Evaporation of groundwater from arid playas measured by C-band SAR

Salt-bearing groundwater evaporates within arid playas and roughens the surface during the summer months. We show that it is possible to infer the rates of evaporation of groundwater using radar backscatter measurements. Using an empirically derived relationship between backscatter coefficient and surface roughness, a model of how surface roughness changes with a continuous process of halite crystal efflorescence, and an assumed value for the salinity of the regional groundwater, we calculate the volumetric rate at which the groundwater must have evaporated. The method is illustrated with data from the European Remote Sensing 1 satellite synthetic aperture radar sensor that imaged the Chott el Djerid playa in southern Tunisia from 1992 to 1993. Independent measurements and calculations indicate that the radar method overestimates the rate of evaporation. The reasons for this probably lie in the assumptions about the salt budget of the groundwater or the model of how the salt crust roughens with time.