Great Plains and Prairies - Water

Irrigation in the United States is usually associated with the dry region of the far West. Yet the benefits derived from irrigation may be higher in many semihumid or even humid areas--in terms of the level of increased production per dollar invested--because irrigation water may be used either as a supplement in dry times to maximize yields for crops already grown in the area or to grow crops for which the available moisture is not quite sufficient.

There are a number of Great Plains areas where large-scale irrigation developments are important. Perhaps the most noteworthy of these is on the High Plains from Colorado and Nebraska to Texas. The area is underlain by the Oglala aquifer, a vast underground geologic reservoir under 250,000 square kilometers of the area that contains an estimated 2 billion acre-feet of water. (An acre-foot is the volume of irrigation water that covers 0.4 hectares to a depth of 0.3 meters.) This is "fossil" water, much of it deposited more than a million years ago. About a quarter of the aquifer's area is irrigated, almost entirely with Oglala water. The High Plains is a major agricultural region, providing, for example, two-fifths of America's sorghum, one-sixth of its wheat, and one-quarter of its cotton. Irrigated lands here produce 45 percent more wheat, 70 percent more sorghum, and 135 percent more cotton than neighboring nonirrigated areas. Groundwater withdrawals have more than tripled since 1950, to more than 20 million acre-feet annually.

Early in the 20th century, the area centered on Lubbock, Texas, became a significant region of cotton production. Irrigated farming, using water from wells drilled into the water-bearing sands that underlie much of the southern High Plains, gradually replaced the early dry-farming approach. Today, the region is the most important area of cotton production in the United States. More than 50,000 wells supply irrigation water in the area.

The second major irrigated area on the Plains is in northeastern Colorado, with sugar beets the primary specialty crop. The area has long been irrigated from wells and from the waters of the South Platte River. The federal government covers the cost of construction, and those who use irrigation pay for the water. Because these waters are no longer adequate to meet demands, the government funded the Big Thompson River project, which is designed to carry water from the west slope of the Front Range of the Rocky Mountains to the east slope and the irrigated lands beyond. The most striking technological feature of this project is a 33-kilometer tunnel, lying 1,200 meters below the Continental Divide in Rocky Mountain National Park.

The largest of the water impoundment projects on the Plains is the Missouri Valley project. The project was an outgrowth of two different sets of needs. People living at the lower end of the Mississippi Valley, including those in Kansas City and St. Louis, needed an effective system of flood control. About 100 centimeters of precipitation falls on this area each year. In contrast, people in the upper Missouri Valley, especially the Dakotas and Montana, needed a system to provide ample water for irrigation. The resultant system is composed of a series of large earth-fill dams on the upper Missouri, as well as numerous dams on many of the tributaries of the river.

These and many other smaller irrigation projects and individual wells have allowed a great expansion in the diversity of Plains agriculture. Throughout the central and northern Plains, alfalfa--the premier hay crop of the West--claims the largest irrigated hectarage. Sugar beets are important in the Arkansas River Valley of eastern Colorado and western Kansas, and along the South Platte in northeastern Colorado. Arkansas valley growers also take great pride in the quality of their cantaloupes, while corn, usually irrigated from wells, is a major crop in south-central Nebraska.