ALBERT

Description: We investigate how the choice of injection mode impacts transport properties in kilometer-scale three-dimensional discrete fracture networks (DFN). The choice of injection mode, resident or flux-weighted, is designed to mimic different physical phenomena. It has been hypothesized that solute plumes injected under resident conditions evolve to behave similarly to solutes injected under flux-weighted conditions. Previously, computational limitations have prohibited the large scale simulations required to investigate this hypothesis. We investigate this hypothesis by using a high performance DFN suite, dfnWorks , to simulate flow in kilometer-scale three-dimensional DFNs based on fractured granite at the Forsmark site in Sweden, and adopt a Lagrangian approach to simulate transport therein. Results show that after traveling through a pre-equilibrium region both injection methods exhibit linear scaling of the first moment of travel time and power law scaling of the breakthrough curve with similar exponents, slightly larger than two. The physical mechanisms behind this evolution appear to be the combination of in-network channeling of mass into larger fractures, which offer reduced resistance to flow, and in-fracture channeling, which results from the topology of the DFN. This article is protected by copyright. All rights reserved.

Description: Hydrologic ensemble forecasts driven by atmospheric ensemble prediction systems need statistical post-processing in order to account for systematic errors in terms of both location and spread. Runoff is an inherently multivariate process with typical events lasting from hours in case of floods to weeks or even months in case of droughts. This calls for multivariate post-processing techniques that yield well calibrated forecasts in univariate terms and ensure a realistic temporal dependence structure at the same time. To this end, the univariate ensemble model output statistics (EMOS) post-processing method is combined with two different copula approaches that ensure multivariate calibration throughout the entire forecast horizon. The domain of this study covers three sub-catchments of the river Rhine that represent different sizes and hydrological regimes: the Upper Rhine up to the gauge Maxau, the river Moselle up to the gauge Trier, and the river Lahn up to the gauge Kalkofen. In this study the two approaches to model the temporal dependence structure are ensemble copula coupling (ECC), which preserves the dependence structure of the raw ensemble, and a Gaussian copula approach (GCA), which estimates the temporal correlations from training observations. The results indicate that both methods are suitable for modelling the temporal dependencies of probabilistic hydrologic forecasts. This article is protected by copyright. All rights reserved.

Description: A three-dimensional mathematical model that describes transport of contaminant in a horizontal aquifer with simultaneous diffusion into a fractured clay formation is proposed. A group of semi-analytical solutions is derived based on specific initial and boundary conditions as well as various source functions. The analytical model solutions are evaluated by numerical Laplace inverse transformation and analytical Fourier inverse transformation. The model solutions can be used to study the fate and transport in a three-dimensional spatial domain in which a non-aqueous phase liquid exists as a pool atop a fractured low permeability clay layer. The non-aqueous phase liquid gradually dissolves into the groundwater flowing past the pool, while simultaneously diffusing into the fractured clay formation below the aquifer. Mass transfer of the contaminant into the clay formation is demonstrated to be significantly enhanced by the existence of the fractures, even though the volume of fractures is relatively small compared to the volume of the clay matrix. The model solution is a useful tool in assessing contaminant attenuation processes in a confined aquifer underlain by a fractured clay formation. This article is protected by copyright. All rights reserved.

Description: The objective of this study is to incorporate a time-dependent SCS CN method (SMA_CN) in Soil and Water Assessment Tool (SWAT) and compare its performance with the existing CN method in SWAT by simulating the hydrology of two agricultural watersheds in Indiana, United States. Results show that fusion of the SMA_CN method causes decrease in runoff volume and increase in profile soil moisture content, associated with larger groundwater contribution to the streamflow. In addition, the higher amount of moisture in the soil profile slightly elevates the actual evapotranspiration. The SMA-based SWAT configuration consistently produces improved goodness of fit scores and less uncertain outputs with respect to streamflow during both calibration and validation. The SMA_CN method exhibits better match with the observed data for all flow regimes, thereby addressing issues related to peak and low flow predictions by SWAT in many past studies. Comparison of the calibrated model outputs with field-scale soil moisture observations reveal that the SMA overhauling enables SWAT to represent soil moisture condition more accurately, with better response to the incident rainfall dynamics. While the results from the modification of the SCS method in SWAT are promising, more studies including watersheds with various physical and climatic settings are needed to validate the proposed approach. This article is protected by copyright. All rights reserved.

Description: We adapted Newton's Law of Cooling to model downstream water temperature change in response to stream-adjacent forest harvest on small and medium streams (average 327 ha in size) throughout the Oregon Coast Range, USA. The model requires measured stream gradient, width, depth and upstream control reach temperatures as inputs and contains two free parameters which were determined by fitting the model to measured stream temperature data. This model reproduces the measured downstream temperature responses to within 0.4 C ° for 15 of the 16 streams studied and provides insight into the physical sources of site-to-site variation among those responses. We also use the model to examine how the pre-to-post harvest change in daily maximum stream temperature depends on distance from the harvest reach. The model suggests that the pre-to-post harvest temperature change approximately 300  m downstream of the harvest will range from roughly 82% to less than 1% of that temperature change which occurred within the harvest reach, depending primarily on the downstream width, depth, and gradient. Using study-averaged values for these channel characteristics the model suggests that for a stream representative of those in the study, the temperature change approximately 300  m downstream of the harvest will be 56% of the temperature change which occurred within the harvest reach. This adapted Newton's Law of Cooling procedure represents a highly practical means for predicting stream temperature behavior downstream of timber harvests relative to conventional heat budget approaches, and is informative of the dominant processes affecting stream temperature. This article is protected by copyright. All rights reserved.

Description: Digital elevation models (DEMs) that are used in hydrological applications must be processed to remove sinks, mainly topographic depressions. Flow enforcement techniques include filling methods, which raise elevations within depressions, breaching, which carves channels through blockages, and hybrid methods. Despite previous research demonstrating the large impact to DEMs and subsequent analyses of depression filling, it is common practice apply this technique to flow enforcement. This is partly due to the greater efficiency of depression filling tools compared to breaching counterparts, which often limits breaching to applications of small- to moderate-sized DEMs. A new hybrid flow enforcement algorithm is presented in this study. The method can be run in complete breaching, selective breaching (either breached or filled), or constrained breaching (partial breaching) modes, allowing for greater flexibility in how practitioners enforce continuous flow paths. Algorithm performance was tested with DEMs of varying topography, spatial extents, and resolution. The sites included three moderate sized DEMs (52,000,000 to 190,000,000 cells) and three massive DEMs of the Iberian Peninsula, and the Amazon and Nile River basins, the largest containing nearly one billion cells. In complete breaching mode, the new algorithm required 87% of the time needed by a filling method to process the test DEMs, while the selective breaching and constrained breaching modes, operating with maximum breach depth constraints, increased run times by 8% and 27% respectively. Therefore, the new algorithm offers comparable performance to filling and the ability to process massive topographic data sets, while giving practitioners greater flexibility and lowering DEM impact. This article is protected by copyright. All rights reserved.

Description: Mountain snowpacks provide most of the annual discharge of western U.S. rivers, but the future of water resources in the western U.S. is tenuous, as climatic changes have resulted in earlier spring melts that have exacerbated summer droughts. Compounding changes to the physical environment are biotic disturbances including the mountain pine beetle (MPB), which has decimated millions of acres of western North American forests. At the watershed scale, MPB disturbance increases the peak hydrograph, and at the stand scale the ‘gray’ phase of MPB canopy disturbance decreases canopy snow interception, increases snow albedo, increases net shortwave radiation and decreases net longwave radiation versus the ‘red’ phase. Fewer studies have been conducted on the red phase of MPB disturbance, and in the mixed coniferous stands that may follow MPB-damaged forests. We measured the energy balance of four snowpacks representing different stages of MPB damage, management, and recovery: a lodgepole pine stand, a MPB-infested stand in the red phase, a mixed coniferous stand (representing one successional trajectory), and a clearcut (representing reactive management) in the Tenderfoot Creek Experimental Forest in Montana, USA. Net longwave radiation was lower in the MPB-infested stand despite higher basal area and plant area index of the other forests, suggesting that the dessicated needles serve as a less effective thermal buffer against longwave radiative losses. Eddy covariance observations of sensible and latent heat flux indicate that they are of similar but opposite magnitude, on the order of 20 MJ m −2 during the melt period. Further analyses reveal that net turbulent energy fluxes were near zero due to the temperature and atmospheric vapor pressure encountered during the melt period. Future research should place snow science in the context of forest succession and management, and address important uncertainties regarding the timing and magnitude of needlefall events. This article is protected by copyright. All rights reserved.

Description: This work focuses on the implementation of a Shallow Water-Exner model for compound natural channels with complex geometry and movable bed within the finite volume framework. The model is devised for compound channels modeling: cross-section overbanks are treated with fixed bed conditions, while the main channel is left free to modify its morphology. A capacitive approach is used for bedload transport modeling, in which the solid flow rates are estimated with bedload transport formulas. The model equations pose some numerical issues in the case of natural channels, where bedload transport may occur for both subcritical and supercritical flows and geometry varies in space. An explicit path-conservative scheme, designed to overcome all these issues, is presented in the paper. The scheme solves liquid and solid phases dynamics in a coupled manner, in order to correctly model near critical currents/channel interactions and is well-balanced, that is able to properly reproduce steady states. The Roe and Osher Riemann solvers are implemented, so as to take into account the spatial geometry variations of natural channels. The scheme reaches up to 2 nd order accuracy. Validation is performed with fixed and movable bed test cases whose analytical solution is known, and with flume experimental data. An application of the model to a real case study is also shown. This article is protected by copyright. All rights reserved.

Description: In this paper we use a physical modelling approach to explore the effect of lateral confinement on gravel bed river planform style, bed morphology, and sediment transport processes. A set of 27 runs was performed in a large flume (25 m long, 2.9 m wide), with constant longitudinal slope (0.01) and uniform grain size (1 mm), changing the water discharge (1.5 to 2.5 l/s) and the channel width (0.15 m to 1.5 m) to model a wide range of channel configurations, from narrow, straight, embanked channels to wide braided networks. The outcomes of each run were characterized by a detailed digital elevation model describing channel morphology, a map of dry areas and areas actively transporting sediment within the channel, and continuous monitoring of the amount of sediment transported through the flume outlet. Analysis reveals strong relationships between unit stream power and parameters describing the channel morphology. In particular, a smooth transition is observed between narrow channels with an almost rectangular cross section profile (with sediment transport occurring across the entire channel width) and complex braided networks where only a limited proportion (30%) of the bed is active. This transition is captured by descriptors of the bed elevation frequency distribution, e.g. standard deviation, skewness and kurtosis. These summary statistics represent potentially useful indicators of bed morphology that are compared with other commonly used summary indicators such as the braiding index and the type and number of bars. This article is protected by copyright. All rights reserved.

Description: We believe that there are too many models in hydrology and we should ask ourselves the question, if we are currently wasting time and effort in developing another model again instead of focusing on the development of a community hydrological model. In other fields this kind of models have been quite successful, but due to several reasons, no single community model has been developed in the field of hydrology yet. The concept, strength and weakness of a community model was discussed at the Chapman Conference on Catchment Spatial Behaviour and Complex Organisation held in Luxembourg in September 2014. This discussion as well as out own opinions about the potential of a community models, or at least the necessary discussion to establish one are debated in this commentary. This article is protected by copyright. All rights reserved.

Description: Soil surface sealing is a widespread natural process occurring frequently in bare soil areas between vegetation patches. The low hydraulic conductivity that characterizes the seal layer reduces both infiltration and evaporation fluxes from the soil, and thus has the potential to affect local vegetation water uptake (VWU). This effect is investigated here using experimental data, 2D physically based modelling and a long-term climatic dataset from three dry sites presenting a climatic gradient in the Negev Desert, Israel. The Feddes VWU parameters for the dominant shrub at the study site ( Sarcopoterium spinosum ) were acquired using lysimeter experiments. The results indicate that during the season surface sealing could either increase or decrease VWU depending on initial soil water content, rainfall intensity, and the duration of the subsequent drying intervals. These factors have a marked effect on inter-annual variability of the seal layer effect on VWU, which on average was found to be 26% higher under sealed conditions than in the case of unsealed soil surfaces. The seal layer was found to reduce the period where the vegetation was under water stress by 31% compared with unsealed conditions. This effect was more pronounced for seasons with total rainfall depth higher than 10 cm/y, and was affected by interseasonal climatic variability. These results shed light on the importance of surface sealing in dry environments and its contribution to the resilience of woody vegetation. This article is protected by copyright. All rights reserved.

Description: As a result of climate change/variation and its aggravation by human activities over the past several decades, the hydrological conditions in the middle Yellow River in China have dramatically changed, which has led to a sharp decrease of streamflow and the drying up of certain tributaries. This paper simulated and analysed the impact of sediment-trapping dams (STDs, a type of large-sized check dam used to prevent sediment from entering the Yellow River main stem) on hydrological processes, and the study area was located in the 3,246 km 2 Huangfuchuan (HFC) River basin. Changes in the hydrological processes were analysed, and periods of natural and disturbed states were defined. Subsequently, the number and distribution of the STDs were determined based on data collected from statistical reports and identified from remote sensing images, and the topological relationships between the STDs and high-resolution river reaches were established. A hydrological model, the Digital Yellow River Integrated Model, was used to simulate the STD impact on the hydrological processes, and the maximum STD impact was evaluated through a comparison between the simulation results with and without the STDs, which revealed that the interception effect of the STDs contributed to the decrease of the streamflow by approximately 39%. This paper also analysed the relationship between the spatial distribution of the STDs and rainfall in the HFC River basin and revealed that future soil and water conservation measures should focus on areas with a higher average annual rainfall and higher number of rainstorm hours. This article is protected by copyright. All rights reserved.

Description: Female salmonids bury and lay their eggs in streambeds by digging a pit, which is then covered with sediment from a second pit. The spawning process alters streambed topography, winnows fine sediment, and mixes sediment in the active layer. The resulting egg nests (redds) contain coarser and looser sediments than those of unspawned streambed areas, and display a dune-like shape with an amplitude and length that vary with fish size, substrate conditions, and flow conditions. Redds increase local bed surface roughness (〈10 −1 channel width, W ), but may reduce the size of macro-bedforms by eroding reach scale topography (10 ° -10 1 W ). Research has suggested that spawning may increase flow resistance due to redd form drag, resulting in lower grain shear stress and less particle mobility. Spawning however also prevents streambed armoring through surface and subsurface material mixing, potentially increasing particle mobility. Here, we use 2-dimensional hydraulic modeling with detailed pre- and post-spawning bathymetries and field observations to test the effect of small spawning salmonids on sediment transport. Our results show that topographical roughness added by small-bodied salmon redds has negligible effects on shear stress at the reach-unit scale, and limited effects at the local scale. Conversely, our results indicate sediment mixing reduces armoring and enhances sediment mobility, which increases potential bed load transport by subsequent floods. River restoration in fish-bearing streams should take into consideration the effects of redd excavation on channel stability. This is particularly important for streams that historically supported salmonids, and at present are the focus of habitat restoration actions. This article is protected by copyright. All rights reserved.

Description: Floods are a natural hazard that affect communities worldwide, but to date the vast majority of flood hazard research and mapping has been undertaken by wealthy developed nations. As populations and economies have grown across the developing world, so too has demand from governments, businesses and NGOs for modelled flood hazard data in these data-scarce regions. We identify six key challenges faced when developing a flood hazard model that can be applied globally, and present a framework methodology that leverages recent cross-disciplinary advances to tackle each challenge. The model produces return period flood hazard maps at ∼90 m resolution for the whole terrestrial land surface between 56˚S and 60˚N, and results are validated against high resolution government flood hazard datasets from the UK and Canada. The global model is shown to capture between two thirds and three quarters of the area determined to be at risk in the benchmark data without generating excessive false positive predictions. When aggregated to ∼1 km, mean absolute error in flooded fraction falls to ∼5%. The full complexity global model contains an automatically parameterised subgrid channel network, and comparison to both a simplified 2D only variant and an independently developed pan-European model shows the explicit inclusion of channels to be a critical contributor to improved model performance. Whilst careful processing of existing global terrain datasets enables reasonable model performance in urban areas, adoption of forthcoming next-generation global terrain datasets will offer the best prospect for a step-change improvement in model performance. This article is protected by copyright. All rights reserved.

Description: Understanding how channel bed morphology affects flow conditions (and vice versa) is important for a wide range of fluvial processes and practical applications. We investigated interactions between bed roughness and flow velocity in a steep, glacier-fed mountain stream (Riedbach, Ct. Valais, Switzerland) with almost flume-like boundary conditions. Bed gradient increases along the 1-km study reach by roughly one order of magnitude ( S =3-41%), with a corresponding increase in streambed roughness, while flow discharge and width remain approximately constant due to the glacial runoff regime. Streambed roughness was characterized by semi-variograms and standard deviations of point clouds derived from terrestrial laser scanning. Reach-averaged flow velocity was derived from dye tracer breakthrough curves measured by 10 fluorometers installed along the channel. Commonly used flow resistance approaches (Darcy-Weisbach equation and dimensionless hydraulic geometry) were used to relate the measured bulk velocity to bed characteristics. As a roughness measure, D 84 yielded comparable results to more laborious measures derived from point clouds. Flow resistance behavior across this large range of steep slopes agreed with patterns established in previous studies for both lower-gradient and steep reaches, regardless of which roughness measures were used. We linked empirical critical shear stress approaches to the variable power equation for flow resistance to investigate the change of bed roughness with channel slope. The predicted increase in D 84 with increasing channel slope was in good agreement with field observations. This article is protected by copyright. All rights reserved.

Description: Large wood governs channel morphology, as well as the availability of in-stream habitat, in many forested streams. In this paper we use a stochastic, physically based model to simulate wood recruitment and in-stream geomorphic processes, in order to explore the influence of disturbance history on the availability of aquatic habitat. Specifically, we consider the effects of fire on a range of stream sizes by varying the rate of tree toppling over time in a simulated forest characterized by a tree height of 30 m. We also consider the effects of forest harvesting with various riparian buffer sizes, by limiting the lateral extent of the riparian stand. Our results show that pulsed inputs of wood increase the availability and variability of physical habitat in the post-fire period; reach-averaged pool area and deposit area double in small streams, while side-channels increase by over 50% in intermediate-sized channels. By contrast, forest harvesting reduces the availability of habitat within the reach, though the effects diminish with increasing buffer size or stream width; in laterally stable streams the effects are minimal so long as buffer width is large enough for key pieces to be recruited to the reach. This research emphasizes the importance of natural disturbance in creating and maintaining habitat heterogeneity and shows that scenario-based numerical modeling provides a useful tool for assessing the historical range of variability associated with natural disturbance, as well as changes in habitat relevant to fish. It can be also used to inform forest harvesting and management. This article is protected by copyright. All rights reserved.

Description: Spreading of conservative solutes in groundwater due to aquifer heterogeneity is quantified by the macrodispersivity, which was found to be scale dependent. It increases with travel distance, stabilizing eventually at a constant value. However, the question of its asymptotic behaviour at very large scale is still a matter of debate. It was surmised in the literature that macrodispersivity scales up following a unique scaling law. Attempts to define such a law were made by fitting a regression line in the log-log representation of an ensemble of macrodispersivities from multiple experiments. The functional relationships differ among the authors, based on the choice of data. Our study revisits the data basis, used for inferring unique scaling, through a detailed analysis of literature marcodispersivities. In addition, values were collected from the most recent tracer tests reported in the literature. We specified a system of criteria for reliability and re-evaluated the reliability of the reported values. The final collection of reliable estimates of macrodispersivity does not support a unique scaling law relationship. On the contrary, our results indicate, that the field data can be explained as a collection of macrodispersivities of aquifers with varying degree of heterogeneity where each exhibits its own constant asymptotic value. Our investigation concludes that transport, and particularly the macrodispersivity, is formation-specific, and that modeling of transport cannot be relegated to a unique scaling law. Instead, transport requires characterization of aquifer properties, e.g. spatial distribution of hydraulic conductivity, and the use of adequate models. This article is protected by copyright. All rights reserved.

Description: For the past few decades, heat has been used to estimate river-aquifer exchange flux at discrete locations by comparison of river and groundwater temperature. In recent years, heat has also been employed to estimate reach-scale river-aquifer exchange flux based only on river temperature. However, there are many more parameters that govern heat exchange and transport in surface water than in groundwater. In this study, we analyzed the sensitivities of surface water temperature to various parameters and assessed the accuracy of temperature-based estimates of exchange flux in two synthetic rivers and in a field setting. For the large synthetic river with a flow rate of 63 m 3 s −1 (i.e., 5.44 × 10 6 m 3 d −1 ), the upper and lower bounds of the groundwater inflow rate can be determined when the actual groundwater inflow is around 100 m 2 d −1 . For higher and lower fluxes, only minimum and maximum bounds respectively can be determined. For the small synthetic river with the flow rate of 0.63 m 3 s −1 (i.e., 5.44 × 10 4 m 3 d −1 ), the bounds of the groundwater inflow rate can only be estimated when the actual groundwater inflow rate is near 10 m 2 d −1 . In the field setting, results show that the inflow rate must be less than 100 m 2 d −1 , but a lower bound for groundwater inflow cannot be determined. The large ranges of estimated groundwater inflow rates in both theoretical and field settings indicate the need to reduce parameter errors and combine heat measurements with other isotopic and/or chemical methods. This article is protected by copyright. All rights reserved.

Description: Climate state can be an important predictor of future hydrologic conditions. In ensemble streamflow forecasting, where historical weather inputs or streamflow observations are used to generate the ensemble, climate index weighting is one way to represent the influence of climate state. Using a climate index, each forecast variable member of the ensemble is selectively weighted to reflect the climate state at the time of the forecast. A new approach to climate index weighting of ensemble forecasts is presented. The method is based on a sampling-resampling approach for Bayesian updating. The original hydrologic ensemble members define a sample drawn from the prior distribution; the relationship between the climate index and the ensemble member forecast variable is used to estimate a likelihood function. Given an observation of the climate index at the time of the forecast, the estimated likelihood function is then used to assign weights to each ensemble member. The weights define the probability of each ensemble member outcome given the observed climate index. The weighted ensemble forecast is then used to estimate the posterior distribution of the forecast variable conditioned on the climate index. The Bayesian climate index weighting approach is easy to apply to hydrologic ensemble forecasts; its parameters do not require calibration with hindcasts, and it adapts to the strength of the relation between climate and the forecast variable, defaulting to equal weighting of ensemble members when no relationship exists. A hydrologic forecasting application illustrates the approach and contrasts it with traditional climate index weighting approaches. This article is protected by copyright. All rights reserved.

Description: Distributed, continuous hydrologic models promote better understanding of hydrology and enable integrated hydrologic analyses by providing a more detailed picture of water transport processes across the varying landscape. However, such models are not widely used in routine modeling practices, due in part to the extensive data input requirements, computational demands, and complexity of routing algorithms. We developed a two-dimensional continuous hydrologic model, HYSTAR, using a time-area method within a grid-based spatial data model with the goal of providing an alternative way to simulate spatiotemporally varied watershed-scale hydrologic processes. The model calculates the direct runoff hydrograph by coupling a time-area routing scheme with a dynamic rainfall excess sub-model implemented here using a modified curve number method with an hourly time step, explicitly considering downstream ‘reinfiltration’ of routed surface runoff. Soil moisture content is determined at each time interval based on a water balance equation, and overland and channel runoff is routed on time-area maps, representing spatial variation in hydraulic characteristics for each time interval in a storm event. Simulating runoff hydrographs does not depend on unit hydrograph theory or on solution of the Saint Venant equation, yet retains the simplicity of a unit hydrograph approach and the capability of explicitly simulating two-dimensional flow routing. The model provided acceptable performance in predicting daily and monthly runoff for a 6-year period for a watershed in Virginia (USA) using readily available geographic information about the watershed landscape. Spatial and temporal variability in simulated effective runoff depth and time area maps dynamically show the areas of the watershed contributing to the direct runoff hydrograph at the outlet over time, consistent with the variable source area overland flow generation mechanism. The model offers a way to simulate watershed processes and runoff hydrographs using the time-area method, providing a simple, efficient, and sound framework that explicitly represents mechanisms of spatially and temporally varied hydrologic processes. This article is protected by copyright. All rights reserved.

Description: Given the importance of groundwater temperature to the biogeochemical health of aquatic ecosystems, a floodplain study was implemented to improve understanding of rural land use impacts on shallow groundwater (SGW) temperature. Study sites included a historic agricultural field (Ag) and bottomland hardwood forest (BHF), each with nine piezometers in an 80 × 80 m grid. Piezometers were equipped with pressure transducers to monitor SGW temperature and level at 30 minute intervals during the 2011, 2012, 2013, and 2014 water years. The study is one of the first to utilize long-term, continuous, automated, in situ monitoring to investigate rural land use impacts on shallow groundwater temperatures. Average SGW temperature during the study period was 11.1 and 11.2 °C at the Ag and BHF sites, respectively. However, temperature range at the Ag site was 72% greater than at the BHF site. Results indicate a greater responsiveness to seasonal climate fluctuations in Ag site SGW temperature related to absence of forest canopy. Patterns of intra-site groundwater temperature differences at both study sites illustrate the influence of stream-aquifer thermal conduction and occasional baseflow reversals. Considering similar surface soil temperature amplitudes and low average groundwater flow values at both sites, results suggest that contrasting rates of plant water use, groundwater recharge, and subsurface hydraulic conductivity are likely mechanistic causes for the observed SGW temperature differences. Results highlight the long-term impact of forest removal on subsurface hydrology and groundwater temperature regime. This article is protected by copyright. All rights reserved.

Description: Field hydrology is on the decline. Meanwhile, the need for new field-derived insight into the age, origin and pathway of water in the headwaters, where most runoff is generated, is more needed than ever. Water Resources Research (WRR) has included some of the most influential papers in field-based runoff process understanding, particularly in the formative years when the knowledge base was developing rapidly. Here, we take advantage of this 50 th anniversary of the journal to highlight a few of these important field-based papers and show how field scientists have posed strong and sometimes outrageous hypotheses—approaches so needed in an era of largely model-only research. We chronicle the decline in field work and note that it is not only the quantity of field work that is diminishing but its character is changing too: from discovery science to data collection for model parameterisation. While the latter is a necessary activity, the loss of the former is a major concern if we are to advance the science of watershed hydrology. We outline a vision for field research to seek new fundamental understanding, new mechanistic explanations of how watershed systems work, particularly outside the regions of traditional focus. This article is protected by copyright. All rights reserved.

Description: Growing demand on groundwater resources and the semi-arid climate in the North China Plain (NCP) highlight the need for improved understanding of connections between regional climate change and groundwater recharge. Hydrologic time series of precipitation and groundwater levels were analyzed in three representative geographical zones throughout the NCP for the period of 1960-2008 using trend analysis and spectral analysis methods. A significant change point around 1975 is followed by a long term decline trend in precipitation time series, which coincides with the Pacific Decadal Oscillation (PDO) positive phase. However, the magnitudes of groundwater levels variability due to heavily pumping overwhelm the low-frequency signal of groundwater levels. Nonlinear trends that related to long-term climatic variability and anthropogenic activities are removed by using the Singular Spectrum Analysis (SSA) method. Spectral analyses of the detrended residuals demonstrate significant short-term oscillations at the frequencies of 2–7 years, which have strong correlations with the El Niño-Southern Oscillation (ENSO) modes. This study contributes to improved understanding of dynamic relationship between groundwater and climate variability modes in the NCP, and demonstrates the importance of reliable detrending methods for groundwater levels that are affected greatly by pumping. This article is protected by copyright. All rights reserved.

Description: On September 3, 1998, a glacial lake outburst flood (GLOF) that originated from Tam Pokhari occurred in the Hinku valley of the eastern Nepal Himalaya. This study analyzes the lake's geomorphic and hydrologic conditions prior to the outburst, and evaluates the conditions that could contribute to a future flood through photogrammetric techniques. We processed high-resolution Corona KH-4A (2.7 m) and ALOS PRISM (2.5 m) stereo-images taken before and after the GLOF event, and produced detailed topographic maps (2-m contour interval) and DEMs (5 m × 5 m). We (re-) constructed lake water surfaces before (4410 ± 5 m) and after (4356 ± 5 m) the outburst, and reliably estimated the lake water surface lowering (54 ± 5 m) and the water volume released (19.5 ± 2.2 × 10 6  m 3 ) from the lake, showing good agreement with the results obtained from ground-based measurements. The most relevant conditions that may have influenced the catastrophic drainage of Tam Pokhari in 1998 include the presence of: i) a narrow (75 ± 6 m), steep (up to 50°) and high (120 ± 5 m) moraine dam; ii) high lake level (8 ± 5 m of freeboard); and iii) a steep overhanging glacier (〉40°). The lake outburst substantially altered the immediate area, creating a low and wide (〉500 m) outwash plain below the lake, a wide lake outlet channel (~50 m) and a gentle channel slope (~3–5°). Our new data suggest that the likelihood of a future lake outburst is low. Our results demonstrate that the datasets produced by photogrammetric techniques provide an excellent representation of micro-landform features on moraine dams, lake water surfaces and the changes in both over time, thereby allowing highly accurate pre- and post-GLOF (volumetric) change analysis of glacial lakes. Furthermore, it enables precise measurement of several predictive variables of GLOFs that can be useful for identifying potentially dangerous glacial lakes or prioritizing them for detailed field investigations. This article is protected by copyright. All rights reserved.

Description: Although the importance to account for microrelief in the calculation of specific yields for shallow groundwater systems is well recognized, the microrelief influence is often treated very simplified, which can cause considerable errors. We provide a general one-dimensional expression that correctly represents the effect of a microrelief on the total specific yield that is composed of the soil and surface specific yield. The one-dimensional expression can be applied for different soil hydraulic parameterizations and soil surface elevation frequency distributions. Applying different van Genuchten parameters and a simple linear microrelief model, we demonstrate that the specific yield is influenced by the microrelief not only when surface storage directly contributes to specific yield by (partial) inundation but also when water levels are lower than the minimum surface elevation. Compared to a simplified representation of the soil specific yield, in which a mean soil surface is assumed for the calculation of soil specific yield, the correct representation can lead to lower as well as higher soil specific yields depending on the specific interaction of the soil water retention characteristics and the microrelief. The new equation can be used to obtain more accurate evapotranspiration estimates from water level fluctuations and to account for the effect of microtopographic subgrid variability on simulated water levels of spatially-distributed hydrological models. This article is protected by copyright. All rights reserved.

Description: Globally, Dissolved Inorganic Carbon (DIC) accounts for more than half the annual flux of carbon exported from terrestrial ecosystems via rivers. Here we assess the relative influences of biogeochemical and hydrological processes on DIC fluxes exported from a tropical river catchment characterized by distinct land cover, climate and geology transition from the wet tropical mountains to the low lying savanna plains. Processes controlling changes in river DIC were investigated using dissolved organic carbon (DOC), particulate organic carbon (POC) and DIC concentrations and stable isotope ratios of DIC (δ 13 C DIC ) at two time scales; seasonal and diel. The recently developed Isotopic Continuous Dissolved Inorganic Carbon Analyser (ISO-CADICA) was used to measure diel DIC concentration and δ 13 C DIC changes at a 15 minute temporal resolution. Results highlight the predominance of biologically mediated processes (photosynthesis and respiration) controlling diel changes in DIC. These resulted in DIC concentrations varying between 3.55-3.82 mg/L, and δ 13 C DIC values ranging from -19.7 ± 0.31 to -17.1 ± 0.08 ‰. In contrast, at the seasonal scale we observe wet season DIC variations predominantly from mixing processes, and dry season DIC variations due to both mixing processes and biological processes. The observed wet season increases in DIC concentrations (by 6.81 mg/L) and δ 13 C DIC values of river water (by 5.4 ‰) largely result from proportional increases in subsurface inflows from the savanna plains (C 4 vegetation) region relative to inflows from the rainforest (C 3 vegetation) highlands. The high DIC river load during the wet season results in the transfer of 97% of the annual river carbon load. Therefore, in this gaining river there are significant seasonal variations in both the hydrological and carbon cycles, and there is evidence of substantial coupling between the carbon cycles of the terrestrial and the fluvial environments. Recent identification of a substantial savanna carbon sink in wetter years in the recent past does not take into account the possibility of a substantial, rapid, lateral flux of carbon to rivers and back to the atmosphere. This article is protected by copyright. All rights reserved.

Description: Various remote-sensing methods are available to estimate soil moisture, but few address the fine spatial resolutions (e.g., 30 m grid cells) and root-zone depth requirements of agricultural and other similar applications. One approach that has been previously proposed to estimate fine-resolution soil moisture is to first estimate the evaporative fraction from an energy balance that is inferred from optical and thermal remote-sensing images (e.g., using the ReSET algorithm) and then estimate soil moisture through an empirical relationship to evaporative fraction. A similar approach has also been proposed to estimate the degree of saturation. The primary objective of this study is to evaluate these methods for estimating soil moisture and degree of saturation, particularly for a semiarid grassland with relatively dry conditions. Soil moisture was monitored at twenty-eight field locations in southeastern Colorado with herbaceous vegetation during the summer months of three years. In-situ soil moisture and degree of saturation observations are compared with estimates calculated from Landsat imagery using the ReSET algorithm. The in-situ observations suggest that the empirical relationships with evaporative fraction that have been proposed in previous studies typically provide overestimates of soil moisture and degree of saturation in this region. However, calibrated functions produce estimates with an accuracy that may be adequate for various applications. The estimates produced by this approach are more reliable for degree of saturation than for soil moisture, and the method is more successful at identifying temporal variability than spatial variability in degree of saturation for this region. This article is protected by copyright. All rights reserved.

Description: Groundwater movements in volcanic mountains and their effects on streamflow discharge and representative elementary area (REA) have remained largely unclear. We surveyed the discharge and chemical composition of spring and stream water in two catchments: the Hontani river (NR) catchment (6.6 km 2 ) and the Hosotani river (SR) catchment (4.0 km 2 ) at the southern part of Daisen volcano, Japan. Daisen volcano is a young volcano (17 × 10 3  years) at an early stage of erosion. Our study indicated that deep groundwater that moved through thick lava and pyroclastic flows and that could not be explained by shallow movements controlled by surface topography contributed dominantly to streamflow at larger catchment areas. At the NR catchment, the deep groundwater contribution clearly increased at a catchment boundary defined by an area of 3.0 km 2 and an elevation of 800 m. At the SR catchment, the contribution deep groundwater to the stream also increased suddenly at a boundary threshold of 2.0 km 2 and 700 m. Beyond these thresholds, the contributions of deep bedrock groundwater remained constant, indicating that the REA is between 2 and 3 km 2 at the observed area. These results indicate that the hydrological conditions of base flow were controlled mainly by the deep bedrock groundwater that moved through thick lava and pyroclastic flows in the undissected volcanic body of the upper part of the catchment. Our study demonstrates that deep and long groundwater movements via a deep bedrock layer including thick deposits of volcanic materials at the two catchments on Daisen volcano strongly determined streamflow discharge instead of the mixing of small-scale hydrological conditions. This article is protected by copyright. All rights reserved.

Description: We propose a novel technique for improving a long-term multi-step-ahead streamflow forecasts. A model based on wavelet decomposition and a multivariate Bayesian machine learning approach is developed for forecasting the streamflow three, six, nine and twelve months ahead simultaneously. The inputs of the model utilize only the past monthly streamflow records. They are decomposed into components formulated in terms of wavelet multiresolution analysis. It is shown that the model accuracy can be increased by using the wavelet boundary rule introduced in this study. A simulation study is performed to evaluate the effects of different wavelet boundary rules using synthetic and real streamflow data from the Yellowstone River in the Uinta Basin in Utah. The model based on the combination of wavelet and Bayesian machine learning regression techniques is compared to the wavelet and artificial neural networks based model. The robustness of the models is evaluated. This article is protected by copyright. All rights reserved.

Description: The combined use of water erosion models and geographic information systems (GIS) has facilitated soil loss estimation at the watershed scale. Tools such as the Geo-spatial interface for the Water Erosion Prediction Project (GeoWEPP) model provide a convenient spatially-distributed soil loss estimate but require discretization to identify hillslopes and channels. In GeoWEPP, the TOpographic PArameteriZation (TOPAZ) model is used as an automated procedure to extract a watershed boundary, hillslopes and channels from a digital elevation model (DEM). Previous studies in small watersheds have shown that the size of the hillslopes and the channel distribution affect the model estimates, but in large watersheds the effects on the soil loss estimates have yet to be tested. Therefore, the objective of this study was to evaluate the effect of discretization on the hillslope sediment yield estimates using GeoWEPP in two large watersheds (〉10 km 2 ). The watersheds were selected and discretized varying the TOPAZ parameters (CSA – Critical Source Area, and MSCL – Minimum Source Channel Length) in a 30-m resolution DEM. The drainage networks built with TOPAZ were compared among each other using the drainage density index. The results showed that the discretization affected hillslope sediment yield estimates and their spatial distribution more than total runoff. The drainage density index and the hillslope sediment yield were proportional but inversely related, thus soil loss estimates were highly affected by the spatial discretization. As a result of this analysis, a method to choose the CSA and MSCL values that generates the greatest fraction of hillslopes having profile lengths less than 200 m was developed. This slope length condition is particularly crucial when using the WEPP and GeoWEPP models, in order for them to produce realistic estimates of sheet and rill erosion. Finally, and as a result of this analysis, a more reliable method was developed for selecting the TOPAZ channel network parameters (CSA and MSCL).

Description: Linkages between the controls on surface storage and catchment streamflow response were examined in a wetland dominated basin in the Canadian Prairie Pothole region. Snowmelt, surface storage, water table elevation, atmospheric fluxes, and streamflow were monitored during spring snowmelt and summer in a 1 km 2 sub-catchment containing a semi-permanent pond complex connected via an intermittent stream. Snow accumulation in the basin in spring of the 2013 study year was the largest in the 24-year record. Rainfall totals in 2013 were close to the long term average, though June was an anomalously wet month. The water budget of the pond complex indicates that there was a significant subsurface contribution to surface storage. Activation of an effective transmission zone occurred between uplands and the stream network where the water table was located near the ground surface, which allowed significant lateral movement of subsurface water into the stream network. This was also important for maintaining and re-establishing surface connectivity and streamflow during rainfall events. The observed period of surface-water connectivity was one of the longest on record in the catchment due to unusually wet conditions; nevertheless, the results of this study have implications for how contributing area and runoff should be considered in monitoring and modelling studies in the region, as inclusion of more frequent and varied runoff processes will be essential to understanding changing streamflow regimes. This article is protected by copyright. All rights reserved.

Description: Stream-subsurface exchange plays a significant role in the fate and transport of contaminants in streams. It has been modeled explicitly by considering fundamental processes such as hydraulic exchange, colloid filtration, and contaminant interactions with streambed sediments and colloids. The models have been successfully applied to simulate the transport of inorganic metals and nutrients. In this study, laboratory experiments were conducted in a recirculating flume to investigate the exchange of a hydrophobic organic contaminant (HOC), p,p′ -DDE, between a stream and a quartz sand bed. A previously developed process-based multiphase exchange model was modified by accounting for the p,p′ -DDE kinetic adsorption to and desorption from the bed sediments/colloids and was applied to interpret the experimental results. Model input parameters were obtained by conducting independent small-scale batch experiments. Results indicate that the immobilization of p,p′ -DDE in the quartz sand bed can occur under representative natural stream conditions. The observed p,p′ -DDE exchange was successfully simulated by the process-based model. The model sensitivity analysis results show that the exchange of p,p′ -DDE can be sensitive to either the sediment sorption/desorption parameters or colloidal parameters depending on the experimental conditions tested. For the experimental conditions employed here, the effect of colloids on contaminant transport is expected to be minimal and the stream-subsurface exchange of p,p′ -DDE is dominated by the interaction of p,p′ -DDE with bed sediment. The work presented here contributes to a better mechanistic understanding of the complex transport process that HOCs undergo in natural streams, and to the development of reliable, predictive models for the assessment of impacted streams. This article is protected by copyright. All rights reserved.

Description: The physical and hydrological conditions in extracted peatlands often act as barriers to the regeneration of the keystone peat-forming genus Sphagnum . Although previous work has suggested that Sphagnum mosses regenerating on cutover peat surfaces quickly become vulnerable to water stress as the thickness of the regenerated layer increases, uncertainties regarding the storage and transmission properties of this layer and how these might evolve over time have made this assertion difficult to evaluate. This study investigates the hydrophysical properties and hydrological behaviour of regenerating Sphagnum layers ranging from 3-43 years in age using both field and laboratory methods. The 〉40 year old regenerated layers had significantly (p 〈 0.001) higher bulk density and retention capacity in the 5 cm thick basal layer directly overlying the cutover peat than the newer (〈10 year old) regenerated layers. Capillarity was a much stronger control on surficial water content (θ) than precipitation, which was poorly retained in the Sphagnum canopy, suggesting that regulation of water table position is an effective method of controlling θ as a means of optimizing productivity. In general, the θ sustained at a given water table position decreased as regenerated layer thickness increased. Analysis of water table position relative to the former cutover peat surface in different areas of the site suggests that the soil water dynamics of the 〉40 year old regenerated layers may be becoming increasingly similar to those of a natural bog peatland. This article is protected by copyright. All rights reserved.

Description: We present a novel inverse modeling strategy to estimate spatially distributed parameters of nonlinear models. The maximum a posteriori (MAP) estimators of these parameters are based on a likelihood functional, which contains spatially discrete measurements of the system parameters and spatio-temporally discrete measurements of the transient system states. The piecewise continuity prior for the parameters is expressed via Total Variation (TV) regularization. The MAP estimator is computed by minimizing a non-quadratic objective equipped with the TV operator. We apply this inversion algorithm to estimate hydraulic conductivity of a synthetic confined aquifer from measurements of conductivity and hydraulic head. The synthetic conductivity field is composed of a low-conductivity heterogeneous intrusion into a high-conductivity heterogeneous medium. Our algorithm accurately reconstructs the location, orientation and extent of the intrusion from the steady-state data only. Addition of transient measurements of hydraulic head improves the parameter estimation, accurately reconstructing the conductivity field in the vicinity of observation locations. This article is protected by copyright. All rights reserved.

Description: Human societies are increasingly altering the water and biogeochemical cycles to both improve ecosystem productivity and reduce risks associated with the unpredictable variability of climatic drivers. These alterations, however, often cause large negative environmental consequences, raising the question as to how societies can ensure a sustainable use of natural resources for the future. Here we discuss how ecohydrological modeling may address these broad questions with special attention to agroecosystems. The challenges related to modeling the two-way interaction between society and environment are illustrated by means of a dynamical model in which soil and water quality supports the growth of human society but is also degraded by excessive pressure, leading to critical transitions and sustained societal growth-collapse cycles. We then focus on the coupled dynamics of soil water and solutes (nutrients or contaminants), emphasizing the modeling challenges, presented by the strong nonlinearities in the soil and plant system and the unpredictable hydro-climatic forcing, that need to be overcome to quantitatively analyze problems of soil water sustainability in both natural and agricultural ecosystems. We discuss applications of this framework to problems of irrigation, soil salinization, and fertilization and emphasize how optimal solutions for large-scale, long-term planning of soil and water resources in agroecosystems under uncertainty could be provided by methods from stochastic control, informed by physically and mathematically sound descriptions of ecohydrological and biogeochemical interactions. This article is protected by copyright. All rights reserved.

Description: Water resource management (WRM) through dams or reservoirs is worldwide necessary to support key human-related activities, ranging from hydropower production to water allocation and flood risk mitigation. Designing of reservoir operations aims primarily to fulfil the main purpose (or purposes) for which the structure has been built. However, it is well known that reservoirs strongly influence river geomorphic processes, causing sediment deficits downstream, altering water and sediment fluxes, leading to river bed incision and causing infrastructure instability and ecological degradation. We propose a framework that, by combining physically based modelling, surrogate modelling techniques and Multi-Objective (MO) optimization, allows to include fluvial geomorphology into MO optimization whose main objectives is the maximization of hydropower revenue and the minimization of river bed degradation. The case study is a run-of-the-river power plant on the River Po (Italy). A 1D mobile-bed hydro-morphological model simulated the river bed evolution over a ten year horizon for alternatives operation rules of the power plant. The knowledge provided by such a physically based model is integrated into a MO optimization routine via surrogate modelling using the response surface methodology. Hence, this framework overcomes the high computational costs that so far hindered the integration of river geomorphology into WRM. We provided numerical proof that river morphologic processes and hydropower production are indeed in conflict, but that the conflict may be mitigated with appropriate control strategies. This article is protected by copyright. All rights reserved.

Description: This paper addresses how much flood water can be conserved for use after the flood season through the operation of reservoir by taking into account the residual flood control capacity (the difference between flood conveyance capacity and the expected inflow in a lead time). A two-stage model for dynamic control of the flood limited water level (the maximum allowed water level during the flood season, DC-FLWL) is established considering forecast uncertainty and acceptable flood risk. It is found that DC-FLWL is applicable when the reservoir inflow ranges from small to medium levels of the historical records, while both forecast uncertainty and acceptable risk in the downstream affect the feasible space of DC-FLWL. As forecast uncertainty increases (under a given risk level) or as acceptable risk level decreases (under a given forecast uncertainty level), the minimum required safety margin for flood control increases, and the chance for DC-FLWL decreases. The derived hedging rules from the modeling framework illustrate either the dominant role of water conservation or flood control or the tradeoff between the two objectives under different levels of forecast uncertainty and acceptable risk. These rules may provide useful guidelines for conserving water from flood, especially in the area with heavy water stress. The analysis is illustrated via a case study with a real-world reservoir in northeastern China. This article is protected by copyright. All rights reserved.

Description: Nutrient concentrations in stream water, rainfall, throughfall, stem flow, surface flow and ground water were compared before, during, and after strip thinning (intensive 50%) in plantation forested watersheds in Tochigi, Japan. Influences were evaluated comparing four thinning-applied and two reference basins for one year before, six months during and one year after the thinning. Results show that this strip thinning significantly increased DTP, TP and DOC (DTP: 0.01 mg l -1 , TP: 0.04 mg l -1 , DOC: 0.53 mg l -1 ) during the thinning period and DTN and TN (DTN: 0.34 mg l -1 , TN: 0.46 mg l -1 ) after the thinning in stream waters relative to the unthinned basins. The increased phosphorus during thinning indicated ground disturbances by the strip thinning, with a concomitant increase in DOC. Changes in biotic and abiotic conditions resulted in increased nitrogen after the thinning, particularly in the dissolved pool. Changes in hydrological processes due to thinning, e.g. a change in flow distributions (less high nutrient stem flow and more low nutrient throughfall) and an increase in water discharge in stream water, possibly weakened the direct influences of thinning on nutrient concentrations. This article is protected by copyright. All rights reserved.

Description: Soil water repellency can impact soil hydrology, overland flow generation and associated soil losses. However, current hydrological models do not take it into account, which creates a challenge in repellency-prone regions. This work focused on the adaptation for soil water repellency of a daily water balance model. Repellency is estimated from soil moisture content using site-specific empirical relations, and used to limit maximum soil moisture. This model was developed and tested using c. 2 years of data from one long-unburned and two recently burned eucalypt plantations in northern Portugal, all of which showed strong seasonal soil water repellency cycles. Results indicated important improvements for the burned plantations, with the Nash-Sutcliffe efficiency increasing from -0.55 and -0.49 to 0.55 and 0.65. For the unburned site, model performance was already good without the modification and efficiency only improved slightly from 0.71 to 0.74, mostly due to the better simulation of delayed soil wetting after dry periods. Results suggested that even a simple approach to simulate soil water repellency can markedly improve the performance of hydrological models in eucalypt forests, especially after fire. This article is protected by copyright. All rights reserved.

Description: We present a co-evolutionary view of hydrologic systems, revolving around feedbacks between environmental and social processes operating across different time scales. This brings to the fore an emphasis on emergent phenomena in changing water systems, such as the levee effect, adaptation to change, system lock-in, and system collapse due to resource depletion. Changing human values play a key role in the emergence of these phenomena and should therefore be considered as internal to the system. Guidance is provided for the framing and modeling of these phenomena to test alternative hypotheses about how they arose. A plurality of co-evolutionary models, from stylized to comprehensive system-of-system models, may assist strategic water management for long time scales through facilitating stakeholder participation, exploring the possibility space of alternative futures, and helping to synthesize the observed dynamics in a wide range of case studies. Future research opportunities lie in exploring emergent phenomena arising from time scale interactions through historical, comparative and process studies of human-water feedbacks. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Multiphase-fluid distribution and flow is inherent in numerous areas of hydrology. Yet, pore-scale characterization of transitions between two and three immiscible-fluids is limited. The objective of this study was to examine the impact of such transitions on the pore-scale configuration of organic liquid in a multi-fluid system comprising natural porous media. Three-dimensional images of an organic liquid (trichloroethene) in two-phase (organic-liquid/water) and three-phase (air/organic-liquid/water) systems were obtained using X-ray microtomography before and after drainage and imbibition. Upon transition from a two-phase to a three-phase system, a significant portion of the organic liquid (intermediate wetting fluid) was observed to exist as lenses and films in contact with air (nonwetting fluid). In these cases, the air was either encased by or contiguous to the organic liquid. The presence of air resulted in an increase in the surface-area-to-volume ratios for the organic-liquid blobs. Upon imbibition, the air was displaced downgradient, and concomitantly, the morphology of the organic-liquid blobs no longer in contact with air reverted to that characteristic of a two-phase distribution (i.e., more spherical blobs and ganglia). This change in morphology resulted in a reduction in the surface-area-to-volume ratio. These results illustrate the impact of transitions between two-phase and three-phase conditions on fluid configuration, and they demonstrate the malleable nature of fluid configuration under dynamic, multiphase-flow conditions. The results have implications for characterizing and modeling pore-scale flow and mass-transfer processes. This article is protected by copyright. All rights reserved.

Description: We performed power-spectral analyses on 133 globally distributed lake-level time series after removing annual variability. Lake-level power spectra are found to be power-law functions of frequency over the range of 20 days -1 to 27 years -1 , suggesting that lake levels are globally a f -β -type noise. The spectral exponent (β), i.e. the best-fit slope of the logarithm of the power spectrum to the logarithm of frequency, is a nonlinear function of lake surface area, indicating that lake size is an important control on the magnitude of water-level variability over the range of time scales we considered. A simple cellular model for lake-level fluctuations that reproduces the observed spectral-scaling properties is presented. The model (an adaptation of a surface-growth model with random deposition and relaxation) is based on the equations governing flow in an unconfined aquifer with stochastic inputs and outputs of water (e.g. random storms). The agreement between observation and simulation suggests that lake surface area, spatio-temporal stochastic forcing, and diffusion of the groundwater table are the primary factors controlling lake water-level variability in natural (unmanaged) lakes. Water-level variability is generally considered to be a manifestation of climate trends or climate change, yet our work shows that an input with short or no memory (i.e. weather) gives rise to a long-memory non-stationary output (lake water-level). This work forms the basis for a null hypothesis of lake water-level variability that should be disproven before water-level trends are to be attributed to climate. This article is protected by copyright. All rights reserved.

Description: Describing convective nonwetting phase flow in unsaturated porous media requires knowledge of the nonwetting phase relative permeability. This study was conducted to formulate and derive a generalized expression for the nonwetting phase relative permeability via combining with the Kosugi water retention function. This generalized formulation is then used to flexibly investigate the Burdine, Mualem and Alexander and Skaggs models' prediction accuracy for relative nonwetting phase permeability. The model and data comparison results show that these three permeability models, if used in their original form, but applied to the nonwetting phase, could not predict the experimental data well. The optimum pore tortuosity and connectivity value is thus obtained for the improved prediction of relative nonwetting phase permeability. As a result, the effective parametrization of (α,β,η) parameters in the modified Burdine, modified Mualem and modified Alexander and Skaggs permeability models were found to be (2.5, 2, 1), (2, 1, 2) and (2.5, 1, 1), respectively. These three suggested models display the highest accuracy among the nine relative permeability models investigated in this study. However, the corresponding discontinuous nonwetting phase and the liquid film flow should be accounted for in future for the improved prediction of nonwetting phase relative permeability at very high and very low water saturation range, respectively. This article is protected by copyright. All rights reserved.

Description: A new optical remote sensing technique for estimating water depth from an oblique camera view is described. The water surface and the bed were imaged simultaneously to create time-dependent maps of the water surface velocities and the bed elevations that can be used to validate numerical models at high spatial and temporal resolution. The technique was applied in a sandy meander bend at the University of Minnesota Saint Anthony Falls Laboratory Outdoor StreamLab. The root mean square differences between optical estimates of the bed and in situ observations ranged between 0.01 and 0.03 m. Mean bedform wavelength was 0.73 m and mean crest height was 0.07 m, but both varied with distance around the meander bend. Bedform classification varied with distance downstream, and sinuosity of bedforms varied with local radius of curvature. Bedform roughness scaled similarly to other natural riverine environments although wavelength and height magnitude and variability were larger than predicted by empirical formulations for straight reaches. Bedform translation rate varied between 1 and 5 mm s −1 . Estimates of velocity from particle image velocimetry (PIV) on the water surface were ∼10% higher than in situ observations collected ∼0.05 m below the water surface. Using the PIV observations to drive simple equations for bedload sediment flux, we explained up to 72% of the observed variance in downstream sediment flux. The new methodology described here provides non-intrusive, high spatial and temporal resolution measurements of both the bed and the flow. This article is protected by copyright. All rights reserved.

Description: We study the influence of topography on groundwater fluxes and water table depths across the Contiguous United States (CONUS). Groundwater tables are often conceptualized as subdued replicas of topography. While it is well known that groundwater configuration is also controlled by geology and climate, nonlinear interactions between these drivers within large real world systems are not well understood and are difficult to characterize given sparse groundwater observations. We address this limitation using the fully integrated physical hydrology model ParFlow to directly simulate groundwater fluxes and water table depths within a complex heterogeneous domain that incorporates all three primary groundwater drivers. Analysis is based on a first of its kind, continental scale, high-resolution (1km), groundwater-surface water simulation spanning more than 6.3 million km 2 . Results show that groundwater fluxes are most strongly driven by topographic gradients (as opposed to gradients in pressure head) in humid regions with small topographic gradients or low conductivity. These regions are generally consistent with the topographically controlled groundwater regions identified in previous studies. However, we also show that areas where topographic slopes drive groundwater flux do not generally have strong correlations between water table depth and elevation. Nonlinear relationships between topography and water table depth are consistent with groundwater flow systems that are dominated by local convergence and could also be influenced by local variability in geology and climate. One of the strengths of the numerical modeling approach is its ability to evaluate continental scale groundwater behavior at a high resolution not possible with other techniques. This article is protected by copyright. All rights reserved.

Description: Hydrology is an integrative discipline linking the broad array of water-related research with physical, ecological, and social sciences. The increasing breadth of hydrological research, often where subdisciplines of hydrology partner with related sciences, reflects the central importance of water to environmental science, while highlighting the fractured nature of the discipline itself. This lack of coordination among hydrologic subdisciplines has hindered the development of hydrologic theory and integrated models capable of predicting hydrologic partitioning across time and space. The recent development of the concept of the critical zone (CZ), an open system extending from the top of the canopy to the base of groundwater, brings together multiple hydrological subdisciplines with related physical and ecological sciences. Observations obtained by CZ researchers provide a diverse range of complementary process and structural data to evaluate both conceptual and numerical models. Consequently, a cross-site focus on “critical zone hydrology” has potential to advance the discipline of hydrology and to facilitate the transition of CZ observatories into a research network with immediate societal relevance. Here we review recent work in catchment hydrology and hydrochemistry, hydrogeology, and ecohydrology that highlights a common knowledge gap in how precipitation is partitioned in the critical zone: “ how is the amount, routing, and residence time of water in the subsurface related to the biogeophysical structure of the CZ? ” Addressing this question will require coordination among hydrologic subdisciplines and interfacing sciences, and catalyze rapid progress in understanding current CZ structure and predicting how climate and land cover changes will affect hydrologic partitioning. This article is protected by copyright. All rights reserved.

Description: In the last decades significant technological advances together with improved modeling capabilities fostered a rapid development of geophysical monitoring techniques in support of hydrological modeling. Geophysical monitoring offers the attractive possibility to acquire spatially distributed information on state variables. These provide complementary information about the functioning of the hydrological system to that provided by standard hydrological measurements, which are either intrinsically local or the result of a complex spatial averaging process. Soil water content is an example of state variable, which is relatively simple to measure pointwise (locally) but with a vanishing constraining effect on catchment-scale modeling, while streamflow data, the typical hydrological measurement, offer limited possibility to disentangle the controlling processes. The objective of this work is to analyze the advantages offered by coupling traditional hydrological data with unconventional geophysical information in inverse modeling of hydrological systems. In particular, we explored how the use of time-lapse, spatially distributed microgravity measurements may improve the conceptual model identification of a topographically complex Alpine catchment (the Vermigliana catchment, South-Eastern Alps, Italy). The inclusion of microgravity data resulted in a better constraint of the inversion procedure and an improved capability to identify limitations of concurring conceptual models to a level that would be impossible relying only on streamflow data. This allowed for a better identification of model parameters and a more reliable description of the controlling hydrological processes, with a significant reduction of uncertainty in water storage dynamics with respect to the case when only streamflow data are used. This article is protected by copyright. All rights reserved.

Description: This study describes the use of linearly modulated optically stimulated luminescence (LM-OSL) to distinguish surface-soil derived sediments from those derived from channel bank erosion. LM-OSL signals from quartz extracted from fifteen surface-soil and five channel bank samples were analysed and compared to signals from samples collected from two downstream river sites. Discriminant analysis showed that the detrapping probabilities of fast, first slow and second slow components of the LM-OSL signal can be used to differentiate between the samples collected from the channel bank and surface-soil sources. We show that for each of these source end members these components are all normally distributed. These distributions are then used to estimate the relative contribution of surface-soil derived and channel bank derived sediment to the river bed sediments. The results indicate that channel bank derived sediments dominate the sediment sources at both sites, with 90.1 ± 3% and 91.9 ± 1.9% contributions. These results are in agreement with a previous study which used measurements of 137 Cs and 210 Pb ex fallout radionuclides to estimate the relative contribution from these two sources. This result shows that LM-OSL may be a useful method, at least in the studied catchment, to estimate the relative contribution of surface soil and channel erosion to river sediments. However, further research in different settings is required to test the difference of OSL signals in distinguishing these sediment sources. And if generally acceptable, this technique may provide an alternative to the use of fallout radionuclides for source tracing. This article is protected by copyright. All rights reserved.

Description: The vadose zone plays a crucial role in the water cycle for storing water, providing water to vegetation and transporting solutes or degrading contaminants. Earth scientists have long acknowledged the importance of the vadose zone and numerous methods have been developed to better understand and predict hydrological processes within this “critical zone”. For several decades, stable isotopes ( 18 O and 2 H) of pore water are used as environmental tracers to gain insights into vadose zone water movement and other processes. To determine the pore water stable isotopic composition various sampling procedures have been developed. We present the procedure and the accompanied advantages and drawbacks of each method. We further discuss possible opportunities and limitations regarding the scale of interest and the pore space that is sampled. The methodological review reveals that the choice of the sampling method is crucial for the interpretation of pore water stable isotopes in the vadose zone, but a thorough comparison between the different methods is yet missing. Spiking experiments, where water of known isotopic composition is added to oven-dried soil, have shown to be questionable, since the extracted water is usually depleted compared to the standard water. A comparative study analyzing soil samples with the recently developed direct water-vapor equilibration method and the widely used cryogenic extraction shows deviations, which can only be partly explained, but discloses the need for a more thorough experimental comparative study. Especially promising are developments of continuous isotope measurements based on laser-based spectrometry that will open up new opportunities of analyzing pore water isotopes with higher temporal and spatial resolutions, revealing new insights into hydrological processes across various temporal and spatial scales. This article is protected by copyright. All rights reserved.

Description: In the Vietnamese Mekong Delta (VMD), water levels at some stations have increased. However, the factors that cause this rise in the VMD have not been identified. We considered four factors that may have contributed to the water level rise: (1) increased runoff from upstream, (2) sea-level rise, (3) land subsidence, and (4) decrease in flood mitigation function due to construction of high dykes. We analyzed daily maximum and minimum water levels, and mean daily water levels from 24 monitoring stations from 1987 to 2006. Using daily and annual water level differences, we classified the delta into two groups; one is dominated by flows from upstream, while the other is tide-dominated. We then tested the trends of annual maximum and minimum water levels using the Mann-Kendall test, and identified the slope of the trend using the method of Sen. The areas of dyke construction were estimated using the Enhanced Vegetation Index (EVI) and Land Surface Water Index (LSWI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) data. Results show (1) river inflow has little impact on rising water levels in the VMD, (2) the influence of high dykes on water level rise could not be quantified in this study, (3) both maximum and minimum water levels significantly increased in the tide-dominated area. Trend of annual minimum water level can be considered as the sum sea-level rise and land subsidence. Therefore, we attribute 6.05 mm year −1 (80%) to land subsidence and 1.42 mm year −1 (20%) to sea level rise, indicating inundations have been severe in the VMD, caused primarily by land subsidence. This article is protected by copyright. All rights reserved.

Description: Groundwater is the principal water resource in semi-arid and arid environments. Therefore, quantitative estimates of its replenishment rate are important for managing groundwater systems. In dry regions, karst outcrops often show enhanced recharge rates compared to other surface and sub-surface conditions. Areas with exposed karst features like sinkholes or open shafts allow point recharge, even from single rainfall events. Using the example of the As Sulb plateau in Saudi Arabia, this study introduces a cost-effective and robust method for recharge monitoring and modelling in karst outcrops. The measurement of discharge of a representative small catchment (4.0·10 4  m 2 ) into a sinkhole, and hence the direct recharge into the aquifer, was carried out with a time lapse camera. During the monitoring period of two rainy seasons (autumn 2012 to spring 2014) four recharge events were recorded. Afterwards, recharge data as well as proxy data about the drying of the sediment cover are used to set up a conceptual water balance model. The model was run for 17 years (1971 to 1986 and 2012 to 2014). Simulation results show highly variable seasonal recharge-precipitation ratios between zero and 0.27. In addition to the amount of seasonal precipitation, this ratio is influenced by the interannual distribution of rainfall events. Overall, an average annual groundwater recharge for the doline (sinkhole) catchment on As Sulb plateau of 5.1 mm a −1 has estimated for the simulation period. This article is protected by copyright. All rights reserved.

Description: We present new measurements of bedload tracer transport in a mountain stream over several snowmelt seasons. Cumulative displacements were measured using passive tracers, which consisted of gravel and cobbles embedded with radio frequency identification tags. The timing of bedload motion during eleven transporting events was quantified with active tracers, i.e., accelerometer-embedded cobbles. Probabilities of cobble transport increased with discharge above a threshold, and exhibited slight to moderate hysteresis during snowmelt hydrographs. Dividing cumulative displacements by the number of movements recorded by each active tracer constrained average step lengths. Average step lengths increased with discharge, and distributions of average step lengths and cumulative displacements were thin-tailed. Distributions of rest times followed heavy-tailed power law scaling. Rest time scaling varied somewhat with discharge and with the degree to which tracers were incorporated into the stream bed. The combination of thin-tailed displacement distributions and heavy-tailed rest time distributions predict superdiffusive dispersion. This article is protected by copyright. All rights reserved.

Description: Gas transfer processes are fundamental to the biogeochemical and water quality functions of wetlands, yet there is limited knowledge of the rates and pathways of soil - atmosphere exchange for gases other than oxygen and methane (CH 4 ). In this study we use a novel push-pull technique with sulfur hexafluoride (SF 6 ) and helium (He) as dissolved gas tracers to quantify the kinetics of root-mediated gas transfer, which is a critical efflux pathway for gases from wetland soils. This tracer approach disentangles the effects of physical transport from simultaneous reaction in saturated, vegetated wetland soils. We measured significant seasonal variation in first-order gas exchange rate constants, with smaller spatial variations between different soil depths and vegetation zones in a New Jersey tidal marsh. Gas transfer rates for most biogeochemical trace gases are expected to be bracketed by the rate constants for SF 6 and He, which ranged from ∼10 −2 to 2x10 −1 h −1 at our site. A modified Damköhler number analysis is used to evaluate the balance between biochemical reaction and root-driven gas exchange in governing the fate of environmental trace gases in rooted, anaerobic soils. This approach confirmed the importance of plant gas transport for CH 4 , and showed that root-driven transport may affect nitrous oxide (N 2 O) balances in settings where N 2 O reduction rates are slow This article is protected by copyright. All rights reserved.

Description: Measuring vertically nested temperatures at the streambed interface poses practical challenges that are addressed here with a new discrete subsurface temperature profiling probe. We describe a new temperature probe and its application for heat as a tracer investigations to demonstrate the probe's utility. Accuracy and response time of temperature measurements made at 6 discrete depths in the probe were analyzed in the laboratory using temperature bath experiments. We find the temperature probe to be an accurate and robust instrument that allows for easily installation and long-term monitoring in highly variable environments. Because the probe is inexpensive and versatile, it is useful for many environmental applications that require temperature data collection for periods of several months in environments that are difficult to access or require minimal disturbance. This article is protected by copyright. All rights reserved.

Description: This paper presents an analytical model for describing the tidal effects in a two-dimensional leaky confined aquifer system in an estuarine delta where ocean and river meet. This system has an unconfined aquifer on top, a confined aquifer on the bottom with an aquitard in between the two. The unconfined and confined aquifers interact with each other through leakage. It was assumed that the aquitard storage was negligible, and that the leakage was linearly proportional to the head difference between the unconfined and confined aquifers. This model's solution was based on the separation of variables method. Two existing solutions that deal with the head fluctuation in one-dimensional or two-dimensional leaky confined aquifers are shown as special cases in the present solution. Based on this new solution, the dynamic effect of the water table's fluctuations can be clearly explored, as well as the influence of leakage on the behavior of fluctuations in groundwater levels in the leaky aquifer system. This article is protected by copyright. All rights reserved.

Description: A portable Wavelength Scanned-Cavity Ring-Down Spectrometer (Picarro L2120) fitted with a diffusion sampler (DS-CRDS) was used for the first time to continuously measure δ 18 O and δ 2 H of stream water. The experiment took place during a storm event in a wet tropical agricultural catchment in north-eastern Australia. At a temporal resolution of one minute, the DS-CRDS measured 2160 δ 18 O and δ 2 H values continuously over a period of 36 hours with a precision of ± 0.08 and 0.5 ‰ for δ 18 O and δ 2 H, respectively. Four main advantages in using high temporal resolution stream δ 18 O and δ 2 H data during a storm event are highlighted from this study. Firstly, they enabled us to separate components of the hydrograph, which was not possible using high temporal resolution electrical conductivity data that represented changes in solute transfers during the storm event rather than physical hydrological processes. The results from the hydrograph separation confirm fast groundwater contribution to the stream, with the first 5 hours of increases in stream discharge comprising over 70% pre-event water. Secondly, the high temporal resolution stream δ 18 O and δ 2 H data allowed us to detect a short-lived reversal in stream isotopic values (δ 18 O increase by 0.4 ‰ over 9 minutes), which was observed immediately after the heavy rainfall period. Thirdly, δ 18 O values were used to calculate a time lag of 20 minutes between the physical and chemical stream responses during the storm event. Finally, the hydrograph separation highlights the role of event waters in the runoff transfers of herbicides and nutrients from this heavily cultivated catchment to the Great Barrier Reef. This article is protected by copyright. All rights reserved.

Description: Nanoparticle deposition behavior observed at the Darcy scale represents an average of the processes occurring at the pore scale. Hence, the effect of various pore-scale parameters on nanoparticle deposition can be understood by studying nanoparticle transport at pore scale and upscaling the results to the Darcy scale. In this work, correlation equations for the deposition rate coefficients of nanoparticles in a cylindrical pore are developed as a function of nine pore-scale parameters: the pore radius, nanoparticle radius, mean flow velocity, solution ionic strength, viscosity, temperature, solution dielectric constant, and nanoparticle and collector surface potentials. Based on dominant processes, the pore space is divided into three different regions, namely, bulk, diffusion, and potential regions. Advection-diffusion equations for nanoparticle transport are prescribed for the bulk and diffusion regions, while the interaction between the diffusion and potential regions is included as a boundary condition. This interaction is modeled as a first-order reversible kinetic adsorption. The expressions for the mass transfer rate coefficients between the diffusion and the potential regions are derived in terms of the interaction energy profile. Among other effects, we account for nanoparticle-collector interaction forces on nanoparticle deposition. The resulting equations are solved numerically for a range of values of pore-scale parameters. The nanoparticle concentration profile obtained for the cylindrical pore is averaged over a moving averaging volume within the pore in order to get the 1D concentration field. The latter is fitted to the 1D advection-dispersion equation with an equilibrium or kinetic adsorption model to determine the values of the average deposition rate coefficients. In this study, pore-scale simulations are performed for three values of Péclet number, Pe = 0.05, 5 and 50. We find that under unfavorable conditions, the nanoparticle deposition at pore scale is best described by an equilibrium model at low Péclet numbers ( Pe = 0.05), and by a kinetic model at high Péclet numbers ( Pe = 50). But, at an intermediate Pe (e.g., near Pe = 5), both equilibrium and kinetic models fit the 1D concentration field. Correlation equations for the pore-averaged nanoparticle deposition rate coefficients under unfavorable conditions are derived by performing a multiple-linear regression analysis between the estimated deposition rate coefficients for a single pore and various pore-scale parameters. The correlation equations, which follow a power law relation with nine pore-scale parameters, are found to be consistent with the column-scale and pore-scale experimental results, and qualitatively agree with the colloid filtration theory. These equations can be incorporated into pore network models to study the effect of pore-scale parameters on nanoparticle deposition at larger length scales such as Darcy scale. This article is protected by copyright. All rights reserved.

Description: Safe drinking water is critical to human health and development. In rural sub-Saharan Africa, most improved water sources are boreholes with handpumps; studies suggest that up to one third of these handpumps are non-functional at any given time. This work presents findings from a secondary analysis of cross-sectional data from 1509 water sources in 570 communities in the rural Greater Afram Plains (GAP) region of Ghana; one of the largest studies of its kind. 79.4% of enumerated water sources were functional when visited; in multivariable regressions, functionality depended on source age, management, the number of other sources in the community, and the district. A Bayesian network (BN) model developed using the same dataset found strong dependencies of functionality on implementer, pump type, management, and the availability of tools, with synergistic effects from management determinants on functionality, increasing the likelihood of a source being functional from a baseline of 72% to more than 97% with optimal management and available tools. We suggest that functionality may be a dynamic equilibrium between regular breakdowns and repairs, with management a key determinant of repair rate. Management variables may interact synergistically in ways better captured by BN analysis than by logistic regressions. These qualitative findings may prove generalizable beyond the study area, and may offer new approaches to understanding and increasing handpump functionality and safe water access. This article is protected by copyright. All rights reserved.

Description: Process controls on water, sediment, nutrient and organic carbon exports from the landscape through runoff are not fully understood. This paper provides analyses from 446 sites worldwide to evaluate the impact of environmental factors (MAP and MAT: mean annual precipitation and temperature; CLAY and BD: soil clay content and bulk density; S: slope gradient and LU: land use) on annual exports (R C : runoff coefficients; SL: sediment loads; TOC L : organic carbon losses; TN L : nitrogen losses and TP L : phosphorus losses) from different spatial scales. R C was found to increase, on average, from 18% at local scale (in headwaters), 25% at micro and subcatchment scale (mid-reaches) to 41% at catchment scale (lower reaches of river basins) in response to multiple factors. SL increased from microplots (468 g m −2 yr −1 ) to plots (901 g m −2 yr −1 ), accompanied by decreasing TOC L and TN L . Climate was a major control masking the effects of other factors. For example, R C , SL, TOC L , TN L and TP L tended to increase with MAP at all spatial scales. These variables, however, decreased with MAT. The impact of CLAY, BD, LU and S on erosion variables was largely confined to the hillslope scale, where R C, SL and TOC L decreased with CLAY, while TNL and TP L increased. The results contribute to better understanding of water, nutrient and carbon cycles in terrestrial ecosystems, and should inform river basin modelling and ecosystem management. The important role of spatial climate variability points to a need for comparative research in specific environments at nested spatio-temporal scales. This article is protected by copyright. All rights reserved.

Description: Quantifying the linkages between vegetation disturbance by fire and the changes in hydrologic processes leading to post-fire erosional response remains a challenge. We measured the influence of fire severity, defined as vegetation disturbance (using a satellite derived vegetation disturbance index, VDI), landscape features that organize hydrologic flow pathways (relief and elongation ratios), and pre-fire vegetation type on the probability of the occurrence of post-fire gully rejuvenation (GR). We combined field surveys across 270 burned low-order catchments (112 occurrences of GR) and geospatial analysis to generate a probabilistic model through logistic regression. VDI alone discriminated well between catchments where GR did and did not occur (area under curve = 0.78, model accuracy = 0.72). The strong effect of vegetation disturbance on GR suggests that vegetation exerts a primary influence on the occurrence of infiltration excess runoff and post-fire erosion and that major gully erosion will not occur until fire consumes above ground biomass. Other topographic and local factors also influenced GR response, including catchment elongation, percent pre-fire shrub, mid-slope riparian vegetation, armored headwaters, firehose effects, and concentration of severe burn in source areas. These factors highlight the need to consider vegetation effects in concert with local topography and site conditions to understand the propensity for flow accumulation leading to GR. We present a process-based conceptual hydrologic model where vegetation loss from fire decreases rainfall attenuation and surface roughness, leading to accelerated flow accumulation and erosion; these effects are also influenced by interactions between fire severity and landscape structure. The VDI metric provides a continuous measure of vegetation disturbance and, when placed in a hydrologic context, may improve quantitative analysis of burned-area susceptibility to erosive rainfall, hazard prediction, ecological effects of fire, landform evolution, and sensitivity to climate change. This article is protected by copyright. All rights reserved.

Description: There is considerable interest in naturalizing flow regime on managed rivers to slow the spread of saltcedar ( Tamarix ramosissima ) invasion in southwestern United States or to preserve riparian forests dominated by saltcedar and other species in northwestern China. However, little is known about the responses of established saltcedar in water sources to frequent intra-annual fluctuation of water table resulting from this new, more dynamic flow regime. This study investigates how saltcedar at a riparian site in the middle reaches of the Heihe River, northwest China responds in water sources use to intra-annual water table fluctuations. Stable oxygen isotope was employed to determine accurate depth at which saltcedar obtains its water supply, and soil moisture monitoring was used to determine sources of plant-available soil water. We found that the primary zone of water uptake by saltcedar was stable at 25-60 cm depth, but the water sources used by saltcedar switched between groundwater and soil moisture with the water table fluctuations. Saltcedar derived its water from groundwater when water table was at depth less than 60 cm, but switched to soil moisture at 25-60 cm depth when water table declined. It is supposed that the well-developed clay layer at 60–80 cm depth constrained lateral roots of saltcedar to the soil layers above 60 cm, while the fine-textured soils at this site, which was periodically re-saturated by rising groundwater before the stored soil moisture had become depleted, provided an important water reservoir for saltcedar when groundwater dropped below the primary zone of fine roots. The root distribution of saltcedar may also be related to local groundwater history. The quick decline in water table in the early 1980s when the riparian saltcedar had established may strand its roots in the shallow unsaturated zone. We suggested that raising the water table periodically instead of maintaining it invariably above the rooting depth could sustain desired facultative phreatophytes while maximizing water deliveries. This article is protected by copyright. All rights reserved.

Description: The wettability of CO 2 -brine-rock systems will have a major impact on the management of carbon sequestration in subsurface geological formations. Recent contact angle measurement studies have reported sensitivity in wetting behaviour of this system to pressure, temperature and brine salinity. We report observations of the impact of reservoir conditions on the capillary pressure characteristic curve and and relative permeability of a single Berea sandstone during drainage - CO 2 displacing brine - through effects on the wetting state. Eight reservoir condition drainage capillary pressure characteristic curves were measured using CO 2 and brine in a single fired Berea sandstone at pressures (5 to 20 MPa), temperatures (25 to 50°C) and ionic strengths (0 to 5 mol kg −1 NaCl). A ninth measurement using a N 2 -water system provided a benchmark for capillarity with a strongly water wet system. The capillary pressure curves from each of the tests were found to be similar to the N 2 -water curve when scaled by the interfacial tension. Reservoir conditions were not found to have a significant impact on the capillary strength of the CO 2 -brine system during drainage through a variation in the wetting state. Two steady-state relative permeability measurements with CO 2 and brine and one with N 2 and brine similarly show little variation between conditions, consistent with the observation that the CO 2 -brine-sandstone system is water wetting and multiphase flow properties invariant across a wide range of reservoir conditions. This article is protected by copyright. All rights reserved.

Description: Hillslope-scale rainfall-runoff processes leading to a fast catchment response are not explicitly included in land surface models (LSMs) for use in earth system models (ESMs) due to computational constraints. This study presents a hybrid-3D hillslope hydrological model (h3D) that couples a 1D vertical soil column model with a lateral pseudo-2D saturated zone and overland flow model for use in ESMs. By representing vertical and lateral responses separately at different spatial resolutions, h3D is computationally efficient. The h3D model was first tested for three different hillslope planforms (uniform, convergent and divergent). We then compared h3D (with single and multiple soil columns) with a complex physically-based 3D model and a simple 1D soil moisture model coupled with an unconfined aquifer (as typically used in LSMs). It is found that simulations obtained by the simple 1D model vary considerably from the complex 3D model and are not able to represent hillslope-scale variations in the lateral flow response. In contrast, the single soil column h3D model shows a much better performance and saves computational time by 2-3 orders of magnitude compared with the complex 3D model. When multiple vertical soil columns are implemented, the resulting hydrological responses (soil moisture, water table depth, and baseflow along the hillslope) from h3D are nearly identical to those predicted by the complex 3D model, but still saves computational time. As such, the computational efficiency of the h3D model provides a valuable and promising approach to incorporating hillslope-scale hydrological processes into continental and global-scale ESMs. This article is protected by copyright. All rights reserved.

Description: We investigated potential source areas of dissolved organic carbon (DOC) in headwater streams by examining DOC concentrations in lysimeter, shallow well, and streamwater samples from a reference catchment at the Hubbard Brook Experimental Forest. These observations were then compared to high frequency temporal variations in fluorescent dissolved organic matter (FDOM) at the catchment outlet and the predicted spatial extent of shallow groundwater in soils throughout the catchment. While near-stream soils are generally considered a DOC source in forested catchments, DOC concentrations in near-stream groundwater were low (mean = 2.4 mg/L, standard error = 0.6 mg/L), less than hillslope groundwater farther from the channel (mean = 5.7 mg/L, standard error = 0.4 mg/L). Furthermore, water tables in near-stream soils did not rise into the carbon rich upper B or O horizons even during events. In contrast, soils below bedrock outcrops near channel heads where lateral soil formation processes dominate had much higher DOC concentrations. Soils immediately downslope of bedrock areas had thick eluvial horizons indicative of leaching of organic materials, Fe, and Al and had similarly high DOC concentrations in groundwater (mean = 14.5 mg/L, standard error = 0.8 mg/L). Flow from bedrock outcrops partially covered by organic soil horizons produced the highest groundwater DOC concentrations (mean = 20.0 mg/L, standard error = 4.6 mg/L) measured in the catchment. Correspondingly, streamwater in channel heads sourced in part by shallow soils and bedrock outcrops had the highest stream DOC concentrations measured in the catchment. Variation in FDOM concentrations at the catchment outlet followed water table fluctuations in shallow to bedrock soils near channel heads. We show that shallow hillslope soils receiving runoff from organic matter-covered bedrock outcrops may be a major source of DOC in headwater catchments in forested mountainous regions where catchments have exposed or shallow bedrock near channel heads. This article is protected by copyright. All rights reserved.

Description: ABSTRACT Multiple scenarios of upward CO 2 migration driven by both injection-induced pressure and buoyancy force were investigated in a horizontally and vertically stratified core utilizing a core-flooding system with a 2D X-ray scanner. Two reservoir type scenarios were considered: (1) the terrestrial reservoir scenario (10 MPa and 50°C), where CO 2 exists in a supercritical state and (2) the deep-sea sediment reservoir scenario (28 MPa and 25°C), where CO 2 is stored in the liquid phase. The core-flooding experiments showed a 36% increase in migration rate in the vertical core setting compared with the horizontal setting, indicating the significance of the buoyancy force under the terrestrial reservoir scenario. Under both reservoir conditions, the injected CO 2 tended to find a preferential flow path (low capillary entry pressure and high-permeability (high- k ) path) and bypass the unfavorable pathways, leaving low CO 2 saturation in the low-permeability (low- k ) layers. No distinctive fingering was observed as the CO 2 moved upward, and the CO 2 movement was primarily controlled by media heterogeneity. The CO 2 saturation in the low- k layers exhibited a more sensitive response to injection rates, implying that the increase in CO 2 injection rates could be more effective in terms of storage capacity in the low- k layers in a stratified reservoir. Under the deep-sea sediment condition, the storage potential of liquid CO 2 was more than twice as high as that of supercritical CO 2 under the terrestrial reservoir scenario. In the end, multiphase transport simulations were conducted to assess the effects of heterogeneity on the spatial variation of pressure build-up, CO 2 saturation and CO 2 flux. Finally, we showed that a high gravity number () tended to be more influenced by the heterogeneity of the porous media. This article is protected by copyright. All rights reserved.

Description: Lake water storage change (Δ S w ) is an important indicator of the hydrologic cycle and greatly influences lake expansion/shrinkage over the Tibetan Plateau (TP). Accurate estimation of Δ S w will contribute to improved understanding of lake variations in the TP. Based on a water balance, this study explored the variations of Δ S w for the Lake Selin Co (the largest closed lake on the TP) during 2003-2012 using the Water and Energy Budget-based Distributed Hydrological Model (WEB-DHM) together with two different evapotranspiration (ET) algorithms (the Penman-Monteith method and a simple sublimation estimation approach for water area in unfrozen and frozen period). The contributions of basin discharge and climate causes to the Δ S w are also quantitatively analyzed. The results showed that WEB-DHM could well reproduce daily discharge, the spatial pattern and basin-averaged values of MODIS land surface temperature (LST) during nighttime and daytime. Compared with the ET reference values estimated from the basin-wide water balance, our ET estimates showed better performance than three global ET products in reproducing basin-averaged ET. The modeled ET at point scale matches well with short-term in situ daily measurements (RMSE = 0.82 mm/day). Lake inflows and precipitation over the water area had stronger relationships with Δ S w in the warm season and monthly scale, whereas evaporation from the water area had remarkable effects on Δ S w in the cold season. The total contribution of the three factors to Δ S w was about 90%, and accounting for 49.5%, 22.1% and 18.3%, respectively. This article is protected by copyright. All rights reserved.

Description: Predicting hydro(geo)logical or environmental systems is subject to high levels of uncertainties, especially if appropriate data for model calibration are lacking. For subsurface systems, where data acquisition is cost intensive and time demanding, it is especially important to collect only those data that provide the largest amount of relevant information. The high expenses call for optimal experimental design, which is widely recognized for maximizing the efficiency of experiments. In model-based design of experiments, the analysis of the design efficiency and the resulting optimal design are based on the initial state of knowledge about the modeled system. Joint optimization of multi-measurement designs is a well known challenge and the usefulness of global optimization approaches is widely recognized in this context. However, we will show that the benefit for such global optimization becomes questionable when measurement data become available sequentially. Instead, the optimization effort should be invested within an interactive design approach. Today's fast telecommunication, global connectivity and high-performance computing allow to consider such interactive coupling. This study will use a synthetic case study to compare the standard en-bloc global optimization approach to two interactive design approaches. The approaches are implemented in a Bayesian framework and are compared based on their complexity and overall performance. The key conclusion confirms a previously untested presumption: for models that trigger nonlinear parameter inference problems, interaction (which may come at a loss of global optimization) is more beneficial than global optimization based on the initial state of knowledge (which typically implies the impossibility of interactivity). This article is protected by copyright. All rights reserved.

Description: ABSTRACT Environmental tracers provide information on groundwater age, recharge conditions, and flow processes which can be helpful for evaluating groundwater sustainability and vulnerability. Dissolved noble gas data have proven particularly useful in mountainous terrain because they can be used to determine recharge elevation. However, tracer-derived recharge elevations have not been utilized as calibration targets for numerical groundwater flow models. Herein we constrain and calibrate a regional groundwater flow model with noble-gas-derived recharge elevations for the first time. Tritium and noble gas tracer results improved the site conceptual model by identifying a previously uncertain contribution of mountain-block recharge from the Coast Mountains to an alluvial coastal aquifer in humid southwestern British Columbia. The revised conceptual model was integrated into a three-dimensional numerical groundwater flow model and calibrated to hydraulic head data in addition to recharge elevations estimated from noble gas recharge temperatures. Recharge elevations proved to be imperative for constraining hydraulic conductivity, recharge location and bedrock geometry, and thus minimizing model non-uniqueness. Results indicate that 45% of recharge to the aquifer is mountain-block recharge. A similar match between measured and modeled heads was achieved in a second numerical model that excludes the mountain block (no mountain block recharge), demonstrating that hydraulic head data alone are incapable of quantifying mountain block recharge. This result has significant implications for understanding and managing source water protection in recharge areas, potential effects of climate change, the overall water budget, and ultimately ensuring groundwater sustainability. This article is protected by copyright. All rights reserved.

Description: The probability that new hydraulically fractured wells drilled within the area of New York underlain by the Marcellus Shale will intersect existing an wellbore is calculated using a statistical model, which incorporates: the depth of a new fracturing well, the vertical growth of induced fractures, and the depths and locations of existing nearby wells. The model first calculates the probability of encountering an existing well in plan view and combines this with the probability of an existing well being at sufficient depth to intersect the fractured region. Average probability estimates for the entire region of New York underlain by the Marcellus Shale range from 0.00% to 3.45% based upon the input parameters used. The largest contributing parameter on the probability value calculated is the nearby density of wells meaning that due diligence by oil and gas companies during construction in identifying all nearby wells will have the greatest effect in reducing the probability of interwellbore communication. This article is protected by copyright. All rights reserved.

Description: The idea of complementary evaporative fluxes, first advanced by Bouchet in 1963 is reformulated as a general polynomial, satisfying boundary conditions based on strictly physical considerations. Experimental evidence supports the validity of the imposed constraints. Earlier complementary relationships are shown to be special cases which satisfy only one of the necessary conditions. The new formulation provides a more rigorous base for the complementary principle. This article is protected by copyright. All rights reserved.

Description: A levee failure occurred along the Secchia River, Northern Italy, on January 19, 2014, resulting in flood damage in excess of $500 Million. In response to this failure, immediate surveillance of other levees in the region led to the identification of a second breach developing on the neighboring Panaro River, where rapid mitigation efforts were successful in averting a full levee failure. The paired breach events that occurred along the Secchia and Panaro Rivers provided an excellent window on an emerging levee failure mechanism. In the Secchia River, by combining the information content of photographs taken from helicopters in the early stage of breach development and 10-cm resolution aerial photographs taken in 2010 and 2012, animal burrows were found to exist in the precise levee location where the breach originated. In the Panaro River, internal erosion was observed to occur at a location where a crested porcupine den was known to exist and this erosion led to the collapse of the levee top. This paper uses detailed numerical modeling of rainfall, river flow, and variably saturated flow in the levee to explore the hydraulic and geotechnical mechanisms that were triggered along the Secchia and Panaro Rivers by activities of burrowing animals leading to levee failures. As habitats become more fragmented and constrained along river corridors it is possible that this failure mechanism could become more prevalent and, therefore, will demand greater attention in both the design and maintenance of earthen hydraulic structures as well as in wildlife management. This article is protected by copyright. All rights reserved.

Description: Future water demand is a main consideration in water system management. Consequently, water demand models (WDMs) have evolved in past decades, identifying principal demand-generating factors and modeling their influence on water demand. Regional water systems serve consumers of various types (e.g., municipalities, farmers, industrial regions) and consumption patterns. Thus, one of the challenges in regional water demand modeling is the heterogeneity of the consumers served by the water system. When a high-resolution, regional WDM is desired, accounting for this heterogeneity becomes all the more important. This paper presents a novel approach to regional water demand modeling. The two-step approach includes aggregating the dataset into groups of consumers having similar consumption characteristics, and developing a WDM for each homogeneous group. The development of WDMs is widely applied in the literature and thus, the focus of this paper is to discuss the first step of data aggregation. The research hypothesis is that water consumption records in their original or transformed form can provide a basis for aggregating the dataset into groups of consumers with similar consumption characteristics. This paper presents a methodology for water consumption data clustering by comparing several data representation methods (termed Feature Vectors): monthly normalized average, monthly consumption coefficient of variation, a combination of the monthly average and monthly variation, and the autocorrelation coefficients of the consumption time-series. Clustering using solely normalized monthly average provided homogeneous and distinct clusters with respect to monthly consumption, which succeed in capturing different consumer characteristics (water use, geographical location) that were not specified a-priori. Clustering using the monthly coefficient of variation provided different, yet homogeneous clusters, clustering consumers characterized by similar variation trends that were closely related to consumer water use type. The concatenation of these two Feature Vectors provided further insight into the relationship between consumption patterns and variability of consumers. An autocorrelation Feature Vector provided results that can form a basis for constructing a time-series model that is based on a group of resembling time-series. The approaches presented here are steps towards utilizing the increasing amount of available water consumption data and data analysis techniques to facilitate the modeling of water demands in larger and heterogeneous regions with sufficient resolution. This article is protected by copyright. All rights reserved.

Description: Most of the human appropriation of freshwater resources is for agriculture. Water availability is a major constraint to mankind's ability to produce food. The notion of virtual water content ( VWC ), also known as crop water footprint, provides an effective tool to investigate the linkage between food and water resources as a function of climate, soil and agricultural practices. The spatial variability in the virtual water content of crops is here explored, disentagling its dependency on climate and crop yields, and assessing the sensitivity of VWC estimates to parameter variability and uncertainty. Here we calculate the virtual water content of four staple crops (i.e., wheat, rice, maize, and soybean) for the entire world developing a high-resolution (5 by 5 arc minute) model, and we evaluate the VWC sensitivity to input-parameters. We find that food production almost entirely depends on green water (〉90%), but, when applied, irrigation makes crop production more water efficient, thus requiring less water. The spatial variability of the VWC is mostly controlled by the spatial patterns of crop yields with an average correlation coefficient of 0.83. The results of the sensitivity analysis show that wheat is most sensitive to the length of the growing period, rice to reference evapotranspiration, maize and soybean to the crop planting date. The VWC sensitivity varies not only among crops, but also across the harvested areas of the world, even at the sub-national scale. This article is protected by copyright. All rights reserved.

Description: The dissolution rate of non-aqueous phase liquid (NAPL) often governs the remediation time frame at subsurface hazardous waste sites. Most formulations for estimating this rate are empirical and assume that the NAPL is the non-wetting fluid. However, field evidence suggests that some waste sites might be organic-wet. Thus, formulations that assume the NAPL is non-wetting may be inappropriate for estimating the rates of NAPL dissolution. An exact solution to the Young-Laplace equation, assuming NAPL resides as pendular rings around the contact points of porous media idealized as spherical particles in a hexagonal close packing arrangement, is presented in this work to provide a theoretical prediction for NAPL-water interfacial area. This analytic expression for interfacial area is then coupled with an exact solution to the advection-diffusion equation in a capillary tube assuming Hagen-Poiseuille flow to provide a theoretical means of calculating the mass transfer rate coefficient for dissolution at the NAPL-water interface in an organic-wet system. A comparison of the predictions from this theoretical model with predictions from empirically-derived formulations from the literature for water-wet systems showed a consistent range of values for the mass transfer rate coefficient, despite the significant differences in model foundations (water-wetting vs NAPL-wetting, theoretical vs. empirical). This finding implies that, under these system conditions, the important parameter is interfacial area, with a lesser role played by NAPL configuration. This article is protected by copyright. All rights reserved.

Description: A number of important candidate CO 2 reservoirs exhibit sedimentary architecture reflecting fluvial deposition. Recent studies have led to new conceptual and quantitative models for sedimentary architecture in fluvial deposits over a range of scales that are relevant to CO 2 injection and storage. We used a geocellular modelling approach to represent this multi-scaled and hierarchical sedimentary architecture. With this model, we investigated the dynamics of CO 2 plumes, during and after injection, in such reservoirs. The physical mechanism of CO 2 trapping by capillary trapping incorporates a number of related processes, i.e. residual trapping, trapping due to hysteresis of the relative permeability, and trapping due to hysteresis of the capillary pressure. Additionally CO 2 may be trapped due to differences in capillary entry pressure for different textural sedimentary facies (e.g. coarser- vs. finer-grained cross-sets). The amount of CO 2 trapped by these processes depends upon a complex system of non-linear and hysteretic characteristic relationships including how relative permeability and capillary pressure vary with brine and CO 2 saturation. The results strongly suggest that representing small-scale features (decimeter to meter), including their organization within a hierarchy of larger-scale features, and representing their differences in characteristic relationships, can all be critical to understanding trapping processes in some important candidate CO 2 reservoirs. This article is protected by copyright. All rights reserved.

Description: Under changing environments, not only univariate but also multivariate hydrological series might become nonstationary. Nonstationarity, in forms of change-point or trend, has been widely studied for univariate hydrological series, while it attracts attention only recently for multivariate hydrological series. For multivariate series, two types of change-point need to be distinguished, i.e. change-point in marginal distributions and change-point in the dependence structure among individual variables. In this paper, a three-step framework is proposed to separately detect two types of change-point in multivariate hydrological series, i.e. change-point detection for individual univariate series, estimation of marginal distributions, and change-point detection for dependence structure. The last step is implemented using both the Cramér-von Mises statistic (CvM) method and the copula-based likelihood-ratio test (CLR) method. For CLR, three kinds of copula model (symmetric, asymmetric, and pair-copula) are employed to construct the dependence structure of multivariate series. Monte Carlo experiments indicate that CLR is far more powerful than CvM in detecting the change-point of dependence structure. This framework is applied to the trivariate flood series composed of annual maxima daily discharge (AMDD), annual maxima 3-day flood volume and annual maxima 15-day flood volume of the Upper Hanjiang River, China. It is found that each individual univariate flood series has a significant change-point; and the trivariate series presents a significant change-point in dependence structure due to the abrupt change in the dependence structure between AMDD and annual maxima 3-day flood volume. All these changes are caused by the construction of the Ankang Reservoir. This article is protected by copyright. All rights reserved.

Description: An output-feedback control strategy for pollution mitigation in combined sewer networks is presented. The proposed strategy provides means to apply model-based predictive control to large-scale sewer networks, in-spite of the lack of measurements at most of the network sewers. In previous works, the authors presented a hybrid linear control-oriented model for sewer networks together with the formulation of Optimal Control Problems (OCP) and State Estimation Problems (SEP). By iteratively solving these problems, preliminary Receding Horizon Control with Moving Horizon Estimation (RHC/MHE) results, based on flow measurements, were also obtained. In this work, the RHC/MHE algorithm has been extended to take into account both flow and water level measurements and the resulting control loop has been extensively simulated to assess the system performance according different measurement availability scenarios and rain events. All simulations have been carried out using a detailed physically-based model of a real case-study network as virtual reality. This article is protected by copyright. All rights reserved.

Description: This paper investigates nonpayment behavior in Guatemala. Determinants of nonpayment behavior are identified through zero-inflated negative binomial regression models in order to take into account particular distributional characteristics of the amount of outstanding payments. Findings indicate that nonpayment behavior is a demonstration of consumer dissatisfaction with current water services. The amount of outstanding bill payments also responds to system unreliability. Results also suggest that nonpayment behaviors are more prominent in community-managed systems than in municipal systems. No evidence was found on a potential relationship between nonpayment behavior and household income. Policy implications are discussed. This article is protected by copyright. All rights reserved.

Description: Agricultural practices have altered watershed-scale dissolved organic matter (DOM) dynamics, including in-stream concentration, biodegradability, and total catchment export. However, mechanisms responsible for these changes are not clear, and field-scale processes are rarely directly linked to the magnitude and quality of DOM that is transported to surface water. In a small (12 ha) agricultural catchment in eastern Washington State, we tested the hypothesis that hydrologic connectivity in a catchment is the dominant control over the concentration and quality of DOM exported to surface water via artificial subsurface drainage. Concentrations of dissolved organic carbon (DOC) and humic-like components of DOM decreased while the Fluorescence Index and Freshness Index increased with depth through the soil profile. In drain discharge, these characteristics were significantly correlated with drain flow across seasons and years, with drain DOM resembling deep sources during low flow and shallow sources during high flow, suggesting that DOM from shallow sources bypasses removal processes when hydrologic connectivity in the catchment is greatest. Assuming changes in streamflow projected for the Palouse River (which contains the study catchment) under the A1B climate scenario (rapid growth, dependence on fossil fuel and renewable energy sources) apply to the study catchment, we project greater interannual variability in annual DOC export in the future, with significant increases in the driest years. This study highlights the variability in DOM inputs from agricultural soil to surface water on daily to interannual timescales, pointing to the need for a more nuanced understanding of agricultural impacts on DOM dynamics in surface water. This article is protected by copyright. All rights reserved.

Description: In basement catchments of sub-humid West Africa, baseflow is the main component of annual streamflow. However, the important heterogeneity of lithology hinders the understanding of baseflow generation processes. Since these processes are linked with water storage changes (WSCs) across the catchment, we propose the use of hybrid gravity data in addition to neutron probe-derived water content and water levels to monitor spatiotemporal WSC of a typical crystalline basement headwater catchment (16 ha) in Benin. These behaviors are shown to provide insights into hydrological processes in terms of water redistribution toward the catchment outlet. Hybrid gravimetry produces gravity change observations from time-lapse microgravity surveys coupled with gravity changes monitored at a base station using a superconducting gravimeter and/or an absolute gravimeter. A dense microgravity campaign (70 surveys of 14 stations) covering three contrasted years was set up with a rigorous protocol, leading to low uncertainties (〈 2.5 µGal) on station gravity determinations (with respect to the network reference station). Empirical orthogonal function analyses of both gravity changes and WSCs from neutron probe data show similar spatial patterns in the seasonal signal. Areas where storage and water table show a capping behavior (when data reach a plateau during the wet season), suggesting threshold-governed fast subsurface redistribution, are identified. This observed storage dynamics, together with geological structures investigated by electrical resistivity tomography and drill log analysis make it possible to derive a conceptual model for the catchment hydrology. This article is protected by copyright. All rights reserved.

Description: Groundwater and surface water contain interfaces across which hydrologic functions are discontinuous. Thin elements with high hydraulic conductivity in a porous media focus groundwater, which flows through such inhomogeneities and causes an abrupt change in stream function across their interfaces, and elements with low conductivity retards flow with discontinuous head. Baseflow interactions at the interface between groundwater and surface water transport water between these stores and generate a discontinuous normal component of flow. Thin objects in surface water with Kutta condition generates circulation by the discontinuous tangential component of flow across their interface. These discontinuities across hydrologic interfaces are quantified and visualized using the Analytic Element Method, where slit elements are formulated using the Joukowsky transformation with Laurent series and new influence functions to represent sinks and circulation, and methods are developed for these applications expressing discontinuities as Fourier series. The specific geometries illustrate solutions for a randomly generated heterogeneous porous media with non-intersecting inhomogeneities, for groundwater/surface water interaction in a synthetic river network, and for a slender body with geometry similar to the wings of the Wright Brothers. The mathematical details are reduced to series solutions and matrix multiplications, which are easily extensible to other geometries and applications. This article is protected by copyright. All rights reserved.

Description: Both flow field heterogeneity and mass transfer between mobile and immobile domains have been studied separately for explaining observed anomalous transport. Here, we investigate non-Fickian transport using high-resolution 3D X-ray micro-tomographic images of Berea sandstone containing microporous cement with pore size below the setup resolution. Transport is computed for a set of representative elementary volumes and results from advection and diffusion in the resolved macroporosity (mobile domain) and diffusion in the microporous phase (immobile domain) where the effective diffusion coefficient is calculated from the measured local porosity using a phenomenological model that includes a porosity threshold ( ɸ ϴ ) below which diffusion is null and the exponent n that characterizes tortuosity-porosity power-law relationship. We show that both flow field heterogeneity and microporosity trigger anomalous transport. Breakthrough curve (BTC) tailing is positively correlated to microporosity volume and mobile-immobile interface area. The sensitivity analysis showed that the BTC tailing increases with the value of ɸ ϴ , due to the increase of the diffusion path tortuosity until the volume of the microporosity becomes negligible. Furthermore, increasing the value of n leads to an increase in the standard deviation of the distribution of effective diffusion coefficients, which in turn results in an increase of the BTC tailing. Finally, we propose a continuous time random walk upscaled model where the transition time is the sum of independently distributed random variables characterized by specific distributions. It allows modeling a 1D equivalent macroscopic transport honoring both the control of the flow field heterogeneity and the multi-rate mass transfer between mobile and immobile domains. This article is protected by copyright. All rights reserved.

Description: Secondary circulation is the component of three-dimensional (3-D) flow in river channels perpendicular to the primary flow direction. Secondary circulation calculated from ADCP transects is sensitive to the calculation method, and is affected by the transect angle relative to the mean flow direction and variations in the flow direction along a transect. To quantify bounds on transect alignment relative to river flow for field data collection and examine tidal time-scale variability in secondary circulation, the 3-D hydrodynamic model UnTRIM was applied to simulate the hydrodynamics in the lower reach of the Sacramento River (CA, USA). Secondary circulation was calculated using the Rozovskii and the zero net discharge methods on repeated transects extracted from the model results in regions of both relatively uniform and complex flows. When the depth-averaged flow direction along a transect varied by more than about 5 degrees, occurring when the transect was as little as 10 to 20 degrees out of normal to the mean flow direction, the Rozovskii method produced more realistic secondary circulation than the zero net discharge method. Analysis indicated that ADCP transects should be within 20 degrees of perpendicular to the mean flow direction when calculating secondary circulation. Secondary circulation strength around two tidally influenced bends generally increased with increasing flow and broke down near slack water. However, the strength of the secondary circulation was not only a function of the flow magnitude, but also depended on the direction of the water flow, and the transect location relative to the river curvature, which varied with the tidal flow direction. This article is protected by copyright. All rights reserved.

Description: Using airborne thermal infrared imaging and horizontally-resolved in situ temperature monitoring at the lake surface we estimated strength and duration of regular wind-driven upwelling of dense deep water to the lake surface in two small (in terms of Rossby radius) temperate lakes during the initial phase of summer thermal stratification. The onset and duration of the upwelling events correlated well with the balance between stratification (in terms of Schmidt Stability) and wind forcing, as expressed by Lake and Wedderburn Numbers: The period of regular upwelling appearances lasted 7-15 days, identified by Schmidt stabilities around 30 J m −2 and Lake Numbers between 0 and 1, and resulted in persistent temperature gradients of up to 2°C across the lake surface. Our results suggest that spring upwelling should inevitably take place in all freshwater temperate lakes with mean temperatures crossing the maximum density value of freshwater on annual cycle, whereas duration and intensity of the upwelling would vary depending on lake morphometry and weather conditions. Our results suggest major contribution of upwelling in nutrient supply to the upper waters, oxygenation of the deep water column, and air-lake gas exchange, in particular, the release of the sediment-produced methane into the atmosphere. This article is protected by copyright. All rights reserved.

Description: We provide a comprehensive experimental study of steady state, drainage relative permeability curves with CO 2 -brine and N 2 -deionised water, on a single Bentheimer sandstone core with a simple two-layer heterogeneity. We demonstrate that, if measured in the viscous limit, relative permeability is invariant with changing reservoir conditions, and is consistent with the continuum scale multiphase flow theory for water wet systems. Furthermore, we show that under capillary limited conditions, the CO 2 -brine system is very sensitive to heterogeneity in capillary pressure, and by performing core floods under capillary limited conditions, we produce effective relative permeability curves that are flow rate and fluid parameter dependent. We suggest that the major uncertainty in past observations of CO 2 -brine relative permeability curves is due to the interaction of CO 2 flow with pore space heterogeneity under capillary limited conditions and is not due to the effects of changing reservoir conditions. We show that the appropriate conditions for measuring intrinsic or effective relative permeability curves can be selected simply by scaling the driving force for flow by a quantification of capillary heterogeneity. Measuring one or two effective curves on a core with capillary heterogeneity that is representative of the reservoir will be sufficient for reservoir simulation This article is protected by copyright. All rights reserved.

Description: We examine the prediction capability of two approximate models (Multi Rate Mass Transfer, MRMT, and Continuous Time Random Walk, CTRW) of non-Fickian transport, by comparison with accurate 2D and 3D numerical simulations. Both non-local in time approaches circumvent the need to solve the flow and transport equations by using proxy models to advection, providing the breakthrough curves (BTC) at control planes at any x , depending on a vector of 5 unknown parameters. Although underlain by different mechanisms, the two models have an identical structure in the Laplace Transform domain and have the Markovian property of independent transitions. We show that also the numerical BTCs enjoy the Markovian property. Following the procedure recommended in the literature, along a practitioner perspective, we first calibrate the parameters values by a best fit with the numerical BTC at a control plane at x 1 , close to the injection plane, and subsequently use it for prediction at further control planes for a few values of . Due to a similar structure and Markovian property, the two methods perform equally well in matching the numerical BTC. The identified parameters are generally not unique, making their identification somewhat arbitrary. The inverse Gaussian model and the recently developed Multi Indicator model (MIM), which does not require any fitting as it relates the BTC to the permeability structure, are also discussed. The application of the proxy models for prediction requires carrying out transport field tests of large plumes for a long duration. This article is protected by copyright. All rights reserved.

Description: Virus removal during rapid transport in an unconfined, low-temperature (6°C) sand and gravel aquifer was investigated at a riverbank field site, 25 km south of Trondheim in central Norway. The data from bacteriophage MS2 inactivation and transport experiments were applied in a two-site kinetic transport model using HYDRUS-1D, to evaluate the mechanisms of virus removal and whether these mechanisms were sufficient to protect the groundwater supplies. The results demonstrated that inactivation was negligible to the overall removal, and that irreversible MS2 attachment to aquifer grains, coated with iron precipitates, played a dominant role in the removal of MS2; 4.1 log units of MS2 were removed by attachment during 38 m travel distance and less than 2 days residence time. Although the total removal was high, pathways capable of allowing virus migration at rapid velocities were present in the aquifer. The risk of rapid transport of viable viruses should be recognized, particularly for water supplies without permanent disinfection. This article is protected by copyright. All rights reserved.

Description: Upstream source control and Stormwater Treatment Areas (STAs) have reduced phosphorus (P) loads to Water Conservation Area 2A (WCA-2A), a northern Everglades wetland, by three-quarters since year 2000. Nevertheless, large storages of P remain in enriched peat soils and it is unclear how legacy stores will impact spatial and temporal scales of recovery. We re-measured soil P enrichment along a well-studied eutrophication gradient in WCA-2A and applied a profile modeling approach with uncertainty analysis to assess changes in longitudinal soil P gradients 13-years after load reductions. We then analyzed existing internal water P data, using a novel data screening approach, for evidence of lowest possible water P concentrations independent from inflows. We interpret such water P limits as evidence of the strength of internal loading at a location. Results indicate that soil P enrichment persists in the ∼7.5 km long “impacted” zone, with no significant evidence of net advancement or recession, while a large pool of labile P in the flocculent layer consolidated and diminished. There is indeed evidence, both spatial and temporal, that this extensive zone of enriched soil P continues to elevate lowest achievable water P concentrations. The corresponding gradient of elevated water P limits is both receding and diminishing since load reductions, thus providing further evidence toward recovery. However, results also suggest that these “transitory P limits” due to internal loading are likely to persist for decades above water quality targets. These results advance our understanding of recovery in impacted wetlands and are relevant to Everglades restoration. This article is protected by copyright. All rights reserved.

Description: Numerical modeling approaches with varying complexity were explored to investigate coupled groundwater flow and geochemical processes in saline basins. Long-term model simulations of a playa system gain insights into the complex feedback mechanisms between density driven flow and the spatio-temporal patterns of precipitating evaporites and evolving brines. Using a reactive multicomponent transport model approach the simulations reproduced, for the first time in a numerical study, the evaporite precipitation sequences frequently observed in saline basins (“bull's eyes”). Playa-specific flow, evapo-concentration and chemical divides were found to be the primary controls for the location of evaporites formed, and the resulting brine chemistry. Comparative simulations with the computationally far less demanding surrogate single-species transport models showed that these were still able to replicate the major flow patterns obtained by the more complex reactive transport simulations. However, the simulated degree of salinization was clearly lower than in reactive multicomponent transport simulations. For example, in the late stages of the simulations, when the brine becomes halite-saturated, the non-reactive simulation overestimated the solute mass by almost 20%. The simulations highlight the importance of the consideration of reactive transport processes for understanding and quantifying geochemical patterns, concentrations of individual dissolved solutes and evaporite evolution. This article is protected by copyright. All rights reserved.

Description: Flooding associated with tropical storms can cause extreme perturbations in riverine and coastal ecosystems. Measuring isotope variability of tropical storm events can help investigate the impacts of flooding. We measured the water isotope composition (δD and δ 18 O) of rain and associated floodwater collected during two storms and subsequent major and minor flooding events in the subtropical coast of eastern Australia. Compared to baseline regional rainfall isotope values of -15.0 ± 1.9‰ for δD and -3.3 ± 0.2‰ for δ 18 O, floodwater had lower values with -33.8 ± 2.5‰ δD and -5.1 ± 0.4‰ δ 18 O for the major flood and -29.4 ± 1.0‰ δD and -4.6 ± 0.1‰ δ 18 O for the minor flood. The low isotope composition of the floodwater was associated with the transport of large quantities of suspended sediments, with sediment loads 30 to 70 times larger than during base flow conditions. Floods carried up to 35% of the annual phosphorus and up to 208% of the currently calculated average annual nitrogen load of the Brisbane River. The dramatic changes caused by a rapid increase in discharge from 2 to 2,015 m 3 s -1 over two days in the major flood would have major consequences in riverine and coastal ecosystems of the region. These changes could potentially be traced using the isotope composition of the floodwaters. This article is protected by copyright. All rights reserved.

Description: Large river floods are a key water source for many lakes in fluvial periglacial settings. Where permeable sediments occur, the distribution of permafrost may play an important role in the routing of floodwaters across a floodplain. This relationship is explored for lakes in the discontinuous permafrost of Yukon Flats, interior Alaska, using an analysis that integrates satellite-derived gradients in water surface elevation, knowledge of hydrogeology, and hydrologic modeling. We observed gradients in water surface elevation between neighboring lakes ranging from 0.001 to 0.004. These high gradients, despite a ubiquitous layer of continuous shallow gravel across the flats, are consistent with limited groundwater flow across lake basins resulting from the presence of permafrost. Permafrost impedes the propagation of floodwaters in the shallow subsurface and constrains transmission to “fill-and-spill” over topographic depressions (surface sills), as we observed for the Twelvemile-Buddy Lake pair following a May 2013 ice-jam flood on the Yukon River. Model results indicate that permafrost table deepening of 1–11 m in gravel, depending on watershed geometry and subsurface properties, could shift important routing of floodwater to lakes from overland flow (fill-and-spill) to shallow groundwater flow (“fill-and-seep”). Such a shift is possible in the next several hundred years of ground surface warming, and may bring about more synchronous water level changes between neighboring lakes following large flood events. This relationship offers a potentially useful tool, well-suited to remote sensing, for identifying long-term changes in shallow groundwater flow resulting from thawing of permafrost. This article is protected by copyright. All rights reserved.

Description: Imperviousness, considered as a critical indicator of the hydrologic impacts of urbanization, has gained increasing attention both in the research field and in practice. However, the effectiveness of imperviousness on rainfall-runoff dynamics has not been fully determined in a fine spatiotemporal scale. In this study, 69 drainage subareas 〈 1 ha of a typical residential catchment in Beijing were selected to evaluate the hydrologic impacts of imperviousness, under a typical storm event with a three-year return period. Two metrics, total impervious area (TIA) and effective impervious area (EIA), were identified to represent the impervious characteristics of the selected subareas. Three runoff variables, total runoff depth (TR), peak runoff depth (PR), and lag time (LT), were simulated by using a validated hydrologic model. Regression analyses were developed to explore the quantitative associations between imperviousness and runoff variables. Then, three scenarios were established to test the applicability of the results in considering the different infiltration conditions. Our results showed that runoff variables are significantly related to imperviousness. However, the hydrologic performances of TIA and EIA were scale-dependent. Specifically, with finer spatial scale and the condition heavy rainfall, TIA rather than EIA was found to contribute more to TR and PR. EIA tended to have a greater impact on LT and showed a negative relationship. Moreover, the relative significance of TIA and EIA was maintained under the different infiltration conditions. These findings may provide potential implications for landscape and drainage design in urban areas, which help to mitigate the runoff risk. This article is protected by copyright. All rights reserved.

Description: ABSTRACT A method is presented for using dissolved CFCs or SF 6 to estimate the apparent age of stream base flow by indirectly estimating the mean concentration of the tracer in the inflowing groundwater. The mean value is estimated simultaneously with the mean residence times of the gas and water in the stream by sampling the stream for one or both age tracers, along with dissolved nitrogen and argon at a single location over a period of approximately 12-14 hours. The data are fitted to an equation representing the temporal in-stream gas exchange as it responds to the diurnal temperature fluctuation. The efficacy of the method is demonstrated by collecting and analyzing samples at six different stream locations across parts of northern Virginia, USA. The studied streams drain watersheds with areas of between 2 and 122 km 2 during periods when the diurnal stream temperature ranged between 2 and 5°C. The method has the advantage of estimating the mean groundwater residence time of discharge from the watershed to the stream without the need for the collection of groundwater infiltrating to streambeds or local groundwater sampled from shallow observation wells near the stream. This article is protected by copyright. All rights reserved.

Description: ABSTRACT A between-groups experimental design examined public acceptance for managed aquifer recharge of stormwater for indirect potable and non-potable reuse; acceptance was based on five policy-related variables (fairness, effectiveness, trust, importance of safety assurances, and importance of communication activities). Results showed that public acceptance (N = 408) for managed aquifer recharge of stormwater was higher for non-potable applications, as was the importance of safety assurances. Analyses of variance also showed that perceptions of fairness and effectiveness were higher for a non-potable scheme, but not trust. A three-step hierarchical regression (Step 1: age, gender, education, income; Step 2: type of use; Step 3: fairness, effectiveness, trust, safety assurance, communication activities) demonstrated that type of stormwater use and the policy related factors accounted for 73% of the variance in acceptance of stormwater ( R 2 = .74, adjusted R 2 = .74, F (10, 397) = 113.919, p  〈 .001). Age, type of use and three of the five policy-related factors were also significant individual predictors of acceptance. The most important predictors were perceptions of trust in water authorities, perceptions of effectiveness, and perceptions of fairness. Interestingly, while safety assurance was important in attitudinal acceptance of managed aquifer recharge based on type of use, safety assurance was not found to be significant predictor of acceptance. This research suggests that policy-makers should look to address matters of greater public importance and drive such as fairness, trust and effectiveness of stormwater programs and advocate these at the forefront of their policies, rather than solely on education campaigns. This article is protected by copyright. All rights reserved.

Description: Observations at the field, catchment, and continental scales across a range of arid and semiarid climates and latitudes reveal aspect-controlled patterns in soil properties, vegetation types, ecohydrologic fluxes, and hillslope morphology. Although the global distribution of solar radiation on earth's surface and its implications on vegetation dynamics are well documented, we know little about how variation of solar radiation across latitudes influence landscape evolution and resulting geomorphic difference. Here, we used a landscape evolution model that couples the continuity equations for water, sediment, and aboveground vegetation biomass at each model element in order to explore the controls of latitude and mean annual precipitation (MAP) on the development of hillslope asymmetry (HA). In our model, asymmetric hillslopes emerged from the competition between soil creep and vegetation-modulated fluvial transport, driven by spatial distribution of solar radiation. Latitude was a primary driver of HA because of its effects on the global distribution of solar radiation. In the Northern Hemisphere, north-facing slopes (NFS), which support more vegetation cover and have lower transport efficiency, get steeper toward the North Pole while south-facing slopes (SFS) get gentler. In the Southern Hemisphere, the patterns are reversed and SFS get steeper towards the South Pole. For any given latitude, MAP is found to have minor control on HA. Our results underscore the potential influence of solar radiation as a global control on the development of asymmetric hillslopes in fluvial landscapes. This article is protected by copyright. All rights reserved.

Description: Temporal dynamics of sediment transport in steep channels using two experiments performed in a steep flume (8%) with natural sediment composed of 12 grain sizes are studied. High-resolution (1 sec) time series of sediment transport were measured for individual grain size classes at the outlet of the flume for different combinations of sediment input rates and flow discharges. Our aim in this paper is to quantify (a) the relation of discharge and sediment transport, and (b) the nature and strength of memory in grain-size dependent transport. None of the simple statistical descriptors of sediment transport (mean, extreme values, quantiles) display a clear relation with water discharge, in fact a large variability between discharge and sediment transport is observed. Instantaneous transport rates have probability density functions with heavy tails and bedload bursts have a coarser grain-size distribution than that of the entire experiment. We quantify the strength and nature of memory in sediment transport rates by estimating the Hurst exponent and the autocorrelation coefficient of the time series for different grain sizes. Our results show the presence of the Hurst phenomenon in transport rates, indicating long-term memory which is grain-size dependent. The short-term memory in coarse-grain transport increases with temporal aggregation and this reveals the importance of the sampling duration of bedload transport rates in natural streams, especially for large fractions. This article is protected by copyright. All rights reserved.