Satellite derived actual evapotranspiration and groundwater modelling, Botswana

Evapotranspiration losses over an aquifer during a dry season in Botswana were determined with the multi-step Surface Energy Balance Algorithm for Land (SEBAL), using sequential Landsat TM and NOAA-AVHRR data, with concurrent meteorological field data for atmospheric correction and calibration. The SEBAL method yielded high actual evapotranspiration (E-a) rates (<0.5 to 4 mm day(-1)), if relatively dense savanna vegetation was present, even when groundwater was over 40 in deep. The association between vegetation pattern and E-a has been interpreted as due to significant uptake of water by deep roots. The mapping of the groundwater contribution to the evapotranspiration was provided through MODFLOW groundwater modelling using the evapotranspiration package with an extinction depth of 60 in. It is believed that a major part of the transpired water originates from the lower vadose zone. The validity of the satellite-derived E-a flux is discussed.