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The evaporation (EV) method is a widely used experimental approach to determine unsaturated hydraulic conductivity (UHC) and soil water retention (SWR) functions. However, the EV method has some limitations in evaluating accurate UHC values near the wet range. This restricts the working range of the models that utilize these UHC values for predicting soil water content and water fluxes for problems involving wet conditions (e.g., infiltration under ponding conditions). Our experimental study shows that when using the EV method, tensiometer offset errors as low as 1 cm can cause problems with wet-range measurements. Current methods to overcome these errors are expensive, inefficient, or cumbersome. In this study, we developed an improved EV method that utilizes a simple experimental protocol for standardizing the soil saturation procedure and a robust calibration method to quantify tensiometer offset errors. The proposed approach utilizes existing EV method hardware without requiring additional specialized instrumentation. Experiments performed using a silt loam soil sample show that our approach can extend the working range of the EV method to the wet range. Therefore, reliable estimates of the UHC and SWR functions can now be obtained over the entire range of soil suction heads. Our study also contributes to a better understanding of the EV method and uses this knowledge for collecting reliable and reproducible SWR and UHC data. The improved EV method is simple, effective, and cost-efficient. This method has the potential to greatly improve soil water content predictions, leading to better-informed decisions for groundwater resources management, sustainable agriculture, and environmental protection. [ABSTRACT FROM AUTHOR]

The spatial variability of soil hydraulic properties has to be considered to provide realistic predictions of unsaturated flow and transport at field scale. Steady unsaturated flow through randomly heterogeneous soils is analyzed and upscaled in this study. Numerical experiments of unsaturated flow have been performed on single realizations of unsaturated soil parameters generated by using threedimensional turning band random generator. In the first part of the study a linear relation for unsaturated hydraulic conductivity as a function of soil suction (ψ) given by K = Kseβψ is used, with constant β and saturated hydraulic conductivity (Ks) as perfectly correlated random space functions. In the first part of the study, linear conductivity relation given by Gardner is used, with Gardner's constant (β) and saturated conductivity (Ks) as perfectly correlated random space functions. The steady flow fields generated are analyzed to explore the effect of variability of the product (β): the pore-size distribution parameter whose reciprocal represents the characteristic capillary length (hcap) and the correlation length (λZ) of the generated soil properties. A set of one-dimensional simulations are performed, with various values of the product βλZ ratio of correlation length to capillary dispersion length (λz/hcap = 1, »1 and «1) used to compare the moisture distribution tendency. The resulting steady pressure fields are compared and analyzed. In some cases, the soil behaves as homogeneous, and in other cases as stratified. The second part of the study seeks to identify simple upscaling laws for block-scale nonlinear constitutive relationships under high tension, with the parameters of their measurement scale counterparts represented by the van Genuchten model. Random fields are generated to represent layering of perfect and imperfectly stratified soils, and numerical simulations are performed in flows that are perpendicular and parallel to stratification. Upscaling of hydraulic conductivity curve KðψÞ and moisture retention curve θðψÞ is performed for perfectly stratified and imperfectly stratified soils, and anisotropy is also studied for both types of layering considered. It is demonstrated that the upscaled nonlinear curves can be represented by simple exponential relations and/or by linear relations obtained by piecewise linearization. The linear relations obtained can be expressed in equivalent terms that can be used in analytical solutions to directly obtain effective hydraulic conductivity values. [ABSTRACT FROM AUTHOR]

Surface microtopography influences both spatial and temporal distributions of a series of hydrologic processes including infiltration, surface runoff, and unsaturated flow. Importantly, surface roughness and depressions also affect the fate and transport of pollutants throughout surface and subsurface environments. The objective of this study was to experimentally investigate the effects of surface microtopography on overland flow, infiltration, and tracer bromide (Br) runoff, and leaching into the subsurface system under simulated rainfall. Both rough and smooth surfaces were created by using sieved soil and scanned by an instantaneous-profile laser scanner to acquire high-resolution digital elevation models (DEMs) of the soil surfaces. A mold surface, featuring a number of depressions of varied sizes and relationships, was used to create the rough soil surface. The Windows-based puddle delineation (PD) software was applied to characterize surface microtopography, determine flow directions and accumulations, and compute maximum depression storage (MDS) and maximum ponding area (MPA). Two overland flow and tracer transport experiments were conducted using the smooth and rough soil surfaces. Runoff water and soil samples were analyzed for Br. The experimental results indicated that the smooth surface seemed to have earlier and greater water and tracer runoff than the rough surface. Water discharge and tracer loading from the rough surface with varying depressions exhibited a unique, microtopography-controlled threshold behavior. It was found that spatial distributions of the tracer in the subsurface system matched the surface microtopographic features (e.g., depressions, peaks/mounds, and ridges). Enhanced infiltration and tracer leaching under depressions of the rough surface were observed, which resulted in higher Br levels in deeper soil. In contrast, the dominant surface runoff and tracer transport along the horizontal direction for the smooth surface yielded higher Br concentrations in shallow soil. This preliminary study highlighted the important role of surface microtopography in tracer transport in a coupled overland and unsaturated flow system, and demonstrated that surface microtopography influenced the location and timing of ponding, runoff, and infiltration. Increased understanding of the effect of soil microtopography on hydrologic processes is expected to yield improved ability to simulate soil water and solute movement. [ABSTRACT FROM AUTHOR]

One-dimensional infiltration tests were conducted to investigate the effects of rainfall intensity and initial water content on the transient infiltration pattern of unsaturated silty sand. For a constant rainfall intensity greater than the saturated permeability but smaller than the maximum infiltration capacity of soil, the infiltration curve is divided into preponding and postponding stages. The time to ponding reduces with increasing rainfall intensity and initial water content. After ponding, the infiltration rate reduces with time and its reducing rate increases with rainfall intensity but does not vary significantly with initial water content. The infiltration pattern can be simulated qualitatively by solving Richard’s equation numerically with the use of the flux and head type boundary conditions at the soil surface in the preponding and postponding stages, respectively. [ABSTRACT FROM AUTHOR]

The accuracy of soil moisture vertical profiles simulated by a conceptual model during erratic rainfall is investigated through laboratory and field experiments. Simulations were performed by a conceptual model earlier formulated for the estimate of local infiltration into vertically homogeneous soils. Laboratory and field experiments were set up. The results indicated that the conceptual model appropriately simulated the observed vertical profiles of soil water content during the stages of infiltration and reinfiltration produced by a wide variety of rainfall patterns. Thus, the conceptual model investigated may be considered a suitable component of watershed hydrological models as well as of models coupling soil and atmosphere. [ABSTRACT FROM AUTHOR]

The aquifer system, which is the resource of water for the city of Milano (Italy), is a multilayered aquifer, characterized by sandy and sandy-gravel units, connected by discontinuous aquitards. The recharge area of the phreatic aquifer is close to the prealpine area, some tens of kilometers north of the city; nevertheless the local recharge given by rain infiltration through the soil is not negligible and contributes to the mass balance of the aquifer. A field campaign was carried out for 16 months in the suburban area of the Lambro Park to evaluate the local recharge due to the rainfall infiltration through the unsaturated zone; the water table in this area lies about 15 m below the ground surface. Standard meteorological data (atmospheric pressure, rainfall, humidity, wind velocity and air temperature) were collected, along with incident and net radiation, to evaluate potential and actual evaporation from the bare soil using three methods (Bowen ratio, Penman, and Hamon equations). Simultaneously, the volumetric soil water content at different depths (down to 76 cm) was measured with time domain reflectrometry probes. A finite-difference model, which solves the one-dimensional Richards equation in transient conditions, was developed to simulate the flow through the unsaturated zone, to evaluate the characteristic time of recharge and the mass balance. The water flow rate through the ground surface was assigned as the upper boundary condition and was evaluated from the mass and energy balance at the atmosphere–soil interface with different approaches based on the meteorological data and the actual soil water content. The lower boundary condition is given by the saturation condition at the water table. Characteristic retention curves were estimated in laboratory using Richards apparatus. In order to calibrate the model, the numerical results were compared with experimental data for two different periods: A dry period (July 2001) and a wet period (October 2001). Finally, the calibrated model was used to simulate the infiltration and the flow through the unsaturated zone for a 15-year-long period (1988–2003) and to estimate the total phreatic aquifer recharge. Moreover, the time lag between a variation of the infiltration/exfiltration rate and the corresponding variation of the recharge rate was evaluated; the values of the delay time of recharge are longer than those computed with models that approximate the transmission zone as a unique cell. [ABSTRACT FROM AUTHOR]

Finite-element and finite-difference solutions to different forms of the Richards equation can exhibit stability problems as well as mass balance errors. These problems are more pronounced for sharp wetting fronts in soils with very dry initial conditions and at material interfaces for layered soil profiles. The pressure head form can suffer from large mass balance errors while the moisture content form conserves mass perfectly but has difficulty handling material boundaries in layered soil profiles and near saturation conditions. This paper presents a conservative mixed formulation for the solution of the Richards equation of unsaturated flow by the finite-element method in which the discharge velocity (volumetric flux) and pressure head are the primary field variables. Solution techniques for various boundary conditions including prescribed constant pressure head and constant flux are also presented. The ability of the formulation to handle variably saturated domain and material heterogeneity is also established. Example solutions for infiltration into homogeneous and heterogeneous unsaturated and variably saturated soil profiles are compared with finite-element and finite-difference solutions of pressure head and mixed form of Richards equation. Results indicate that the alternative formulation is mass conservative, stable, and can better handle heterogeneous boundaries and variably saturated flow domains. [ABSTRACT FROM AUTHOR]

Unsaturated flow transforms streams from constant head boundaries to constant flux boundaries, impacting the quantity of stream recharge to groundwater. The objective of this research was to analyze the impact of unsaturated flow on stream/aquifer exchange by incorporating a three-regime, saturated/unsaturated flow, stream recharge model into MODFLOW, the most commonly used numerical flow model in ground water hydrology. This three-regime model extends prior research by accounting for a transition regime between fully saturated and gravity-driven unsaturated flow. Accounting for unsaturated flow between a stream and aquifer is necessary to adequately simulate stream depletion. Sensitivity analyses of steady stream recharge versus drawdown demonstrated that the transition regime became more important for cases with small stream stages, smaller streambed thicknesses, and larger entry pressure heads. Based on coarse sand and fine sand hypothetical scenarios, the transition regime in the proposed three-regime model could generally be neglected with minor error for most stream/aquifer scenarios. However, the transition regime became important when: (1) the drawdown stabilized near the range of drawdowns that represented the transition regime; and (2) there must be a significant difference (i.e., 0.2 m) in the entry and ultimate capillary pressure heads, which occurred with larger entry pressure heads (i.e., equal to or greater than 1.0 m) and a smaller pore size distribution index (i.e., approximately 1.0). [ABSTRACT FROM AUTHOR]

Erosion by lateral, subsurface flow is known to erode streambank sediment in numerous geographical locations; however, the role of seepage erosion on mass failure of streambanks is not well understood. In the absence of an established sediment transport model for seepage erosion, the objectives of this research were to investigate the mechanisms of erosion due to concentrated, lateral subsurface flow and develop an empirical sediment transport model for seepage erosion of noncohesive sediment on near-vertical streambanks. Laboratory experiments were performed using a two-dimensional soil lysimeter of a reconstructed streambank profile packed with three different soil layers to mimic seepage erosion occurring at Little Topashaw Creek (LTC) in northern Mississippi. Soil samples from LTC streambanks indicated considerable hydraulic conductivity contrast between an overlying silt loam layer (SiL), highly permeable loamy sand, and confining clay loam layer. Lysimeter experiments were conducted with various upstream water table heads, overburden heights, and lysimeter slopes. Bank failure occurred prior to the total release of negative pore-water pressures in the SiL layer suggesting that such a mechanism was not critical for bank collapse due to seepage erosion. A seepage erosion transport model for conductive, noncohesive soil layers was derived based on a dimensionless sediment discharge and dimensionless seepage flow shear stress. The advantage of this sediment transport model is that it relates sediment flux to seepage discharge from the streambank. [ABSTRACT FROM AUTHOR]

Verification and application results are presented for a new saturated-unsaturated 3D groundwater flow model (SU3D) developing in Dogan and Motz, which can be used to calculate the pressure distribution over the entire groundwater flow domain in response to rainfall and evapotranspiration. SU3D solves the nonlinear 3D, modified mixed form of the Richards equation continuously throughout the groundwater flow domain, including both the unsaturated and saturated zones. The block-centered finite-difference method, a modified Picard iteration scheme, and the preconditioned conjugate gradient method are used to solve the governing partial differential equations in the new model. SU3D can simulate evaporation from land surface and transpiration from the root zone. Potential evapotranspiration is partitioned into potential evaporation and potential transpiration as a function of the leaf area index; actual evaporation and actual transpiration are then calculated individually. As shown in this paper, SU3D has been verified by reproducing the results of five analytical and numerical studies and a 3D unconfined-aquifer pumping test from the published literature. [ABSTRACT FROM AUTHOR]

A new saturated-unsaturated three-dimensional (3D) groundwater flow model (SU3D) has been developed that calculates the pressure distribution over the entire groundwater flow domain in response to rainfall and evapotranspiration. Recent advances in solving the saturated-unsaturated groundwater flow equation are incorporated into SU3D, which solves the nonlinear, 3D, modified mixed form of the Richards equation continuously throughout the groundwater flow domain. The block-centered, finite-difference method with a variably sized grid is employed to solve the governing partial differential equation. The non-linear terms of the governing equation are linearized using a modified Picard iteration scheme, and the preconditioned conjugate gradient method is used to solve the linearized system of equations. SU3D can simulate the effects of pumping from an aquifer, and it has an option to calculate potential evapotranspiration (PET) from meteorological data. The PET is partitioned into potential evaporation and potential transpiration as a function of the leaf area index, and then the actual evaporation and transpiration are calculated. Overland flow and seepage face calculations are not included in SU3D. [ABSTRACT FROM AUTHOR]