Evaluation and application of the modified van Genuchten function for unsaturated porous media

Abstract Proper soil hydraulic functions of the water retention curve (the WRC) and relative hydraulic conductivity (the RHC) are essential for studies of unsaturated groundwater flow in soils. Various mathematical expressions of these functions have been proposed, including the widely used van Genuchten-Mualem model combining the van Genuchten formula for the WRC and the Mualem formula for the RHC. Here we validate a model based on the van Genuchten function for the both WRC and RHC (termed the MVG model), against published experimental data covering a wide range of soil textures from sand to silt clay loam. The results show that the MVG model is more accurate and flexible than existing soil hydraulic functions, especially for predicting the RHC. The fitting of the model to the data also reveals a relationship of model parameters between the MVG-WRC and MVG-RHC functions. Both functions can be integrated and differentiated to attain simple analytical expressions for characteristic parameters of soil water flow, for example, the wetting front pressure ψ f used in the Green and Ampt model for ponded infiltration analysis and the characteristic length L c as a key factor in determining various flow processes (e.g., drainage or evaporation) in unsaturated soils.