|Van Genuchten, Martinus|
Submitted to: Geoderma
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/1993
Publication Date: N/A
Citation: Interpretive Summary: Computer models for simulating the flow of water and the transport of dissolved chemicals in the unsaturated zone between the soil surface and the ground water table require that accurate values for the unsaturated hydraulic properties are available. Because of the natural variability of most field soils and because of the problems and limitations on experiments in the field, these properties are often estimated or measured in the laboratory on soil samples. This paper presents results from a technique to accurately and conveniently measure the unsaturated hydraulic properties on "undisturbed" samples of a silt loam in the laboratory with minitensiometers and Time Domain Reflectometry (TDR). The experimental data points were described with mathematical functions using a curve fitting program. It may be preferable to limit the number of fitting parameters by obtaining parameters from independent measurements. We investigated the sensitivity of the mathematical function as well as the simulation of drainage of a soil profile with this function. Two parameters in the function, viz. the saturated water content and saturated conductivity, should be included in the fitting procedure and can not obtained from independent measurements; the cumulative drainage was overpredicted in the latter case. Two other parameters, the residual water content and a geometry factor, may be estimated independently.
Technical Abstract: Accurate in situ determination of the unsaturated soil hydraulic properties is often not feasible because of the natural variability of most field soils, and because of the instrumental limitations. This paper presents water retention and hydraulic conductivity data obtained with a relatively new technique involving "undisturbed" soil samples instrumented with minitensiometers and Time Domain Reflectometry (TDR) miniprobes, and subjected to evaporation. Laboratory results for a silt loam were compared with available field data obtained with the instantaneous profile method. The models by Mualem and van Genuchten (MvG) were used to describe the hydraulic properties. The laboratory method allows a high spatial and temporal resolution; this facilitates an investigation of some of the assumptions made when fitting the MvG models to hydraulic data. A reasonably good description of the hydraulic data was obtained when setting gthe residual water content, to 0 and the pore connectivity factor, to 0.5. However, a poor fit resulted when the saturated water content, was equated to the porosity, and the saturated hydraulic conductivity, to its independently measured value. Values derived from field measurements were somewhat higher than those obtained from laboratory samples. The cumulative drainage from an initially saturated soil column was simulated with different sets of hydraulic parameters estimated from field and laboratory data. Parameters derived from the laboratory results consistently yielded lower predictions of cumulative drainage compared to hydraulic parameters derived from field measurements. However, the differences were relatively small when an initial water content corresponding to 60 hPa suction (field capacity) was used.