The Southeast Anatolia Development Project (known as GAP) is a multifaceted development project for agriculture and water resources within the Turkish portions of the Euphrates and Tigris river basins. Through this project, the vulnerability of the region to drought has been investigated in both temporal and spatial terms. On completion of the project, 28% of the total water potential of Turkey will be brought under control through facilities on the Euphrates and Tigris rivers, which have a joint flow of more than 50 billion m3 (GAP Regional Development Administration, 1997). The GAP project aims to irrigate 8.5 million hectares of land in Southeast Anatolia, which is 19% of the total economically irrigable lands in Turkey. A project of such magnitude inevitably is of major importance to the region’s water resources and agricultural potential. It is therefore important to establish reasonable expectations of water use in the GAP region, since agriculture is going to be a critical component of the region’s economy in coming decades. The GAP area is located in the continental Mediterranean rainfall region, and its annual precipitation varies between 400 and 800 mm. Annual precipitation decreases from north to south in the region, and the greatest portion of the annual precipitation falls in winter, December and January being the wettest months. Summers in the region are very dry, with high temperatures