ALBERT

Description: ABSTRACT Hydrology has advanced considerably as a scientific discipline since its recognized inception in the mid-20th century. Modern water resource related questions have forced adaptation from exclusively physical or engineering science viewpoints toward a deliberate interdisciplinary context. Over the past few decades, many of the eventual manifestations of this evolution were foreseen by prominent expert hydrologists. However, their narrative descriptions have lacked substantial quantification. This study addressed that gap by measuring the prevalence of and analyzing the relationships between the terms most frequently used by hydrologists to define and describe their research. We analyzed 16,591 journal article titles from 1965-2015 in Water Resources Research , through which the scientific dialogue and its time-sensitive progression emerged. Our word frequency and term co-occurrence network results revealed the dynamic timing of the lateral movement of hydrology across multiple disciplines as well as the deepening of scientific discourse with respect to traditional hydrologic questions. The conversation among water resource scientists surrounding the hydrologic sub-disciplines of catchment-hydrology, hydro-meteorology, socio-hydrology, hydro-climatology and eco-hydrology all gained statistically significant momentum in the analyzed time period, while hydro-geology and contaminant-hydrology experienced periods of increase followed by significant decline. This study concludes that formerly exotic disciplines can potentially modify hydrology, prompting new insights and inspiring unconventional perspectives on old questions that may have otherwise become obsolete. This article is protected by copyright. All rights reserved.

Description: Threshold estimation in the Peaks Over Threshold (POT) method, and the impact of the estimation method on the calculation of high return period quantiles and their uncertainty (or confidence intervals) are issues that are still unresolved. In the past, methods based on goodness-of-fit tests and EDF-statistics have yielded satisfactory results, but their use has not yet been systematized. This paper proposes a methodology for automatic threshold estimation, based on the Anderson-Darling EDF-statistic and goodness-of-fit test. When combined with bootstrapping techniques, this methodology can be used to quantify both the uncertainty of threshold estimation and its impact on the uncertainty of high return period quantiles. This methodology was applied to several simulated series and to four precipitation/riverflow data series. The results obtained confirmed its robustness. For the measured series, the estimated thresholds corresponded to those obtained by non-automatic methods. Moreover, even though the uncertainty of the threshold estimation was high, this did not have a significant effect on the width of the confidence intervals of high return period quantiles. This article is protected by copyright. All rights reserved.

Description: The response of hillslope processes to changes in precipitation may drive the observed changes in the solute geochemistry of rivers with discharge. This conjecture is most robust when variations in the key environmental factors that affect hillslope processes (e.g., lithology, erosion rate, and climate) are minimal across a river's catchment area. For rivers with heterogenous catchments, temporal variations in the relative contributions of different tributary sub-catchments may modulate variations in solute geochemistry with runoff. In the absence of a dense network of hydrologic gauging stations, alternative approaches are required to distinguish between the different drivers of temporal variability in river solute concentrations. In this contribution, we apportion the water and solute fluxes of a reach of the Madre de Dios River (Peru) between its four major tributary sub-catchments during two sampling campaigns (wet and dry seasons) using spatial variations in conservative tracers. Guided by the results of a mixing model, we identify temporal variations in solute concentrations of the mainstem Madre de Dios that are due to changes in the relative contributions of each tributary. Our results suggest that variations in tributary mixing are, in part, responsible for the observed concentration-discharge (C-Q) relationships. The implications of these results are further explored by re-analyzing previously published C-Q data from this region, developing a theoretical model of tributary mixing, and, in a companion paper, comparing the C-Q behavior of a suite of major and trace elements in the Madre de Dios River system. This article is protected by copyright. All rights reserved.

Description: Solving inverse problems in a complex, geologically realistic, and discrete model space and from a sparse set of observations is a very challenging task. Extensive exploration by Markov chain Monte Carlo (McMC) methods often results in considerable computational efforts. Most optimization methods, on the other hand, are limited to linear (continuous) model spaces and the minimization of an objective function, what often proves to be insufficient. To overcome these problems, we propose a new ensemble based exploration scheme for geostatistical prior models generated by a multiple-point statistics (MPS) tool. The principle of our method is to expand an existing set of models by using posterior facies information for conditioning new MPS realizations. The algorithm is independent of the physical parametrization. It is tested on a simple synthetic inverse problem. When compared to two existing McMC methods (Iterative Spatial Resampling (ISR) and Interrupted Markov chain Monte Carlo (IMcMC)) the required number of forward model runs was divided by a factor of 8-12. This article is protected by copyright. All rights reserved.

Description: Although vegetation is present in many rivers, the bulk of past work concerned with modeling the influence of vegetation on flow has considered vegetation to be morphologically simple, and has generally neglected the complexity of natural plants. Here we report on a combined flume and numerical model experiment which incorporates time-averaged plant posture, collected through Terrestrial Laser Scanning, into a Computational Fluid Dynamics model to predict flow around a submerged riparian plant. For three depth-limited flow conditions (Reynolds number = 65 000 – 110 000), plant dynamics were recorded through high-definition video imagery, and the numerical model was validated against flow velocities collected with an acoustic Doppler velocimeter. The plant morphology shows an 18% reduction in plant height and a 14% increase in plant length, compressing and reducing the volumetric canopy morphology as the Reynolds number increases. Plant shear layer turbulence is dominated by Kelvin–Helmholtz type vortices generated through shear instability, the frequency of which is estimated to be between 0.20 and 0.30 Hz, increasing with Reynolds number. These results demonstrate the significant effect that the complex morphology of natural plants has on in-stream drag, and allows a physically determined, species-dependent drag coefficient to be calculated. Given the importance of vegetation in river corridor management, the approach developed here demonstrates the necessity to account for plant motion when calculating vegetative resistance. This article is protected by copyright. All rights reserved.

Description: The vast majority of continental sediment delivered to the world's oceans moves by suspension in rivers. Depth- or point-integrated bottle sampling are the traditional methods used to determine the mean concentration of suspended sediment in rivers. While there has been some investigation of the error associated with depth-integrated sampling, the representativeness of a point-integrated bottle sample has not been addressed in the literature. Here, we analyze continuous hour-long measurements of suspended sediment and grain size fractions collected using a LISST-SL in the sand-bed portion of the Fraser River, British Columbia to determine an appropriate sampling time. The 2σ uncertainty range of individual 30 s samples varied from ±3% to ±33% about the observed mean, with a systematic increase toward the streambed. Mean concentrations for suspended sediment and grain size fractions were computed over increasing time periods and compared with a long duration mean concentration to determine when a sample becomes representative. A cumulative probability distribution was generated from multiple iterations of this process. All suspended sediment load and grain size fractions bear a low probability of representing the actual mean concentration over standard bottle sample durations. A probability 〉90% of representing the mean concentration and grain-size of various fractions requires ∼570 seconds (9.5 minutes) of sampling. Sampling for a shorter period of 264 seconds (4.4 minutes) can yield a sample with 73% probability of representing the mean concentration. This article is protected by copyright. All rights reserved.

Description: Variations in riverine solute chemistry with changing runoff are used to interrogate catchment hydrology and to investigate chemical reactions in Earth's critical zone. This approach requires some understanding of how spatial and temporal averaging of solute-generating reactions affect the dissolved load of rivers and streams. In this study, we investigate the concentration-runoff (C-Q) dynamics of a suite of major (Na, Mg, Ca, Si, K, and SO 4 ) and trace (Al, Ba, Cd, Co, Cr, Cu, Fe, Ge, Li, Mn, Mo, Nd, Ni, Rb, Sr, U, V, and Zn) elements in nested catchments of variable size, spanning the geomorphic gradient from the Andes mountains to the Amazon foreland-floodplain. The major elements exhibit various degrees of dilution with increasing runoff at all sites, whereas the concentrations of most trace elements either increase or show no relationship with increasing runoff in the three larger catchments (160 to 28 000 km 2 area). We show that the observed mainstem C-Q dynamics are influenced by variable mixing of tributaries with distinct C-Q relationships. Trace element C-Q relationships are more variable among tributaries relative to major elements, which could be the result of variations in geomorphology, lithology, and hydrology of the sub-catchments. Certain trace metals are also lost from solution during in-channel processes (possibly related to colloidal size-partitioning), which may exert an additional control on C-Q dynamics. Overall, we suggest that aggregation effects should be assessed in heterogeneous catchments before C-Q or ratio-Q relationships can be interpreted as reflecting catchment-wide solute generation processes and their relationship to hydrology. This article is protected by copyright. All rights reserved.

Description: As modeling capabilities at regional and global scales improve, questions remain regarding the appropriate process representation required to accurately simulate multichannel river hydraulics. This study uses the hydrodynamic model LISFLOOD-FP to simulate patterns of water surface elevation (WSE), depth, and inundation extent across a ∼90 km, anabranching reach of the Tanana River, Alaska. To provide boundary conditions, we collected field observations of bathymetry and WSE during a two-week field campaign in summer 2013. For the first time at this scale, we test a simple, raster-based model's capabilities to simulate 2D, in-channel patterns of WSE and inundation extent. Additionally, we compare finer resolution (≤ 25 m) 2D models to four other models of lower dimensionality and coarser resolution (100–500 m) to determine the effects of simplifying process representation. Results indicate that simple, raster-based models can accurately simulate 2D, in-channel hydraulics in the Tanana. Also, the fine-resolution, 2D models produce lower errors in spatiotemporal outputs of WSE and inundation extent compared to coarse-resolution, 1D models: 22.6 cm vs. 56.4 cm RMSE for WSE, and 90% vs. 41% Critical Success Index values for simulating inundation extent. Incorporating the anabranching channel network using subgrid representations for smaller channels is important for simulating accurate hydraulics and lowers RMSE in spatially distributed WSE by at least 16%. As a result, better representation of the converging and diverging multichannel network by using subgrid solvers or downscaling techniques in multichannel rivers is needed to improve errors in regional to global scale models. This article is protected by copyright. All rights reserved.

Description: Attempts to estimate the influence of erosion on the carbon (C) cycle are limited by difficulties in accounting for the fate of mobilized organic material and for the uncertainty associated with land management practices. This study proposes a method to quantify the uncertainty introduced by the influence of land management on soil organic C (SOC) generation and decomposition at eroding soils. The framework is implemented in tRIBS-ECO (Triangulated Irregular Network-based Real-time Integrated Basin Simulator-Erosion and Carbon Oxidation). tRIBS-ECO is a spatially- and depth-explicit model of C dynamics coupled with a process-based hydro-geomorphic model. We assess the impact of soil erosion on the net soil-atmosphere CO 2 exchange at the Calhoun Critical Zone Observatory, one of the most severely agriculturally eroded regions in the U.S. Measurements of SOC storage are used from different catena positions. We demonstrate that the spatio-temporal variations of land management practices introduce significant uncertainty in estimates of the erosion-induced CO 2 exchange with the atmosphere. Observations and simulations suggest that a substantial portion of eroded organic material is buried in alluvial sediments at the study site. According to results, recent reforestation led to a partial decline in soil and SOC erosion rates. It is suggested that the representation of the fine spatio-temporal variability of the dynamics of eroded C is important in the computation of C budgets in regional and global scales. This article is protected by copyright. All rights reserved.

Description: A 3D refractive-index matching Lagrangian particle tracking (3D-RIM-LPT) system was developed to study the filtration and clogging process inside a homogeneous porous medium. A small subset of particles flowing through the porous medium was dyed and tracked. As this subset was randomly chosen, its dynamics is representative of all the rest. The statistics of particle locations, number, and velocity vectors were obtained as functions of different volumetric concentrations. It is found that, in our system, the clogging time decays with particle concentration following a power law relationship. As the concentration increases, there is a transition from depth filtration to cake filtration. At high concentration, more clogged pores lead to frequent flow redirections and more transverse migrations of particles. In addition, the velocity distribution of the transverse direction is symmetrical around zero, and it is slightly more intermittent than the random Gaussian curve due to particle-particle interactions and particle-grain interactions. In contrast, as clogging develops, the longitudinal velocity of particles along the main flow direction has peak near zero because of those trapped particles. But at the same time, the remaining open pores will experience larger pressure and, as a result, particles through those pores will have a larger longitudinal velocity. This article is protected by copyright. All rights reserved.

Description: Air temperature is correlated with precipitation oxygen isotope (δ 18 O prcp ) variability for much of the eastern and central United States, but the nature of this δ 18 O prcp -temperature relationship is largely based on data coarsely aggregated at a monthly resolution. We constructed a database of 6177 weeks of isotope and precipitation-day air temperature data from 25 sites to determine how more precise data change our understanding of this classic relationship. Because the δ 18 O prcp -temperature relationship is not perfectly linear, trends in the regression residuals suggest the influence of additional environmental factors such as moisture recycling and extratropical cyclone interactions. Additionally, the temporal relationships between δ 18 O prcp and temperature observed in the weekly data at individual sites can explain broader spatial patterns observed across the study region. For 20 of 25 sites, the δ 18 O prcp -temperature relationship slope is higher for colder precipitation than for warmer precipitation. Accordingly, northern and western sites with relatively more cold precipitation events have steeper overall relationships with higher slope values than southeastern sites that have more warm precipitation events. Although the magnitude of δ 18 O prcp variability increases to the north and west, the fraction of δ 18 O prcp variability explained by temperature increases due to wider annual temperature ranges, producing stronger relationships in these regions. When our δ 18 O prcp -temperature data is grouped by month, we observe significant variations in the relationship from month to month. This argues against a principal causative role for temperature and suggests the existence of an alternative environmental control on δ 18 O prcp values that simply co-varies seasonally with temperature.

Description: Immiscible fluid-fluid displacement in permeable media is important in many subsurface processes, including enhanced oil recovery and geological CO 2 sequestration. Controlled by capillary and viscous forces, displacement patterns of one fluid displacing another more viscous one exhibit capillary and viscous fingering, and crossover between the two. Although extensive studies investigated viscous and capillary fingering in porous media, a few studies focused on the crossover in rough fractures, and how viscous and capillary forces affect the crossover remains unclear. Using a transparent fracture-visualization system, we studied how the two forces impact the crossover in a horizontal rough fracture. Drainage experiments of water displacing oil were conducted at seven flow rates (capillary number log 10 Ca ranging from −7.07 to −3.07) and four viscosity ratios ( M =1/1000,1/500,1/100 and 1/50). We consistently observed lower invading fluid saturations in the crossover zone. We also proposed a phase diagram for the displacement patterns in a rough fracture that is consistent with similar studies in porous media. Based on real-time imaging and statistical analysis of the invasion morphology, we showed that the competition between capillary and viscous forces is responsible for the saturation reduction in the crossover zone. In this zone, finger propagation toward the outlet (characteristic of viscous fingering) as well as void-filling in the transverse/backward directions (characteristic of capillary fingering), are both suppressed. Therefore, the invading fluid tends to occupy larger apertures with higher characteristic front velocity, promoting void-filling toward the outlet with thinner finger growth and resulting in a larger volume of defending fluid left behind.

Description: Inter-annual changes in low, median and high regimes of streamflow have important implications for flood control, irrigation, and ecologic and human health. The Gravity Recovery and Climate Experiment (GRACE) satellites record global terrestrial water storage anomalies (TWSA), providing an opportunity to observe, interpret, and potentially utilize the complex relationships between storage and full-flow-regime streamflow. Here we show that utilizable storage-streamflow correlations exist throughout vastly different climates in the continental US (CONUS) across low to high flow regimes. A panoramic framework, the storage-streamflow correlation spectrum (SSCS), is proposed to examine macroscopic gradients in these relationships. SSCS helps form, corroborate or reject hypotheses about basin hydrologic behaviors. SSCS patterns vary greatly over CONUS with climate, land surface and geologic conditions. Data mining analysis suggests that for catchments with hydrologic settings that favor storage over runoff, e.g., a large fraction of precipitation as snow, thick and highly permeable soil, SSCS values tend to be high. Based on our results, we form the hypotheses that groundwater flow dominates streamflows in Southeastern CONUS and Great Plains, while thin soils in a belt along the Appalachian Mountains impose a limit on water storage. SSCS also suggests shallow water table caused by high-bulk density soil and flat terrain induces rapid runoff in several regions. Our results highlight the importance of subsurface properties and groundwater flow in capturing flood and drought. We propose that SSCS can be used as a fundamental hydrologic signature to constrain models and to provide insights that lead us to better understand hydrologic functioning.

Description: Construction of predictive subsurface flow models involves subjective interpretation and interpolation of spatially limited data, often using imperfect modeling assumptions. Hence, the process can introduce significant uncertainty and bias in predicting the flow and transport behavior of these systems. In particular, the uncertainty in the facies distribution in complex geologic environments, such as alluvial/fluvial channels, can be consequential for forecasting the dynamic response of these systems to perturbations due to pumping and development activities. Conventional model calibration techniques that are designed to update continuous model parameters cannot be used to estimate discrete parameters from flow and pressure data. We present a distance transform approach for converting discrete facies models to continuous parameters that can be updated using continuous model calibration methods. Distance transforms are widely used in discrete image processing, where the discrete values in each pixel are replaced with their distance (i.e., a continuous variable) to the nearest boundary cell. After updating the continuous distance maps during model calibration, a back-transformation is applied to retrieve the updated facies maps. To preserve large-scale facies connectivity, truncated singular value decomposition (SVD) parametrization may be used to represent the distance maps with low-rank parameters. A variant of the ensemble smoother, ES-MDA is used to update the continuous parameters of the inversion (either distance maps or their SVD coefficients if used). The distance transform method addresses an important problem in facies model calibration where model updating can result in losing facies connectivity and discreteness.

Description: Introducing water harvesting technology is expected to be more effective and last longer if farm households are involved in their design. The main objective of this study is to inform policymakers in Ethiopia about the most important terms and conditions to incentivize farmers to enter into a contractual agreement to invest in water harvesting on their land. In order to test the influence of the way the specific contractual terms and conditions are communicated to farm households, many of whom are illiterate, a split sample approach is applied with and without visual aids for technical, institutional and economic contract characteristics. Both samples generate significantly different results, highlighting the importance of how information is conveyed to farm households. This pattern is confirmed when examining the self-reported importance attached to the various contract characteristics. Equality constrained latent class models show that contract characteristics for which visual aids were developed are considered more attentively, emphasizing the importance of adequate communication tools in a developing country context where literacy rates are limited to increase water technology innovation uptake and reduce farm household vulnerability to droughts.

Description: The critical condition of incipient motion of cohesive sediments was investigated in a laboratory study. One hundred experimental runs were performed with sediment samples by varying the yield stress to determine the relationship between the critical condition of incipient motion and the rheological properties of the cohesive sediments. The results indicate that yield stress is a factor that has a major influence on the incipient motion of cohesive sediments. In addition, the critical Shields parameter is found to be exponentially proportional to the yield stress and inversely proportional to the median grain size. The effect of yield stress on the critical Shields parameter is significant for the cohesive sediments and becomes progressively weaker with increasing median grain size. Furthermore, an empirical formula for calculating the critical Shields parameter of cohesive sediments that includes a rheological term and a gravity term is proposed by introducing the yield stress. According to this formula, a modified Shields diagram is obtained in which the values of the critical Shields parameter for cohesive sediments vary within a band that contains countless curves (instead of on a single line) to reflect the influence of the yield stress. This modification of the traditional Shields curve is effective for fine sediments, but the effects tend to vanish for coarse sediments as the behavior of sediments changes from cohesive to non-cohesive. Finally, potential further investigations are discussed.

Description: ABSTRACT This study is focused on the water-agriculture-environment nexus as it played out in the Murrumbidgee River Basin, eastern Australia, and how co-evolution of society and water management actually transpired. Over 100 years of agricultural development the Murrumbidgee Basin experienced a “pendulum swing” in terms of water allocation, initially exclusively for agriculture production changing over to reallocation back to the environment. In this paper, we hypothesize that in the competition for water between economic livelihood and environmental wellbeing, economic diversification was the key to swinging community sentiment in favor of environmental protection, and triggering policy action that resulted in more water allocation to the environment. To test this hypothesis, we developed a socio-hydrology model to link the dynamics of the whole economy (both agriculture and industry composed of manufacturing and services) to the community's sensitivity towards the environment. Changing community sensitivity influenced how water was allocated and governed and how the agricultural sector grew relative to the industrial sector (composed of manufacturing and services sectors). In this way we show that economic diversification played a key role in influencing the community's values and preferences with respect to the environment and economic growth. Without diversification, model simulations show that the community would not have been sufficiently sensitive and willing enough to act to restore the environment, highlighting the key role of sectoral transformation in achieving the goal of sustainable agricultural development.

Description: Drainage is known to affect peatland natural hydrology and water quality, but peatland restoration is considered to ameliorate peatland degradation. Using a replicated BACIPS (Before-After-Control-Impact Paired Series) design, we investigated 24 peatlands, all drained for forestry and subsequently restored, and 19 pristine control boreal peatlands with high temporal and spatial resolution data on hydroclimate and pore water quality. In drained conditions, total nitrogen (N tot ), total phosphorus (P tot ), and dissolved organic carbon (DOC) in pore water were several-fold higher than observed at pristine control sites, highlighting the impacts of long-term drainage on pore water quality. In general, pore water DOC and N tot decreased after restoration measures, but still remained significantly higher than at pristine control sites, indicating long time lags in restoration effects. Different peatland classes and trophic levels (vegetation gradient) responded differently to restoration, primarily due to altered hydrology and varying acidity levels. Sites that were hydrologically over-restored (inundated) showed higher P tot , N tot and DOC than well or insufficiently restored sites, indicating the need to optimize natural-like hydrological regimes when restoring peatlands drained for forestry. Rich fens (median pH 6.2-6.6) showed lower pore water P tot , N tot , and DOC than intermediate and poor peats (pH 4.0-4.6) both before and after restoration. Nutrients and DOC in pore water increased in the first year post-restoration, but decreased thereafter. The most important variables related to pore water quality were trophic level, peatland class, watertable level, and soil and air temperature.

Description: Hydrologic exchange fluxes (HEFs) vary significantly along river corridors due to spatio-temporal changes in discharge and geomorphology. This variability results in the emergence of biogeochemical hot-spots and hot-moments that ultimately control solute and energy transport and ecosystem services from the local to the watershed scales. In this work, we use a reduced-order model to gain mechanistic understanding of river bank storage and sinuosity-driven hyporheic exchange induced by transient river discharge. This is the first time that a systematic analysis of both processes is presented and serves as an initial step to propose parsimonious, physics-based models for better predictions of water quality at the large watershed scale. The effects of channel sinuosity, alluvial valley slope, hydraulic conductivity, and river stage forcing intensity and duration are encapsulated in dimensionless variables that can be easily estimated or constrained. We find that the importance of perturbations in the hyporheic zone's flux, residence times, and geometry is mainly explained by two dimensionless variables representing the ratio of the hydraulic time constant of the aquifer and the duration of the event (Γ d ) and the importance of the ambient groundwater flow (Δ h *). Our model additionally shows that even systems with small sensitivity, resulting in small changes in the hyporheic zone extent, are characterized by highly variable exchange fluxes and residence times. These findings highlight the importance of including dynamic changes in hyporheic zones for typical HEF models such as the transient storage model.

Description: The upcoming Surface Water and Ocean Topography (SWOT) mission will measure water surface heights and widths for rivers wider than 100 m. At its native resolution, SWOT height errors are expected to be on the order of meters, which prevents the calculation of water surface slopes and the use of slope-dependent discharge equations. To mitigate height and width errors, the high-resolution measurements will be grouped into reaches (∼5 to 15 km), where slope and discharge are estimated. We describe three automated river segmentation strategies for defining optimum reaches for discharge estimation: 1) arbitrary lengths, 2) identification of hydraulic controls, 3) sinuosity. We test our methodologies on 9 and 14 simulated SWOT overpasses over the Sacramento and the Po Rivers respectively, which we compare against hydraulic models of each river. Our results show that generally, height, width, and slope errors decrease with increasing reach length. However, the hydraulic controls and the sinuosity methods led to better slopes and often height errors that were either smaller or comparable to those of arbitrary reaches of compatible sizes. Estimated discharge errors caused by the propagation of height, width, and slope errors through the discharge equation were often smaller for sinuosity (on average 8.5% for the Sacramento and 6.9% for the Po) and hydraulic controls (Sacramento: 7.3% and Po: 5.9%) reaches than for arbitrary reaches of comparable lengths (Sacramento: 8.6% and Po: 7.8%). This analysis suggests that reach definition methods that preserve the hydraulic properties of the river network may lead to better discharge estimates.

Description: In semiarid regions, where water resources are limited and precipitation dynamics are changing, understanding land surface-atmosphere interactions that regulate the coupled soil moisture-precipitation system is key for resource management and planning. We present a modeling approach to study soil moisture and albedo controls on planetary boundary layer height ( PBL h ). We used Santa Rita Creosote Ameriflux and Tucson Airport atmospheric sounding data to generate empirical relationships between soil moisture, albedo and PBL h . Empirical relationships showed that ∼50% of the variation in PBL h can be explained by soil moisture and albedo with additional knowledge gained by dividing the soil profile into two layers. Therefore, we coupled these empirical relationships with soil moisture estimated using a two-layer bucket approach to model PBL h under six precipitation scenarios. Overall we observed that decreases in precipitation tend to limit the recovery of the PBL at the end of the wet season. However, increases in winter precipitation despite decreases in summer precipitation may provide opportunities for positive feedbacks that may further generate more winter precipitation. Our results highlight that the response of soil moisture, albedo, and the PBL h will depend not only on changes in annual precipitation, but also on the frequency and intensity of this change. We argue that because albedo and soil moisture data are readily available at multiple temporal and spatial scales, developing empirical relationships that can be used in land surface – atmosphere applications have great potential for exploring the consequences of climate change.

Description: Unsaturated flow is an important process in baseflow recessions and its effect is rarely investigated. A mathematical model for a coupled unsaturated-saturated flow in a horizontally unconfined aquifer with time-dependent infiltrations is presented. The effects of the lateral discharge of the unsaturated zone and aquifer compressibility are specifically taken into consideration. Semi-analytical solutions for hydraulic heads and discharges are derived using Laplace transform and Cosine transform. The solutions are compared with solutions of the linearized Boussinesq equation (LB solution) and the linearized Laplace equation (LL solution), respectively. A larger dimensionless constitutive exponent κ D (a smaller retention capacity) of the unsaturated zone leads to a smaller discharge during the infiltration period and a larger discharge after the infiltration. The lateral discharge of the unsaturated zone is significant when κ D ≤1, and becomes negligible when κ D κD≥100. The compressibility of the aquifer has a non-negligible impact on the discharge at early times. For late times, the power index b of the recession curve - dQ / dt ∼ aQ b , is 1 and independent of κ D , where Q is the baseflow and a is a constant lumped aquifer parameter. For early times, b is approximately equal to 3 but it approaches infinity when t 0. The present solution is applied to synthetic and field cases. The present solution matched the synthetic data better than both the LL and LB solutions, with a minimum relative error of 16% for estimate of hydraulic conductivity. The present solution was applied to the observed streamflow discharge in Iowa, and the estimated values of the aquifer parameters were reasonable.

Description: This paper explores the impacts of a water right's allocative priority―as an indicator of farmers' risk-bearing ability―on land irrigation under water supply uncertainty. We develop and use an economic model to simulate farmers' land irrigation decision and associated economic returns in eastern Idaho. Results indicate that the optimal acreage of land irrigated increases with water right priority when hydroclimate risk exhibits a negatively-skewed or right-truncated distribution. Simulation results suggest that prior appropriation enables senior water rights holders to allocate a higher proportion of their land to irrigation, six times as much as junior rights holders do, creating a gap in the annual expected net revenue reaching up to $141.4 acre −1 or $55,800 per farm between the two groups. The optimal irrigated acreage, expected net revenue, and shadow value of a water right's priority are subject to substantial changes under a changing climate in the future, where temporal variation in water supply risks significantly affects the profitability of agricultural land use under the priority-based water sharing mechanism.

Description: The application of heat as a tracer for assessing river-aquifer exchanges has been mainly limited to vertical flow through the riverbed. Lateral river-aquifer exchanges become more important than vertical riverbed exchanges if the river is deeply incised into an aquifer. Few studies have examined lateral river-aquifer exchanges and the ability of heat to constrain such exchanges. This study aims to perform a robust assessment of the limits of heat as a tracer to quantify lateral river-aquifer exchanges. The study is based on a section of the Meuse River in Belgium, a river predominantly gaining in the studied area and becoming intermittently losing in the winter time. A calibrated transect model shows that river temperature can affect groundwater temperature up to 9 m into the aquifer. An accompanying synthetic modelling investigation using Monte Carlo simulation shows that heat data for distances between 4 and 9 m from the river can reduce the uncertainty of river-aquifer exchanges for conditions similar to those of the transect model. The ability of heat to reduce the river-aquifer exchange uncertainty improves with distance from the river because of the reduction in the number of acceptable model realizations. The optimal distance is 8 m from the river where the groundwater temperature is no longer affected by the river temperature. The synthetic modelling also indicates that heat alone cannot constrain river-aquifer exchanges better than the commonly used hydraulic head. However, when combined with hydraulic head, heat can significantly reduce the uncertainty of river-aquifer lateral exchanges under gaining conditions.

Description: Estimating spatially distributed parameters remains one of the biggest challenges for large domain hydrologic modeling. Many large domain modeling efforts rely on spatially inconsistent parameter fields, e.g., patchwork patterns resulting from individual basin calibrations, parameter fields generated through default transfer functions that relate geophysical attributes to model parameters, or spatially constant, default parameter values. This paper provides an initial assessment of a multi-scale parameter regionalization (MPR) method over large geographical domains to derive seamless parameters in a spatially consistent manner. MPR applies transfer functions at the native scale of the geophysical data, and then scales these model parameters to the desired model resolution. We developed a stand-alone framework called MPR-flex for multi-model use and applied MPR-flex to the Variable Infiltration Capacity model to produce hydrologic simulations over the contiguous USA (CONUS). We first independently calibrate 531 basins across the CONUS to obtain a performance benchmark for each basin. To derive the CONUS parameter fields, we perform a joint MPR calibration using all but the poorest behaved basins to obtain a single set of transfer function parameters that are applied to the entire CONUS. Results show that the CONUS-wide calibration has similar performance compared to previous simulations using a patchwork quilt of partially calibrated parameter sets, but without the spatial discontinuities in parameters that characterize some previous CONUS-domain model simulations. Several avenues to improve CONUS-wide calibration remain, including selection of calibration basins, objective function formulation, as well as MPR-flex improvements including transfer function formations and scaling operator optimization.

Description: Hyporheic exchange induced by periodic river fluctuations leads to important biogeochemical processes, particularly nitrogen cycling, in riparian zones (RZs) where chemically distinct surface water and groundwater mix. We developed a two-dimensional coupled flow, reactive transport model to study the role of bank storage induced by river fluctuations on removing river-borne nitrate. Sensitivity analyses were conducted to quantify the effects of river amplitude, sediment hydraulic conductivity and dispersivity, and ambient groundwater flow on nitrate removal rate. The simulations showed that nitrification occurred in the shallower zone adjacent to the bank where oxic river water and groundwater interacted while denitrification occurred deeper into the aquifer and in the riverbed sediments where oxygen was depleted. River fluctuations greatly increased the amount of nitrate being removed; the nitrate removal rate increased as river amplitude increased. Similarly, increasing hydraulic conductivity increased overall nitrate removal since it expanded the denitrifying zone but decreased efficiency. In contrast, increasing sediment dispersivity increased the removal efficiency of nitrate because it promoted mixing between electron acceptors and donors. The presence and direction of ambient groundwater flow had a significant impact on nitrate removal rate when compared to neutral conditions. A losing river showed a larger nitrate removal rate, whereas a gaining river showed a smaller nitrate removal rate. Our results demonstrated that daily river fluctuations created denitrification hot spots within the RZ that would not otherwise exist under naturally neutral or gaining conditions.

Description: Estimating hydraulic conductivity from particle size distribution (PSD) is an important issue for various engineering problems. Classical models such as Hazen model, Beyer model and Kozeny-Carman model usually regard the grain diameter at 10% passing ( d 10 ) as an effective grain size and the effects of particle size uniformity (in Beyer model) or porosity (in Kozeny-Carman model) are sometimes embedded. This technical note applies the dimensional analysis (Buckingham's ∏ theorem) to analyze the relationship between hydraulic conductivity and particle size distribution (PSD). The porosity is regarded as a dependent variable on the grain size distribution in unconsolidated conditions. It indicates that the coefficient of grain size uniformity and a dimensionless group representing the gravity effect, which is proportional to the mean grain volume, are the main two determinative parameters for estimating hydraulic conductivity. Regression analysis is then carried out on a database comprising 431 samples collected from different depositional environments and new equations are developed for hydraulic conductivity estimation. The new equation, validated in specimens beyond the database, shows an improved prediction comparing to using the classic models.

Description: ABSTRACT We derive the early-time solution of the Boussinesq equation for the horizontal unconfined aquifer in the build-up phase under constant recharge and zero inflow. The solution is expressed as a power series of a suitable similarity variable, which is constructed so that to satisfy the boundary conditions at both ends of the aquifer, that is, it is a polynomial approximation of the exact solution. The series turns out to be asymptotic and it is regularized by re-summation techniques that are used to define divergent series. The outflow rate in this regime is linear in time, and the (dimensionless) coefficient is calculated to eight significant figures. The local error of the series is quantified by its deviation from satisfying the self-similar Boussinesq equation at every point. The local error turns out to be everywhere positive, hence, so is the integrated error, which in turn quantifies the degree of convergence of the series to the exact solution.

Description: Vegetation acclimation resulting from elevated atmospheric CO 2 concentration, along with response to increased temperature and altered rainfall pattern, is expected to result in emergent behavior in ecologic and hydrologic functions. We hypothesize that microtopographic variability, which are landscape features typically of the length scale of the order of meters, such as topographic depressions, will play an important role in determining this dynamics by altering the persistence and variability of moisture. To investigates these emergent ecohydrologic dynamics, we develop a modeling framework, Dhara , which explicitly incorporates the control of microtopographic variability on vegetation, moisture, and energy dynamics. The intensive computational demand from such a modeling framework that allows coupling of multi-layer modeling of the soil-vegetation continuum with 3-D surface-subsurface flow processes is addressed using hybrid CPU-GPU parallel computing framework. The study is performed for different climate change scenarios for an intensively managed agricultural landscape in central Illinois, U.S.A., which is dominated by row crop agriculture, primarily soybean ( Glycine max ) and maize ( Zea mays ). We show that rising CO 2 concentration will decrease evapotranspiration, thus increasing soil moisture and surface water ponding in topographic depressions. However, increased atmospheric demand from higher air temperature overcomes this conservative behavior resulting in a net increase of evapotranspiration, leading to reduction in both soil moisture storage and persistence of ponding. These results shed light on the linkage between vegetation acclimation under climate change and microtopography variability controls on ecohydrologic processes.

Description: ABSTRACT Large hydrological systems aggregate compositionally different waters derived from a variety of pathways. In the case of continental-scale rivers, such aggregation occurs noticeably at confluences between tributaries. Here we explore how such aggregation can affect solute concentration-discharge ( C-Q ) relationships and thus obscure the message carried by these relationships in terms of weathering properties of the Critical Zone. We build up a simple model for tributary mixing to predict the behavior of C-Q relationships during aggregation. We test a set of predictions made in the context of the largest world's river, the Amazon. In particular, we predict that the C-Q relationships of the rivers draining heterogeneous catchments should be the most “dilutional” and should display the widest hysteresis loops. To check these predictions, we compute 10 day-periodicity time series of Q and major solute (Si, Ca 2+ , Mg 2+ , K + , Na + , Cl - , SO 4 2- ) C and fluxes ( F ) for 13 gauging stations located throughout the Amazon basin. In agreement with the model predictions, throughout the Amazon Basin C-Q relationships of most solutes shift from fairly a “chemostatic” behavior (nearly constant C ) at the Andean mountain front and in pure lowland areas, to more “dilutional” patterns (negative C-Q relationship) towards the system mouth. More prominent C-Q hysteresis loops are also observed at the most downstream stations. Altogether, this study suggests that mixing of water and solutes between different flowpaths exerts a strong control on C-Q relationships of large-scale hydrological systems.

Description: We consider transport of a conservative solute through an aquifer as determined: i) by the advective velocity, which depends upon the hydraulic conductivity K , and ii) by the local spreading due to the pore - scale dispersion (PSD). The flow is steady, and it takes place in a porous formation where, owing to its erratic spatial variations, the hydraulic log-conductivity Y ≡ ln K is modeled as a stationary Gaussian random field. The relative effect of the above mechanisms i)-ii) is quantified by the Peclet number (Pe) which, in most of the previous studies, was considered infinite (i.e. no PSD) due to the overtake of advective heterogeneities upon the PSD. Here, we aim at generalizing such studies by accounting for the impact of finite Pe on conservative transport. Previous studies on the topic required extensive numerical computations [see, e.g. Fiori , 1996]. In the present note we remove the computational burden by adopting the rational approximate expression of Dagan and Cvetkovic [1993] for the covariance of the velocity field. This allows one to obtain closed form expressions for the quantities characterizing the longitudinal plume's dispersion. Transport can be straightforwardly investigated by dealing with a modified Peclet number ( ) incorporating both the PSD and the aquifer's anisotropy. The satisfactory match to Cape Cod field data suggests that the present theoretical results lend themselves as a useful tool to assess the impact of the PSD upon conservative transport through heterogeneous porous formations.

Description: Critical zone science seeks to develop mechanistic theories that describe critical zone structure, function and long-term evolution. One postulate is that hydrogeochemical controls on critical zone evolution can be inferred from solute discharges measured down-gradient of reactive flow paths. These flow paths have variable lengths, interfacial compositions, and residence times, and their mixing is reflected in concentration-discharge (C-Q) relations. Motivation for this special section originates from a U.S. Critical Zone Observatories workshop that was held at the University of New Hampshire, July 20-22, 2015. The workshop focused on resolving mechanistic CZ controls over surface water chemical dynamics across the full range of lithogenic (e.g., non-hydrolyzing and hydrolyzing cations and oxyanions) and bioactive solutes (e.g., organic and inorganic forms of C, N, P, S), including dissolved and colloidal species that may co-occur for a given element. Papers submitted to this special section on “concentration-discharge relations in the critical zone” include those from authors who attended the workshop, as well as others who responded to the open solicitation. Submissions were invited that utilized information pertaining to internal, integrated catchment function (relations between hydrology, biogeochemistry and landscape structure) to help illuminate controls on observed C-Q relations.

Description: Fluid flow in porous media is very important in a wide range of science and engineering applications. The entire establishment of fluid flow application in porous media is based on the use of an experimental law proposed by Darcy in 1856. There are evidences in the literature that the flow of a fluid in consolidated and unconsolidated porous media does not follow Darcy law at very low fluxes, which is called pre-Darcy flow. In this paper, the unsteady flow regimes of a slightly compressible fluid under the linear and radial pre-Darcy flow conditions are modeled and the corresponding highly nonlinear diffusivity equations are solved analytically by aid of a generalized Boltzmann transformation technique. The influence of pre-Darcy flow on the pressure diffusion for homogenous porous media is studied in terms of the nonlinear exponent and the threshold pressure gradient. In addition, the pressure gradient, flux, and cumulative production per unit area are compared with the classical solution of the diffusivity equation based on Darcy flow. The presented results advance our understanding of fluid flow in low permeability media such as shale and tight formations where pre-Darcy is the dominant flow regime.

Description: We used paired continuous nitrate (NO 3 - ) measurements along a tidally-affected river receiving wastewater discharge rich in ammonium (NH 4 + ) to quantify rates of change in NO 3 - concentration ( ) and estimate nitrification rates. NO 3 - sensors were deployed 30 km apart in the Sacramento River, California (USA), with the upstream station located immediately above the regional wastewater treatment plant (WWTP). We used a travel-time model to track water transit between the stations and estimated every 15-minutes (October 2013-September 2014). Water temperature was strongly related to changes in NO 3 - concentration. In the presence of wastewater, was generally positive, ranging from about 7 µM d −1 in the summer to near zero in the winter. Numerous periods when the WTTP halted discharge allowed the to be estimated under no-effluent conditions, and revealed that in the absence of effluent net gains in NO 3 - were substantially lower but still positive in the summer and negative (net sink) in the winter. Nitrification rates of effluent derived NH 4 ( R Nitrific_E ) were estimated from the difference between measured in the presence versus absence of effluent, and ranged from 1.5-3.4 µM d −1 , which is within literature values but ten-fold greater than recently reported for this region. R Nitrific_E was generally lower in winter (∼2 µM d −1 ) than summer (∼3 µM d −1 ). This in situ, high frequency approach provides advantages over traditional discrete sampling, incubation, and tracer methods, and allows measurements to be made over broad areas for extended periods of time. Incorporating this approach into environmental monitoring programs will facilitate our ability to protect and manage aquatic systems.

Description: Building reservoir release schedules to manage engineered river systems can involve costly tradeoffs between storing and releasing water. As a result, the design of release schedules requires metrics that quantify the benefit and damages created by releases to the downstream ecosystem. Such metrics should support making operational decisions under uncertain hydrologic conditions, including drought and flood seasons. This study addresses this need and develops a reservoir operation rule structure and method to maximize downstream environmental benefit while meeting human water demands. The result is a general approach for hedging downstream environmental objectives. A multi-stage stochastic mixed-integer non-linear program with Markov Chains, identifies optimal "environmental hedging," releases to maximize environmental benefits subject to probabilistic seasonal hydrologic conditions, current, past, and future environmental demand, human water supply needs, infrastructure limitations, population dynamics, drought storage protection, and the river's carrying capacity. Environmental hedging ‘hedges bets' for drought by reducing releases for fish, sometimes intentionally killing some fish early to reduce the likelihood of large fish kills and storage crises later. This approach is applied to Folsom reservoir in California to support survival of fall-run Chinook salmon in the Lower American River for a range of carryover and initial storage cases. Benefit is measured in terms of fish survival; maintaining self-sustaining native fish populations is a significant indicator of ecosystem function. Environmental hedging meets human demand and outperforms other operating rules, including the current Folsom operating strategy, based on metrics of fish extirpation and water supply reliability.

Description: Risk management and climate adaptation literature focuses mainly on reducing the impacts of, exposure to and vulnerability to extreme events such as floods and droughts. Posttraumatic stress disorder is one of the most important impacts related to these events, but also a relatively under-researched topic outside original psychopathological contexts. We conduct a survey to investigate the mental stress caused by floods. We focus on hydrological, individual and collective drivers of posttraumatic stress. We assess stress with flood-specific health scores and the GHQ-12 General Health Questionnaire. Our findings show that the combination of water depth and flood velocity measured via a Hazard Class Index is an important stressor; and that mental health resilience can be significantly improved by providing the population with adequate information. More specifically, the paper shows that psychological distress can be reduced by i ) coordinating awareness of flood risks and flood protection and prevention behaviour; ii ) developing the ability to protect oneself from physical, material and intangible damage; iii ) designing simple insurance procedures and protocols for fast recovery; and iv ) learning from previous experiences.

Description: ABSTRACT Transport of fine-grained sediment from unpaved forest roads into streams is a concern due to the potential negative effects of additional suspended sediment on aquatic ecosystems. Here, we compared turbidity and suspended sediment concentration (SSC) dynamics in five non-fish bearing coastal Oregon streams above and below road crossings, during three consecutive time periods (‘before', ‘after road construction/improvement', and ‘after forest harvest and hauling'). We hypothesized that the combined effects of road construction/improvement and the hauling following forest harvest would increase turbidity and SSC in these streams. We tested whether the differences between paired samples from above and below road crossing exceeded various biological thresholds, using literature values of biological responses to increases in SSC and turbidity. Overall, we found minimal increases of both turbidity and SSC after road improvement, forest harvest, and hauling. Because flow is often used as a surrogate for turbidity or SSC we examined these relationships using data from locations above road crossings that were unaffected by roads or forest harvest and hauling. In addition, we examined the association between turbidity and SSC for these background locations. We found a positive, but in some cases weak association between flow and turbidity, and between flow and SSC; the relationship between turbidity and SSC was more robust, but also inconsistent among sites over time. In these low order streams, the concentrations and transport of suspended sediment seems to be highly influenced by the variability of local conditions. Our study provides an expanded understanding of current forest road management practice effects on fine-grained sediment in streams and introduces alternative metrics using multiple thresholds to evaluate potential indicators of biological relevance.

Description: Organic matter and nutrient export from drained peatlands is affected by complex hydrological and biogeochemical interactions. Here, partial least squares regression (PLSR) was used to relate various soil and catchment characteristics to variations in chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) concentrations in runoff. Peat core samples and water quality data were collected from 15 peat extraction sites in Finland. PLSR models constructed by cross-validation and variable selection routines predicted 92, 88, and 95% of the variation in mean COD, TN, and TP concentration in runoff, respectively. The results showed that variations in COD were mainly related to net production (temperature and water-extractable dissolved organic carbon (DOC)), hydrology (topographical relief), and solubility of dissolved organic matter (peat sulfur (S) and calcium (Ca) concentrations). Negative correlations for peat S and runoff COD indicated that acidity from oxidation of organic S stored in peat may be an important mechanism suppressing organic matter leaching. Moreover, runoff COD was associated with peat aluminum (Al), P, and sodium (Na) concentrations. Hydrological controls on TN and COD were similar (i.e., related to topography), whereas degree of humification, bulk density, and water-extractable COD and Al provided additional explanations for TN concentration. Variations in runoff TP concentration were attributed to erosion of particulate P, as indicated by a positive correlation with suspended sediment concentration (SSC), and factors associated with metal-humic complexation and P adsorption (peat Al, water-extractable P, and water-extractable iron (Fe)).

Description: Hydraulic conductivity ( K ) and specific yield ( S y ) are important aquifer parameters, pertinent to groundwater resources management and protection. These parameters are commonly estimated through a traditional cross-well pumping test. Employing the traditional approach to obtain detailed spatial distributions of the parameters over a large area is generally formidable. For this reason, this study proposes a stochastic method that integrates hydraulic head and time-lapse gravity based on hydraulic tomography (HT) to efficiently derive the spatial distribution of K and S y over a large area. This method is demonstrated using several synthetic experiments. Results of these experiments show that the K and S y fields estimated by joint inversion of the gravity and head data set from sequential injection tests in unconfined aquifers are superior to those from the HT based on head data alone. We attribute this advantage to the mass constraint imposed on HT by gravity measurements. Besides, we find that gravity measurement can detect the change of aquifer's groundwater storage at kilometer scale, as such they can extend HT's effectiveness over greater volumes of the aquifer. Furthermore, we find that the accuracy of the estimated fields is improved as the number of the gravity stations is increased. The gravity station's location, however, has minor effects on the estimates if its effective gravity integration radius covers the well field.

Description: In this study, we present the improved Rangeland Hydrology and Erosion Model (RHEM V2.3), a process-based erosion prediction tool specific for rangeland application. The article provides the mathematical formulation of the model and parameter estimation equations. Model performance is assessed against data collected from 23 runoff and sediment events in a shrub-dominated semiarid watershed in Arizona, USA. To evaluate the model, two sets of primary model parameters were determined using the RHEM V2.3 and RHEM V1.0 parameter estimation equations. Testing of the parameters indicated that RHEM V2.3 parameter estimation equations provided a 76% improvement over RHEM V1.0 parameter estimation equations. Second, the RHEM V2.3 model was calibrated to measurements from the watershed. The parameters estimated by the new equations were within the lowest and highest values of the calibrated parameter set. These results suggest that the new parameter estimation equations can be applied for this environment to predict sediment yield at the hillslope scale. Furthermore, we also applied the RHEM V2.3 to demonstrate the response of the model as a function of foliar cover and ground cover for 124 data points across Arizona and New Mexico. The dependence of average sediment yield on surface ground cover was moderately stronger than that on foliar cover. These results demonstrate that RHEM V2.3 predicts runoff volume, peak runoff, and sediment yield with sufficient accuracy for broad application to assess and manage rangeland systems.

Description: Landslides typically alter hillslope topography, but may also change the hydrologic connectivity and subsurface water-storage dynamics. In settings where mobile materials are not completely evacuated from steep slopes, influences of landslide disturbances on hillslope hydrology and susceptibility to subsequent failures remain poorly characterized. Since landslides often recur at the site of previous failures, we examine differences between a stable vegetated hillslope (VH) and a recent landslide (LS). These neighboring hillslopes exhibit similar topography and are situated on steep landslide-prone coastal bluffs of glacial deposits along the northeastern shore of Puget Sound, Washington. Our control hillslope, VH, is mantled by a heterogeneous colluvium, supporting a dense forest. In early 2013, our test hillslope, LS, also supported a forest before a landslide substantially altered the topography and disturbed the hillslope. In 2015, we observed a clay-rich landslide deposit at LS with sparse vegetation and limited root reinforcement, soil structures, and macropores. Our characterization of the sites also found matrix porosity and hydraulic conductivity are both lower at LS. Continuous monitoring during 2015-2016 revealed reduced effective precipitation at VH (due to canopy interception), an earlier seasonal transition to near-saturated conditions at LS, and longer persistence of positive pore pressures and slower drainage at LS (both seasonally and between major storm events). These differences, along with episodic, complex slope failures at LS support the hypothesis that, despite a reduced average slope, other disturbances introduced by landsliding may promote the hydrologic conditions leading to slope instability, thus contributing to the persistence of landslide hazards.

Description: In humid tropical watersheds, the hydrologic flow paths taken by rain event waters and how they interact with groundwater and soil matrix water to form streamflow are poorly understood. Preferential flow paths (PFPs) confound storm infiltration processes, especially in the humid tropics where PFPs are common. This work applies germanium (Ge) and silicon (Si) as natural flow path tracers in conjunction with water stable isotopes and electrical conductivity to examine the rapid delivery of shallow soil water, the activation of PFPs, and event water partitioning in an experimental catchment in central Panama. We employed a three component mixing model for hydrograph separation using the following end-member waters: (i) baseflow (high [Si], low ≥, low Ge/Si ratio), (ii) dilute canopy throughfall (low [Si], low ≥), and (iii) shallow (〈15 cm) soil matrix water (low [Si], high ≥, high Ge/Si ratio). These three end-members bounded all observed Ge/Si streamflow ratios. During small rain events (〈 ∼24 mm), baseflow and dilute canopy throughfall components dominated stormflow. During larger precipitation events (〉 ∼35 mm), we detected the third shallow soil water component with an elevated ≥ and Ge/Si ratio. This component reached its maximum during the hydrograph's receding limb coincident with the maximum event fraction, and increased proportionally to the total storm rainfall exceeding ∼35 mm. Only shallow (〈15 cm) soil matrix water exhibited elevated Ge concentrations and high Ge/Si ratios. This third component represents rapidly-delivered soil matrix water combined with shallow lateral PFP activation through which event waters interact with soil minerals.

Description: California's Central Valley region has been called the “bread-basket” of the United States. The region is home to one of the most productive agricultural systems on the planet. Such high levels of agricultural productivity require large amounts of fresh water for irrigation. However, the long-term availability of water required to sustain high levels of agricultural production is being called into question following the latest drought in California. In this paper, we use Bayesian multilevel spatiotemporal modeling techniques to examine the influence of the structure of surface water rights in the Central Valley on agricultural production during the recent drought. California is an important place to study these dynamics as it is the only state to recognize the two dominant approaches to surface water management in the United States: riparian and appropriative rights. In this study, Bayesian spatiotemporal modeling is employed to account for spatial processes that have the potential to influence the effects of water right structures on agricultural production. Results suggest that, after accounting for spatiotemporal dependencies in the data, seniority in surface water access significantly improves crop health and productivity on cultivated lands, but does not independently affect the ability to maintain cultivated extent. In addition, agricultural productivity in watersheds with more junior surface water rights show less sensitivity to cumulative drought exposure than other watersheds, however the extent of cultivation in these same watersheds is relatively more sensitive to cumulative drought exposure.

Description: Characterizing and understanding the multi-decadal variations of the continental hydrological cycle is a challenging issue given the limitation of observed data-sets. In this paper, a new approach to derive 20th century hydrological reconstructions over France with an hydrological model is presented. The method combines the results of long-term atmospheric reanalyses downscaled with a stochastic statistical method and homogenized station observations to derive the meteorological forcing needed for hydrological modeling. Different methodological choices are tested and evaluated. We show that using homogenized observations to constrain the results of statistical downscaling help to improve the reproduction of precipitation, temperature and river flows variability. In particular, it corrects some unrealistic long-term trends associated with the atmospheric reanalyses. Observationally-constrained reconstructions therefore constitute a valuable dataset to study the multi-decadal hydrological variations over France. Thanks to these reconstructions, we confirm that the multi-decadal variations previously noted in French river flows have mainly a climatic origin. Moreover, we show that multi-decadal variations exist in other hydrological variables (evapotranspiration, snow cover and soil moisture). Depending on the region, the persistence from spring to summer of soil moisture or snow anomalies generated during spring by temperature and precipitation variations may explain river flows variations in summer, when no concomitant climate variations exist.

Description: How does the variability of topography structure the spatial heterogeneity of nutrient dynamics? In particular, what role does micro-topographic depression play in the spatial and temporal dynamics of nitrate, ammonia, and ammonium? We explore these questions using the 3D simulation of their joint dynamics of concentration and age. To explicitly resolve micro-topographic variability and its control on moisture, vegetation, and carbon-nitrogen dynamics, we use a high-resolution LiDAR data over an agricultural site under a corn-soybean rotation in the Intensively Managed landscapes Critical Zone Observatory in the U.S. Midwest. We utilize a hybrid CPU-GPU parallel computing architecture to reduce the computational cost associated with such high-resolution simulations. Our results show that in areas that present closed topographic depressions, relatively lower nitrate concentration and age are observed compared to elsewhere. The periodic ponding in depressions increases the downward flux of water that carries more dissolved nitrate to the deeper soil layer. However, the variability in the depressions is relatively higher as a result of the episodic ponding pattern. When aggregate efflux from the soil domain at the bottom of the soil is considered, we find a gradual decrease in the age on the rising limb of nitrate efflux and a gradual increase on the falling limb. In addition, the age of the nitrate efflux ranges from 4 to 7 years. These are significantly higher as compared to the ages associated with a non-reactive tracer indicating that they provide an inaccurate estimate of residence time of a reactive constituent through the soil column.

Description: Inspired by the work of Newton, Darwin and Wegener, this paper tracks the drivers and dynamics that have shaped the growth of hydrological understanding over the last century. On the basis of an interpretation of this history the paper then speculates about what kind of future is in store for hydrology and how we can better prepare for it. The historical narrative underpinning this analysis indicates that progress in hydrological understanding is brought about by changing societal needs and technological opportunities: new ideas are generated by hydrologists through addressing societal needs with the technologies of their time. We suggest that progress in hydrological understanding over the last century has expressed itself through repeated cycles of euphoria and disenchantment, which have served as stimuli for the progress. The progress, for it to happen, also needed inspirational leaders as well as a supportive scientific community that provided the backdrop to major advances in the field. The paper concludes that, in a similar way to how Newton, Darwin and Wegener conducted their research, hydrology too can benefit from synthesis activities aimed at “connecting the dots”.

Description: We introduce a simple method to measure the relative permeability of supercritical CO 2 in low-permeability rocks. The method is built on the assumption of the stability of formed CO 2 percolation pathway under lowered pressure drops. Initially, a continuous CO 2 flow pathway is created under a relatively high-pressure drop. Then, several subsequent steps of lowered pressure drops are performed while monitoring the associated flow rates. When the pressure drop is lower than a threshold value, the created flow pathway is assumed to be adequately stable and does not vary significantly during successive flows, with the average saturation and flow rate achieving a quasi-steady state. The relative permeability of CO 2 is then calculated from the relationship between the pressure drop and flow rate at several lowered pressure drops according to the extended form of Darcy's law. We demonstrate this method using both numerical modeling and an experimental test using X-ray CT imaging. The results indicate the validity of the assumption for the stability of flow pathway under lowered pressure drops. A linear relationship between the lowered pressure drops and the corresponding CO 2 flow rate is found. Furthermore, the measurement results suggest that the relative permeability of CO 2 can still be high in low-permeability rocks if the CO 2 saturation is higher than the threshold value required to build a flow pathway. The proposed method is important for measuring the pathway-flow relative permeability of non-wetting fluids in low-permeability rocks.

Description: We tested an empirical modelling approach using relatively low-cost continuous records of turbidity and discharge as proxies to estimate phosphorus (P) concentrations at a sub-hourly time step for estimating loads. The method takes into account non-linearity and hysteresis effects during storm events, and hydrological conditions variability. High-frequency records of total P and reactive P originating from four contrasting European agricultural catchments in terms of P loads were used to test the method. The models were calibrated on weekly grab sampling data combined with 10 storms surveyed sub-hourly per year ( weekly+ survey) and then used to reconstruct P concentrations during all storm events for computing annual loads. For total P, results showed that this modelling approach allowed the estimation of annual loads with limited uncertainties (≈ -10% ± 15%), more reliable than estimations based on simple linear regressions using turbidity, based on interpolated weekly+ data without storm event reconstruction, or on discharge weighted calculations from weekly series or monthly series. For reactive P, load uncertainties based on the non-linear model were similar to uncertainties based on storm event reconstruction using simple linear regression (≈ 20% ± 30%), and remained lower than uncertainties obtained without storm reconstruction on weekly or monthly series, but larger than uncertainties based on interpolated weekly+ data (≈ -15% ± 20%). These empirical models showed we could estimate reliable P exports from non-continuous P time series when using continuous proxies, and this could potentially be very useful for completing time-series datasets in high-frequency surveys, even over extended periods.

Description: Water utilities are increasingly relying on water efficiency and conservation to extend the availability of supplies. Despite spatial and institutional inter-dependency of many utilities, these demand-side management initiatives have traditionally been tackled by individual utilities operating in isolation. In this study, we introduce a policy framework for water conservation credits that enables collaboration at the regional scale. Under the proposed approach, utilities have the flexibility to invest in water conservation measures that are appropriate for their specific service area. When utilities have insufficient capacity for local cost-effective measures, they may opt to purchase credits, contributing to fund subsidies for utilities that do have that capacity and can provide the credits, while the region as a whole benefits from more reliable water supplies. This work aims to provide insights on the potential impacts of a water conservation credit policy framework when utilities are given the option to collaborate in their efforts. We model utility decisions as rational cost-minimizing actors subject to different decision-making dynamics and water demand scenarios, and demonstrate the institutional characteristics needed for the proposed policy to be effective. We apply this model to a counterfactual case study of water utility members of the Bay Area Water Supply and Conservation Agency in California during the drought period of June 2015 to May 2016. Our scenario analysis indicates that, when the institutional structure and incentives are appropriately defined, water agencies can achieve economic benefits from collaborating in their conservation efforts, especially if they coordinate more closely in their decision-making.

Description: The geographic variability in the partitioning of precipitation into surface runoff (Q) and evapotranspiration (ET) is fundamental to understanding regional water availability. The Budyko equation suggests this partitioning is strictly a function of aridity, yet observed deviations from this relationship for individual watersheds impede using the framework to model surface water balance in ungauged catchments and under future climate and land use scenarios. A set of climatic, physiographic, and vegetation metrics were used to model the spatial variability in the partitioning of precipitation for 211 watersheds across the contiguous United States (CONUS) within Budyko's framework through the free parameter ω (Fu, 1981). A generalized additive model found that four widely available variables, precipitation seasonality, the ratio of soil water holding capacity to precipitation, topographic slope, and the fraction of precipitation falling as snow, explained 81.2% of the variability in ω . The ω model applied to the Budyko equation explained 97% of the spatial variability in long-term Q for an independent set of watersheds. The ω model was also applied to estimate the long-term water balance across the CONUS for both contemporary and mid-21 st century conditions. The modeled partitioning of observed precipitation to Q and ET compared favorably across the CONUS with estimates from more sophisticated land-surface modeling efforts. For mid-21 st century conditions, the model simulated an increase in the fraction of precipitation used by ET across the CONUS with declines in Q for much of the eastern CONUS and mountainous watersheds across the western US.

Description: ABSTRACT Mountain pine beetle outbreaks in western North America have led to extensive forest mortality, justifiably generating interest in improving our understanding of how this type of ecological disturbance affects hydrological cycles. While observational studies and simulations have been used to elucidate the effects of mountain beetle mortality on hydrological fluxes, an ecologically-mechanistic model of forest evapotranspiration (ET) evaluated against field data has yet to be developed. In this work, we use the Terrestrial Regional Ecosystem Exchange Simulator (TREES) to incorporate the ecohydrological impacts of mountain pine beetle disturbance on ET for a lodgepole pine-dominated forest equipped with an eddy covariance tower. An existing degree-day model was incorporated that predicted the life cycle of mountain pine beetles, along with an empirically-derived submodel that allowed sap flux to decline as a function of temperature-dependent blue stain fungal growth. The eddy covariance footprint was divided into multiple cohorts for multiple growing seasons, including representations of recently attacked trees and the compensatory effects of regenerating understory, using two different spatial scaling methods. Our results showed that using a multiple cohort approach matched eddy covariance-measured ecosystem-scale ET fluxes well, and showed improved performance compared to model simulations assuming a binary framework of only areas of live and dead overstory. Cumulative growing season ecosystem-scale ET fluxes were 8 – 29% greater using the multi-cohort approach during years in which beetle attacks occurred, highlighting the importance of including compensatory ecological mechanism in ET models.

Description: Steep mountain streams have higher resistance to flow and lower sediment-transport rates than expected by comparison with low gradient rivers, and often these differences are attributed to reduced near-bed flow velocities and stresses associated with form drag on channel forms and immobile boulders. However, few studies have directly measured drag and lift forces acting on bed sediment for shallow flows over coarse sediment, which ultimately control sediment transport rates and grain-scale flow resistance. Here we report on particle lift and drag force measurements in flume experiments using a planar, fixed cobble bed over a wide range of channel slopes (0.004 〈  S  〈 0.3) and water discharges. Drag coefficients are similar to previous findings for submerged particles ( C D ∼ 0.7), but increase significantly for partially submerged particles. In contrast, lift coefficients decrease from near unity to zero as the flow shallows, and are strongly negative for partially submerged particles, indicating a downward force that pulls particles towards the bed. Fluctuating forces in lift and drag decrease with increasing relative roughness, and they scale with the depth-averaged velocity squared rather than the bed shear stress. We find that, even in the absence of complex bed topography, shallow flows over coarse sediment are characterized by high flow resistance because of grain drag within a roughness layer that occupies a significant fraction of the total flow depth, and by heightened critical Shields numbers and reduced sediment fluxes because of reduced lift forces and reduced turbulent fluctuations.

Description: Pumping tests interpretation is an art that involves dealing with noise coming from multiple sources and conceptual model uncertainty. Interpretation is greatly helped by diagnostic plots, which include drawdown data and their derivative with respect to log-time, called log-derivative. Log-derivatives are especially useful to complement geological understanding in helping to identify the underlying model of fluid flow because they are sensitive to subtle variations in the response to pumping of aquifers and oil reservoirs. The main problem with their use lies in the calculation of the log-derivatives themselves, which may display fluctuations when data are noisy. To overcome this difficulty, we propose a variational regularization approach based on the minimization of a functional consisting of two terms: one ensuring that the computed log-derivatives honor measurements and one that penalizes fluctuations. The minimization leads to a diffusion-like differential equation in the log-derivatives, and boundary conditions that are appropriate for well hydraulics (i.e., radial flow, wellbore storage, fractal behavior, etc.). We have solved this equation by finite differences. We tested the methodology on two synthetic examples showing that a robust solution is obtained. We also report the resulting log-derivative for a real case.

Description: Floodplains provide a variety of hydrological and ecological functions and are therefore of great importance. The flooding frequency, as well as the height and duration of inundations are particularly relevant for ecosystem states and are dependent on the exchange between surface water and groundwater. In this study, we developed a fully distributed model approach to simulate distributed groundwater levels in a floodplain in Hesse, Germany (14.8 km 2 ). To overcome the problem of large computation times we simplified the surface water equation. Thus, the water surface of flooding is at the same level everywhere and the dynamic effect of the flooding is ignored. In this way, it was possible to run the model 5,000 times and investigate its parameter uncertainty using Latin hypercube sampling. Behavioral model runs were selected based on a threshold criterion of a mean root mean square error that was smaller than 0.26 m. All the simulated groundwater wells show an individual RMSE between 0.17 and 0.41 m for the calibration period. Regarding the parameterization, the model shows rather large variance in parameters that are capable of generating good simulations: a range of saturated conductivity of 2,793 m/day, porosity of 0.4 m3/m3, residual wetness of soil of 0.2 m3/m3/soil and range of soil thickness of 2.9 m.

Description: The equivalent permeability, k eq of stratified fractured porous rocks and its anisotropy is important for hydrocarbon reservoir engineering, groundwater hydrology, and subsurface contaminant transport. However, it is difficult to constrain this tensor property as it is strongly influenced by infrequent large fractures. Boreholes miss them and their directional sampling bias affects the collected geostatistical data. Samples taken at any scale smaller than that of interest truncate distributions and this bias leads to an incorrect characterization and property upscaling. To better understand this sampling problem, we have investigated a collection of outcrop-data based Discrete Fracture and Matrix (DFM) models with mechanically constrained fracture aperture distributions, trying to establish a useful Representative Elementary Volume (REV). Finite-element analysis and flow-based upscaling have been used to determine k eq eigenvalues and anisotropy. While our results indicate a convergence towards a scale-invariant k eq REV with increasing sample size, k eq magnitude can have multi-modal distributions. REV size relates to the length of dilated fracture segments as opposed to overall fracture length. Tensor orientation and degree of anisotropy also converge with sample size. However, the REV for k eq anisotropy is larger than that for k eq magnitude. Across scales, tensor orientation varies spatially, reflecting inhomogeneity of the fracture patterns. Inhomogeneity is particularly pronounced where the ambient stress selectively activates late- as opposed to early (through-going) fractures. While we cannot detect any increase of k eq with sample size as postulated in some earlier studies, our results highlight a strong k eq anisotropy that influences scale dependence.

Description: The STIC (Stream Temperature, Intermittency, and Conductivity) electrical conductivity (EC) logger as presented by Chapin et al . [2014] serves as an inexpensive (∼50 USD) means to assess relative EC in freshwater environments. This communication demonstrates the calibration of the STIC logger for quantifying EC, and provides examples from a month long field deployment in the High Arctic. Calibration models followed multiple non-linear regression and produced calibration curves with high coefficient of determination values (R 2 = 0.995 – 0.998; n = 5). Percent error of mean predicted specific conductance at 25°C (SpC) to known SpC ranged in magnitude from -0.6% to 13% (mean = -1.4%), and mean absolute percent error (MAPE) ranged from 2.1% to 13% (mean = 5.3%). Across all tested loggers we found good accuracy and precision, with both error metrics increasing with increasing SpC values. During ten, month-long field deployments, there were no logger failures and full data recovery was achieved. Point SpC measurements at the location of STIC loggers recorded via a more expensive commercial electrical conductivity logger followed similar trends to STIC SpC records, with 1:1.05 and 1:1.08 relationships between the STIC and commercial logger SpC values. These results demonstrate that STIC loggers calibrated to quantify EC are an economical means to increase the spatiotemporal resolution of water quality investigations.

Description: Sea level rise has serious consequences for harbor infrastructure, storm drains and sewer systems, and many other issues. Adapting to sea level rise requires comparing different possible adaptation strategies, comparing the cost of different actions (including no action), and assessing where and at what point in time the chosen strategy should be implemented. All these decisions must be made under considerable uncertainty–in the amount of sea level rise, in the cost and prioritization of adaptation actions, and in the implications of no action. Here we develop two illustrative examples: for Bergen on Norway's west coast and for Esbjerg on the west coast of Denmark, to highlight how technical efforts to understand and quantify uncertainties in hydrologic projections can be coupled with concrete decision-problems framed by the needs of the end-users using statistical formulations. Different components of uncertainty are visualized. We demonstrate the value of uncertainties and show for example that failing to take uncertainty into account can result in the median projected damage costs being an order of magnitude smaller.

Description: In this work we investigate methods for gaining greater insight from hydrological model runs conducted for uncertainty quantification and model differentiation. We frame the sensitivity analysis questions in terms of the main purposes of sensitivity analysis: parameter prioritization, trend identification and interaction quantification. For parameter prioritization, we consider variance-based sensitivity measures, sensitivity indices based on the L 1 -norm, the Kuiper metric and the sensitivity indices of the DELSA methods. For trend identification, we investigate insights derived from graphing the one-way ANOVA sensitivity functions, the recently introduced CUSUNORO plots and derivative scatterplots. For interaction quantification, we consider information delivered by variance-based sensitivity indices. We rely on the so-called given-data principle, in which results from a set of model runs are used to perform a defined set of analyses. One avoids using specific designs for each insight, thus controlling the computational burden. The methodology is applied to a hydrological model of a river in Belgium simulated using the well established Framework for Understanding Structural Errors (FUSE) on five alternative configurations. The findings show that the integration of the chosen methods provide insights unavailable in most other analyses.

Description: Carbon dioxide (CO 2 ) geological sequestration plays an important role in mitigating CO 2 emissions for climate change. Understanding interactions of the injected CO 2 with network fractures and hydrocarbons is key for optimizing and controlling CO 2 geological sequestration and evaluating its risks to ground water. However, there is a well-known, difficult process in simulating the dynamic interaction of fracture-matrix, such as dynamic change of matrix porosity, unsaturated processes in rock matrix and effect of rock mineral properties. In this paper, we develop an explicit model of the fracture-matrix interactions using multi-layer bounce-back treatment as a first attempt to simulate CO 2 reactive transport in network fractured media through coupling the Dardis's LBM porous model for a new interface treatment. Two kinds of typical fracture networks in porous media are simulated: straight cross network fractures and interleaving network fractures. The reaction rate and porosity distribution are illustrated and well-matched patterns are found. The species concentration distribution and evolution with timesteps are also analyzed and compared with different transport properties. The results demonstrate the capability of this model to investigate the complex processes of CO 2 geological injection and reactive transport in network fractured media, such as dynamic change of matrix porosity.

Description: Capillary trapping of a non-wetting phase arising from two-phase immiscible flow in sedimentary rocks is critical to many geoscience scenarios, including oil and gas recovery, aquifer recharge and, with increasing interest, carbon sequestration. Here we demonstrate the successful use of low field 1 H Nuclear Magnetic Resonance (NMR) to quantify capillary trapping; specifically we use transverse relaxation time ( T 2 ) time measurements to measure both residual water (wetting phase) content and the surface-to-volume ratio distribution (which is proportional to pore size) of the void space occupied by this residual water. Critically we systematically confirm this relationship between T 2 and pore size by quantifying inter-pore magnetic field gradients due to magnetic susceptibility contrast, and demonstrate that our measurements at all water saturations are unaffected. Diffusion in such field gradients can potentially severely distort the T 2 -pore size relationship, rendering it unusable. Measurements are performed for nitrogen injection into a range of water-saturated sandstone plugs at reservoir conditions. Consistent with a water-wet system, water was preferentially displaced from larger pores while relatively little change was observed in the water occupying smaller pore spaces. The impact of cyclic wetting/non-wetting fluid injection was explored and indicated that such a regime increased non-wetting trapping efficiency by the sequential occupation of the most available larger pores by nitrogen. Finally the replacement of nitrogen by CO 2 was considered; this revealed that dissolution of paramagnetic minerals from the sandstone caused by its exposure to carbonic acid reduced the in situ bulk fluid T 2 relaxation time on a timescale comparable to our core flooding experiments. The implications of this for the T 2 -pore size relationship are discussed.

Description: ABSTRACT Green infrastructure (GI) is an approach to stormwater management that promotes natural processes of infiltration and evapotranspiration, reducing surface runoff to conventional stormwater drainage infrastructure. As more urban areas incorporate GI into their stormwater management plans, greater understanding is needed on the effects of spatial configuration of GI networks on hydrological performance, especially in the context of potential subsurface and lateral interactions between distributed facilities. In this research, we apply a three-dimensional, coupled surface-subsurface, land-atmosphere model, ParFlow.CLM, to a residential urban sewershed in Washington DC that was retrofitted with a network of GI installations between 2009 and 2015. The model was used to test nine additional GI and imperviousness spatial network configurations for the site and was compared with monitored pipe-flow data. Results from the simulations show that GI located in higher flow-accumulation areas of the site intercepted more surface runoff, even during wetter and multi-day events. However, a comparison of the differences between scenarios and levels of variation and noise in monitored data suggests that the differences would only be detectable between the most and least optimal GI/imperviousness configurations.

Description: ABSTRACT Wave-induced water exchange and groundwater flows in beach aquifers impact the fate of contaminants including nutrients, fecal bacteria and non-aqueous phase liquids (NAPLs). Waves induce high frequency fluxes in shallow beach sediments. In addition, the phase-averaged effect of waves (wave setup) drives deeper flow recirculations through a beach aquifer. Field data of shallow instantaneous and time-averaged vertical head gradients (fluxes) are first compared with deeper time-averaged fluxes over a period of varying wave conditions. The time-averaged fluxes are equivalent to that which would be simulated assuming a phase-averaged water surface (i.e. wave setup). Based on this comparison, the need to simulate phase-resolved wave motion versus the simplified phase-averaged water surface in predicting contaminant fate is evaluated. While high frequency fluxes cause large surface water volumes to filter through beach sediments, the exchanging water has a short residence time (〈1 – 70 seconds). The time-averaged flow behavior captures exchanging water with longer residence time (hours to months) and deeper flow paths. Therefore consideration of the time-averaged behavior may be sufficient for evaluating dissolved reactive constituents. In contrast, calculations indicate that instantaneous fluxes may need to be considered in evaluating colloidal contaminants (e.g. particulate organic matter and fecal bacteria) as sediment interactions affect their transport and residence time. Finally, multi-phase simulations illustrate the differential effect of considering instantaneous versus time-averaged fluxes on the downward migration of NAPL in beach sediments. This study provides an important foundation for future field and modeling efforts focused on understanding and predicting contaminant transport in wave-influenced beaches.

Description: Green stormwater infrastructure has been demonstrated as an innovative water resources management approach that addresses multiple challenges facing urban environments. However, there is little consensus on what policy strategies can be used to best incentivize green infrastructure adoption by private landowners. Game theory, an analysis framework that has historically been under-utilized within the context of stormwater management, is uniquely suited to address this policy question. We used a cooperative game theory framework to investigate the potential impacts of different policy strategies used to incentivize green infrastructure installation. The results indicate that municipal regulation leads to the greatest reduction in pollutant loading. However, the choice of the ‘best' regulatory approach will depend on a variety of different factors including politics and financial considerations. Large, downstream agents have a disproportionate share of bargaining power. Results also reveal that policy impacts are highly dependent on agents' spatial position within the stormwater network, leading to important questions of social equity and environmental justice.

Description: Gas ebullition of river impoundments plays an increasingly significant role, particularly in transporting methane CH 4 from their sediments to the atmosphere, and contributing to the global carbon budget and global warming. Quantifying stochastic and episodic nature of gas ebullition is complicated especially when conventionally conducted by using coverage-limited gas traps. Current knowledge of seasonality in a reservoir's gas ebullition is lacking in the literature. For this reason, advanced acoustic surveying was intensively applied to determine spatiotemporal distributions of gas ebullition in a European water-storage reservoir for two years. Additionally, the sampling was accompanied with gas collecting for analyzing gas composition. The gas released from the reservoir was primarily composed of CH 4 (on average 52%, up to 94%). The longitudinal distribution of gas ebullition was mainly determined by a proximity to the river inflow as a source of organic matter. A magnitude of ebullitive fluxes within the reservoir varied up to 1,300 mL m −2 d −1 (30 mmol CH 4 m −2 d −1 ). The most significant period of ebullition has turned out to be in fall, on average reaching a sevenfold ebullitive flux (70 mL m −2 d −1 , 1.6 mmol CH 4 m −2 d −1 ) higher than in the rest of the season. A substantial contribution to the fall peak was induced by an expansion of gas ebullition into greater depths, covering two thirds of the reservoir in late fall. The study demonstrates that the ebullitive fluxes of the temperate water storage reservoir were correlated to season, depth, and inflow proximity.

Description: Reservoir operations may alter the characteristics of Flood Frequency Curve (FFC) and challenge the basic assumption of stationarity used in flood frequency analysis. This paper presents a combined data-modeling analysis of reservoir as a nonlinear filter of runoff routing that alters the FFCs. A dimensionless Reservoir Impact Index (RII), defined as the total upstream reservoir storage capacity normalized by the annual streamflow volume, is used to quantify reservoir regulation effects. Analyses are performed for 388 river stations in the contiguous U.S. using the first two moments of the FFC, mean annual maximum flood (MAF) and coefficient of variations (CV), calculated for the pre- and post-dam periods. It is found that MAF generally decreases with increasing RII but stabilizes when RII exceeds a threshold value, and CV increases with RII until a threshold value beyond which CV decreases with RII. Hence depending on the magnitude of RII, reservoir regulation acts as a filter to increase or reduce the nonlinearity of the natural runoff routing process and alters flood characteristics. The non-linear relationships of MAF and CV with RII can be captured by three reservoir models with different levels of complexity, suggesting that they emerge from the basic flood control function of reservoirs. However, the threshold RII values in the nonlinear relationships depend on the more detailed reservoir operations and objectives that can only be captured by the more complex reservoir models. Our conceptual model may help improve flood-risk assessment and mitigation in regulated river systems at the regional scale.

Description: Inverse problem permits to map the subsurface properties from a few observed data. The inverse problem can be physically constrained by a priori information on the property distribution in order to limit the non-uniqueness of the solution. The geostatistical information are often chosen as a priori information, however when the field properties present a spatial locally-distributed high variability the geostatistical approach becomes inefficient. Therefore, we propose a new method adapted for fields presenting linear structures (such as a fractured field). The Cellular Automata-based Deterministic Inversion (CADI) method is, as far as we know when this paper is produced, the first inversion method which permits a deterministic inversion based on a Bayesian approach and using a dynamic optimization to generate different linear structures iteratively. The model is partitioned in cellular automaton subspaces, each one controlling a different zone of the model. A cellular automata subspace structures the properties of the model in two units (‘structure' and ‘background') and control their dispensing direction and their values. The partitioning of the model in subspaces permits to monitor a large-scale structural model with only a few pilot-parameters and to generate linear structures with local direction changes. Thereby, the algorithm can easily handle with large-scale structures, and a sensitivity analysis is possible on these structural pilot-parameters, which permits to considerably accelerate the optimization process in order to find the best structural geometry. The algorithm has been successfully tested on simple, to more complex, theoretical models with different inversion techniques by using seismic and hydraulic data. This article is protected by copyright. All rights reserved.

Description: A restored riparian zone was characterized to understand the effects of flooding on subsurface hydrological flow paths and nitrate removal in groundwater. Field and laboratory investigations were combined with numerical modeling of dynamic flow and reactive nitrate transport. Flooding enhances nitrate removal in groundwater primarily by two mechanisms. First, by creating a stagnant flow zone beneath the flooded area thereby increasing the residence time and leaving more time for nitrate removal. Secondly, nitrate removal is increased by enhancing upward flow into the highly reactive organic-rich top layers. Flooding therefore contributes to nitrate removal in “hot spots”, where nitrate is transported to the peat and during “hot moments”, when flow is stagnant. The permeability of the capping peat layer relative to the aquifer is important as it controls both mechanisms. The model shows that the deep-seated nitrate removal is greater than projected from the laboratory nitrate reduction experiments. This article is protected by copyright. All rights reserved.

Description: Analyzing the dissolution of rocks and other porous materials is simplified by the large disparity between mineral and reactant concentrations. In essence, the porosity remains frozen on the time scale of the reactant transport, which can then be treated as a quasi-stationary process. This conceptual idea can be derived mathematically using asymptotic methods, which show that the length scales in the system are, to a first approximation, independent of the ratio of reactant and mineral concentrations. Nevertheless, in a growing number of papers on dissolutional instabilities, the reactant-mineral concentration ratio has been incorrectly linked to the thickness of the dissolution front. In this paper we critically review the application of asymptotic methods to the reaction-infiltration instability. In particular we discuss the limited validity of the thin-front or “Stefan” limit, where the interface between dissolved and undissolved mineral is sharp. This article is protected by copyright. All rights reserved.

Description: Modern hydrology places nearly all its emphasis on science-as-knowledge, the hypotheses of which are increasingly expressed as physical models, whose predictions are tested by correspondence to quantitative data sets. Though arguably appropriate for applications of theory to engineering and applied science, the associated emphases on truth and degrees of certainty are not optimal for the productive and creative processes that facilitate the fundamental advancement of science as a process of discovery. The latter requires an investigative approach, where the goal is uberty, a kind of fruitfulness of inquiry, in which the abductive mode of inference adds to the much more commonly acknowledged modes of deduction and induction. The resulting world-directed approach to hydrology provides a valuable complement to the prevailing hypothesis- (theory-) directed paradigm. This article is protected by copyright. All rights reserved.

Description: Variability and nonstationarity in flood regimes of cold regions are examined using data from hydrometric reference streamflow gauging stations from 27 natural watersheds in Canada and adjacent areas of the United States. Choosing stations from reference networks with nearly 100 years of data allows for the investigation of changes that span several phases of some of the atmospheric drivers that may influence flood behaviour. The reference hydrologic networks include only stations considered to have good quality data and were screened to avoid the influences of regulation, diversions, or land use change. Changes and variations in flood regimes are complex and require a multifaceted approach to properly characterize the types of changes that have occurred and are likely to occur in the future. Peaks over threshold (POT) data are extracted from daily flow data for each watershed and changes to the magnitude, timing, frequency, volume and duration of threshold exceedences are investigated. Seasonal statistics are used to explore changes in the nature of the flood regime based on changes in the timing of flood threshold exceedences. A variety of measures are developed to infer flood regime shifts including from a nival regime to a mixed regime and a mixed regime to a more pluvial-dominated regime. The flood regime at many of the watersheds demonstrates increased prominence of rainfall floods and decreased prevalence of snowmelt contributions to flood responses. While some individual stations show a relationship between flood variables and climate indices, these relationships are generally weak. This article is protected by copyright. All rights reserved.

Description: The influence of the length of the calibration period and observation frequency on the predictive uncertainty in time series modeling of groundwater dynamics is investigated. Studied series are from deltaic regions with predominantly shallow groundwater tables in a temperate maritime climate where heads vary due to precipitation and evaporation. Response times vary over a wide range from ∼60 days to ∼1200 days. A Transfer Function-Noise model is calibrated with the Markov Chain Monte Carlo method to both synthetic series and measured series of heads. The model fit and uncertainty are evaluated for various calibration periods and observation frequencies. It is often assumed that the required length of the calibration period is related to the response time of the system. In this study, no strong relationship was observed. Results indicate, however, that the required length of the calibration period is related to the decay time of the noise. Furthermore, the length of the calibration period was much more important than the total number of observations. For the measured series, the credible intervals could commonly be reduced to ∼10% of the measured head range and the prediction intervals to ∼50% of the measured head range with calibration periods of 20 years with ∼2 observations per month. This article is protected by copyright. All rights reserved.

Description: Advances in hydrology are greatly needed and approaches that employ hypotheses to guide research have the potential to contribute to future advances. In this context, hypotheses can serve a range of purposes. Overarching hypotheses can provide a common integrating framework for collaborative research and can be revised as research progresses over time. Hypotheses that attempt to explain unexpected field observations or experimental results can provide a guide for designing further field studies. Focused testable hypotheses can facilitate effective presentation of proposed research, and clarify alternative hypotheses. Finally, the value of employing a hypothesis-based approach depends upon the research environment, which can act as an “environmental filter” in determining successful research outcomes. This article is protected by copyright. All rights reserved.

Description: The hydraulics of steep mountain streams differ from lower gradient rivers due to shallow and rough flows, energetic subsurface flow, and macro-scale form drag from immobile boulders and channel- and bed-forms. Heightened flow resistance and reduced sediment transport rates in steep streams are commonly attributed to macro-scale form drag; however, little work has explored steep river hydrodynamics in the absence of complex bed geometries. Here we present theory for the vertical structure of flow velocity in steep streams with planar, rough beds that couples surface and subsurface flow. We test it against flume experiments using a bed of fixed cobbles over a wide range of bed slopes (0.4 – 30%). Experimental flows have a nearly logarithmic velocity profile far above the bed; flow velocity decreases less than logarithmically towards the bed and is non-zero at the bed surface. Velocity profiles match theory derived using a hybrid eddy-viscosity model, in which the mixing length is a function of height above the bed and bed roughness. Subsurface flow velocities are large (〉 1 m/s) and follow a modified Darcy-Brinkman-Forchheimer relation that accounts for channel slope and shear from overlying surface flow. Near-bed turbulent fluctuations decrease for shallow, rough flows and scale with the depth-averaged flow velocity rather than bed shear velocity. Flow resistance for rough, planar beds closely matches observations in natural steep streams despite the lack of bed- or channel-forms in the experiments, suggesting that macro-scale form drag is smaller than commonly assumed in stress partitioning models for sediment transport. This article is protected by copyright. All rights reserved.

Description: The use of ‘off-the-shelf' acoustic Doppler velocity profilers (ADCPs) to estimate suspended sediment concentration and grain size in rivers requires robust methods to estimate sound attenuation by suspended sediment. Theoretical estimates of sediment attenuation require a priori knowledge of the concentration and grain size distribution (GSD), making the method impractical to apply in routine monitoring programs. In-situ methods use acoustic backscatter profile slope to estimate sediment attenuation, and are a more attractive option. However, the performance of in-situ sediment attenuation methods has not been extensively compared to theoretical methods. We used three collocated horizontally mounted ADCPs in the Fraser River at Mission, British Columbia and 298 observations of concentration and GSD along the acoustic beams to calculate theoretical and in-situ sediment attenuation. Conversion of acoustic intensity from counts to decibels is influenced by the instrument noise floor, which affects the backscatter profile shape and therefore in-situ attenuation. We develop a method that converts counts to decibels to maximize profile length, which is useful in rivers where cross-channel acoustic profile penetration is a fraction of total channel width. Nevertheless, the agreement between theoretical and in-situ attenuation is poor at low concentrations because cross-stream gradients in concentration, sediment size or GSD can develop, which affect the backscatter profiles. We establish threshold concentrations below which in-situ attenuation is unreliable in Fraser River. Our results call for careful examination of cross-stream changes in suspended sediment characteristics and acoustic profiles across a range of flows before in-situ attenuation methods are applied in river monitoring programs.

Description: Interactive Genetic Algorithms (IGA) are advanced human-in-the-loop optimization methods that enable humans to give feedback, based on their subjective and unquantified preferences and knowledge, during the algorithm's search process. While these methods are gaining popularity in multiple fields, there is a critical lack of data and analyses on (a) the nature of interactions of different humans with interfaces of decision support systems (DSS) that employ IGA in water resources planning problems and on (b) the effect of human feedback on the algorithm's ability to search for design alternatives desirable to end-users. In this paper, we present results and analyses of observational experiments in which different human participants ( surrogates and stakeholders ) interacted with an IGA-based, watershed DSS called WRESTORE to identify plans of conservation practices in a watershed. The main goal of this paper is to evaluate how the IGA adapts its search process in the objective space to a user's feedback, and identify whether any similarities exist in the objective space of plans found by different participants. Some participants focused on the entire watershed, while others focused only on specific local subbasins. Additionally, two different hydrology models were used to identify any potential differences in interactive search outcomes that could arise from differences in the numerical values of benefits displayed to participants. Results indicate that stakeholders , in comparison to their surrogates , were more likely to use multiple features of the DSS interface to collect information before giving feedback, and dissimilarities existed among participants in the objective space of design alternatives.

Description: Fracture network geometry is crucial for transport in hard rock aquifers, but it can only be approximated in models. While fracture orientation, spacing and intensity can be obtained from borehole logs, core images and outcrops, the characterization of in-situ fracture network geometry requires the interpretation of spatially distributed hydraulic and transport experiments. In this study we present a novel concept using a transdimensional inversion method (reversible jump Markov Chain Monte Carlo, rjMCMC) to invert a two-dimensional cross-well discrete fracture network (DFN) geometry from tracer tomography experiments. The conservative tracer transport is modelled via a fast finite difference model neglecting matrix diffusion. The proposed DFN inversion method iteratively evolves DFN variants by geometry updates to fit the observed tomographic data evaluated by the Metropolis-Hastings-Green acceptance criteria. A main feature is the varying dimensions of the inverse problem, which allows for the calibration of fracture geometries and numbers. This delivers an ensemble of thousands of DFN realizations that can be utilized for probabilistic identification of fractures in the aquifer. In the presented hypothetical and outcrop-based case studies, cross-sections between boreholes are investigated. The procedure successfully identifies major transport pathways in the investigated domain and explores equally probable DFN realizations, which are analyzed in fracture probability maps and by multidimensional scaling.

Description: The evolution of the joint distribution of groundwater age, velocity and arrival times based on a Markov model for the velocities of fluid particles in heterogeneous porous media has been quantified. An explicit evolution equation for the joint distribution of age, arrival time and particle velocity is derived, which is equivalent to a continuous time random walk for age, velocity and arrival time. The approach is fully parameterized by the correlation model and the distribution of groundwater flow velocities. The transition probability for subsequent particle velocities along streamlines is implemented by a Copula, which is an efficient method to generate a correlated velocity series with prescribed marginal distribution. We discuss different solution methods based on finite-differences and random walk particle tracking. The latter is based on continuous time random walks, whose transition times are obtained kinematically from the flow velocities. Specifically, we discuss a renormalization scheme to accelerate the particle tracking simulations based on the definition of aggregate particle transitions while at the same time renormalizing velocity correlation. The impact of velocity correlation and velocity distribution on the evolution of age at different distances from the inlet plane is also studied. At distances of the order of the correlation length, persistent particle velocities give the same behavior as stochastic streamtube models. For velocity distributions which give rise to transition times with finite variance, the age distributions evolve towards an inverse Gaussian. For heavy-tailed weighting times, they evolve towards stable distribution as the distance from the inlet increases.

Description: On behalf of the journal, the American Geophysical Union, and the scientific community, the editors would like to sincerely thank those who reviewed manuscripts for Water Resources Research in 2016. Their time spent reading and commenting on manuscripts not only improves the manuscripts themselves but also increases the scientific rigor of future research in the field. Many of those listed below went above and beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics. Together, they contributed 3674 individual reviews of manuscripts submitted to Water Resources Research for consideration, of which 562 were eventually published. Thank you again. We look forward to a 2017 of exciting advances in the field and communicating those advances to our community and to the broader public.

Description: Groundwater discharge along streams exerts an important influence on biogeochemistry and thermal regimes of aquatic ecosystems. A common approach for predicting locations of shallow lateral groundwater discharge is to use digital elevation models (DEMs) combined with upslope contributing area algorithms. We evaluated a topography-based prediction of subsurface discharge zones along a 1500 m headwater stream reach using temperature and water isotope tracers. We deployed fibre-optic distributed temperature sensing instrumentation to monitor stream temperature at 0.25 m intervals along the reach. We also collected samples of stream water for the analysis of its water isotope composition at 50 m intervals on five occasions representing distinct streamflow conditions before, during and after a major rain event. The combined tracer evaluation showed that topography-predicted locations of groundwater discharge were generally accurate; however, predicted magnitude of groundwater inflows estimated from upslope contributing area did not always agree with tracer estimates. At the catchment scale, lateral inflows were an important source of streamflow at baseflow and peak flow during a major rain event; however, water from a headwater lake was the dominant water source during the event hydrograph recession. Overall, this study highlights potential utility and limitations of predicting locations and contributions of lateral groundwater discharge zones using topography-based approaches in humid boreal regions.

Description: Despite many efforts to develop evapotranspiration (ET) models with improved parametrizations of resistance terms for water vapor transfer into the atmosphere, estimates of ET and its partitioning remain prone to bias. Much of this bias could arise from inadequate representations of physical interactions near non-uniform surfaces from which localized heat and water vapor fluxes emanate. This study aims to provide a mechanistic bridge from land-surface characteristics to vertical transport predictions, and proposes a new physically based ET model that builds on a recently developed bluff-rough bare soil evaporation model incorporating coupled soil moisture-atmospheric controls. The newly developed ET model explicitly accounts for (1) near-surface turbulent interactions affecting soil drying and (2) soil-moisture-dependent stomatal responses to atmospheric evaporative demand that influence leaf (and canopy) transpiration. Model estimates of ET and its partitioning were in good agreement with available field-scale data, and highlight hidden processes not accounted for by commonly used ET schemes. One such process, nonlinear vegetation-induced turbulence (as a function of vegetation stature and cover fraction) significantly influences ET-soil moisture relationships. Our results are particularly important for water resources and land use planning of semiarid sparsely vegetated ecosystems where soil surface interactions are known to play a critical role in land-climate interactions. This study potentially facilitates a mathematically tractable description of the strength (i.e., the slope) of the ET-soil moisture relationship, which is a core component of models that seek to predict land-atmosphere coupling and its feedback to the climate system in a changing climate.

Description: We focus on the measurement of the bed-elevation of braided-networks in flume experiments. In particular, the effect of the survey frequency on the measurement accuracy is studied. To this aim, an innovative measurement system is adopted. It consists of a laser-ultrasonic sensor and can survey the bed elevation under owing water. This measurement system was used to profile a flume transect with a frequency of 4 minutes, without stopping the water discharge. By this technique, the topography of a single transect was continuously acquired during the evolution of a braided river model. Twelve braided rivers generated with different experimental conditions were studied. The main results are: (i) there exists a threshold survey-frequency (4-8 minutes in our analysis) which guarantees that the morphological evolution of the braiding channels is fully measured; (ii) if this threshold frequency of survey is exceeded, significant errors occur in the balance of the eroded/deposited sediments and in the evaluation of the bed-elevation dynamics; and (iii) these errors depend on the river stream-power.

Description: We study the effect of spatially-correlated heterogeneity on isothermal drying of porous media. We combine a minimal pore-scale model with microfluidic experiments with the same pore geometry. Our simulated drying behavior compare favorably with experiments, considering the large sensitivity of the emergent behavior to the uncertainty associated with even small manufacturing errors. We show that increasing the correlation length in particle sizes promotes preferential drying of clusters of large pores, prolonging liquid connectivity and surface wetness and thus higher drying rates for longer periods. Our findings improve our quantitative understanding of how pore-scale heterogeneity impacts drying, which plays a role in a wide range of processes ranging from fuel cells to curing of paints and cements to global budgets of energy, water and solutes in soils.

Description: The operation of most engineered hydrogeological systems relies on simulating physical processes using numerical models with uncertain parameters and initial conditions. Predictions by such uncertain models can be greatly improved by Kalman-filter techniques that sequentially assimilate monitoring data. Each assimilation constitutes a nonlinear optimization, which is solved by linearizing an objective function about the model prediction and applying a linear correction to this prediction. However, if model parameters and initial conditions are uncertain, the optimization problem becomes strongly nonlinear and a linear correction may yield unphysical results. In this paper, we investigate the utility of one-step ahead smoothing, a variant of the traditional filtering process, to eliminate non-physical results and reduce estimation artifacts caused by nonlinearities. We present the smoothing-based compressed state Kalman filter (sCSKF), an algorithm that combines one step ahead smoothing, in which current observations are used to correct the state and parameters one step back in time, with a non-ensemble covariance compression scheme, that reduces the computational cost by efficiently exploring the high-dimensional state and parameter space. Numerical experiments show that when model parameters are uncertain and the states exhibit hyperbolic behavior with sharp fronts, as in CO 2 storage applications, one-step ahead smoothing reduces overshooting errors and, by design, gives physically consistent state and parameter estimates. We compared sCSKF with commonly used data assimilation methods and showed that for the same computational cost, combining one step ahead smoothing and non-ensemble compression is advantageous for real time characterization and monitoring of large-scale hydrogeological systems with sharp moving fronts.

Description: Fine particles (1-100 µm), including particulate organic carbon (POC) and fine sediment, influence stream ecological functioning because they may contain or have a high affinity to sorb nitrogen and phosphorus. These particles are immobilized within stream storage areas, especially hyporheic sediments and benthic biofilms. However, fine particles are also known to remobilize under all flow conditions. This combination of downstream transport and transient retention, influenced by stream geomorphology, controls the distribution of residence times over which fine particles influence stream ecosystems. The main objective of this study was to quantify immobilization and remobilization rates of fine particles in a third-order sand-and-gravel bed stream (Difficult Run, Virginia, USA) within different geomorphic units of the stream (i.e., pool, lateral cavity, thalweg). During our field injection experiment, a thunderstorm-driven spate allowed us to observe fine particle dynamics during both baseflow and in response to increased flow. Solute and fine particles were measured within stream surface waters, porewaters, sediment cores, and biofilms on cobbles. Measurements were taken at four different subsurface locations with varying geomorphology and at multiple depths. Approximately 68% of injected fine particles were retained during baseflow until the onset of the spate. Retention was evident even after the spate, with 15.4% of the baseflow-deposited fine particles retained within benthic biofilms on cobbles and 14.9% within hyporheic sediment after the spate. Thus, through the combination of short-term remobilization and long-term retention, fine particles can serve as sources of carbon and nutrients to downstream ecosystems over a range of timescales.

Description: The complexity of hyporheic flow paths requires reach-scale models of solute transport in streams that are flexible in their representation of the hyporheic passage. We use a model that couples advective-dispersive in-stream transport to hyporheic exchange with a shape-free distribution of hyporheic travel times. The model also accounts for two-site sorption and transformation of reactive solutes. The coefficients of the model are determined by fitting concurrent stream-tracer tests of conservative (fluorescein) and reactive (resazurin/resorufin) compounds. The flexibility of the shape-free models give rise to multiple local minima of the objective function in parameter estimation, thus requiring global-search algorithms, which is hindered by the large number of parameter values to be estimated. We present a local-in-global optimization approach, in which we use a Markov-Chain Monte Carlo method as global-search method to estimate a set of in-stream and hyporheic parameters. Nested therein, we infer the shape-free distribution of hyporheic travel times by a local Gauss-Newton method. The overall approach is independent of the initial guess and provides the joint posterior distribution of all parameters. We apply the described local-in-global optimization method to recorded tracer breakthrough curves of three consecutive stream sections, and infer section-wise hydraulic parameter distributions to analyze how hyporheic exchange processes differ between the stream sections.

Description: China continues to deal with severe levels of water scarcity and water pollution. To help address this situation, in 2002 the Chinese central government initiated urban water pricing reforms that emphasized the adoption of increasing block rate (IBR) price structures in place of existing uniform rate structures. ……By combining urban water use records with micro-level data from the Chinese Urban Household Survey, this research investigates the effectiveness of this national policy reform. Specifically, we compare the household water consumption in 28 cities that adopted IBR tariffs during 2002-2009, with that of 110 cities that had not yet done so. Based on difference-in-differences models, our results show that the policy reform reduced annual residential water demand by 3-4% in the short-run and 5% in the longer-term. These relatively modest reductions are consistent with the typically generous nature of the IBR tariffs that Chinese cities have chosen to implement, and imply that more efforts are needed to address China's persistent urban water scarcity challenges.

Description: This work addresses the signatures embedded in the planform geometry of meandering rivers consequent to the formation of floodplain heterogeneities as the river bends migrate. Two geomorphic features are specifically considered: scroll bars produced by lateral accretion of point bars at convex banks and oxbow lake fills consequent to neck cutoffs. The sedimentary architecture of these geomorphic units depends on the type and amount of sediment, and controls bank erodibility as the river impinges on them, favoring or contrasting the river migration. The geometry of numerically generated planforms obtained for different scenarios of floodplain heterogeneity is compared to that of natural meandering paths. Half meander metrics and spatial distribution of channel curvatures are used to disclose the complexity embedded in meandering geometry. Fourier Analysis, Principal Component Analysis, Singular Spectrum Analysis and Multivariate Singular Spectrum Analysis are used to emphasize the subtle but crucial differences which may emerge between apparently similar configurations. A closer similarity between observed and simulated planforms is attained when fully coupling flow and sediment dynamics (fully-coupled models) and when considering self-formed heterogeneities that are less erodible than the surrounding floodplain.

Description: Research gaps in understanding flood changes at the catchment scale caused by changes in forest management, agricultural practices, artificial drainage and terracing are identified. Potential strategies in addressing these gaps are proposed, such as complex systems approaches to link processes across time scales, long-term experiments on physical-chemical-biological process interactions, and a focus on connectivity and patterns across spatial scales. It is suggested that these strategies will stimulate new research that coherently addresses the issues across hydrology, soil and agricultural sciences, forest engineering, forest ecology and geomorphology.

Description: Identification of causal links in the land-atmosphere system is important for construction and testing of land surface and general circulation models. However, the land and atmosphere are highly coupled and linked by a vast number of complex, interdependent processes. Statistical methods, such as Granger causality, can help to identify feedbacks from observational data, independent of the different parameterizations of physical processes and spatiotemporal resolution effects that influence feedbacks in models. However, statistical causal identification methods can easily be misapplied, leading to erroneous conclusions about feedback strength and sign. Here, we discuss three factors that must be accounted for in determination of causal soil moisture-precipitation feedback in observations and model output: seasonal and interannual variability, precipitation persistence, and endogeneity. The effect of neglecting these factors is demonstrated in simulated and observational data. The results show that long timescale variability and precipitation persistence can have a substantial effect on detected soil moisture-precipitation feedback strength, while endogeneity has a smaller effect that is often masked by measurement error and thus is more likely to be an issue when analyzing model data or highly accurate observational data.

Description: Snow and frozen soil are important factors that influence terrestrial water and energy balances through snowpack accumulation and melt and soil freeze-thaw. In this study, a new land surface model (LSM) with coupled snow and frozen soil physics was developed based on a hydrologically improved LSM (HydroSiB2). First, an energy-balance-based 3-layer snow model was incorporated into HydroSiB2 (hereafter HydroSiB2-S) to provide an improved description of the internal processes of the snow pack. Second, a universal and simplified soil model was coupled with HydroSiB2-S to depict soil water freezing and thawing (hereafter HydroSiB2-SF). In order to avoid the instability caused by the uncertainty in estimating water phase changes, enthalpy was adopted as a prognostic variable instead of snow/soil temperature in the energy balance equation of the snow/frozen soil module. The newly developed models were then carefully evaluated at two typical sites of the Tibetan Plateau (TP) (one snow-covered and the other snow-free, both with underlying frozen soil). At the snow-covered site in northeastern TP (DY), HydroSiB2-SF demonstrated significant improvements over HydroSiB2-F (same as HydroSiB2-SF but using the original single-layer snow module of HydroSiB2), showing the importance of snow internal processes in 3-layer snow parameterization. At the snow-free site in southwestern TP (Ngari), HydroSiB2-SF reasonably simulated soil water phase changes while HydroSiB2-S did not, indicating the crucial role of frozen soil parameterization in depicting the soil thermal and water dynamics. Finally, HydroSiB2-SF proved to be capable of simulating upward moisture fluxes towards the freezing front from the underlying soil layers in winter.

Description: Storm events dominate riverine loads of dissolved organic carbon (DOC) and nitrate, and are expected to increase in frequency and intensity in many regions due to climate change. We deployed three high-frequency (15-minute) in-situ absorbance spectrophotometers to monitor DOC and nitrate concentration for 126 storms in three watersheds with agricultural, urban, and forested land use/land cover. We examined intrastorm hysteresis and the influences of seasonality, antecedent conditions, storm size, and dominant land use/land cover on storm DOC and nitrate loads. DOC hysteresis was generally anti-clockwise at all sites, indicating distal and plentiful sources for all three streams despite varied DOC character and sources. Nitrate hysteresis was generally clockwise for urban and forested sites, but anti-clockwise for the agricultural site, indicating an exhaustible, proximal source of nitrate in the urban and forested sites, and more distal and plentiful sources of nitrate in the agricultural site. The agricultural site had significantly higher storm nitrate yield per water yield and higher storm DOC yield per water yield than the urban or forested sites. Seasonal effects were important for storm nitrate yield in all three watersheds and farm management practices likely caused complex interactions with seasonality at the agricultural site. Hysteresis indices did not improve predictions of storm nitrate yields at any site. We discuss key lessons from using high-frequency in-situ optical sensors.

Description: Disturbances such as fire or flood, in addition to changing the local magnitude of ecological, hydrological, or biogeochemical processes, can also change their functional connectivity—how those processes interact in space. Complex networks offer promise for quantifying functional connectivity in watersheds. The approach resolves connections between nodes in space based on statistical similarities in perturbation signals (derived from solute time series) and is sensitive to a wider range of timescales than traditional mass-balance modeling. We use this approach to test hypotheses about how fire and flood impact ecological and biogeochemical dynamics in a wetland (Everglades, FL, USA) that was reconnected to its floodplain. Reintroduction of flow pulses after decades of separation by levees fundamentally reconfigured functional connectivity networks. The most pronounced expansion was that of the calcium network, which reflects periphyton dynamics and may represent an indirect influence of elevated nutrients, despite the comparatively smaller observed expansion of phosphorus networks. With respect to several solutes, periphyton acted as a “biotic filter,” shifting perturbations in water-quality signals to different timescales through slow but persistent transformations of the biotic community. The complex-networks approach also revealed portions of the landscape that operate in fundamentally different regimes with respect to dissolved oxygen, separated by a threshold in flow velocity of 1.2 cm/s, and suggested that complete removal of canals may be needed to restore connectivity with respect to biogeochemical processes. Fire reconfigured functional connectivity networks in a manner that reflected localized burn severity, but had a larger effect on the magnitude of solute concentrations.

Description: The health of the Chesapeake Bay ecosystem has been declining for several decades due to high levels of nutrients and sediments largely tied to agricultural production systems. Therefore, monitoring of agricultural water use and hydrologic connections between crop lands and Bay tributaries has received increasing attention. Remote sensing retrievals of actual evapotranspiration (ET) can provide valuable information in support of these hydrologic modeling efforts, spatially and temporally describing consumptive water use by crops and natural vegetation and quantifying response to expansion of irrigated area occurring with Bay watershed. In this study, a multi-sensor satellite data fusion methodology, combined with a multi-scale ET retrieval algorithm, was applied over the Choptank River watershed located within the Lower Chesapeake Bay region on the Eastern Shore of Maryland, USA to produce daily 30-m resolution ET maps. ET estimates directly retrieved on Landsat satellite overpass dates have high accuracy with relative error (RE) of 9%, as evaluated using flux tower measurements. The fused daily ET time series have reasonable errors of 18% at the daily time step - an improvement from 27% errors using standard Landsat-only interpolation techniques. Annual water consumption by different land cover types was assessed, showing reasonable distributions of water use with cover class. Seasonal patterns in modeled crop transpiration and soil evaporation for dominant crop types were analyzed, and agree well with crop phenology at field scale. Additionally, effects of irrigation occurring during a period of rainfall shortage were captured by the fusion program. These results suggest that the ET fusion system will have utility for water management at field and regional scales over the Eastern Shore. Further efforts are underway to integrate these detailed water use datasets into watershed-scale hydrologic models to improve assessments of water quality and inform best management practices to reduce nutrient and sediment loads to the Chesapeake Bay.

Description: We examine a suite of journal-level productivity and citation statistics for six leading hydrology journals in order to help authors understand the robustness and meaning of journal impact factors. The main results are (1) the probability distribution of citations is remarkably homogenous across hydrology journals; (2) hydrology papers tend to have a long-lasting impact, with a large fraction of papers cited after the 2 year window used to calculate the journal impact factor; and (3) journal impact factors are characterized by substantial year-to-year variability (especially for smaller journals), primarily because a small number of highly cited papers have a large influence on the journal impact factor. Consequently, the ranking of hydrology journals with respect to the journal impact factor in a given year does not have much information content. These results highlight problems in using citation data to evaluate hydrologic science. We hope that this analysis helps authors better understand journal-level citation statistics, and also helps improve research assessments in institutions and funding agencies.

Description: A systematic investigation of multiscale pore structure in organic-rich shale by means of the combination of various imaging techniques is presented, including the state-of-the-art Helium-Ion-Microscope (HIM). The study achieves insight into the major features at each scale and suggests the affordable techniques for specific objectives from the aspects of resolution, dimension and cost. The pores, which appear to be isolated, are connected by smaller pores resolved by higher resolution imaging. This observation provides valuable information, from the microscopic perspective of pore structure, for understanding how gas accumulates and transports from where it is generated. A comprehensive workflow is proposed based on the characteristics acquired from the multiscale pore structure analysis to simulate the gas transport process. The simulations are completed with three levels: the microscopic mechanisms should be taken into consideration at level I; the spatial distribution features of organic matter, inorganic matter, and macropores constitute the major issue at level II; and the micro-fracture orientation and topological structure are dominant factors at level III. The results of apparent permeability from simulations agree well with the values acquired from experiments. By means of the workflow, the impact of various gas transport mechanisms at different scales can be investigated more individually and precisely than conventional experiments.

Description: Water-related hazards such as floods, droughts and disease cause damage to an economy through the destruction of physical capital including property and infrastructure, the loss of human capital and the interruption of economic activities, like trade and education. The question for policy makers is whether the impacts of water-related risk accrue to manifest as a drag on economic growth at a scale suggesting policy intervention. In this study, the average drag on economic growth from water-related hazards faced by society at a global level is estimated. We use panel regressions with various specifications to investigate the relationship between economic growth and hydroclimatic variables at the country-river basin level. In doing so, we make use of surface water runoff variables never used before. The analysis of the climate variables shows that water availability and water hazards have significant effects on economic growth, providing further evidence beyond earlier studies finding that precipitation extremes were at least as important or likely more important than temperature effects. We then incorporate a broad set of variables representing the areas of infrastructure, institutions and information to identify the characteristics of a region that determine its vulnerability to water-related risks. The results identify water scarcity, governance and agricultural intensity as the most relevant measures affecting vulnerabilities to climate variability effects.

Description: We investigate the energy balance and ablation regimes of glaciers in high-elevation, dry environments using glacio-meteorological data collected on six glaciers in the semiarid Andes of North-Central Chile (29-34°S, 3127-5324 m). We use a point-scale physically-based energy balance (EB) model and an enhanced Temperature-Index (ETI) model that calculates melt rates only as a function of air temperature and net shortwave radiation. At all sites, the largest energy inputs are net shortwave and incoming longwave radiation, which are controlled by surface albedo and elevation, respectively. Turbulent fluxes cancel each other out at the lower sites, but as elevation increases, cold, dry and wind-exposed conditions increase the magnitude of negative latent heat fluxes, associated with large surface sublimation rates. In midsummer (January), ablation rates vary from 67.9 mm w.e. d − 1 at the lowest site (∼100% corresponding to melt), to 2.3 mm w.e. d − 1 at the highest site (〉85% corresponding to surface sublimation). At low-elevation, low-albedo, melt-dominated sites, the ETI model correctly reproduces melt using a large range of possible parameters, but both the performance and parameter transferability decrease with elevation for two main reasons: i) the air temperature threshold approach for melt onset does not capture the diurnal variability of melt in cold and strong irradiated environments and ii) energy losses decrease the correlation between melt and net shortwave radiation. We summarize our results by means of an elevation profile of ablation components that can be used as reference in future studies of glacier ablation in the semiarid Andes.