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Soil Physics

Comparison of Unimodal Analytical Expressions for the Soil-Water Retention Curve

^a Dep. Soil Management and Soil Care, Ghent Univ., Coupure links 653, B-9000 Ghent, Belgium
^b Institute of Forestry and Game Management, Ministry of the Flemish Community, Gaverstraat 4, B-9500 Geraardsbergen, Belgium

^* Corresponding author (wim.cornelis{at}UGent.be)

This study was conducted to evaluate ten closed-form unimodal analytical expressions to describe the soil-water retention curve, in terms of their accuracy, linearity, Akaike Information Criterion (AIC), and prediction potential. The latter was evaluated by correlating the model parameters to basic soil properties. Soil samples were taken in duplicate from 48 horizons of 24 soil series in Flanders, Belgium. All sample locations were under forest and hence the samples had, besides their difference in texture, a high variety in bulk density (_b) and organic matter content (OM). The van Genuchten model with m as a free parameter showed the highest overall performance in terms of goodness-of-fit. It had the highest accuracy, the highest degree of linearity, and the lowest AIC value. However, it had a low prediction potential. Imposing the constraint m = 1 – 1/n and hence reducing the number of model parameters by one, increased the prediction potential of the model significantly, without loosing much of the model's accuracy and linearity. A high degree of accuracy and linearity was also observed for the two Kosugi models tested. Restricting the bubbling pressure to be equal to zero resulted in a rather high prediction potential, which was not the case when keeping the bubbling pressure as a free parameter. A major drawback of van Genuchten and Kosugi type models is that they do not define the soil-water retention curve beyond the residual water content. We further demonstrated that the performance of all but one model in terms of their match to the data increased with increasing clay content and decreasing sand content, which is contradictory to the deterministic character of these models. Bulk density and OM did not have a significant effect on the accuracy of most models.

Abbreviations: A1, Assouline et al. (1998) with five free parameters • A2, Assouline et al. (1998) with four free parameters • AIC, Akaike Information Criterion • BC, Brooks and Corey (1964) • K1, Kosugi (1994) • K2, Kosugi (1996, 1997) • OM, organic matter content • PTF, pedotransfer function • R, Russo (1988) • RMSE, root of mean squared error • RN, Rossi and Nimmo (1994) • SWRC, soil-water retention curve • T, Tani (1982) • VG1, van Genuchten (1980) with five free parameters • VG2, van Genuchten (1980) with four free parameters

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