ESTIMATING THE UNSATURATED HYDRAULIC CONDUCTIVITY FROM THEORETICAL-MODELS USING SIMPLE SOIL PROPERTIES

The knowledge of the unsaturated hydraulic conductivity is one of the prerequisites to describe water flow and solute transport in soils. In this paper we examined the quality of 11 different theoretical models to predict the unsaturated hydraulic conductivity for a wide variety of soils. The hydraulic conductivity models were fitted to 44 measured curves using a quasi Newton parameter estimation method minimizing an ordinary least squared objective function. The optimized parameter set for the best model having the lowest overal mean squared error was related to basic soil properties such as texture, organic carbon, bulk density and the measured saturated hydraulic conductivity. The model evaluation showed that theoretical models can only describe the measured data succesfully when the tortuosity and the pore size interaction term are allowed to vary across different soils. Models with completely fixed parameters obtained from literature were not able to describe the measured data. The generalized theoretical models as described by Mualem and Dagan were equally flexible. The Mualem model was choosen to derive estimation or pedotransfer functions between the tortuosity (b), the pore size interaction term (x) and soil properties. Different regression equations have been derived depending upon the available information. Soil texture alone is not sufficient to provide reasonable estimates, resulting in a coefficient of determination of 30% for x and 11% for b. The best estimates where obtained when using information from the moisture retention characteristic and the measured saturated hydraulic conductivity (K-sat). Introducing K-sat in the regression equation improved the coefficient of determination with 7% for x and 56% for b.