Martinus Th. van Genuchten

Estimating Unsaturated Soil Hydraulic Properties From Multiple Tension Disc Infiltrometer Data

In a previous study, we showed that the cumulative infiltration rate measured with a tension disc infiltrometer at one particular tension does not provide enough information to estimate van Genuchtens soil-hydraulic parameters by numerical inversion of the Richards equation. In this paper we analyze the possibility of using cumulative infiltration rates obtained at several consecutive tensions for the purpose of estimating soil hydraulic parameters. We also investigate whether additional, easily obtainable information improves identifiability of the unknown parameters. The study is carried out using numerically generated data. The uniqueness problem was analyzed by studying the behavior of response surfaces in the optimized parameter planes. Our parameter estimation procedure combines the Levenberg-Marquardt nonlinear parameter optimization method with a quasi three-dimensional numerical model, HYDRUS-2D, which solves the variably-saturated flow equation. We found that the combination of multiple tension cumulative infiltration data with measured values of the initial and final water contents yielded unique solutions of the inverse problem for the unknown parameters.

Impacts of the 2004 tsunami on groundwater resources in Sri Lanka

The 26 December 2004 tsunami caused widespread destruction and contamination of coastal aquifers across southern Asia. Seawater filled domestic open dug wells and also entered the aquifers via direct infiltration during the first flooding waves and later as ponded seawater infiltrated through the permeable sands that are typical of coastal aquifers. In Sri Lanka alone, it is estimated that over 40,000 drinking water wells were either destroyed or contaminated. From February through September 2005, a team of United States, Sri Lankan, and Danish water resource scientists and engineers surveyed the coastal groundwater resources of Sri Lanka to develop an understanding of the impacts of the tsunami and to provide recommendations for the future of coastal water resources in south Asia. In the tsunami-affected areas, seawater was found to have infiltrated and mixed with fresh groundwater lenses as indicated by the elevated groundwater salinity levels. Seawater infiltrated through the shallow vadose zone as well as entered aquifers directly through flooded open wells. Our preliminary transport analysis demonstrates that the intruded seawater has vertically mixed in the aquifers because of both forced and free convection. Widespread pumping of wells to remove seawater was effective in some areas, but overpumping has led to upconing of the saltwater interface and rising salinity. We estimate that groundwater recharge from several monsoon seasons will reduce salinity of many sandy Sri Lankan coastal aquifers. However, the continued sustainability of these small and fragile aquifers for potable water will be difficult because of the rapid growth of human activities that results in more intensive groundwater pumping and increased pollution. Long-term sustainability of coastal aquifers is also impacted by the decrease in sand replenishment of the beaches due to sand mining and erosion.

Estimating hysteresis in the soil water retention function from cone permeameter experiments

Data obtained from modified cone penetrometer experiments were used to estimate the hysteretic soil hydraulic properties with a parameter estimation technique which combined a numerical solution of the Richards equation with Marquardt-Levenberg optimization. The modified cone penetrometer was designed to inject water into a soil through a cylindrical screen, measure the infiltration rate with time, and track the movement of the wetting front using two tensiometer rings positioned above the screen. After reaching relatively stable tensiometer readings during the experiments, the source of water was cut off and pressure head readings measured while water in the soil profile redistributed. Cumulative inflow and pressure head readings for two experiments with different supply pressures were analyzed to obtain estimates of the soil water retention and hydraulic conductivity functions. Analysis of flow responses obtained during the infiltration period, and of those obtained during the combined infiltration and redistribution phases, demonstrated the importance of hysteresis of the soil hydraulic functions. We found that the redistribution phase could not be described accurately when hysteresis was neglected. Hysteresis in the soil hydraulic functions was modeled using a relatively simple empirical model in which wetting scanning curves are scaled from the main wetting curve and drying scanning curves are scaled from the main drying curve. This model was deemed adequate for our examples. Optimization results for various combinations of unknown soil hydraulic parameters were compared to results of standard laboratory and in situ methods. Estimates of the saturated hydraulic conductivity were well within the range of in situ measurements. The estimated main hysteretic loops of the soil water retention curve were for the most part situated between the wetting and drying curves obtained with standard methods.

Groundwater Recharge at Five Representative Sites in the Hebei Plain, China

Accurate estimates of groundwater recharge are essential for effective management of groundwater, especially when supplies are limited such as in many arid and semiarid areas. In the Hebei Plain, China, water shortage is increasingly restricting socioeconomic development, especially for agriculture, which heavily relies on groundwater. Human activities have greatly changed groundwater recharge there during the past several decades. To obtain better estimates of recharge in the plain, five representative sites were selected to investigate the effects of irrigation and water table depth on groundwater recharge. At each site, a one-dimensional unsaturated flow model (Hydrus-1D) was calibrated using field data of climate, soil moisture, and groundwater levels. A sensitivity analysis of evapotranspirative fluxes and various soil hydraulic parameters confirmed that fine-textured surface soils generally generate less recharge. Model calculations showed that recharge on average is about 175 mm/year in the piedmont plain to the west, and 133 mm/year in both the central alluvial and lacustrine plains and the coastal plain to the east. Temporal and spatial variations in the recharge processes were significant in response to rainfall and irrigation. Peak time-lags between infiltration (rainfall plus irrigation) and recharge were 18 to 35 days in the piedmont plain and 3 to 5 days in the central alluvial and lacustrine plains, but only 1 or 2 days in the coastal plain. This implies that different time-lags corresponding to different water table depths must be considered when estimating or modeling groundwater recharge.

Estimating unsaturated soil hydraulic properties from laboratory tension disc infiltrometer experiments

Four tension disc infiltration experiments were carried out on a loamy soil in the laboratory for the purpose of estimating the unsaturated soil hydraulic properties. Sixteen tensiometers were installed in pairs at the following coordinate (r, z) positions: (10, 2.5), (10, 5), (10, 10), (15, 5), (15, 10), (15, 15), (15, 20), and (15, 30), where r represents the distance from the axis of symmetry and z is the location below the soil surface. A time domain reflectometry (TDR) probe was used to measure water contents at a depth of 2 cm directly below the tension disc. The first three experiments involved supply pressure heads at the disc of 220, 210, 25, and 21 cm, with the experiment lasting for ;5 hours. The same supply pressure heads were also used for the fourth experiment, which lasted 6.25 days so as to reach steady state at each applied tension. The measured data were analyzed using Woodings (1968) analytical solution and by numerical inversion. The parameter estimation method combined a quasi three-dimensional numerical solution of the Richards equation with the Marquardt-Levenberg optimization scheme. The objective function for the parameter estimation analysis was defined using different combinations of the cumulative infiltrated volume, TDR readings, and tensiometer measurements. The estimated hydraulic properties were compared against results obtained with an evaporation experiment as analyzed with Winds (1968) method. Water contents in the retention curves were underestimated when both transient and quasi steady state experiments were analyzed by parameter estimation. Unsaturated hydraulic conductivities obtained by parameter estimation and using Woodings (1968) analysis corresponded well. Drying branches of the hydraulic conductivity function determined by parameter estimation also corresponded well with those obtained with the evaporation method.

Modeling the coupled effects of pore space geometry and velocity on colloid transport and retention

[1]xa0Recent experimental and theoretical work has demonstrated that pore space geometry and hydrodynamics can play an important role in colloid retention under unfavorable attachment conditions. Conceptual models that only consider the average pore water velocity and a single attachment rate coefficient are therefore not always adequate to describe colloid retention processes, which frequently produce nonexponential profiles of retained colloids with distance. In this work, we highlight a dual-permeability model formulation that can be used to account for enhanced colloid retention in low-velocity regions of the pore space. The model accounts for different rates of advective and dispersive transport, as well as first-order colloid retention and release in fast and slow velocity regions of the pore space. The model also includes provisions for the exchange of colloids from fast to slow regions in the aqueous phase and/or on the solid phase. A sensitivity analysis performed with the dual-permeability model parameters indicated that low rates of advective transport to low-velocity regions had a pronounced influence on colloid retention profiles, especially near the inlet. The developed model provided a good description of measured colloid breakthrough curves and retention profiles that were collected for a variety of conditions.

Exact analytical solutions for contaminant transport in rivers 1. The equilibrium advection-dispersion equation

Abstract Analytical solutions of the advection-dispersion equation and related models are indispensable for predicting or analyzing contaminant transport processes in streams and rivers, as well as in other surface water bodies. Many useful analytical solutions originated in disciplines other than surface-water hydrology, are scattered across the literature, and not always well known. In this two-part series we provide a discussion of the advection-dispersion equation and related models for predicting concentration distributions as a function of time and distance, and compile in one place a large number of analytical solutions. In the current part 1 we present a series of one- and multi-dimensional solutions of the standard equilibrium advection-dispersion equation with and without terms accounting for zero-order production and first-order decay. The solutions may prove useful for simplified analyses of contaminant transport in surface water, and for mathematical verification of more comprehensive numerical transport models. Part 2 provides solutions for advective- dispersive transport with mass exchange into dead zones, diffusion in hyporheic zones, and consecutive decay chain reactions.

Performance of Pitcher Irrigation System

Pitcher irrigation is an ancient, but very efficient irrigation system used in many arid and semiarid regions. Small pitchers are often used because they are less expensive than large ones. However, questions exist about whether the patterns and extent of soil wetting obtained with small pitchers are comparable to those achieved with larger pitchers. This work addresses these questions through a combination of experimental and simulation studies involving three pitcher sizes, identified here as large (20 L), medium (15 L), and small (11 L). Saturated hydraulic conductivities of the pitcher materials were measured using a constant head method; the measured values ranged from 0.07 cm d−1 for the large pitcher to 0.14 cm d−1 for the smaller sizes. To determine the zone of wetting, the pitchers were buried down to their necks in a sandy loam soil and filled with water. Water content distributions were determined after 1 and 10 days at locations 20, 40, and 60 cm away from the pitcher center at soil depths of 0, 20, 40, and 60 cm. Moisture distributions predicted with the HYDRUS-2D simulation model were found to be in close agreement with the experimental results, showing root-mean-square-error values between 0.004 and 0.023. The close agreement suggests that HYDRUS-2D is a suitable tool for investigating and designing pitcher irrigation systems. Experimental and numerical results showed that a small pitcher half the size of a larger one, but with double the hydraulic conductivity, will produce approximately the same wetting front as the larger pitcher. Simulations for the large pitcher further showed, as expected, more horizontal spreading of water in a fine-texture soil as compared with a coarse-texture soil.

A complete soil hydraulic model accounting for capillary and adsorptive water retention, capillary and film conductivity, and hysteresis

A soil hydraulic model that considers capillary hysteretic and adsorptive water retention as well as capillary and film conductivity covering the complete soil moisture range is presented. The model was obtained by incorporating the capillary hysteresis model of Parker and Lenhard into the hydraulic model of Peters-Durner-Iden (PDI) as formulated for the van Genuchten (VG) retention equation. The formulation includes the following processes: capillary hysteresis accounting for air entrapment, closed scanning curves, nonhysteretic sorption of water retention onto mineral surfaces, a hysteretic function for the capillary conductivity, a nonhysteretic function for the film conductivity, and a nearly nonhysteretic function of the conductivity as a function of water content (θ) for the entire range of water contents. The proposed model only requires two additional parameters to describe hysteresis. The model was found to accurately describe observed hysteretic water retention and conductivity data for a dune sand. Using a range of published data sets, relationships could be established between the capillary water retention and film conductivity parameters. Including vapor conductivity improved conductivity descriptions in the very dry range. The resulting model allows predictions of the hydraulic conductivity from saturation until complete dryness using water retention parameters.

Selected HYDRUS modules for modeling subsurface flow and contaminant transport as influenced by biological processes at various scales

A large number of modifications or special modules of the HYDRUS software packages have been developed during the past several years to evaluate the effects of a range of biohydrological processes on subsurface water flow and the transport of various chemicals and contaminants. The objective of this manuscript is to briefly review the different modules that were included, and to present various applications illustrating the effects of biological processes on water flow and solute transport and reactions in variably-saturated media.