Modeling soil water fluxes is an important part of many agricultural and environmental investigations and applications. A prerequisite for accurate modeling results is a sound parameterization of the soil hydraulic functions. The aim of this study was to investigate whether the unimodal retention function is sufficient to simulate the soil water regime of an Acrisol in Northern Thailand. The van Genuchten parameterization and its modification by Vogel and Cislerova were compared with the bimodal van Genuchten parameterization of Durner. Parameters of the soil hydraulic functions were optimized by solving the inverse problem of a one-dimensional transient flow field experiment. Simulations using the bimodal approach resulted in the best agreement between measured and simulated data. Results with the modified van Genuchten function had a lower degree of accuracy but were still sufficient to predict the soil water balance. In case of the original van Genuchten approach, unrealistically high saturated hydraulic conductivities were required to obtain satisfactory simulation results. Hence, in fine-textured soils those optimized Ks values should be considered as fitting and not as physical parameters.