M. Van Genuchten - Academia.edu (original) (raw)

Papers by M. Van Genuchten

Canopies of forested and agricultural ecosystems can significantly alter rainfall patterns into s... more Canopies of forested and agricultural ecosystems can significantly alter rainfall patterns into separate stemflow
and throughfall areas. These two areas often have also different organic matter contents and soil compaction
properties, and hence also soil hydraulic properties, thus causing further differences in the local infiltration
rates close to and away fromtrees. In this studywe analyzed possible differences in the unsaturated soil hydraulic
properties of the stemflow and throughfall areas below an oil palmtree. Tension disc infiltrometer experiments
were carried out underneath the canopy and in the interspace area of an oil palm tree plantation at successive
tensions of 5, 2, and 0 cm. Soil hydraulic properties were estimated inversely from the measured data using
the HYDRUS-2D/3D software package. Four van Genuchten soil hydraulic parameters (i.e., the residual water
content, θr, the shape factors α and n, and the saturated hydraulic conductivity, Ks) were optimized. Saturated
water contents, θs, were fixed at their laboratory-measured values. Initial estimates of the optimized parameters
were set according to Wooding's solution, which ensured rapid convergence of the inverse solution. The
stemflow and throughfall regions exhibited contrasting hydraulic properties as indicated by the estimated
hydraulic parameters. Values of θs, α, n and Ks for the stemflow area were all found to be higher as compared
to those of the throughfall area. The inverse solution using tension disc infiltrometer data proved to be very useful
for rapid characterization of hydraulic properties of soil under the oil palm trees.

Water Resources Research, 1997

Modeling water flow in macroporous field soils near saturation has been a major challenge in vado... more Modeling water flow in macroporous field soils near saturation has been a major challenge in vadose zone hydrology. Using in situ and laboratory measurements, we developed new piecewise-continuous soil water retention and hydraulic conductivity functions to describe preferential flow in tile drains under a flood-irrigated agricultural field in Las Nutrias, New Mexico. After incorporation into a two-dimensional numerical flow code, CHAIN_2D, the performance of the new piecewise-continuous hydraulic functions was compared with that of the unimodal van Genuchten-Mualem model and with measured tile-flow data at the field site during a number of irrigation events. Model parameters were collected/estimated by site characterization (e.g., soil texture, surface/ subsurface saturated/unsaturated soil hydraulic property measurements), as well as by local and regional-scale hydrologic monitoring (including the use of groundwater monitoring wells, piezometers, and different surface-irrigation and subsurface-drainage measurement systems). Comparison of numerical simulation results with the observed tile flow indicated that the new piecewise-continuous hydraulic functions generally predicted preferential flow in the tile drain reasonably well following all irrigation events at the field site. Also, the new bimodal soil water retention and hydraulic conductivity functions performed better than the unimodal van Genuchten-Mualem functions in terms of describing the observed flow regime at the field site.

Soil Science Society of America Journal, 1992

Solute transport through the vadose zone can be described with the one-dimensional convection-dis... more Solute transport through the vadose zone can be described with the one-dimensional convection-dispersion equation (CDE). Analytical solutions are widely available for the classical formulation of the CDE that accounts for convection, diffusion and dispersion, linear equilibrium adsorption, zero-order production, and first-order decay or degradation processes. In the root zone, transport may be affected by additional processes, including depth dependent rates of degradation and the uptake of water and solute by roots. We formulate transport equations accounting for these processes and present analytical and numerical solutions for various boundary conditions.

Colloid transport in porous media has traditionally been assumed to be controlled by chemical int... more Colloid transport in porous media has traditionally been assumed to be controlled by chemical interactions between the colloids and the solid-water and air-water interfaces. The influence of pore space geometry, interface configuration, and system hydrodynamics in classical colloid retention models has largely been neglected. Recent experimental and theoretical work, however, has demonstrated that these factors can also play important roles in colloid retention under unfavorable attachment conditions. In particular, hydrodynamic forces can funnel weakly associated colloids to grain-grain contacts and air-water-solid triple points, as well as to hydrodynamically isolated low velocity (eddy) regions. The extent to which colloid mass transfer and retention will occur to/in these locations is a function of the balance of adhesive, diffusion, and hydrodynamic forces. One consequence of enhanced colloid retention in low velocity regions is that the colloid retention profile may not be exp...

Water Resources Research, 1984

Water Resources Research, 1993

Water Resources Research, 2002

1] Many models used in land surface hydrology, vadose zone hydrology, and hydroclimatology requir... more 1] Many models used in land surface hydrology, vadose zone hydrology, and hydroclimatology require an accurate representation of soil properties. Unfortunately, existing soil property databases are limited in terms of reliability, precision, and their usefulness in evolving soil-vegetationatmosphere-transfer (SVAT) schemes of general circulation models (GCMs) or regional-scale hydrologic models. Furthermore, not many site-specific, comprehensive soil property measurement campaigns have been carried out concurrently with large-scale remote sensing hydrologic campaigns. To better understand the complex and interdependent geophysical processes in the near surface, we conducted an extensive soil property measurement campaign during the Southern Great Plains 1997 (SGP97) Hydrology Experiment. We measured soil physical, hydraulic, and thermal properties across the SGP97 study region. The resulting soil property database not only is useful for evaluating the SVAT schemes in GCMs and other hydrologic models but also can be used as a basis for transfer function modeling, extrapolating point estimates of soil properties to larger spatial scales, testing point and nonpoint source pollution modeling, and evaluating evolving hypotheses in water and energy transfer across the land-atmosphere boundary. The complete data report and raw data are available upon request from the George E. Brown Salinity Laboratory. Summarized data are given by Mohanty et al. [1999].

Water Resources Research, 1998

A comprehensive field experiment was conducted near Las Nutrias, New Mexico, to study field-scale... more A comprehensive field experiment was conducted near Las Nutrias, New Mexico, to study field-scale flow and transport in the vadose zone. The field data were analyzed in terms of a two-dimensional numerical model based on the Richards equation for variably saturated water flow, convection-dispersion equations with first-order chemical decay chains for solute transport, and bimodal piecewise-continuous unsaturated hydraulic functions to account for preferential flow of water and nitrate-nitrogen (NO 3 -N; loosely used as NO 3 Ϫ ) following flood irrigation events at the experimental site. The model was tested against measured NO 3

Water Resources Research, 1983

Water Resources Research, 1994

Vadose Zone Journal, 2005

Vadose Zone Journal, 2008

Vadose Zone Journal, 2007

In regulatory and risk management analyses of environmental contaminants, the vadose zone may be ... more In regulatory and risk management analyses of environmental contaminants, the vadose zone may be treated as a subcomponent within a larger environmental modeling framework. For the complexity of the larger system model to remain at manageable levels, it is desirable that subcomponent models be relatively simple and require few input parameters. In this work, we develop an advective-dispersive solute transport equation that includes plant uptake of water and solute and present an analytical solution. Assumptions underlying the transport model include linear solute sorption, fi rst-order uptake, and a uniform soil water content. We examine the latter assumption in detail and demonstrate the effects of rooting depth, soil texture, and leaching fraction on the uniformity of the root-zone water content. The new analytical advection-dispersion model should be useful for estimating the transport and uptake of strongly sorbing and persistent contaminants, where the timescale relevant for assessing environmental impacts is long (decades) and short-term fl uctuations caused by, for example, precipitation can be averaged. As an illustration, model predictions are made for the uptake of cadmium (Cd) by wheat (Triticum aestivum L.) grown in sludgeamended soil. The predictions are compared with those of a "one-compartment" model that has been proposed previously for risk analysis and regulatory studies. The comparison shows that the one-compartment model overestimates the long-term, steady-state Cd concentration in harvested wheat grain. The analytical advection-dispersion model is recommended as a tool for environmental risk assessment of strongly sorbing, persistent contaminants.

Vadose Zone Journal, 2003

used to interpret results from solute transport experiments. For certain conditions (e.g., neglig... more used to interpret results from solute transport experiments. For certain conditions (e.g., negligible matrix In two-domain schematizations of macroporous soils or fractured domain flux, steady-state water flow and instantaneous rock systems, lateral mass exchange between macropores and the soil matrix is generally modeled as an apparent first-order process. With adsorption), the transfer rate coefficients for solute (␣ s ) respect to lateral diffusion, the system is thus characterized by a single can be determined from moment analysis of the concenparameter, the transfer rate coefficient, which is difficult to estimate tration breakthrough curve (BTC) (e.g., ).