Water resources in the desertification-threatened Messara Valley of Crete: estimation of potential lake evaporation

Solar and terrestrial radiation flux models are used with long-term monthly-averaged climatological data to estimate the seasonal variation in downward surface radiation fluxes in central Crete. Assuming a water surface, Fresnel reflection is used to compute the albedo of the surface in order to estimate the net heating and hence potential lake evaporation. The long-term monthly variation in the surface radiation budget is then compared with measurements by recently installed automatic weather stations, one on the north coast and another in the Messara Valley in the south. As expected, over monthly timescales the surface radiation budget does not differ appreciably between the two locations and good agreement is found between the predictions of the models and the measured fluxes. An estimate of the monthly variation of potential evaporation is obtained and it is shown that a water surface in the Valley undergoes relatively large variations in potential evaporation, from 2 mm per day during its typically Mediterranean wet winter to 7 mm per day during its dry summer. A simple first estimate of the Valleyʹs actual evapotranspiration gives about 2 mm per day average during the wet winter months and about 2–3 mm per day during the dry summer months for drip-irrigated olive groves, where water supply is limited by the pumping of the available groundwater.