Water resources in the desertification-threatened Messara Valley of Crete: estimation of the annual water budget using a rainfall-runoff model

A simple rainfall-runoff model (Vardavas, I.M., 1988. A simple water balance daily rainfall-runoff model with application to the tropical Magela Creek catchment. Ecol. Model 42, 245–264) developed and applied to the tropical wet–dry Magela catchment in the Northern Territory of Australia has been modified and applied to the Mediterranean wet–dry Messara Valley catchment of Crete. The Messara Valley constitutes the most important agricultural region of Crete and is threatened by desertification due to falling groundwater levels. The steep topography of the Messara Valley necessitated the introduction of a two-component subsurface flow in the rainfall-runoff model, with the slow component representing deep sub-surface flow from the mountains forming the north and south boundaries of the catchment. The original model was also modified to include estimation of the groundwater level fluctuations, and recharge in order to look at possible future exploitation scenarios. While the model was designed for catchments with distinct wet–dry periods, it has been successfully applied to the River Pang catchment in the UK GRAPES (GRAPES, 2000. GRAPES Technical Report, European Commission, ENV4 CT95-0186, 250 pp, March 2000). The model indicates that the Valley’s surface and groundwater resources are very sensitive to climatic variations, with a natural drop in groundwater levels of about 10 m and little surface runoff being possible during drought years. The 20 m drop in the groundwater level over the past 10 years is due to the increased irrigation pumping in conjunction with drought years. The model also indicates that the Valley might have gone through cycles of near zero groundwater net recharge every 3–4 years starting in 1982 (an El Nino year).