Jianting Zhu

Spatial Averaging of van Genuchten Hydraulic Parameters for Steady-State Flow in Heterogeneous Soils: A Numerical Study

For meso- or regional-scale Soil–Vegetation–Atmosphere Transfer (SVAT) schemes in hydroclimatic models, pixel dimensions may range from several hundred square meters to several hundred square kilometers. Pixel-scale soil hydraulic parameters and their accuracy are critical for the success of hydroclimatic and soil hydrologic models. This study tries to answer a major question: What will be the effective and average hydraulic properties for the entire pixel (or footprint of a remote sensor) consisting of several textures if the soil hydraulic properties can be estimated for each individual texture? In this study, we examined the impact of areal heterogeneity in soil hydraulic parameters on soil ensemble behavior for steady-state evaporation and infiltration. Using the widely used van Genuchten model and hydraulic parameter statistics obtained from neural network–based pedotransfer functions (PTFs) for various soil textural classes, we address the impact of areal hydraulic property heterogeneity on ensemble behavior and uncertainty in steady-state vertical flow in large-scale heterogeneous fields. The various averaging schemes of van Genuchten parameters are compared with “effective parameters” calculated by conceptualizing the areally heterogeneous soil formation as an equivalent homogeneous medium that will discharge approximately the same amount of ensemble flux of the heterogeneous soil. The impact of boundary conditions and parameter correlation on the effective parameters, as well as the accuracy and uncertainty of the averaging schemes for the hydraulic parameters, are investigated and discussed. In light of our results, we suggest the following guidelines for van Genuchten hydraulic parameter averaging: arithmetic means for Ks and n , a value between arithmetic and geometric means for α when Ks and α are highly correlated, and a value between geometric and harmonic means for α when Ks and α are poorly correlated.

Spatial Averaging of van Genuchten Hydraulic Parameters for Steady-State Flow in Heterogeneous Soils

For meso- or regional-scale Soil–Vegetation–Atmosphere Transfer (SVAT) schemes in hydroclimatic models, pixel dimensions may range from several hundred square meters to several hundred square kilometers. Pixel-scale soil hydraulic parameters and their accuracy are critical for the success of hydroclimatic and soil hydrologic models. This study tries to answer a major question: What will be the effective and average hydraulic properties for the entire pixel (or footprint of a remote sensor) consisting of several textures if the soil hydraulic properties can be estimated for each individual texture? In this study, we examined the impact of areal heterogeneity in soil hydraulic parameters on soil ensemble behavior for steady-state evaporation and infiltration. Using the widely used van Genuchten model and hydraulic parameter statistics obtained from neural network–based pedotransfer functions (PTFs) for various soil textural classes, we address the impact of areal hydraulic property heterogeneity on ensemble behavior and uncertainty in steady-state vertical flow in large-scale heterogeneous fields. The various averaging schemes of van Genuchten parameters are compared with “effective parameters” calculated by conceptualizing the areally heterogeneous soil formation as an equivalent homogeneous medium that will discharge approximately the same amount of ensemble flux of the heterogeneous soil. The impact of boundary conditions and parameter correlation on the effective parameters, as well as the accuracy and uncertainty of the averaging schemes for the hydraulic parameters, are investigated and discussed. In light of our results, we suggest the following guidelines for van Genuchten hydraulic parameter averaging: arithmetic means for Ks and n , a value between arithmetic and geometric means for α when Ks and α are highly correlated, and a value between geometric and harmonic means for α when Ks and α are poorly correlated.

Effective Hydraulic Parameters in Horizontally and Vertically Heterogeneous Soils for Steady-State Land–Atmosphere Interaction

In this study, the authors investigate effective soil hydraulic parameter averaging schemes for steady-state flow in heterogeneous shallow subsurfaces useful to land–atmosphere interaction modeling. “Effective” soil hydraulic parameters of the heterogeneous shallow subsurface are obtained by conceptualizing the soil as an equivalent homogeneous medium. It requires that the effective homogeneous soil discharges the same mean surface moisture flux (evaporation or infiltration) as the heterogeneous media. Using the simple Gardner unsaturated hydraulic conductivity function, the authors derive the effective value for the saturated hydraulic conductivity Ks or the shape factor under various hydrologic scenarios and input hydraulic parameter statistics. Assuming one-dimensional vertical moisture movement in the shallow unsaturated soils, both scenarios of horizontal (across the surface landscape) and vertical (across the soil profile) heterogeneities are investigated. The effects of hydraulic parameter statistics, surface boundary conditions, domain scales, and fractal dimensions in case of nested soil hydraulic property structure are addressed. Results show that the effective parameters are dictated more by the heterogeneity for the evaporation scenario and mainly by Ks variability for the infiltration scenario. Also, heterogeneity orientation (horizontal or vertical) of soil hydraulic parameters impacts the effective parameters. In general, an increase in both the fractal dimension and the domain scale enhances the heterogeneous effects of the parameter fields on the effective parameters. The impact of the domain scale on the effective hydraulic parameters is more significant as the fractal dimension increases.

Soil Hydraulic Parameter Upscaling for Steady-State Flow with Root Water Uptake

In this study we investigate effective soil hydraulic parameter averaging schemes for steady-state flow with plant root water uptake in heterogeneous soils. “Effective” soil hydraulic parameters of a heterogeneous soil formation are obtained by conceptualizing the soil as an equivalent homogeneous medium. The “effective” homogeneous medium is only required to discharge the same ensemble-mean flux across the soil surface. One-dimensional flow at the local scale has been used as an approximation for various simplified problems under investigation (e.g., a shallow subsurface dominated by vertical flows). The domain is assumed to be composed of homogeneous one-dimensional soil columns without mutual interactions. Using Gardner9s unsaturated hydraulic conductivity model, we derive the effective value for the parameter α. While root water uptake influences the overall water budget, its impact on the effective hydraulic parameter averaging scheme was found to be secondary. Results show that the arithmetic mean of Gardner9s α is usually too large to serve as an effective parameter. Deviations of the effective parameter from the arithmetic mean become larger as the surface suction increases; that is, the flow scenario switches from infiltration to evaporation. The results consistently show a smaller effective parameter for evaporation scenarios than for infiltration scenarios. The effective parameter α eff decreases with an increase in the mean value of α. Spatial variability in α also decreases the effective value of α eff . Alternative root water uptake distributions do not produce significant differences in both the water budget and the averaging scheme as long as total water loss to the plant roots remains the same.

Stochastic simulations of NAPL mass transport in variably saturated heterogeneous porous media.

A multiphase flow and transport numerical model is developed to study the effects of porous media heterogeneities on residual NAPL mass partitioning and transport of dissolved and/or volatilized NAPL mass in variably saturated media. The results indicate the significance of porous media heterogeneity in influencing the mass transfer processes and NAPL transport in the subsurface. Among the parameters investigated in this study, the heterogeneity of the permeability field has the most significant influence on the NAPL mass partitioning and transport. In general, the heterogeneity of the porous media properties enhances the NAPL mass plume spreading in both the water phase and the gas phase while the influence on the water phase is much more significant. Overall, the porous media property heterogeneities tend to increase the accumulation of NAPL mass in the water phase. The nonequilibrium mass transfer processes result in the expected trend of decreasing the NAPL mass dissipation rate and causing long-term groundwater contamination.

Impact of Climate Change on Extreme Rainfall across the United States

AbstractChanges in climate may alter extreme rainfall intensity in most regions. In order to incorporate potential future changes, tools for planning and design should be capable of considering nonstationary climate conditions. In this study, potential changes in intensity-duration-frequency (IDF) curves, which are often used for assessment of extreme rainfall events, were explored using climate model historic runs, reanalysis runs, and future climate projections. The concept of the adjustment factor, which represents the general ability of climate models in representing the reanalysis data that is a reasonable estimate of actual gridded historical climate at the model grid scale, was applied to investigate potential impacts of climate change on IDF curves. For most of the study regions, future climate projections suggested an increase in the intensity of extreme storms for a given return period and duration with strong regional variations. The results also revealed the dependence of the adjustment factor...

Incorporating layer- and local-scale heterogeneities in numerical simulation of unsaturated flow and tracer transport

This study characterizes layer- and local-scale heterogeneities in hydraulic parameters (i.e., matrix permeability and porosity) and investigates the relative effect of layer- and local-scale heterogeneities on the uncertainty assessment of unsaturated flow and tracer transport in the unsaturated zone of Yucca Mountain, USA. The layer-scale heterogeneity is specific to hydrogeologic layers with layerwise properties, while the local-scale heterogeneity refers to the spatial variation of hydraulic properties within a layer. A Monte Carlo method is used to estimate mean, variance, and 5th, and 95th percentiles for the quantities of interest (e.g., matrix saturation and normalized cumulative mass arrival). Model simulations of unsaturated flow are evaluated by comparing the simulated and observed matrix saturations. Local-scale heterogeneity is examined by comparing the results of this study with those of the previous study that only considers layer-scale heterogeneity. We find that local-scale heterogeneity significantly increases predictive uncertainty in the percolation fluxes and tracer plumes, whereas the mean predictions are only slightly affected by the local-scale heterogeneity. The mean travel time of the conservative and reactive tracers to the water table in the early stage increases significantly due to the local-scale heterogeneity, while the influence of local-scale heterogeneity on travel time gradually decreases over time. Layer-scale heterogeneity is more important than local-scale heterogeneity for simulating overall tracer travel time, suggesting that it would be more cost-effective to reduce the layer-scale parameter uncertainty in order to reduce predictive uncertainty in tracer transport.

Validation of finite water‐content vadose zone dynamics method using column experiments with a moving water table and applied surface flux

Data from laboratory experiments on a 143 cm tall and 14.5 cm diameter column, packed with Wedron sand with varied constant upper boundary fluxes and water table velocities for both falling and rising water tables are used to validate a finite water-content vadose zone simulation methodology. The one-dimensional finite water-content Talbot and Ogden (2008) (T-O) infiltration and redistribution method was improved to simulate groundwater table dynamic effects and compared against the numerical solution of the Richards equation using Hydrus-1D. Both numerical solutions agreed satisfactorily with time series measurements of water content. Results showed similar performance for both methods, with the T-O method on average having higher Nash-Sutcliffe efficiencies and smaller absolute biases. Hydrus-1D was more accurate in predicting deponding times in the case of a falling water table, while Hydrus-1D and the T-O method had similar errors in predicted ponding times in the case of a rising water table in six of nine tests. The improved T-O method was able to predict general features of vadose zone moisture dynamics with moving water table and surface infiltration using an explicit, mass-conservative formulation. The advantage of an explicit formulation is that it is numerically simple, using forward Euler solution methodology, and is guaranteed to converge and to conserve mass. These properties make the improved T-O method presented in this paper a robust and computationally efficient alternative to the numerical solution of the Richards equation in hydrological modeling applications involving groundwater table dynamic effects on vadose zone soil moistures.

Spatiotemporal patterns in nutrient loads, nutrient concentrations, and algal biomass in Lake Taihu, China

Abstract Lake Taihu, Chinas third largest freshwater lake, exemplifies the severity of eutrophication problems in rapidly developing regions. We used long term land use, water quality, and hydrologic data from 26 in-lake and 32 tributary locations to describe the spatiotemporal patterns in nutrient loads, nutrient concentration, algal biomass, measured as chlorophyll a (Chl-a), in Lake Taihu. Point and nonpoint sources, as determined by chemical oxygen demand, contributed approximately 75 and 25% of the total nutrient loads to the lake, respectively. Spatial patterns in total phosphorus (TP) and total nitrogen (TN) concentrations in Lake Taihu strongly corresponded with observed loads from adjoining rivers with high concentrations proximate to densely populated areas. Chl-a concentrations exhibited spatial patterns similar to TP and TN concentrations. Generally, nutrient and Chl-a concentrations were highest in the northwestern region of the lake and lowest in the southeastern region of the lake. Seasonally, the largest nutrient loads occurred during summer. The annual net retention rate of TP and TN in Lake Taihu was approximately 30% of the total load. This study identifies regions of the lake and the watershed that are producing more nutrients to develop targeted management strategies. Reducing external P and N input from both point and nonpoint sources is obviously critical to address water quality issues in the lake. In addition, atmospheric deposition and resuspension of existing lake sediments also likely play a role in eutrophication processes and harmful algal blooms occurrence.

Non-newtonian effects on the drag of creeping flow through packed beds

Abstract Slow flows of non-Newtonian fluids through packed beds of solid particles are studied numerically and analytically using the free-surface cell model to account for the interactions between particles. The flow problem of a Carreau fluid is solved by the finite difference method and that of a second-order fluid by the perturbation method. It is shown that the flow drag decreases with a decrease in the flow behavior index and with an increase in the characteristic time. The degree of this reduction is found to be more significant at low voidages. The numerical results are found to be closer to the lower bounds obtained using variational principles by earlier investigators. The perturbation solutions predict that the second normal stress difference coefficient has a significant influence on the flow resistance. The flow resistance can either increase or decrease with an increase in the Deborah number, according to the values of the second normal stress difference coefficient. The results are found to be in agreement with the experimental findings that the viscoelastic flow through packed beds can exhibit a rapid increase in the flow resistance, over and above that expected for a comparable viscous fluid, in the second normal stress difference coefficient range for most real viscoelastic fluids.