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United States Department of Agriculture

Agricultural Research Service

Title: Closed-Form Expressions for Water Retention and Conductivity Data

Authors
item Leij, Feike - U.C. RIVERSIDE
item Russell, Walter
item Lesch, Scott - U.C. RIVERSIDE

Submitted to: Ground Water
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 4, 1997
Publication Date: N/A

Interpretive Summary: Computer programs used for simulating water and chemical movement from the soil surface to the groundwater rely on accurate estimates for the water retention and hydraulic conductivity curves. These curves are commonly quantified by fitting mathematical expressions to observed data points. Many different functions have been selected for this purpose with different degrees of success. Some functions can only fit the data in a limited range of soil water contents whereas the formulation of some other functions may lead to mathematical problems when fitting retention and conductivity data or during the simulation of subsurface flow and transport. Although the selection of the appropriate function may be quite important, there are surprisingly little comparative studies of different functions. We attempted to fit fourteen retention and eleven conductivity data measured on soil samples to horizons reports in the unsaturate hydraulic database UNSODA. Functions reported by van Genuchten (1980), Globus (1987), and Hutson and Cass (1987) tended to best describe retention data. Different types of conductivity data were well described by a function reported by Gardner (1958) and by functions reported by Brooks and Corey (1964) and van Genuchten (1980), respectively.

Technical Abstract: Closed-form expressions for quantifying the unsaturated soil hydraulic properties are widely used in computer programs to model subsurface flow and transport and to investigate indirect methods for estimating these properties. For example, water retention data, which relate soil water pressure head (h) and effective water saturation (Se), are frequently used to predict the unsaturated hydraulic conductivity (K). The latter is notoriously cumbersome to measure directly. However, the suitability of different functions to describe unsaturated hydraulic data have rarely been investigated comprehensively. We attempted to fit fourteen retention and eleven conductivity functions to 903 sets of water retention and hydraulic conductivity data measured on soil samples or horizons reported in the unsaturated hydraulic database UNSODA. Some of the best mean values for r2 and MSE for fitting Se(h) data were obtained with the retention functions reported by van Genuchten (1980), Globus (1987), and Hutson and Cass (1987). A function reported by Gardner (1958) could describe K(h) data quite well whereas functions reported by Brooks and Corey (1964) and van Genuchten (1980), which are respectively based on the conductivity models by Burdine and Mualem, yielded a relatively good description of K(Se) data.

Last Modified: 12/22/2014
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