Evapotranspiration from a phreatic groundwater table is an important input parameter for hydrological models. This parameter, however, is difficult to determine in the field, since plants take their water from various depths and the penetration depth of direct evaporation is unknown. We present a method to estimate evapotranspiration rates based on diurnal groundwater level fluctuations observed in the Okavango Delta, Botswana. To this end, the regional setting as well as the local processes in the unsaturated zone have to be analysed. Resulting estimates for the evapotranspiration rate range from 0.06 to 4.3 mm/day, for varying local vegetation cover and soil characteristics.