The Ogallala Aquifer, part of the High Plains Aquifer System, is a vast yet shallow underground water table aquifer located beneath the Great Plains in the United States. One of the world's largest aquifers, it covers an area of approximately 174,000 mi² (450,000 km²) in portions of the eight states of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas. It was named in 1898 by N.H. Darton from its type locality near the town of Ogallala, Nebraska.
The Ogallala Formation underlies about 80 percent of the High Plains and is the principal geologic unit forming the High Plains Aquifer. About 27 percent of the irrigated land in the United States overlies this aquifer system, which yields about 30 percent of all ground water used for irrigation in the United States. The aquifer system supplies drinking water to 82 percent of the 2.3 million people (1990 census) who live within the boundaries of the High Plains study area.
The deposition of the aquifer material dates back 2 to 6 million years, to late Miocene to early Pliocene age when the southern Rocky Mountains were still tectonically active. From the uplands to the west, rivers and streams cut channels in a generally west to east or southeast direction. Erosion of the Rockies provided alluvial and aeolian sediment that filled the ancient channels and eventually covered the entire area of the present-day aquifer, forming the water-bearing Ogallala Formation. In that respect, the process is little different from that currently prevailing in other modern rivers of the area, such as the Kansas River and its tributaries. The major differences are time and depth.
The depth of the Ogallala varies with the shape of then-prevailing surface, being deepest where it fills ancient valleys and channels. The Ogallala Formation consists mostly of coarse sedimentary rocks in its deeper sections, which transition upward into finer-grained material.
The water-permeated thickness of the Ogallala Formation ranges from a few feet to more than 1000 feet (300 m) and is generally greater in the northern plains. The depth of the water below the surface of the land ranges from almost 400 feet (120 m) in parts of the north to between 100 and 200 feet (30 and 60 m) throughout much of the south. Present-day recharge of the aquifer with fresh water occurs at an exceedingly slow rate, suggesting that much of the water in its pore spaces is paleowater, dating back to the most recent ice age and probably earlier.
Aquifer water balance
An aquifer is a groundwater storage reservoir in the water cycle. While groundwater is a renewable source, reserves replenish relatively slowly. The USGS has performed several studies of the aquifer, to determine what is coming in (groundwater recharge from the surface), what is leaving (water pumped out and baseflow to streams) and what the net changes in storage are (rise, fall or no change — see figure above).
Withdrawals from the Ogallala Aquifer for irrigation amounted to 26 km3 (21,000,000 acre·ft) in 2000. As of 2005, the total depletion since pre-development amounted to 253,000,000 acre feet (312 km3). Some estimates indicate a remaining volume sufficient for as little as 25 years. Many farmers in the Texas High Plains, which rely particularly on the underground source, are now turning away from irrigated agriculture as they become aware of the hazards of overpumping.
The rate at which recharge water enters the aquifer is limited by several factors. Much of the plains region is semiarid, with steady winds that hasten evaporation of surface water and precipitation. In many locations, the aquifer is overlain, in the vadose zone, with a shallow layer of caliche that is practically impermeable; this limits the amount of water able to recharge the aquifer from the land surface. However, the soil of the playa lakes is different and not lined with caliche, making these some of the few areas where the aquifer can recharge. The destruction of playas by farmers and development decreases the available recharge area. The prevalence of the caliche is partly due to the ready evaporation of soil moisture and the semiarid climate; the aridity increases the amount of evaporation, which in turn increases the amount of caliche in the soil. Both mechanisms reduce the amount of recharge water that reaches the water table.
Recharge in the aquifer ranges from 0.024 inches (0.61 mm) per year in parts of Texas and New Mexico to up to 6 inches (150 mm) per year in south-central Kansas 
The regions overlying the Ogallala aquifer are some of the most productive regions in the United States for ranching livestock, and growing corn, wheat, and soybeans. The success of large-scale farming in areas that do not have adequate precipitation and do not always have perennial surface water for diversion has depended heavily on pumping groundwater for irrigation.
Early settlers of the semiarid High Plains were plagued by crop failures due to cycles of drought, culminating in the disastrous Dust Bowl of the 1930s. It was only after World War II when center pivot irrigation became available, that the land mass of the High Plains aquifer system was transformed into one of the most agriculturally productive regions in the world. During the early years, this source of water was thought to be inexhaustible, and its hydrology a mystery.
Change in groundwater storage
Ground water levels decline when the rate of extraction by irrigation exceeds the rate of recharge. At some places, the water table was measured to drop more than five feet (1.5 m) per year at the time of maximum extraction. In extreme cases, the deepening of wells was required to reach the steadily falling water table. In the 21st century, recognition of the significance of the aquifer has led to increased coverage from regional and international journalists.
The USGS estimated that total water storage was about 2,925,000,000 acre feet (3,608 km3) in 2005. This is a decline of about 253,000,000 acre feet (312 km3), or 9%, since substantial ground-water irrigation development began in the 1950s.
Water conservation practices (terracing and crop rotation), more efficient irrigation methods (center pivot and drip), and reduced area under irrigation have helped to slow depletion of the aquifer, but levels are generally still dropping in areas including southwestern Kansas and the Texas Panhandle. In other areas, such as parts of eastern and central Nebraska and of the region south of Lubbock, Texas, water levels have risen since 1980.
Writer John Miller Morris characterized the increased use of the center pivot irrigation system as part of a profound attitude shift towards modernism (expensive tractors, center-pivot irrigation, dangerous new pesticides) and away from traditional farming that took place in the mid-1970s and 1980s in the United States as the post-WWII generation of go-go farmers with college degrees in agriculture, grandchildren of survivors of the Great Depression, returned to farms. This generation chose high-risk, high-reward crops like irrigated corn or peanuts peanuts which are very costly to produce taking quantities of ground water, fertilizer and chemicals. The new family farm corporation on behalf of their stockholders turned many pastures into new cropland and were more interested in rising land prices than water conservation. 
The center-pivot irrigator was described as the "villian"  in a New York Times article, "Wells Dry, Fertile Plains Turn to Dust" recounting the relentless decline of parts of the Ogallala Aquifer. Sixty years of intensive farming using huge center-pivot irrigators has emptied parts of the High Plains Aquifer. It would take hundreds to thousands of years of rainfall to replace the groundwater in the depleted aquifer. In 1950 irrigated cropland covered 250,000 acres. With the use of center-pivot irrigation, nearly three million acres of land were irrigated.  In some places in the Texas Panhandle, the water table has been drained (dewatered). "Vast stretches of Texas farmland lying over the aquifer no longer support irrigation. In west-central Kansas, up to a fifth of the irrigated farmland along a 100-mile swath of the aquifer has already gone dry."
The center-pivot irrigation system is considered to be a highly efficient system which helps conserve water. It is used by Arrowhead Mills, the largest natural foods wholesaler in the United States, founded by organic farmer and activist Frank Ford from the Texas Panhandle, for example. However, by 2013 it was shown that as the water consumption efficiency of the center-pivot irrigator improved over the years, farmers chose to plant more intensively, irrigate more land and grow thirstier crops not to reduce water consumption. Using treated, recycled sources of water in agriculture is one approach to safeguarding the future of the aquifer. Another method of reducing the amount of water use is changing to crops that require less water, such as sunflowers.
Several rivers, such as the Platte, run below the water level of the aquifer. Because of this, the rivers receive groundwater flow (baseflow), carrying it out of the region rather than recharging the aquifer.
Accelerated decline in aquifer storage
According to a 2013 report by research hydrologist, Leonard F. Konikow, at the United States Geological Survey (USGC), the depletion between 2001–2008, inclusive, is about 32 percent of the cumulative depletion during the entire 20th century (Konikow 2013:22)." In the United States, the biggest users of water from aquifers include agricultural irrigation and oil and coal extraction."Cumulative total groundwater depletion in the United States accelerated in the late 1940s and continued at an almost steady linear rate through the end of the century. In addition to widely recognized environmental consequences, groundwater depletion also adversely impacts the long-term sustainability of groundwater supplies to help meet the Nation’s water needs."
In the 1980s, Texas Panhandle organic foods farmer and spokesman Frank Ford successfully prevented the location of a nuclear waste repository in his native Deaf Smith County. He opposed the site on the grounds that the material could poison the Ogallala Aquifer. The repository was located instead in Yucca Mountain in Nevada.
Proposed Keystone XL Pipeline
In 2008, TransCanada Corporation proposed the construction of the 1,661-mile (2,673 km) Keystone XL pipeline to carry oil from the Athabasca oil sands of Alberta to refineries near Houston, Texas. The proposed route of the pipeline crosses the eastern part of the Nebraska Sandhills; opponents of the route cite the risk to the Ogallala Aquifer posed by the possibility of contamination from spilled dilute bitumen.
As the lead agency in the transboundary pipeline project, the U.S. State Department commissioned an environmental-impact assessment as required by the National Environmental Policy Act of 1969. The Environmental Impact Statement concluded that the project posed little threat of "adverse environmental impacts", the report was drafted by Cardno Entrix, a regular TransCanada subcontractor. While it is "common for companies applying to build government projects to be involved in assigning and paying for the impact analysis", U.S. President Barack Obama cited the apparent conflict of interest in his January 2012 decision to await further environmental review. On February 17, 2013 a rally at the National Mall drew an estimated 40,000 in protest of Keystone XL.
On 1 March 2013, the U.S. State Department released its Keystone pipeline Draft Supplemental Environmental Impact Statement, which concluded that a large crude oil spill from the pipline that reached the Ogallala could spread as far as 1,214 feet, with dissolved components spreading as much as 1,050 feet further. Secretary of State John Kerry is reviewing the permit application and the White House is expected to announce its decision after the Secretary has familiarised himself with the project.
- Darton, N.H. 1898. Preliminary report on the geology and water resources of Nebraska west of the one hundred and third meridian. In: Walcott, C.D. (ed), Nineteenth Annual Report of the United States Geological Survey, 1897-1898, Part IV, pp. 719-785.
- Dennehy, K.F. (2000). "High Plains regional ground-water study: U.S. Geological Survey Fact Sheet FS-091-00". USGS. Retrieved 2008-05-07.
- North Plains Groundwater Conservation District
- High Plains Underground Water Conservation District #1 (Texas) retrieved April 9, 2007.
- Mcguire, V.L. (May 2007). "Changes in Water Levels and Storage in the High Plains Aquifer, Predevelopment to 2005". USGS. Retrieved 2009-08-12.
- "Ogallala aquifer - Water hot spots". BBC News. 2003.
- Geohydrology of the High Plains Aquifer in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. Gutentag, E. D.; Heimes, F. J.; Krothe, N. C.; Luckey, R. R.; Weeks, J. B. 1984.
- "Shrinking aquifer looms as big problem for farms". Nancy Cole, Arkansas Democrat-Gazette. September 24, 2006. Last accessed October 24, 2006.
- Column - Mansel Phillips: "Too many thirsty industries, not nearly enough water". Mansel Phillips, Amarillo Globe News. October 4, 2006. Last accessed October 24, 2006.
- "Another sign of long-term water worries", Lincoln Star Journal, October 8, 2006. Last accessed November 20, 2012
- Daily Telegraph (UK) Saturday Magazine Issue no 48,446 (dated 5 March 2011) pp 26-32 "High and Dry" Report by Charles Lawrence
- Morris, John Miller (2003). Sherry L. Smith, ed. The Future of the Southern Plains. Norman, Oklahoma: University of Oklahoma Press. p. 275. ISBN 0806137355.
- Rainwater, Ken (1 January 2004). "Book Review: The Future of the Southern Plains". Great Plains Quarterly Great Plains Studies (Lincoln, Nebraska: Center for Great Plains Quarterly Great Plains Studies, University of Nebraska).
- Wines, Michael (19 May 2013). "Wells Dry, Fertile Plains Turn to Dust". New York Times.
- "About Arrowhead Mills". Arrowhead Mills.
- Jeremy P. Meyer, "Farmers' tower of power", Denver Post, 2 October 2006. Last accessed October 24, 2006
- Konikow, Leonard F. (PDF). Groundwater Depletion in the United States (1900–2008) (Report). Scientific Investigations Report. Reston, Virginia: U.S. Department of the Interior, U.S. Geological Survey. pp. 63. http://pubs.usgs.gov/sir/2013/5079/SIR2013-5079.pdf.
- Zabarenko, Deborah (20 May 2013). "Drop in U.S. underground water levels has accelerated: USGS". Washington, DC: Reuters.
- Hovey, Art. "TransCanada Proposes Second Oil Pipeline". Lincoln Journal-Star. 2008-06-12. Reproduced at Downstream Today website. Retrieved 2011-08-27.
- "Keystone Pipeline Project". TransCanada. Retrieved 2011-08-27.
- Morton, Joseph, and Paul Hammel. "Report: Sand Hills route best". Omaha World-Herald. 2011-08-27. Retrieved 2011-08-27.
- "Keystone XL Pipeline". Friends of the Earth. Retrieved 2011-08-27.
- O'Meara, Dina, and Sheldon Alberts. "U.S. report clears way for TransCanada's Keystone XL pipeline". Calgary Herald. 2011-08-27. Retrieved 2011-08-27.
- "Pipeline Review Is Faced With Question of Conflict", New York Times, October 7, 2011.
- Rafferty, Andrew. "Thousands rally in D.C. against Keystone Pipeline". NBC News. National Broadcast Company. Retrieved 21 February 2013.
- U.S. Dept. of State, Draft Supplemental Environmental Impact Statement, 1 March 2013, p.4.16-2.
- "The Ogallala Aquifer" Manjula V. Guru, Agricultural Policy Specialist and James E. Horne, President & CEO, The Kerr Center for Sustainable Agriculture, Poteau, Oklahoma
- USGS High Plains Regional Groundwater Study
- A Legal Fight in Texas over the Ogallala Aquifer
- Kansas Geological Survey information on the High Plains / Ogallala Aquifer
- Rapid Recharge of Parts of the High Plains Aquifer Indicated by a Reconnaissance Study in Oklahoma