This article, written by the American Ground Water Trust was originally published in THE AMERICAN WELL OWNER, 2002, Number 3]
WHAT HAVE WE DONE TO THE OGALLALA AQUIFER?
The Ogallala Aquifer is the largest aquifer in the United States and one of the largest aquifers in the world. It is referred to as the High Plains Aquifer in most technical reports because it underlies portions of eight states in America’s central “high” plains (predominantly Nebraska and Kansas, and parts of Oklahoma, Colorado, New Mexico, Texas, South Dakota and Wyoming) encompassing an area of about 174,050 square miles. As with most aquifers it receives recharge [new] water primarily through infiltration of rain and snow melt from the surface. The recharge water fills fractures and interstices (open spaces between rock particles) in the rock.
The Ogallala formation is the main rock unit of the High Plains Aquifer and is named after the town of Ogallala in southwestern Nebraska where the rock is exposed at the surface. It is comprised of clay, silt, sand and gravel that were deposited in streams that drained from the Rocky Mountains during the late Tertiary geologic time period (about 10 to 5 million years ago). The Ogallala is up to 215 meters (705 feet) thick and in the Sand Hills area of central Nebraska the Ogallala formation is covered by sand dunes.
The nomadic hunter-gather culture and later subsistence farming activities of the Native Americans had little impact on the water stored in the Ogallala. In 1854, the Halladay wind pump was introduced to the high plains to provide water for irrigation and, somewhat later, to replenish railroad steam engines. More efficient steel blades replaced wood fan blades in the 1870’s and the withdrawal of water from the Ogallala formation was permanently established. Beginning in the early 1940’s and increasingly after the end of World War II, high discharge gas-powered center-pivot turbine pumps were employed in many areas over the Ogallala aquifer.
These large capacity pumps have withdrawn water at a rate that significantly exceeds the recharge back to the Ogallala aquifer. The United Nations estimated in its 1996 Comprehensive Global Freshwater Assessment that withdrawals from the Ogallala aquifer exceed recharge by approximately 3 to 1.
In 1980, the Ogallala held about 3,250 million acre-feet of drainable water according to the United State Geological Survey (1 acre-foot equals 325,805 gallons). Since then, the aquifer has lost a large volume of this water through withdrawals for agricultural uses. The average annual fall in the water table of the aquifer between 1980 and 1999 according to the USGS was 3.2 feet. The largest declines were in southwest Kansas and the Texas Panhandle. In these two regions, the average decline was near 25 feet, but had maximums greater than 67 feet. In eastern Nebraska, there were local areas where the water table rose by over 30 feet. The areas showing a rise in the water table were much smaller in extent than the areas showing declines.
The average saturated thickness of the Ogallala aquifer in Kansas and Texas is slightly over 100 feet. As the water table falls it becomes more expensive to withdraw water due to increased pumping. Without conservation of the resource, large areas of the aquifer will be depleted in the next two decades. This condition will have significant impacts on society in these areas of the United States and on the agricultural economies that rely on production from these regions.
Further reading: Quick facts (1994 data): http://www-ne.cr.usgs.gov/highplains/hpchar.html Water level change information [1980 to 1999]: http://ne.water.usgs.gov/highplains/hp_99_web_report/FS-029-01.pdf Aerial photos of Southwest Kansas between 1974 and 1989: http://www.cossa.csiro.au/lb/lbbook/agric/ag18.htm Prevailing water requirements of the United States http://www.kerrcenter.com/RDPP/Ogallala2.htm
[© American Ground Water Trust. This article may be reprinted for non-commercial educational purposes provided it is used in its entirety and that reference is made to its source as an article in THE AMERICAN WELL OWNER, 2002, Number 3]