The Euphrates-Tigris watershed (below) is the heartland of one of the oldest and longest-lived civilizations, a testimony to the ingenuity and persistence of people utilizing relatively simple technical means. The landscape ranges from high mountains to fertile piedmont, to monotonously flat alluvial plains and stony desert. A strongly seasonal climate, in which there is a water deficit at least part of the year over most of the land, imposes strict conditions on life. Over the past 8000 years, with improved techniques and massive use of human labor, the water sources have been transformed from naturally flowing rivers to ones that are controlled by dams and reservoirs, and distributed through canals. Rain-fed agriculture is practiced in the Fertile Crescent, but in much of the region irrigation is necessary, and the driest zones are reserved for livestock raising. Despite the long history of use of this landscape and the importance of water economically and politically, neither the hydrologic cycle nor the hydrologic budget have come under close scientific examination.
The watershed encompasses some 1.7 million km2, starting in the Taurus and Zagros mountains of Turkey and Iran. The Tigris and Euphrates flow some 1900 and 2700 km respectively from their headwaters in central Anatolia to the Persian Gulf. Today the watershed occupies parts of Turkey, Iran, Syria and Saudi Arabia - and Iraq lies almost entirely within it. It is probable that close to 50 million people live in the Euphrates-Tigris drainage, 24 million of whom live in Iraq. A large proportion of the population of each country is engaged in farming and raising livestock, occupations directly dependent upon water supply.
Annual variability in the timing and amount of precipitation affects both rain-fed agriculture and irrigation. Precipitation is also the key to the rangeland, the traditional source of food for herds of sheep and goats. As agriculture has encroached on rangeland, and rising demand for meat has encouraged the overstocking of pastures, there has been serious deterioration of the natural vegetation, a problem that is exacerbated during years that are drier than normal.
Water, more than oil, is the key to sustaining civilization in Southwest Asia and there are signs that the supply is being stretched thin to provide for rapidly growing human populations. With modern methods it has been possible to improve the supply and efficiency of water use although such measures are expensive. Before further large-scale investment, however, an accurate assessment of factors that supply precipitation, as well as those that capture and expend it is required. In this way the potential limits of growth and change can be anticipated and some of the problems that have arisen through unrealistic ambition may be avoided.