Water development in the American West has been shaped by the region's geography, legal and institutional arrangements, urban expansion, and the spread of irrigated agriculture.

Geography

From the grasslands of west Texas to the deserts of Arizona, the southwest quadrant of the United States is the most arid part of the country. The southern Great Plains states with their vast farmlands and grazing lands are semiarid and relatively flat. To the west, the Rocky Mountains and other ranges in Colorado and New Mexico rise out of the plains to form the massive peaks of the Continental Divide. These mountains shelter some well-watered valleys before giving way to arid basins and deserts in Nevada, Utah, and southern California. Terrain and local climate thus vary considerably throughout the arid and semiarid West. Of the 1.9 billion acres of land in the continental United States, almost half receive less than 20 inches of precipitation per year (Council on Environmental Quality, 1980). (See Figure 1.1.) Water supplies in this arid and semiarid region are both limited and variable. Precipitation in the form of rainfall and snowfall is unevenly distributed across and within the western states. For example, average rainfall in the mountainous area surrounding Flagstaff, Arizona, measures over 20 inches per year. But in central and southern Arizona, where most of the people live and most of the agricultural areas are located, average rainfall ranges from only 7 inches (in Phoenix) to 11 inches (in Tucson) per year (Ruffner and Bair, 1981).

Equally as important as the quantity of rainfall for western water users is the natural variation in supplies from season to season, year to year, and even decade to decade.