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Title: Performance Evaluation of Models that Describe the Soil Water Retention Curve between Saturation and Oven Dryness

Author
item KHLOSI, MUHAMMED - GHENT UNIVERSITY
item CORNELIS, WIM - GHENT UNIVERSITY
item DOUAIK, AHMED - NAT INST OF AGRICULTURE
item Van Genuchten, Martinus
item GABRIELS, DONALD - GHENT UNIVERSITY

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/24/2007
Publication Date: 1/25/2008
Citation: Khlosi, M., Cornelis, W.M., Douaik, A., Van Genuchten, M.T., Gabriels, D. 2008. Performance Evaluation of Models that Describe the Soil Water Retention Curve between Saturation and Oven Dryness. Vadose Zone Journal. Vol 7:87-96

Interpretive Summary: The unsaturated soil hydraulic properties are key factors governing the partitioning of rainfall and irrigation into soil water storage, evapotranspiration and deep drainage. The hydraulic properties involve the soil-water retention curve which defines the water content of a soil as a function of the pressure head (or matric potential) as the soil dries out, and the hydraulic conductivity function which defined the permeability of the soil to water as a function of the water content. Theoretical analyses of infiltration and water flow in soils are generally facilitated by using relatively simple mathematical expressions for the hydraulic properties. While several functions have been proposed over the years, most have been tested primarily in the wet range close to full saturation (i.e., close to the saturated water content). By comparison, few studies have looked at accurate descriptions of observed water retention data in very dry soils. In this paper we compared eight different mathematical expressions in terms of their ability to describe soil water retention data over the complete range of soil water contents. The comparison was carried out using 137 soil samples from a widely used UNSODA unsaturated soil hydraulic database. All eight models were found to perform well when evaluated for all soils combined. However, when the textural class was taken into account (e.g., sand, sandy loam, clay loam, etc), the eight models performed differently for the sandy clay loam class. One model which accounts for the adsorption of water on the mineral phase in dry soils, was found to be the most consistent for the different soils. We hence recommend this analytical model for reliable modeling of water flow over the whole range of water contents in unsaturated soils. Results of this study are particularly important when trying to predict water flow and contaminant transport processes in very dry soils.

Technical Abstract: The objective of this work was to evaluate eight closed-form unimodal analytical expressions that describe the soil-water retention curve over the complete range of soil water contents. To meet this objective, the eight models were compared in terms of their accuracy (root mean square error, RMSE), linearity (coefficient of determination R2 and adjusted coefficient of determination R2adj), and prediction potential. The latter was evaluated by correlating the model parameters to basic soil properties. Retention data for 137 undisturbed soils from the UNSODA database were used for the model comparison. The samples showed considerable differences in texture, bulk density and organic matter content. All functions were found to provide relatively realistic fits and anchored the curve at zero soil-water content for the coarse-textured soils. The performance criteria were similar when averaged over all data sets. The criteria were found to be statistically different between the eight models only for the sandy clay loam soil textural class. An analysis of the individual data sets separately showed that the performance criteria were statistically different between the models for 17 data sets belonging to six different textural classes. We found that the Khlosi et al. [Water Resour. Res., 2006 42: Art. No. 11501] model with four parameters was the most consistent among different soils. Its prediction potential was also relatively good due to significant correlation between its parameters and basic soil properties.