Ogallala Aquifer

The Ogallala aquifer underlies portions of eight states. This image documents the decline in the level of water in many areas for the period 1980-1995. Source: USGS

The Ogallala Aquifer, or High Plains Aquifer, is a vast yet shallow aquifer located beneath the Great Plains in the United States. One of the world's largest aquifers, it lies under about 174,000 mi² (450,000 km²) in portions of South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas. It was named in 1899 by N.H. Darton from its type locality near the town of Ogallala, Nebraska.

General characteristics

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 somewhat 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 eolian sediment that filled the ancient channels and eventually covered the entire area of the present-day aquifer, forming the water-bearing Ogallala Formation. The depth of the formation varies with the shape of the pre-Ogallala surface, being deepest where it fills ancient valleys and channels. The Ogallala Formation consists mostly of coarse sedimentary rocks in its lower sections, which grade upward into finer-grained lithologies.

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 (122 m) in parts of the north to between 100 to 200 feet (30 to 61 m) throughout much of the North. Present-day recharge of the aquifer with fresh water occurs at a slow rate; this implies that much of the water in its pore spaces is paleowater, dating back to the last ice age.

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 (pumping and baseflow to streams) and what the net changes in storage are (rise, fall or no change — see figure above). Simply put, water in, minus the water out, is equal to the change in water stored in the aquifer. This type of mass-balance "accounting" is how hydrologic budgets are performed, and is a crucial first step in sustainable management of any natural resource. Presently, groundwater in the United States is utilized approximately four times faster than it is naturally replaced.

Groundwater recharge

The rate at which recharge water enters the aquifer is limited by several factors. Much of the plains region is semi-arid 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 then decreases the available recharge area. The prevalence of the caliche is partly due to the ready evaporation of soil moisture and the semi-arid climate; the aridity increases the amount of evaporation which increases the amount of caliche in the soil. Both mechanisms reinforce the difficulty recharge has in reaching the water table.

Groundwater discharge

File:Crops Kansas AST 20010624.jpg

NASA ASTER image of an approx. 557 mi² area of fields (1443 km²) in Kansas which are watered from the Ogallala aquifer with center pivot irrigation systems.

The regions overlying the Ogallala aquifer are some of the most productive regions for ranching livestock, and growing corn, wheat and soybeans in the United States. The success of large-scale farming in areas which do not have adequate precipitation and do not always have perennial surface water for diversion depends heavily on pumping groundwater for irrigation.

Early settlers of the semi-arid High Plains were plagued by crop failures due to cycles of drought, culminating in the disastrous Dust Bowl of the 1930s. The aquifer was first tapped for irrigation in 1911. Large scale use for irrigation began in the 1930s and continued through the 1950s, due to the availability of electric power to rural farming communities and the development of cheap and efficient electric turbine pumps. It was only after World War II that affordable technology became available to substantially extract water. This transformed the High Plains 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. However, because the rate of extraction exceeds the rate of recharge, water level elevations are decreasing. 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; and it has even been drained (dewatered) in some places such as Northern Texas. Today, water is being extracted at rates exceeding one hundred times the natural replacement rate. Utilizing treated recycled sources of water in agriculture is one approach at vouchsafing the future of the aquifer. Another method to reducing the amount of water use is changing to a crop that requires less water, such as sunflowers.

Several of the rivers in the region, such as the Platte, in places are below the water level of the aquifer and therefore the rivers actually receive groundwater flow (baseflow) rather than supply recharge to the aquifer.

Change in groundwater storage

The USGS estimated that total water storage was about 2,925 million acre feet (3,608 km³) in 2005. This is a decline of about 253 million 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 simply reduced area under irrigation have helped to slow depletion of the aquifer, but levels are generally still dropping, particularly in the southern parts at rates exceeding one hundred times the replacement rate. See the figure above for an illustration of the places where large drops in water level have been observed (i.e., the red areas in Texas and southern Kansas). In the more humid areas water levels have actually risen since 1980 (i.e., eastern and central Nebraska and south of Lubbock, Texas). Agricultural dependence on this valuable water source needs to change within a generation in order to save this invaluable groundwater source.

As in the case of oil peak groundwater depletion can be anticipated and estimated. For the aquifer as a whole the peak groundwater depletion rate is estimated to have occurred in 2006.

Other mentions of Ogallala aquifer

The importance of this aquifer has been recognized recently in many works, ranging from increased attention from journalism to a mention in a plot for world domination in the Amazing Spider-Man comic book made by Marvel Comics. Increased oversight and regulation of the aquifer by the federal government does not appear to be forthcoming, though the economic and environmental implications of the potential decline of the region are significant.

See also

• Hydrogeology
• Aquifer and Water table
• Groundwater

External links and further reading

• "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
• "Wells Dry, Fertile Plains Turn to Dust" article by Michael Wines in The New York Times May 19, 2013
• "Peak groundwater depletion in the High Plains Aquifer, projections from 1930 to 2110" article by David R. Steward and Andrew J. Allen in Agricultural Water Management 11 November 2015 doi:10.1016/j.agwat.2015.10.003

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Adapted from the Wikipedia article "Ogallala Aquifer" http://en.wikipedia.org/w/index.php?title=Ogallala_Aquifer&oldid=168263931 released under the GNU Free Documentation License