The Ogallala Aquifer () is a shallow water table aquifer surrounded by sand, silt, clay, and gravel located beneath the Great Plains in the United States.
As one of the world's largest aquifers, it underlies an area of approximately in portions of eight states (South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas). It was named in 1898 by geologist N. H. Darton from its type locality near the town of Ogallala, Nebraska. The aquifer is part of the High Plains Aquifer System, and resides in the Ogallala Formation, which is the principal geologic unit underlying 80% of the High Plains.
Large-scale extraction for agricultural purposes started after World War II due partially to center pivot irrigation and to the adaptation of automotive engines to power groundwater wells. Today about 27% of the irrigated land in the entire United States lies over the aquifer, which yields about 30% of the ground water used for irrigation in the United States. The aquifer is at risk of over-extraction and pollution. Since 1950, agricultural irrigation has reduced the saturated volume of the aquifer by an estimated 9%. Once depleted, the aquifer will take over 6,000 years to replenish naturally through rainfall.
The aquifer system supplies drinking water to 82% of the 2.3 million people (1990 census) who live within the boundaries of the High Plains study area.
General characteristics The deposition of aquifer material dates back two to six million years, from the late Miocene to early Pliocene ages 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.Gustavson, T. C, Winkler, D. A. (1988). Depositional facies of the Miocene-Pliocene Ogallala Formation, northwestern Texas and eastern New Mexico. Geology, 16(3), 203–206. Retrieved from: http://geology.gsapubs.org/content/16/3/203.shortDiffendal, R. F. (1984). Comments on the geologic history of the Ogallala Formation in the southern panhandle of Nebraska. Papers in Natural Resources. Paper 116. Retrieved from http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1117&context=natrespapers In that respect, the process is similar to those 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-saturated thickness of the Ogallala Formation ranges from a few feet to more than . Its deepest part is and is generally greater in the Northern Plains.High Plains Underground Water Conservation District #1 (Texas) retrieved April 9, 2007. The depth of the water below the surface of the land ranges from almost in parts of the north to between 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. An impervious geological layer between the aquifer and surface of the land, combined with an arid climate, prevents much recharge from occurring.Thompson, J., et al. (2018). Legal Control of Water Resources: Cases and Materials, 6th ed. West Academic Publishing. p. 13. ISBN 978-1-68328-983-8.
Groundwater within the Ogallala generally flows from west to east at an average rate of a foot per day. Hydraulic conductivity, or the ability for a fluid (water) to move through porous material, ranges from per day.Gutentag, E. D, Heimes, F. J, Krothe, N. C, Luckey, R. R, Weeks, J. B. (1984). "Geohydrology of the High Plains Aquifer in parts of Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming". U.S. Geological Survey Professional Paper 1400-B. Retrieved from https://pubs.usgs.gov/pp/1400b/report.pdf Water quality within the Ogallala varies with the highest quality for drinking and irrigation in the northern region while the southern region had the poorest.Gurdak, J. J, McMahon, P. B, Dennehy, K, Qi, S. L. (2009). "Water quality in the High Plains Aquifer, Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming". National Water-Quality Assessment Program, USGS Circular 1337. Retrieved from https://pubs.usgs.gov/circ/1337/pdf/C1337.pdf Human and natural processes over the past 60 to 70 years, including irrigation density, climate, and nitrogen applications, have caused higher concentrations of contaminants including nitrates. Nitrate levels generally meet USGS water quality standards, but continue to gradually increase over time. This trend can impact the future groundwater sustainability for portions of the aquifer.
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).
The USGS estimated that total water storage was about in 2005. Withdrawals from the Ogallala Aquifer for irrigation amounted to in 2000. Since major groundwater pumping began in the late 1940s, overdraft from the High Plains Aquifer has amounted to , 85% of the volume of Lake Erie. Many farmers in the Texas High Plains, which rely particularly on groundwater, are now turning away from irrigated agriculture as pumping costs have risen and as they have become aware of the hazards of overpumping.
Groundwater recharge The rate at which groundwater is recharged 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 per year in parts of Texas and New Mexico to per year in south-central Kansas.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.
Groundwater discharge
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. Only after World War II, when center-pivot irrigation became available, was the land mass of the High Plains aquifer system transformed into one of the most agriculturally productive regions in the world.
Change in groundwater storage Ground water levels decline when the rate of extraction by irrigation exceeds the rate of recharge. At places, the water table was measured to drop more than 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."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, 2012Daily Telegraph (UK) Saturday Magazine Issue no 48,446 (dated 5 March 2011) pp 26-32 "High and Dry" Report by Charles Lawrence
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.
The center-pivot irrigator was described as the "villain" in a 2013 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. Hundreds to thousands of years of rainfall would be needed to replace the groundwater in the depleted aquifer. In Kansas in 1950, irrigated cropland covered ; with the use of center-pivot irrigation, nearly 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 of the aquifer has already gone dry."
The center-pivot irrigation system is considered to be a highly efficient system which helps conserve water. However, by 2013, 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 rather than reduce water consumption--an example of the Jevons Paradox in practice. One approach to reducing the amount of groundwater used is to employ treated recycled water for irrigation; another approach is to change to crops that require less water, such as sunflowers.Jeremy P. Meyer, "Farmers' tower of power", Denver Post, 2 October 2006. Last accessed October 24, 2006
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.
The $46.1-million Optima Lake dam in western Oklahoma was rendered useless when the dropping level of the aquifer drastically reduced flow of the Beaver River, the lake's intended source of water.Logan Layden, "If you Want to Build a New Lake in Oklahoma, Forget History", March 28, 2013, StateImpact Oklahoma
Accelerated decline in aquifer storage