Biexponential model for water retention characteristics

Abstract

Water retention characteristic (WRC) is an important soil property used in many hydrologic applications. Although there are numerous models in the literature for its prediction, the models are largely empirical with fundamental derivations based on soil textural pore-spaces. This paper presents a biexponential model for WRC that has its basis on textural and structural soil pore-spaces. The model contains physical parameters related to soil textural and structural pore characteristics and can also navigate through all inflections present in measured WRC. Thus, it adequately represents possible classes of soil pores in WRC. The performance of this model on different soil types from various parts of the world was compared to some of the popular WRC models in the literature. The models tested were those proposed by van Genuchten [van Genuchten, M.T., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soils. Soil Science Society of America Journal, 44, 892–898], Campbell [Campbell, G.S., 1974. A simple method for determining unsaturated conductivity from moisture retention data. Soil Science, 117, 311–314], Brooks and Corey [Brooks, R.H., Corey, A.T., 1964. Hydraulic Properties of Porous Medium. Hydrology Paper Number 3. Colorado State University, USA.], Gardner [Gardner, W.R., 1958. Some steady state solutions of the unsaturated moisture flow equation with application to evaporation from a water table. Soil Science, 85, 228–232], Groenevelt and Grant [Groenevelt, P.H., Grant, C.D., 2004. A new model for the soil-water retention curve hat solves the problem of residual water contents. European Journal of Soil Science, 55, 479–485]. These models were found inadequate in fitting the whole range of measured WRC, are limited to certain geographic regions of the world, and contain parameters not directly linked to soil physical properties. The biexponential model does not only overcome these limitations but also gives the best fit of WRC. Its model parameters have physical significance and have been shown in this paper to be useful in fitting different soil physical conditions. The model is continuous and continuously differentiable throughout the whole range of WRC and is envisaged to perform equally well in fitting unsaturated hydraulic conductivity.