Evaporative climate changes at Bet Dagan, Israel, 1964–1998

Abstract

Analysis of evaporation measurements made between 1964 and 1998 at Bet Dagan in Israel’s central coastal plain shows a small but statistically significant increase in screened Class A pan evaporation, mainly in the dry, summer half of the year. No changes were found in the total open water evaporation or reference crop evapotranspiraton estimated with Penman’s combined heat balance and aerodynamic equation because the decreases found in the radiation balance term were offset by increases in the aerodynamic term. The climatic changes responsible for these opposing trends were, respectively, decreases in global irradiance and increases in water vapor pressure deficit (VPD) and wind speed, the latter associated with changes in wind direction. Increases in windspeed were found to be concentrated in afternoon hours of summer and fall months, while those of VPD were in the late afternoon of fall months. Possible reasons for these changes are discussed. Normalized pan evaporation for dry months showed no significant time trend, but a significant increase was found for wet months. These results support the view that the widespread reductions in potential evaporation that have been reported, although not found at Bet Dagan, were caused by global dimming rather than an increase in the rate of atmospheric moisture cycling due to global warming.