute comparably to solute loading. Sherwood Number could be a non‐monotonic function of mass transfer timescale due to complicated interactions of solute between source zone, mobile zone and immobile zone in the top boundary layer, resulting in accordingly a similar behaviour of the total mass. The initial assessment provides important insights into unstable density‐driven flow and transport in the presence of kinetic mass transfer. 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.
- Utilizing the state of ENSO as a means for season‐ahead predictor
- Authors: Brian G. Zimmerman; Daniel J. Vimont, Paul J. Block
Abstract: This paper introduces the Nino Index Phase Analysis (NIPA) framework for forecasting hydroclimatic variables on a seasonal timescale. Antecedent Sea Surface Temperatures (SSTs) are commonly used in statistical predictive frameworks for seasonal forecasting, however the typical approach of evaluating all the years on record in one bin (‘phase') does not often provide the level of skill required by decision makers. For many locations around the world, the most influential climate signal on the seasonal timescale is the El Nino Southern Oscillation (ENSO), and there are various indices used to capture the state of ENSO and provide this information. NIPA utilizes the state of ENSO to classify the years of record into four phases, operating under the hypothesis that ENSO itself is affecting the ‘mean state' of the atmospheric ‐ oceanic system, and relevant teleconnections depend on and must be selected within these mean states. A case study focused on spring precipitation over the Lower Colorado River Basin (LCRB) in Texas is chosen to illustrate NIPA's potential. Results show that correlations between wintertime SST fields and spring precipitation in the LCRB improve from 0.21 to 0.47 for the typical ‘one phase' and the NIPA ‘four phase' approach, respectively. Even greater improvements are seen across tercile‐based skill scores such as the Heidke Hit Skill Score and Ranked Probability Skill Score; skill is particularly strong for years exhibiting extreme wet or dry conditions. It also outperforms the North American Multi‐Model Ensemble predictions across the LCRB for the selected seasons. This is encouraging as improved predictability through NIPA may translate to better decision‐making for water managers. This article is protected by copyright. All rights reserved.
- Terrestrial freshwater lenses in stable riverine settings: Occurrence and
- Authors: Adrian D. Werner; Tariq Laattoe
Abstract: Rivers in arid and semi‐arid regions often traverse saline aquifers, creating buoyant freshwater lenses in the adjoining riparian and floodplain zones. The occurrence of freshwater lenses where the river is otherwise gaining saline groundwater appears counterintuitive, given that both hydraulic and density forces act towards the river. In this paper, an analytical solution is presented that defines the extent of a stable, sharp‐interface terrestrial freshwater lens (in cross section) in a riverine environment that otherwise contains saline groundwater moving towards the river. The method is analogous to the situation of an island freshwater lens, except in the riverine setting, the saltwater is mobile and the lens is assumed to be stagnant. The solution characterizes the primary controlling factors of riverine freshwater lenses, which are larger for situations involving lower hydraulic conductivities and rates of saltwater discharge to the river. Deeper aquifers, more transmissive riverbeds, and larger freshwater‐saltwater density differences produce more extensive lenses. The analytical solution predicts the parameter combinations that preclude the occurrence of freshwater lenses. The utility of the solution as a screening method to predict the occurrence of terrestrial freshwater lenses is demonstrated by application to parameter ranges typical of the South Australian portion of the River Murray, where freshwater lenses occur in only a portion of the neighboring floodplains. Despite assumptions of equilibrium conditions and a sharp freshwater‐saltwater interface, the solution for predicting the occurrence of riverine freshwater lenses presented in this study has immediate relevance to the management of floodplains in which freshwater lenses are integral to biophysical conditions. This article is protected by copyright. All rights reserved.
- A multivariate Copula‐based framework for dealing with Hazard
Scenarios and Failure Probabilities
- Authors: G. Salvadori; F. Durante, C. De Michele, M. Bernardi, L. Petrella
Abstract: This paper is of methodological nature, and deals with the foundations of Risk Assessment. Several international guidelines have recently recommended to select appropriate/relevant Hazard Scenarios in order to tame the consequences of (extreme) natural phenomena. In particular, the scenarios should be multivariate, i.e. they should take into account the fact that several variables, generally not independent, may be of interest. In this work, it is shown how a Hazard Scenario can be identified in terms of (i) a specific geometry and (ii) a suitable probability level. Several scenarios, as well as a Structural approach, are presented, and due comparisons are carried out. In addition, it is shown how the Hazard Scenario approach illustrated here is well suited to cope with the notion of Failure Probability, a tool traditionally used for design and risk assessment in engineering practice. All the results outlined throughout the work are based on the Copula Theory, which turns out to be a fundamental theoretical apparatus for doing multivariate risk assessment: formulas for the calculation of the probability of Hazard Scenarios in the general multi‐dimensional case (d ≥ 2) are derived, and worthy analytical relationships among the probabilities of occurrence of Hazard Scenarios are presented. In addition, the Extreme Value and Archimedean special cases are dealt with, relationships between dependence ordering and scenario levels are studied, and a counter‐example concerning Tail Dependence is shown. Suitable indications for the practical application of the techniques outlined in the work are given, and two case studies illustrate the procedures discussed in the paper. This article is protected by copyright. All rights reserved.
- A global analysis of the seaward salt marsh extent: The importance of
- Authors: Thorsten Balke; Martin Stock, Kai Jensen, Tjeerd J. Bouma, Michael Kleyer
Abstract: Despite the growing interest in ecosystem services provided by intertidal wetlands, we lack sufficient understanding of the processes that determine the seaward extent of salt marsh vegetation on tidal flats. With the present study, we aim to establish a globally valid demarcation between tidal flats and salt marsh vegetation in relation to tidal range.
By comparing results from a regional GIS study with a global literature search on the salt marsh‐ tidal flat border, we are able to define the global critical elevation, above which salt marsh plants can grow in the intertidal zone. Moreover, we calculate inundation characteristics from global tide gauge records to determine inundation duration and frequency at this predicted salt marsh ‐ tidal flat border depending on tidal range.
Our study shows that the height difference between the lowest elevation of salt‐marsh pioneer vegetation and mean high water increases logarithmically with tidal range when including macrotidal salt marshes. Hence, the potentially vegetated section of the tidal frame below mean high water does not proportionally increase with tidal range.
The data analysis suggests that inundation frequency rather than duration defines the global lower elevational limit of vascular salt marsh plants on tidal flats. This is critical information to better estimate sea level rise and coastal change effects on lateral marsh development. This article is protected by copyright. All rights reserved.
- Identification and quantification of redox and pH buffering processes in a
heterogeneous, low carbonate aquifer during managed aquifer recharge
- Authors: Simone Seibert; Olivier Atteia, S. Ursula Salmon, Adam Siade, Grant Douglas, Henning Prommer
Abstract: Managed aquifer recharge of aerobic water into deep aquifers often induces the oxidation of pyrite, which can lead to groundwater acidification and metal mobilisation. As circumneutral pH is often maintained by the dissolution of sedimentary calcite or high injectant alkalinity little attention is generally paid to potential alternative pH buffering processes. In contrast, this study analysed water quality evolution from a 2 year long groundwater replenishment trial in an anaerobic, mostly carbonate free aquifer. While injection of aerobic, very low salinity water triggered pyrite oxidation, the comprehensive field data showed that in many aquifer zones pH was buffered without substantial release of inorganic carbon. A numerical analysis was performed to test and evaluate different conceptual models and suggested that either proton buffering or the dissolution of aluminosilicates, or a combination thereof, can explain the observed, rapid buffering at locations where carbonates were absent. In contrast to many previous managed aquifer recharge (MAR) studies, the oxidation of sedimentary pyrite by nitrate was found to be of minor importance or negligible. The study also highlights that the depositional history of the aquifer, and the associated differences in mineralogy and geochemistry, need to be considered when estimating groundwater quality evolution during the injection of various water types for aquifer replenishment or other management purposes. This article is protected by copyright. All rights reserved.
- From conservative to reactive transport under diffusion‐controlled
- Abstract: We assess the possibility to use conservative transport information, such as that contained in transit time distributions, breakthrough curves and tracer tests, to predict non‐linear fluid‐rock interactions in fracture/matrix or mobile/immobile conditions. Reference simulated data are given by conservative and reactive transport simulations in several diffusive porosity structures differing by their topological organization. Reactions includes non‐linear kinetically‐controlled dissolution and desorption. Effective Multi‐Rate Mass Transfer models (MRMT) are calibrated solely on conservative transport information without pore topology information and provide concentration distributions on which effective reaction rates are estimated. Reference simulated reaction rates and effective reaction rates evaluated by MRMT are compared, as well as characteristic desorption and dissolution times. Although not exactly equal, these indicators remain very close whatever the porous structure, differing at most by 0.6% and 10% for desorption and dissolution. At shorter times, this close agreement arises from the fine characterization of the diffusive porosity close to the mobile zone that controls fast mobile‐diffusive exchanges. At intermediate to larger times, concentration gradients are strongly reduced by diffusion, and reactivity can be captured by a very limited number of rates. We conclude that effective models calibrated solely on conservative transport information like MRMT can accurately estimate monocomponent kinetically‐controlled non‐linear fluid‐rock interactions. Their relevance might extend to more advanced biogeochemical reactions because of the close representation of conservative concentration distributions, even by parsimonious models (e.g., MRMT with 3‐5 rates). We propose a methodology to estimate reactive transport from conservative transport in mobile‐immobile conditions. This article is protected by copyright. All rights reserved.
- Valuing year‐to‐go hydrologic forecast improvements for a
peaking hydropower system in the Sierra Nevada
- Authors: David E. Rheinheimer; Roger C. Bales, Carlos A. Oroza, Jay R. Lund, Joshua H. Viers
Abstract: We assessed the potential value of hydrologic forecasting improvements for a snow‐dominated high‐elevation hydropower system in the Sierra Nevada of California, using a hydropower optimization model. To mimic different forecasting skill levels for inflow time series, rest‐of‐year inflows from regression‐based forecasts were blended in different proportions with representative inflows from a spatially distributed hydrologic model. The statistical approach mimics the simpler, historical forecasting approach that is still widely used. Revenue was calculated using historical electricity prices, with perfect price foresight assumed. With current infrastructure and operations, perfect hydrologic forecasts increased annual hydropower revenue by $0.14 to $1.6 million, with lower values in dry years and higher values in wet years, or about $0.8 million (1.2%) on average, representing overall willingness‐to‐pay for perfect information. A second, sensitivity analysis found a wider range of annual revenue gain or loss using different skill levels in snow measurement in the regression‐based forecast, mimicking expected declines in skill as climate warms and historical snow measurements no longer represent current conditions. The value of perfect forecasts was insensitive to storage capacity for small and large reservoirs, relative to average inflow, and modestly sensitive to storage capacity with medium (current) reservoir storage. The value of forecasts is highly sensitive to powerhouse capacity, particularly for the range of capacities in the northern Sierra Nevada. The approach can be extended to multi‐reservoir, multi‐purpose systems to help guide investments in forecasting. This article is protected by copyright. All rights reserved.
- Confined aquifer head measurements and storage properties in the San Luis
Valley, Colorado, from spaceborne InSAR observations
- Abstract: Interferometric synthetic aperture radar (InSAR), a remote sensing technique for measuring centimeter‐level surface deformation, is used to estimate hydraulic head in the confined aquifer of the San Luis Valley (SLV), Colorado. Reconstructing head measurements from InSAR in agricultural regions can be difficult, as InSAR phase data are often decorrelated due to vegetation growth. Analysis of 17 L‐band ALOS PALSAR scenes, acquired between January, 2007 and March, 2011, demonstrates that comprehensive InSAR deformation measurements can be recovered over the vegetated groundwater basin with an improved processing strategy. Local skeletal storage coefficients and time delays between the head change and deformation are estimated through a joint InSAR‐well data analysis. InSAR subsidence estimates are transformed to head changes with finer temporal and spatial resolution than is possible using existing well records alone. Both InSAR and well data suggest that little long‐term water‐storage loss occurred in the SLV over the study period and that inelastic compaction was negligible. The seasonal head variations derived from InSAR are consistent with the existing well data at most locations where confined aquifer pumping activity dominates. Our results demonstrate the advantages of InSAR measurements for basin‐wide characterization of aquifer storage properties and groundwater levels over agricultural regions. This article is protected by copyright. All rights reserved.
- Water table‐dependent hydrological changes following peatland
forestry drainage and restoration: Analysis of restoration success
- Abstract: A before‐after‐control approach was used to analyze the impact of peatland restoration on hydrology, based on high temporal resolution water‐table (WT) data from 43 boreal peatlands representative of a south‐boreal to north‐boreal climate gradient. During the study, 24 forestry drained sites were restored and 19 pristine peatlands used as control sites. Different approaches were developed and used to analyze WT changes (mean WT position, WT fluctuation, WT hydrograph, recession, and storage characteristics). Restoration increased WT in most cases but particularly in spruce mires, followed by pine mires and fens. Before restoration, the WT fluctuation (WTF) was large, indicating peat temporary storage gain (SG). After restoration, the WT hydrograph recession limb slopes and SG coefficients (Rc) declined significantly. Drainage or restoration did not significantly affect mean diurnal WT fluctuations, used here as a proxy for evapotranspiration. Overall, the changes in WT characteristics following restoration indicated creation of favorable hydrological conditions for recovery of functional peatland ecosystems in previously degraded peatland sites. This was supported by calculation of bryophyte species abundance thresholds for WT. These results can be used to optimize restoration efforts in different peatland systems and as a qualitative conceptual basis for future restoration operations. This article is protected by copyright. All rights reserved.
- Global‐scale regionalization of hydrologic model parameters
- Authors: Hylke E. Beck; Albert I. J. M. van Dijk, Ad de Roo, Diego G. Miralles, Tim R. McVicar, Jaap Schellekens, L. Adrian Bruijnzeel
Abstract: Current state‐of‐the‐art models typically applied at continental to global scales (hereafter called macro‐scale) tend to use a priori parameters, resulting in suboptimal streamflow (Q) simulation. For the first time, a scheme for regionalization of model parameters at the global scale was developed. We used data from a diverse set of 1787 small‐to‐medium sized catchments (10‐10000 km2) and the simple conceptual HBV model to set up and test the scheme. Each catchment was calibrated against observed daily Q, after which 674 catchments with high calibration and validation scores, and thus presumably good‐quality observed Q and forcing data, were selected to serve as donor catchments. The calibrated parameter sets for the donors were subsequently transferred to 0.5° grid cells with similar climatic and physiographic characteristics, resulting in parameter maps for HBV with global coverage. For each grid cell, we used the ten most similar donor catchments, rather than the single most similar donor, and averaged the resulting simulated Q, which enhanced model performance. The 1113 catchments not used as donors were used to independently evaluate the scheme. The regionalized parameters outperformed spatially‐uniform (i.e., averaged calibrated) parameters for 79% of the evaluation catchments. Substantial improvements were evident for all major Köppen‐Geiger climate types and even for evaluation catchments > 5000 km distant from the donors. The median improvement was about half of the performance increase achieved through calibration. HBV with regionalized parameters outperformed nine state‐of‐the‐art macro‐scale models, suggesting these might also benefit from the new regionalization scheme. The produced HBV parameter maps including ancillary data are available via http://water.jrc.ec.europa.eu/HBV/. This article is protected by copyright. All rights reserved.
- A probabilistic formulation of bed load transport to include spatial
variability of flow and surface grain size distributions
- Authors: Angel Monsalve; Elowyn M. Yager, Jens M. Turowski, Dieter Rickenmann
Abstract: Bed load fluxes are typically calculated as a function of the reach averaged boundary shear stress and a representative bed grain size distribution. In steep, rough channels, heterogeneous bed surface texture and macro‐roughness elements cause significant local deviations from the mean shear stress but this variability is often omitted in bed load calculations. Here we present a probabilistic bed load transport formulation that explicitly includes local variations in the flow field and grain size distribution. The model is then tested in a 10% gradient stream, to evaluate its predictive capability and to explore relations between surface grain size sorting and boundary shear stress. The boundary shear stress field, calculated using a quasi‐3D hydraulic model, displayed substantial variability between patch classes, but the patch mean dimensionless shear stress varied inversely with patch median grain size. We developed an empirical relation between the applied shear stress on each patch class and the reach averaged shear stress and median grain size. Predicted sediment volumes using this relation in our bed load equation were as accurate as those using complete shear stress distributions and more accurate than current bed load transport equations. Our results suggest that when spatially variable grain size distributions (e.g. patches of sediment) are present they must be explicitly included in bed load transport calculations. Spatial variability in shear stress was relatively more important than grain size variations for sediment transport predictions. This article is protected by copyright. All rights reserved.
- Modeling soil evaporation efficiency in a range of soil and atmospheric
conditions using a meta‐analysis approach
- Abstract: A meta‐analysis data‐driven approach is developed to represent the soil evaporative efficiency (SEE) defined as the ratio of actual to potential soil evaporation. The new model is tested across a bare soil database composed of more than 30 sites around the world, a clay fraction range of 0.02‐0.56, a sand fraction range of 0.05‐0.92, and about 30,000 acquisition times. SEE is modeled using a soil resistance (rss) formulation based on surface soil moisture (θ) and two resistance parameters rss,ref and θefolding. The data‐driven approach aims to express both parameters as a function of observable data including meteorological forcing, cut‐off soil moisture value θ1/2 at which SEE=0.5, and first derivative of SEE at θ1/2, named Δθ1/2−1. An analytical relationship between (rss,ref; θefolding) and (θ1/2; Δθ1/2−1) is first built by running a soil energy balance model for two extreme conditions with rss = 0 and rss ∼ ∞ using meteorological forcing solely, and by approaching the middle point from the two (wet and dry) references points. Two different methods are then investigated to estimate the pair (θ1/2; Δθ1/2−1) either from the time series of SEE and θ observations for a given site, or using the soil texture information for all sites. The first method is based on an algorithm specifically designed to accomodate for strongly nonlinear SEE(θ) relationships and potentially large random deviations of observed SEE from the mean observed SEE(θ). The second method parameterizes θ1/2 as a multilinear regression of clay and sand percentages, and sets Δθ1/2−1 to a constant mean value for all sites. The new model significantly outperformed the evaporation modules of ISBA (Interaction Sol‐Biosphére‐Atmosphére), H‐TESSEL (Hydrology‐Tiled ECMWF Scheme for Surface Exchange over Land), and CLM (Community Land Model). It has potential for integration in various land‐surface schemes, and real calibration capabilities using combined thermal and microwave remote sensing data. 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.
- A critical evaluation of the Miller and Miller similar media theory for
application to natural soils
- Authors: Morteza Sadeghi; Bijan Ghahraman, Arthur W. Warrick, Markus Tuller, Scott B. Jones
Abstract: The Miller‐Miller similar media theory is widely applied to characterize the spatial variability of soil hydraulic properties. For a group of soils a distinct scaling factor is commonly assigned to each individual soil to coalesce the soil water characteristic and hydraulic conductivity functions to single curves. It is generally assumed that the Miller‐Miller theory is valid as long as soils are “similar” either with regard to their microscopic pore space geometry or the closely related macroscopic soil hydraulic functions. In this paper it is illustrated that similarity is not the sole required condition for validity of the Miller‐Miller theory. In addition, the interrelation between the soil water characteristic and the hydraulic conductivity functions considered for scaling need to be comparable. The interrelation is dependent not only on the pore space geometry, but also on solid‐liquid interactions. Hence similar interrelation cannot be concluded from similarity of microscopic pore space geometry. A dimensionless parameter termed the “joint scaling factor” was defined and applied to evaluate the soundness of the interrelation condition for 26 soils from the UNSODA database that were grouped into 6 classes of similar soils. Obtained results clearly reveal the crucial importance of the interrelation condition for the Miller‐Miller scaling theory, which has been hidden behind the “similarity” requirement, and contradict the general belief that Miller‐Miller scaling is valid as long as soils are “similar”. This article is protected by copyright. All rights reserved.
- 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
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, then we 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. This article is protected by copyright. All rights reserved.
- Statistical analysis of turbulent super‐streamwise vortices based on
observations of streaky structures near the free surface in the smooth
open channel flow
- Authors: Qiang Zhong; Qigang Chen, Hao Wang, Danxun Li, Xingkui Wang
Abstract: Long streamwise‐elongated high‐ and low‐speed streaks are repeatedly observed near the free surface in open channel flows in natural rivers and lab experiments. Super‐streamwise vortex model has been proposed to explain this widespread phenomenon for quite some time. However, statistical evidence of the existence of the super‐streamwise vortices as one type of coherent structures is still insufficient. Correlation and proper orthogonal decomposition (POD) analysis based on PIV experimental data in the streamwise‐spanwise plane near the free surface in a smooth open channel flow are employed to investigate this topic. Correlation analysis revealed that the streaky structures appear frequently near the free surface and their occurrence probability at any spanwise position is equal. The spanwise velocity fluctuation usually flows from low‐speed streaks toward high‐speed streaks. The average spanwise width and spacing between neighboring low (or high) speed streaks are approximately h and 2h respectively. POD analysis reveals that there are streaks with different spanwise width in the instantaneous flow fields. Typical streamwise rotational movement can be sketched out directly based on the results from statistical analyzes. Point‐by‐point analysis indicates that this pattern is consistent everywhere in the measurement window and is without any inhomogeneity in the spanwise direction, which reveals the essential difference between coherent structures and secondary flow cells. The pattern found by statistical analysis is consistent with the notion that the super‐streamwise vortices exist universally as one type of coherent structure in open channel flows. This article is protected by copyright. All rights reserved.
- Stage‐discharge rating curves based on satellite altimetry and
modeled discharge in the Amazon Basin
- Abstract: In this study, rating curves (RCs) were determined by applying satellite altimetry to a poorly gauged basin. This study demonstrates the synergistic application of remote sensing and watershed modeling to capture the dynamics and quantity of flow in the Amazon River Basin, respectively. Three major advancements for estimating basin‐scale patterns in river discharge are described. The first advancement is the preservation of the hydrological meanings of the parameters expressed by Manning's equation to obtain a dataset containing the elevations of the river beds throughout the basin. The second advancement is the provision of parameter uncertainties and, therefore, the uncertainties in the rated discharge. The third advancement concerns estimating the discharge while considering backwater effects. We analyzed the Amazon Basin using nearly one thousand series that were obtained from ENVISAT and Jason‐2 altimetry for more than 100 tributaries. Discharge values and related uncertainties were obtained from the rain‐discharge MGB‐IPH model. We used a global optimization algorithm based on the Monte Carlo Markov Chain and Bayesian framework to determine the rating curves. The data was randomly allocated into 80% calibration and 20% validation subsets. A comparison with the validation samples produced a Nash‐Sutcliffe efficiency (Ens) of 0.68. When the MGB discharge uncertainties were less than 5%, the Ens value increased to 0.81 (mean). A comparison with the in situ discharge resulted in an Ens value of 0.71 for the validation samples (and 0.77 for calibration). The Ens values at the mouths of the rivers that experienced backwater effects significantly improved when the mean monthly slope was included in the RC. Our RCs were not mission dependent, and the Ens value was preserved when applying ENVISAT rating curves to Jason‐2 altimetry at cross‐overs. The cease‐to‐flow parameter of our RCs provided a good proxy for determining river bed elevation. This proxy was validated against Acoustic Doppler current profiler (ADCP) cross sections with an accuracy of more than 90%. Altimetry measurements are routinely delivered within a few days, and this RC dataset provides a simple and cost‐effective tool for predicting discharge throughout the basin in nearly real time. This article is protected by copyright. All rights reserved.
- Regime‐shifting streamflow processes: Implications for water supply
- Authors: S.W.D. Turner; S. Galelli
Abstract: This paper examines the extent to which regime‐like behavior in streamflow time series impacts reservoir operating policy performance. We begin by incorporating a regime state variable into a well‐established stochastic dynamic programming model. We then simulate and compare optimized release policies—with and without the regime state variable—to understand how regime shifts affect operating performance in terms of meeting water delivery targets. Our optimization approach uses a hidden Markov model to partition the streamflow time series into a small number of separate regime states. The streamflow persistence structures associated with each state define separate month‐to‐month streamflow transition probability matrices for computing penalty cost expectations within the optimization procedure. The algorithm generates a four‐dimensional array of release decisions conditioned on the within‐year time period, reservoir storage state, inflow class, and underlying regime state. Our computational experiment is executed on 99 distinct, hypothetical water supply reservoirs fashioned from the Australian Bureau of Meteorology's Hydrologic Reference Stations. Results show that regime‐like behavior is a major cause of sub‐optimal operations in water supply reservoirs; conventional techniques for optimal policy design may misguide the operator, particularly in regions susceptible to multi‐year drought. Stationary streamflow models that allow for regime‐like behavior can be incorporated into traditional stochastic optimization models to enhance the flexibility of operations. This article is protected by copyright. All rights reserved.
- Understanding satellite‐based monthly‐to‐seasonal
reservoir outflow estimation as a function of hydrologic controls
- Authors: Matthew Bonnema; Safat Sikder, Yabin Miao, Xiaodong Chen, Faisal Hossain, Ismat Ara Pervin, S M. Mahbubur Rahman, Hyongki Lee
Abstract: Growing population and increased demand for water is causing an increase in dam and reservoir construction in developing nations. When rivers cross international boundaries, the downstream stakeholders often have little knowledge of upstream reservoir operation practices. Satellite remote sensing in the form of radar altimetry and multi‐sensor precipitation products can be used as a practical way to provide downstream stakeholders with the fundamentally elusive upstream information on reservoir outflow needed to make important and proactive water management decisions. This study uses a mass balance approach of three hydrologic controls to estimate reservoir outflow from satellite data at monthly and annual time scales: precipitation induced inflow, evaporation, and reservoir storage change. Furthermore, this study explores the importance of each of these hydrologic controls to the accuracy of outflow estimation. The hydrologic controls found to be unimportant could potentially be neglected from similar future studies. Two reservoirs were examined in contrasting regions of the world, the Hungry Horse Reservoir in a mountainous region in northwest U.S. and the Kaptai Reservoir in a low‐lying, forested region of Bangladesh. It was found that this mass balance method estimated the annual outflow of both reservoirs with reasonable skill. The estimation of monthly outflow from both reservoirs was however less accurate. The Kaptai basin exhibited a shift in basin behavior resulting in variable accuracy across the 9‐year study period. Monthly outflow estimation from Hungry Horse Reservoir was compounded by snow accumulation and melt processes, reflected by relatively low accuracy in summer and fall, when snow processes control runoff. Furthermore, it was found that the important hydrologic controls for reservoir outflow estimation at the monthly time scale differs between the two reservoirs, with precipitation induced inflow being the most important control for the Kaptai Reservoir and storage change being the most important for Hungry Horse Reservoir. This article is protected by copyright. All rights reserved.
- 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
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 multi‐salinity 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 0.39 and normalized root mean square errors (NRMSE) of 0.16 and 0.20, 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. This article is protected by copyright. All rights reserved.
- Smart pilot points using reversible‐jump Markov‐chain Monte
- Abstract: Pilot points are typical means for calibration of highly parameterized numerical models. We propose a novel procedure based on estimating not only the pilot point values, but also their number and suitable locations. This is accomplished by a trans‐dimensional Bayesian inversion procedure that also allows for uncertainty quantification. The utilized algorithm, reversible‐jump Markov‐Chain Monte Carlo (RJ‐MCMC), is computationally demanding and this challenges the application for model calibration. We present a solution for fast, approximate simulation through the application of a Bayesian inversion. A fast pathfinding algorithm is used to estimate tracer travel times instead of doing a full transport simulation. This approach extracts the information from measured breakthrough curves, which is crucial for the reconstruction of aquifer heterogeneity. As a result, the “smart pilot points” can be tuned during thousands of rapid model evaluations. This is demonstrated for both a synthetic and a field application. For the selected synthetic layered aquifer, two different hydrofacies are reconstructed. For the field investigation, multiple fluorescent tracers were injected in different well screens in a shallow alluvial aquifer and monitored in a tomographic source‐receiver configuration. With the new inversion procedure, a sand layer was identified and reconstructed with a high spatial resolution in 3‐D. The sand layer was successfully validated through additional slug tests at the site. The promising results encourage further applications in hydrogeological model calibration, especially for cases with simulation of transport. This article is protected by copyright. All rights reserved.
- Tracking multiple sediment cascades at the river network scale identifies
controls and emerging patterns of sediment connectivity
- Authors: Rafael J. P. Schmitt; Simone Bizzi, Andrea Castelletti
Abstract: Sediment connectivity in fluvial networks results from the transfer of sediment between multiple sources and sinks. Connectivity scales differently between all sources and sinks as a function of distance, source grain size and sediment supply, network topology and topography, and hydrologic forcing. In this paper, we address the challenge of quantifying sediment connectivity and its controls at the network scale. We expand the concept of a single, catchment‐scale sediment cascade towards representing sediment transport from each source as a suite of individual cascading processes. We implement this approach in the herein presented CAtchment Sediment ConnectivityAnd DElivery (CASCADE) modeling framework. In CASCADE, each sediment cascade establishes connectivity between a specific source and its multiple sinks. From a source perspective, the fate of sediment is controlled by its detachment and downstream transport capacity, resulting in a specific trajectory of transfer and deposition.
From a sink perspective, the assemblage of incoming cascades defines provenance, sorting, and magnitude of sediment deliveries. At the network scale, this information reveals emerging patterns of connectivity and the location of bottlenecks, where dis‐connectivity occurs.
In this paper, we apply CASCADE to quantitatively analyze the sediment connectivity of a major river system in SE Asia.
The approach provides a screening model that can support analyses of large, poorly monitored river systems. We test the sensitivity of CASCADE to various parameters and identify the distribution of energy between the multiple, simultaneously active sediment cascades as key control behind network sediment connectivity at the network scale. To conclude, CASCADE enables a quantitative, spatially explicit analysis of network connectivity with potential applications in both river science and management. This article is protected by copyright. All rights reserved.
- Issue Information
- Pages: 3285 - 3287
- Unlocking the full potential of Earth observation during the 2015 Texas
- Pages: 3288 - 3293
Abstract: Intense rainfall during late April and early May 2015 in Texas and Oklahoma led to widespread and sustained flooding in several river basins. Texas state agencies relevant to emergency response were activated when severe weather then ensued for 6 weeks from 8 May until 19 June following Tropical Storm Bill. An international team of scientists and flood response experts assembled and collaborated with decision‐making authorities for user‐driven high‐resolution satellite acquisitions over the most critical areas; while experimental automated flood mapping techniques provided daily ongoing monitoring. This allowed mapping of flood inundation from an unprecedented number of spaceborne and airborne images. In fact, a total of 27,174 images have been ingested to the USGS Hazards Data Distribution System (HDDS) Explorer, except for the SAR images used. Based on the Texas flood use case, we describe the success of this effort as well as the limitations in fulfilling the needs of the decision‐makers, and reflect upon these. In order to unlock the full potential for Earth observation data in flood disaster response, we suggest in a call for action (i) stronger collaboration from the onset between agencies, product developers, and decision‐makers; (ii) quantification of uncertainties when combining data from different sources in order to augment information content; (iii) include a default role for the end‐user in satellite acquisition planning; and (iv) proactive assimilation of methodologies and tools into the mandated agencies.
- The impact of information on behavior under an ambient‐based policy
for regulating nonpoint source pollution
- Authors: Haoran Miao; Jacob R. Fooks, Todd Guilfoos, Kent D. Messer, Soni M. Pradhanang, Jordan F. Suter, Simona Trandafir, Emi Uchida
Pages: 3294 - 3308
Abstract: Stemming from Segerson , literature on nonpoint source pollution shows that ambient‐based regulatory policies can induce polluters in a common watershed to comply with an exogenously determined pollution standard. This study uses laboratory economic experiments in a spatially heterogeneous setting to test the effectiveness of an ambient tax/subsidy policy in a setting with realistic in‐stream nutrient transport dynamics when varying levels of sensor information on ambient pollution are available to the agents and the regulator. We find that increasing the frequency of ambient monitoring improves the spatial allocation of emissions reductions. In particular, with more frequent monitoring, the ambient‐based policy induces firms further from the monitoring point to reduce emissions significantly more than downstream firms. Overall, the results suggest that enhanced temporal resolution of monitoring leads to efficiency gains.
- Time domain random walks for hydrodynamic transport in heterogeneous media
- Authors: Anna Russian; Marco Dentz, Philippe Gouze
Pages: 3309 - 3323
Abstract: We derive a general formulation of the time domain random walk (TDRW) approach to model the hydrodynamic transport of inert solutes in complex geometries and heterogeneous media. We demonstrate its formal equivalence with the discretized advection‐dispersion equation and show that the TDRW is equivalent to a continuous time random walk (CTRW) characterized by space‐dependent transition times and transition probabilities. The transition times are exponentially distributed. We discuss the implementation of different concentration boundary conditions and initial conditions as well as the occurrence of numerical dispersion. Furthermore, we propose an extension of the TDRW scheme to account for mobile‐immobile multirate mass transfer. Finally, the proposed TDRW scheme is validated by comparison to analytical solutions for spatially homogeneous and heterogeneous transport scenarios.
- Recharge of low‐arsenic aquifers tapped by community wells in
Araihazar, Bangladesh, inferred from environmental isotopes
- Authors: I. Mihajlov; M. Stute, P. Schlosser, B. J. Mailloux, Y. Zheng, I. Choudhury, K. M. Ahmed, A. van Geen
Pages: 3324 - 3349
Abstract: More than 100,000 community wells have been installed in the 150–300 m depth range throughout Bangladesh over the past decade to provide low‐arsenic drinking water (
- Hydrologic modeling in dynamic catchments: A data assimilation approach
- Authors: S. Pathiraja; L. Marshall, A. Sharma, H. Moradkhani
Pages: 3350 - 3372
Abstract: The transferability of conceptual hydrologic models in time is often limited by both their structural deficiencies and adopted parameterizations. Adopting a stationary set of model parameters ignores biases introduced by the data used to derive them, as well as any future changes to catchment conditions. Although time invariance of model parameters is one of the hallmarks of a high quality hydrologic model, very few (if any) models can achieve this due to their inherent limitations. It is therefore proposed to consider parameters as potentially time varying quantities, which can evolve according to signals in hydrologic observations. In this paper, we investigate the potential for Data Assimilation (DA) to detect known temporal patterns in model parameters from streamflow observations. It is shown that the success of the DA algorithm is strongly dependent on the method used to generate background (or prior) parameter ensembles (also referred to as the parameter evolution model). A range of traditional parameter evolution techniques are considered and found to be problematic when multiple parameters with complex time variations are estimated simultaneously. Two alternative methods are proposed, the first is a Multilayer approach that uses the EnKF to estimate hyperparameters of the temporal structure, based on apriori knowledge of the form of nonstationarity. The second is a Locally Linear approach that uses local linear estimation and requires no assumptions of the form of parameter nonstationarity. Both are shown to provide superior results in a range of synthetic case studies, when compared to traditional parameter evolution techniques.
- Impact of Homeowner Association (HOA) landscaping guidelines on
residential water use
- Authors: Elizabeth A. Wentz; Sandra Rode, Xiaoxiao Li, Elizabeth M. Tellman, B. L. Turner
Pages: 3373 - 3386
Abstract: The association between increasing water intensive land‐cover, such as the use of turf grass and trees, and increasing water use is a growing concern for water‐stressed arid cities. Appropriate regulatory measures addressing residential landscaping, such as those applied by Homeowner Associations (HOAs), may serve to reduce municipal water use, joining other water‐use reducing measures under consideration by arid cities. This research assesses quantitatively the role that Covenants, Conditions, and Restrictions (CCRs) applied to landscaping by HOAs play on water consumption. Statistical comparisons and models of n=1973 parcels in Goodyear, Arizona, USA, reveal that: HOA yards have less vegetation cover and those households use less peak‐season water (July) than those households in non‐HOA neighborhoods. This hold true even though the HOA CCRs regulate only the minimum required front‐yard vegetation and most residents maintain more than the minimum vegetation level. Furthermore, front‐yard landscaping tends to be mimicked in the backyard such that total yard landscaping tracks best with total household water use. Results of the study suggest that HOA landscaping regulations have the potential to reduce peak‐season water use by up to 24% if CCRs were to set maximum vegetation regulations rather than minimum and if compliance were enforced. Lowering residential water consumption in this way potentially involves tradeoffs with the cooling effects of vegetation and its consequences on the urban heat island effect, on energy use, and on home values.
- Integrating cobenefits produced with water quality BMPs into credits
markets: Conceptualization and experimental illustration for EPRI's Ohio
River Basin Trading
- Authors: Pengfei Liu; Stephen K. Swallow
Pages: 3387 - 3407
Abstract: This paper develops a method that incorporates the public value for environmental cobenefits when a conservation buyer can purchase water quality credits based on nonmarket valuation results. We demonstrate this approach through an experiment with adult students in a classroom laboratory environment. Our application contributes to the study of individual preference and willingness to pay for cobenefits associated with the production of water quality credits in relation to the Ohio River Basin Trading Project. We use three different methods to elicit individuals' willingness to pay (WTP), including (1) a hypothetical referendum, (2) a real referendum lacking incentive compatibility, and (3) a real choice with incentive compatibility. Methodologically, our WTP estimates suggest individuals are more sensitive to the cost changes and reveal the lowest value in the real choice with incentive compatibility. Practically, we find individuals value certain cobenefits and credits as public goods. Incorporating public value toward cobenefits may improve the overall efficiency of a water quality trading market. Based on our specification of a planner's welfare function, results suggest a substantial welfare improvement after identifying an optimal allocation of a buyer's budget across credits derived from agricultural management practices producing different portfolios of cobenefits.
- Catchment‐ and reach‐scale controls on the distribution and
expectation of geomorphic channel adjustment
- Authors: Peyton E. Lisenby; Kirstie A. Fryirs
Pages: 3408 - 3427
Abstract: Variability in channel function (behavior) can be assessed by characterizing different forms of adjustment over time. Here, historical channel adjustments in three tributary systems of the Lockyer Valley, Southeast Queensland (SEQ) are analyzed in order to evaluate the range of catchment‐ and reach‐scale controls on channel behavior. Over 300 individual adjustments and 13 forms of adjustment were identified over a ∼130 year time span. We measured the width‐to‐depth ratio (W:D), mean stream power (ω), and basin area (A) at the location of all observed adjustments. The most common forms of adjustment were avulsions, lateral expansion of the channel, and bend adjustments. The tributary systems behave distinctly different from one another according to statistical comparisons between the W:D, ω, and A data for these forms of adjustment. We find that it is possible to develop process domains or typologies for forms of geomorphic adjustment found in the Lockyer Valley. These domains or typologies provide the foundations for synoptic comparisons between catchments and assessing the expectation of channel adjustment (forecasting), which should be included in process‐based river management practice.
- Evaluating the relative air permeability of porous media from their water
- Authors: S. Assouline; A. Tuli, J. W. Hopmans
Pages: 3428 - 3439
Abstract: Accurate modeling of water and air flow in porous media requires the definition of the relevant hydraulic properties, namely, the water retention curve (WRC) and the relative hydraulic conductivity function (RHC), as well as the definition of the relative air permeability function (RAP). Capitalizing on the approach developed previously to represent the RHC, a new model allowing the prediction of RAP based on information resulting from the WRC is proposed. The power value ηa in the model is a decreasing exponential function of the coefficient of variation, ɛ, characterizing the pore size distribution of the porous medium, and derived from its WRC. The model was calibrated using data from 22 disturbed and undisturbed soil samples and was validated using data from eight soil types ranging from quartz sand to silty clay loam. The proposed model provided accurate prediction of the soil RAP and performed in some cases (sandy loam and silty clay loam soils) better than available alternative models.
- Alternating irrigation water quality as a method to control solute
concentrations and mass fluxes below irrigated fields: A numerical study
- Authors: David Russo
Pages: 3440 - 3456
Abstract: The aim of the present numerical study was to extend the data‐driven protocol for the control of soil salinity, to control chloride and nitrate concentrations and mass fluxes below agricultural fields irrigated with treated waste water (TWW). The protocol is based on alternating irrigation water quality between TWW and desalinized water (DSW), guided by solute concentrations at soil depth, zs. Two different schemes, the first requires measurements of soil solution concentrations of chloride and nitrate at zs, while, the second scheme requires only measurements of soil solution EC at zs, were investigated. For this purpose, 3‐D numerical simulations of flow and transport were performed for variably saturated, spatially heterogeneous, flow domains located at two different field sites. The sites differ in crop type, irrigation method, and in their lithology; these differences, in turn, considerably affect the performance of the proposed schemes, expressed in terms of their ability to reduce solute concentrations that drained below the root zone. Results of the analyses suggest that the proposed data‐driven schemes allow the use of low‐quality water for irrigation, while minimizing the consumption of high‐quality water to a level, which, for given climate, soil, crop, irrigation method, and water quality, may be determined by the allowable nitrate and chloride concentrations in the groundwater. The results of the present study indicate that with respect to the diminution of groundwater contamination by chloride and nitrate, the more data demanding, first scheme is superior the second scheme.
- Daytime sensible heat flux estimation over heterogeneous surfaces using
multitemporal land‐surface temperature observations
- Pages: 3457 - 3476
Abstract: Equations based on surface renewal (SR) analysis to estimate the sensible heat flux (H) require as input the mean ramp amplitude and period observed in the ramp‐like pattern of the air temperature measured at high frequency. A SR‐based method to estimate sensible heat flux (HSR‐LST) requiring only low‐frequency measurements of the air temperature, horizontal mean wind speed, and land‐surface temperature as input was derived and tested under unstable conditions over a heterogeneous canopy (olive grove). HSR‐LST assumes that the mean ramp amplitude can be inferred from the difference between land‐surface temperature and mean air temperature through a linear relationship and that the ramp frequency is related to a wind shear scale characteristic of the canopy flow. The land‐surface temperature was retrieved by integrating in situ sensing measures of thermal infrared energy emitted by the surface. The performance of HSR‐LST was analyzed against flux tower measurements collected at two heights (close to and well above the canopy top). Crucial parameters involved in HSR‐LST, which define the above mentioned linear relationship, were explained using the canopy height and the land surface temperature observed at sunrise and sunset. Although the olive grove can behave as either an isothermal or anisothermal surface, HSR‐LST performed close to H measured using the eddy covariance and the Bowen ratio energy balance methods. Root mean square differences between HSR‐LST and measured H were of about 55 W m−2. Thus, by using multitemporal thermal acquisitions, HSR‐LST appears to bypass inconsistency between land surface temperature and the mean aerodynamic temperature. The one‐source bulk transfer formulation for estimating H performed reliable after calibration against the eddy covariance method. After calibration, the latter performed similar to the proposed SR‐LST method.
- The impact of land‐cover change on flood peaks in peatland basins
- Authors: Jihui Gao; Joseph Holden, Mike Kirkby
Pages: 3477 - 3492
Abstract: In headwater peatlands, saturation‐excess overland flow is a dominant source of river discharge. Human modifications to headwater peatlands result in vegetation cover change but there is a lack of understanding about how the spatial distribution of such change impacts flood peaks. A fully distributed version of TOPMODEL with an overland flow velocity module was used to simulate flood response for three upland peat basins. Bare peat strips adjacent to channels resulted in a higher and faster flow peak; for a 20 mm h−1 rainfall event, with bare riparian zones covering 10% of the basin area, peaks were increased, compared to the current hydrograph, by 12.8%, 1.8%, and 19.6% in the three basins. High density Sphagnum ground cover over the same riparian zones reduced flow peaks (e.g., by 10.1%, 1.8%, and 13.4% for the 20 mm h−1 event) compared to the current hydrograph. With similar total areas of land‐cover change, the size of randomly located patches of changed cover had no effect on peak flow for patch sizes up to 40,000 m2. However, cover changes on gentle slope areas generally resulted in a larger change in peak flow when compared with the same changes on steeper slopes. Considering all results for the same proportion of catchment area that undergoes change, land‐cover change along narrow riparian buffer strips had the highest impact on river flow. Thus, the protection and revegetation of damaged riparian areas in upland peat catchments may be highly beneficial for flood management.
- Vertical variation of mixing within porous sediment beds below turbulent
- Authors: I. D. Chandler; I. Guymer, J. M. Pearson, R. van Egmond
Pages: 3493 - 3509
Abstract: River ecosystems are influenced by contaminants in the water column, in the pore water and adsorbed to sediment particles. When exchange across the sediment‐water interface (hyporheic exchange) is included in modeling, the mixing coefficient is often assumed to be constant with depth below the interface. Novel fiber‐optic fluorometers have been developed and combined with a modified EROSIMESS system to quantify the vertical variation in mixing coefficient with depth below the sediment‐water interface. The study considered a range of particle diameters and bed shear velocities, with the permeability Péclet number,
PeK between 1000 and 77,000 and the shear Reynolds number,
Re*, between 5 and 600. Different parameterization of both an interface exchange coefficient and a spatially variable in‐sediment mixing coefficient are explored. The variation of in‐sediment mixing is described by an exponential function applicable over the full range of parameter combinations tested. The empirical relationship enables estimates of the depth to which concentrations of pollutants will penetrate into the bed sediment, allowing the region where exchange will occur faster than molecular diffusion to be determined.
- Valuing water resources in Switzerland using a hedonic price model
- Authors: Diana van Dijk; Rosi Siber, Roy Brouwer, Ivana Logar, Dorsa Sanadgol
Pages: 3510 - 3526
Abstract: In this paper, linear and spatial hedonic price models are applied to the housing market in Switzerland, covering all 26 cantons in the country over the period 2005–2010. Besides structural house, neighborhood and socioeconomic characteristics, we include a wide variety of new environmental characteristics related to water to examine their role in explaining variation in sales prices. These include water abundance, different types of water bodies, the recreational function of water, and water disamenity. Significant spatial autocorrelation is found in the estimated models, as well as nonlinear effects for distances to the nearest lake and large river. Significant effects are furthermore found for water abundance and the distance to large rivers, but not to small rivers. Although in both linear and spatial models water related variables explain less than 1% of the price variation, the distance to the nearest bathing site has a larger marginal contribution than many neighborhood‐related distance variables. The housing market shows to differentiate between different water related resources in terms of relative contribution to house prices, which could help the housing development industry make more geographically targeted planning activities.
- Relation between grid, channel, and Peano networks in
high‐resolution digital elevation models
- Authors: Samuele De Bartolo; Francesco Dell'Accio, Giuseppe Frandina, Giovanni Moretti, Stefano Orlandini, Massimo Veltri
Pages: 3527 - 3546
Abstract: The topological interconnection between grid, channel, and Peano networks is investigated by extracting grid and channel networks from high‐resolution digital elevation models of real drainage basins, and by using a perturbed form of the equation describing how the average junction degree varies with Horton‐Strahler order in Peano networks. The perturbed equation is used to fit the data observed over the Hortonian substructures of real networks. The perturbation parameter, denoted as “uniformity factor,” is shown to indicate the degree of topological similarity between Hortonian and Peano networks. The sensitivities of computed uniformity factors and drainage densities to grid cell size and selected threshold for channel initiation are evaluated. While the topological relation between real and Peano networks may not vary significantly with grid cell size, these networks are found to exhibit the same drainage density only for specific grid cell sizes, which may depend on the selected threshold for channel initiation.
- The importance of base flow in sustaining surface water flow in the Upper
Colorado River Basin
- Authors: Matthew P. Miller; Susan G. Buto, David D. Susong, Christine A. Rumsey
Pages: 3547 - 3562
Abstract: The Colorado River has been identified as the most overallocated river in the world. Considering predicted future imbalances between water supply and demand and the growing recognition that base flow (a proxy for groundwater discharge to streams) is critical for sustaining flow in streams and rivers, there is a need to develop methods to better quantify present‐day base flow across large regions. We adapted and applied the spatially referenced regression on watershed attributes (SPARROW) water quality model to assess the spatial distribution of base flow, the fraction of streamflow supported by base flow, and estimates of and potential processes contributing to the amount of base flow that is lost during in‐stream transport in the Upper Colorado River Basin (UCRB). On average, 56% of the streamflow in the UCRB originated as base flow, and precipitation was identified as the dominant driver of spatial variability in base flow at the scale of the UCRB, with the majority of base flow discharge to streams occurring in upper elevation watersheds. The model estimates an average of 1.8 × 1010 m3/yr of base flow in the UCRB; greater than 80% of which is lost during in‐stream transport to the Lower Colorado River Basin via processes including evapotranspiration and water diversion for irrigation. Our results indicate that surface waters in the Colorado River Basin are dependent on base flow, and that management approaches that consider groundwater and surface water as a joint resource will be needed to effectively manage current and future water resources in the Basin.
- Comment on “Quantifying renewable groundwater stress with
GRACE” by Alexandra S. Richey et al.
- Authors: Sasmita Sahoo; Tess Russo, Upmanu Lall
Pages: 4184 - 4187
- Reply to comment by Sahoo et al. on “Quantifying renewable
groundwater stress with GRACE”
- Pages: 4188 - 4192
- Comment on “Climate and agricultural land use change impacts on
streamflow in the upper midwestern United States” by Satish C. Gupta
- Authors: Dingbao Wang
Pages: 4193 - 4194
- Reply to comment by Dingbao Wang 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
Pages: 4195 - 4198