Remotely sensed actual evapotranspiration : implications for groundwater management in Botswana

In order to determine evapotranspiration losses from the groundwater of an aquifer in Botswana during the dry season, the multi-step Surface Energy Balance Algorithm for Land (SEBAL) was applied using sequential Landsat TM and NOAA-AVHRR data. During satellite overpasses, continuous data on surface temperatures and soil moisture were available from a meteorological tower and field observations for calibration and partial validation of the results. The SEBAL method yielded high actual evapotranspiration (Ea) rates (1.5 – 3 mm/d), if relatively dense savannah vegetation was present, even when the water-table was over 30 m deep, as is the case in the upper part of the aquifer. No relationship between Ea and depth to water-table was found, except in the valleys, where riverine forests are fed by a system of discharging groundwater flow. The patterns on a vegetation map, based on a supervised classification using TM data, including thermal bands, showed similarity with the Ea patterns. The spatial distributions of vegetation types and of Ea have been interpreted as important uptake of water by deep roots; this is supported by increasing evidence from other parts of the world. Sap flow was measured in tall bushes near the tower site. The upper part (2 m) of the soil was dry. The results have implications for the groundwater recharge mechanism and the management of groundwater. Further validation studies have been initiated.