Description: The volume of water released from storage per unit head drop per volume of an REV is a basic quantity in groundwater hydrology, but the details of the process of storage change in the vicinity of a well are commonly overlooked. We characterize storage change in a flat-lying fracture or thin sedimentary bed through the apparent hydraulic compliance, Cf, the change in aperture of the fracture or thickness of the layer per unit change in pressure. The results of theoretical analyses and field measurements show that Cf increases with time near the well during pumping, but it drops suddenly and may become negative at the beginning of recovery during a well test. Profiles of Cf increase with radial distance from a well, but they are marked by a sharp increase and a sharp decrease at the edge of the region affected by the wellbore pressure transient. The conventional view in groundwater hydrology is that storage change at a point is proportional to the local change in pressure, which requires that the hydraulic compliance is uniform and constant. It appears that this conventional view is a simplification of a process that varies in both space and time and can even take on negative values. This simplification may be a source of uncertainty when interpreting well tests and extensometer records or predicting long-term well performance.

Description: Bare soil evaporation is a key process for water exchange between the land and the atmosphere and an important component of the water balance. However, there is no agreement on the best modeling methodology to determine evaporation under different atmospheric boundary conditions. Also, there is a lack of directly measured soil evaporation data for model validation to compare these methods to establish the validity of their mathematical formulations. Thus, a need exists to systematically compare evaporation estimates using existing methods to experimental observations. The goal of this work is to test different conceptual and mathematical formulations that are used to estimate evaporation from bare soils to critically investigate various formulations and surface boundary conditions. Such a comparison required the development of a numerical model that has the ability to incorporate these boundary conditions. For this model, we modified a previously developed theory that allows nonequilibrium liquid/gas phase change with gas phase vapor diffusion to better account for dry soil conditions. Precision data under well-controlled transient heat and wind boundary conditions were generated, and results from numerical simulations were compared with experimental data. Results demonstrate that the approaches based on different boundary conditions varied in their ability to capture different stages of evaporation. All approaches have benefits and limitations, and no one approach can be deemed most appropriate for every scenario. Comparisons of different formulations of the surface boundary condition validate the need for further research on heat and vapor transport processes in soil for better modeling accuracy.

Description: The primary objective of this work is to synthesize components of benthic flux in the Patos Lagoon coastal zone, Rio Grande do Sul, Brazil. Specifically, the component of benthic discharge flux forced by the terrestrial hydraulic gradient is 0.8 m3 d−1; components of benthic discharge and recharge flux associated with the groundwater tidal prism are both 2.1 m3 d−1; components of benthic discharge and recharge flux forced by surface-gravity wave setup are both 6.3 m3 d−1; the component of benthic discharge flux that transports radium-228 is 350 m3 d−1; and components of benthic discharge and recharge flux forced by surface-gravity waves propagating over a porous medium are both 1400 m3 d−1. (All models are normalized per meter shoreline.) Benthic flux is a function of components forced by individual mechanisms and nonlinear interactions that exist between components. Constructive and destructive interference may enhance or diminish the contribution of benthic flux components. It may not be possible to model benthic flux by summing component magnitudes. Geochemical tracer techniques may not accurately model benthic discharge flux or submarine groundwater discharge (SGD). A conceptual model provides a framework on which to quantitatively characterize benthic discharge flux and SGD with a multifaceted approach.

Description: This study presents a quantitative national assessment of urban water availability and vulnerability for 225 U.S. cities with population greater than 100,000. Here, the urban assessments account for not only renewable water flows, but also the extracted, imported, and stored water that urban systems access through constructed infrastructure. These sources represent important hydraulic components of the urban water supply, yet are typically excluded from water scarcity assessments. Results from this hydraulic-based assessment were compared to those obtained using a more conventional method that estimates scarcity solely based on local renewable flows. The inclusion of hydraulic components increased the mean availability to cities, leading to a significantly lower portion of the total U.S. population considered “at risk” for water scarcity (17%) than that obtained from the runoff method (47%). Water vulnerability was determined based on low-flow conditions, and smaller differences were found for this metric between at-risk populations using the runoff (66%) and hydraulic-based (54%) methods. The large increase in the susceptible population between the scarcity measures evaluated using the hydraulic method may better reconcile the seeming contradiction in the United States between perceptions of natural water abundance and widespread water scarcity. Additionally, urban vulnerability measures developed here were validated using a media text analysis. Vulnerability assessments that included hydraulic components were found to correlate with the frequency of urban water scarcity reports in the popular press while runoff-based measures showed no significant correlation, suggesting that hydraulic-based assessments provide better context for understanding the nature and severity of urban water scarcity issues.

Description: Diverging radial flow takes place in a heterogeneous porous medium where the log conductivity Y = ln K is modeled as a stationary random space function (RSF). The flow is steady, and is generated by a fully penetrating well. A linearly sorbing solute is injected through the well envelope, and we aim at computing the average flux concentration (breakthrough curve). A relatively simple solution for this difficult problem is achieved by adopting, similar to Indelman and Dagan (1999), a few simplifying assumptions: (i) a thick aquifer of large horizontal extent, (ii) mildly heterogeneous medium, (iii) strongly anisotropic formation, and (iv) large Peclet number. By introducing an appropriate Lagrangian framework, three-dimensional transport is mapped onto a one-dimensional domain (τ, t) where τ and t represent the fluid travel and current time, respectively. Central for this approach is the probability density function of the RSF τ that is derived consistently with the adopted assumptions stated above. Based on this, it is shown that the travel time can be regarded as a Gaussian random variable only in the far field. The breakthrough curves are analyzed to assess the impact of the hydraulic as well as reactive parameters. Finally, the travel time approach is tested against a forced-gradient transport experiment and shows good agreement.

Description: We present a three-dimensional high-resolution hydrodynamic model for unsteady incompressible flow over an evolving bed topography. This is achieved by using a multilevel Cartesian grid technique that allows the grid to be refined in high-gradient regions and in the vicinity of the river bed. The grid can be locally refined and adapted to the bed geometry, managing the Cartesian grid cells and faces using a hierarchical tree data approach. A ghost-cell immersed-boundary technique is applied to cells intersecting the bed topography. The governing equations have been discretized using a finite-volume method on a staggered grid, conserving second-order accuracy in time and space. The solution advances in time using the fractional step approach. Large-eddy simulation is used as turbulence closure. We validate the model against several experiments and other results from literature. Model results for Stokes flow around a cylinder in the vicinity of a moving wall agree well with Wannier's analytical solution. At higher Reynolds numbers, computed trailing bubble length, separation angle, and drag coefficient compare favorably with experimental and previous computational results. Results for the flow over two- and three-dimensional dunes agree well with published data, including a fair reproduction of recirculation zones, horse-shoe structures, and boiling effects. This shows that the model is suitable for being used as a hydrodynamic submodel in the high-resolution modeling of sediment transport and formation and evolution of subaqueous ripples and dunes.

Description: Satellite-passive microwave remote sensing has been extensively used to estimate snow water equivalent (SWE) in northern regions. Although passive microwave sensors operate independent of solar illumination and the lower frequencies are independent of atmospheric conditions, the coarse spatial resolution introduces uncertainties to SWE retrievals due to the surface heterogeneity within individual pixels. In this article, we investigate the coupling of a thermodynamic multilayered snow model with a passive microwave emission model. Results show that the snow model itself provides poor SWE simulations when compared to field measurements from two major field campaigns. Coupling the snow and microwave emission models with successive iterations to correct the influence of snow grain size and density significantly improves SWE simulations. This method was further validated using an additional independent data set, which also showed significant improvement using the two-step iteration method compared to standalone simulations with the snow model.

Description: There has been a recent debate in the hydrological community about the relative merits of the informal generalized likelihood uncertainty estimation (GLUE) approach to uncertainty assessment in hydrological modeling versus formal probabilistic approaches. Some recent literature has suggested that the methods can give similar results in practice when properly applied. In this note, we show that the connection between formal Bayes and GLUE is not merely operational but goes deeper, with GLUE corresponding to a certain approximate Bayesian procedure even when the “generalized likelihood” is not a true likelihood. The connection we describe relates to recent approximate Bayesian computation (ABC) methods originating in genetics. ABC algorithms involve the use of a kernel function, and the generalized likelihood in GLUE can be thought of as relating to this kernel function rather than to the model likelihood. Two interpretations of GLUE emerge, one as a computational approximation to a Bayes procedure for a certain “error-free” model and the second as an exact Bayes procedure for a perturbation of that model in which the truncation of the generalized likelihood in GLUE plays a role. The intent of this study is to encourage cross-fertilization of ideas regarding GLUE and ABC in hydrologic applications. The connection we outline suggests the possibility of combining a formal likelihood with a kernel based on a generalized likelihood within the ABC framework and also allows advanced ABC computational methods to be used in GLUE applications. The model-based interpretation of GLUE may also be helpful in partially illuminating the implicit assumptions in different choices of generalized likelihood.

Description: In semiarid regions, the rooting strategies employed by vegetation can be critical to its survival. Arid regions are characterized by high variability in the arrival of rainfall, and species found in these areas have adapted mechanisms to ensure the capture of this scarce resource. Vegetation roots have strong control over this partitioning, and assuming a static root profile, predetermine the manner in which this partitioning is undertaken.A coupled, dynamic vegetation and hydrologic model, tRIBS + VEGGIE, was used to explore the role of vertical root distribution on hydrologic fluxes. Point-scale simulations were carried out using two spatially and temporally invariant rooting schemes: uniform: a one-parameter model and logistic: a two-parameter model. The simulations were forced with a stochastic climate generator calibrated to weather stations and rain gauges in the semiarid Walnut Gulch Experimental Watershed (WGEW) in Arizona. A series of simulations were undertaken exploring the parameter space of both rooting schemes and the optimal root distribution for the simulation, which was defined as the root distribution with the maximum mean transpiration over a 100-yr period, and this was identified. This optimal root profile was determined for five generic soil textures and two plant-functional types (PFTs) to illustrate the role of soil texture on the partitioning of moisture at the land surface. The simulation results illustrate the strong control soil texture has on the partitioning of rainfall and consequently the depth of the optimal rooting profile. High-conductivity soils resulted in the deepest optimal rooting profile with land surface moisture fluxes dominated by transpiration. As we move toward the lower conductivity end of the soil spectrum, a shallowing of the optimal rooting profile is observed and evaporation gradually becomes the dominate flux from the land surface. This study offers a methodology through which local plant, soil, and climate can be accounted for in the parameterization of rooting profiles in semiarid regions.

Description: This article focuses on household water use in Spain by analyzing the influence of a detailed set of factors. We find that, although the presence of both water-saving equipment and water-conservation habits leads to water savings, the factors that influence each are not the same. In particular, our results show that those individuals most committed to the adoption of water-saving equipment and, at the same time, less committed to water-conservation habits tend to have higher incomes.