Validation of a simple model to determine regional evapotranspiration and groundwater recharge rates

The spatial distribution of daily evapotranspiration and groundwater recharge rates are determined by the simple bucket model SOWAS. It is shown, that SOWAS has better or as good results as the more complex, Richards' equation based model SWATRE, if it is used in flat catchments. This paper gives a short discription of SOWAS and emphasizes the regionalization and the validation of the model. In order to validate the calibrated model, simulations were performed for 33 measuring points with different crops and soil types over a period of more than one year. Without changing any parameter value, the average deviation in soil moisture between measured and calculated values is less than 4 Vol.-%. Therefore the assumption is justified that only the field capacity, the rooting depth, the layering of the soil and one plant-specific parameter are sufficient to assess evapotranspiration and groundwater recharge in flat areas. Furthermore, the paper demonstrates that regional values and an areal distribution of the fluxes are easy to perform with (carefully chosen) soil-vegetation-classes. (C) 1999 Elsevier Science Ltd. All rights reserved.