Groundwater evaporation from East-Saharian depressions by means of deuterium and oxygen-18 in soil moisture

Estimates of the natural groundwater discharge of the Nubian Aquifer System by evapotranspiration in East Saharian depressions correspond to a mean evapotranspiration rate of 100 mm/a there. This unbalanced loss of fossil groundwater causes an exponential decrease of the hydraulic head in the vast desert area around the depressions, which is equivalent to a mean regional groundwater drawdown rate of 0.5 cm/a at present. At various barren soil sites in East Oweinat, SW Egypt, where groundwater is still close to the ground surface, local evaporation rates ranging between 4 mm/a and 700 mm/a have been derived from vertical profiles of deuterium and oxygen-18 in soil moisture. These evaporation data compare with theoretical estimates of steady state moisture transfer from the groundwater surface to the atmosphere, which occurs by capillary rise of soil water up to the evaporating front followed by an equivalent water vapour flow through the dry soil layer above this front. (author). 12 refs, 6 figs