Regional Hydrologic Cycle
Basic elements of the hydrologic system in the Middle East are shown below. The Tigris and Euphrates watersheds can be divided in to three broad classes of land cover, based on the ratio of precipitation to evaporation. First are all the areas that contribute water to the rivers. These areas include the upland snow fields (2) and broad mid-level terrains that have an excess of precipitation over evaporation (5). The rivers receive this excess and carry it to lower elevation and, in the case of the Tigris and Euphrates watersheds, into a much dryer climate zone. Along these rivers, reservoirs store and modulate the river discharge (4).
The second category is the area that draws water from the rivers, including traditional floodplain irrigation (8) and large new irrigated "districts" (9). One might include small pump-fed agriculture in this category (10), although this water comes from ground aquifers rather than the rivers or surface reservoirs. All these regions evaporate more water than they receive by precipitation.
The third class is composed of areas that neither draw from nor contribute to the river discharge. Vast areas of spring season rain-fed agriculture and steppe grazing land (6) have hydrologic budgets that are nearly in balance. They evaporate in the spring and summer most of the precipitation they receive in the winter and spring. One could include in this category numerous small basins, wadis and seasonal lakes (7). These areas may receive occasional run-off or ground water seepage. Water is stored in these depressions until evaporated by the hot winds of summer.
Schematic of the basic elements of the Euphrates-Tigris catchment hydrologic system. 1. Mountain induced precipitation; 2. High elevation snow storage; 3. River network; 4. Reservoirs; 5. Landscapes that drain into the rivers; 6. Landscapes that do not drain into the rivers; 7. Depressions and transient lakes; 8. Flood plain irrigation; 9. Irrigated districts; 10. Well-fed agriculture; 11. Rain-fed agriculture; 12. Ground water, 13. Water-body source regions (Persian Gulf); and 14. Atmospheric transport of water vapor.