THE OGALLALA AQUIFER

The Ogallala aquifer (pronounced OH-GA-LA-LA) is one of the largest aquifer systems in the world. It stretches across all or portions of eight states generally from north to south to include South Dakota, Nebraska, Wyoming, Colorado, Kansas, Oklahoma, New Mexico, and Texas and underlies about 174,000 square miles. N.H. Darton is credited with describing and naming the formation in 1899 after the town of Ogallala, Nebraska.

The Ogallala aquifer lies relatively near the land surface in most of the above-described area with a maximum thickness of about 1,000 feet with a few hundred feet more the norm. Even in those areas of only a few feet of thickness, the aquifer can almost always be counted on to yield water to a well drilled into it. Some wells yield only a few gallons of water per minute, while others yield 1,000 gallons of water per minute or more. The Ogallala aquifer not only includes the portion of the Ogallala that is saturated with water, but may also include saturated portions of the overlying and underlying formations that are hydraulically connected to the Ogallala. Water in the aquifer on the Southern High Plains flows from northwest to southeast at about 150 feet per year under natural conditions. This rate of movement can be altered by discharge from the aquifer by pumping wells.

Deposition of the Ogallala Formation began 10 to 12 million years ago during late Tertiary (Miocene/Pliocene) geologic time. Sand, gravel, silt, and clay eroded from upland areas to the west and north were deposited over the erosional land surface of the present-day High Plains by primarily eastward flowing streams.   The surface on which the sediments were deposited would have been much like the present area located east of the High Plains escarpment characterized by low hills, relatively shallow valleys, and meandering streams.  As a result of the burial of this land surface by predominantly Ogallala sediments, the Ogallala Formation is thicker where these sediments filled the old stream channels and thinner where hills or upland areas were buried. The uppermost layer of the Ogallala Formation is typically a caliche layer described as the “Caprock Caliche.”  This layer locally varies in thickness and has been reported to be as much as 60 feet in thickness in some areas. The caliche layer formed about one million years ago after the land surface stabilized and soils formed.

The water table is a term used to describe the uppermost surface of sediments that are 100 percent saturated. Saturated thickness describes the thickness of an aquifer. This interval is determined by subtracting the elevation of the base of the aquifer from the elevation of the water table at a point of interest. In some parts of Nebraska, the saturated thickness exceeds 1,000 feet and generally thins to less than 20 feet in thickness in some areas of the Great Plains.

The amount of water that may be recovered in an aquifer, such as the Ogallala, is dependent primarily on the areal extent, the saturated thickness, and the specific yield of the aquifer. Specific yield is a hydrologic parameter related to the volume of water an aquifer will yield as a result of gravity drainage. As an example of this parameter, 15 percent specific yield is an average value that is customarily accepted for the Southern High Plains Ogallala aquifer.  More specifically at 15 percent specific yield each cubic foot of water saturated aquifer volume will yield 0.15 cubic-foot of water as a result of gravity drainage (See graphic depicting specific yield at right).

In 1990, the Ogallala aquifer in the eight-state area of the Great Plains contained 3.270 billion acre-feet of water, of which about 65 percent was located under Nebraska. Texas had about 12 percent of the water in storage or approximately 417 million acre-feet of water. Kansas has 10 percent of the water. About 4 percent was located under Colorado, with 3.5 percent located under Oklahoma. Another 2 percent was under Wyoming. The remaining 1.5 percent of the water was under New Mexico.  A recent estimate of the volume of water in the eight-state Great Plains area was just under 3 billion acre-feet.

Natural recharge to the Ogallala aquifer occurs primarily through the percolation of precipitation through the soils and underlying sediments to the water table. It is generally recognized that playa lakes are the primary points of most natural recharge. The interplaya areas generally contribute a minimum of the recharge, except for areas of exceptional accumulation of precipitation with resultant extensive percolation of water to the water table in locations where streambeds and dune areas are common.  Various studies of natural recharge have historically estimated various ranges of average recharge to the Ogallala aquifer. Recent studies have estimated an average recharge rate for the entire High Plains region of approximately 0.5 of an inch per year.

Before the development of irrigation, the discharge from the aquifer occurred from both saline and fresh water like basins, from streams, and from seeps and springs located primarily along the eastern escarpment. Some of these still flow today; however, most seeps and springs have ceased to flow due primarily to lowering of the water table as discharge has exceeded natural recharge.

Approximately 95 percent of the water pumped from the Ogallala is for irrigation. The High Plains area represents 65 percent of the total irrigated acreage in the United States. The quality of the water pumped from the aquifer is suitable for irrigating; but in some places, the water does not meet U.S. Environmental Protection Agency (USEPA) drinking water quality standards. For example, some constituents identified above EPA standards include sulfate, chloride, selenium, fluoride, nitrate, and total dissolved solids.

The Southern High Plains area is characterized as a semi-arid climate. Average annual rainfall varies across the region. In the Lubbock, Texas area, the average annual rainfall is about 18 inches per year. High evaporation rates are common in the area. The average annual evaporation rate for the Lubbock area is about 80 inches per year.

Monitoring of the depth-to-water in the aquifer’s Southern High Plains revealed rapid declines in the water table in the early 1950s, 1960s, and the 1970s. Declines of a foot or more per year were recorded throughout the 1940s; and during the late 1950s at the peak of irrigation development, some monitoring wells indicated as much as five feet of decline in a single year. The trend of rapid decline started slowing in the mid-1970s. By 1985, the portion of the Ogallala aquifer within the service area of the High Plains Underground Water Conservation District No. 1 began to stabilize. In some limited or unique areas, water level rises have been documented.  A drought began in the area in mid-1992, and continued until late 1996. Agricultural producers, out of necessity, increased pumpage of water for irrigation to supplement precipitation. The increased demand for crop water by irrigation resulted in an increased rate of water level decline during this period. In subsequent years of greater precipitation, decreased needs for pumpage have allowed water level declines to decrease.

Early settlers believed the water supply that lay beneath them was inexhaustible. In the 1930s, people had begun to realize the potential of the vast water supply that lay beneath them. By 1949, about 2 million acres of the Southern High Plains were irrigated. Pumpage for irrigation increased from about 4 million acre-feet in 1949 to nearly 18 million acre-feet in1980.

In the early days of irrigation on the Texas High Plains, very little water conservation equipment or technology was available. As a result, large amounts of water were lost to evaporation and deep percolation. Open, unlined ditches were used to transport the water from the well to the field being irrigated. It was not uncommon to have water losses ranging from 10 to 30 percent per 1,000 feet of ditch. High pressure, hand-moved sprinklers had evaporation losses of up to 50 percent.

Throughout the years, irrigation technology has evolved to allow agricultural producers to apply water much more efficiently without waste.  Irrigation water escaping from fields into road ditches (“irrigation tailwater”) is not as common as it once was in the 1960s and 1970s. Improved technologies, spurred on by incentives such as the Environmental Quality Incentives Program (EQIP) and low interest agricultural water conservation equipment loan programs, have helped improve water use efficiencies.  For example, average water use efficiency within the High Plains Water District service area improved from about 50 percent in the mid 1970s to approximately 75 percent in 1990.  Current state-of-the-art low pressure, full dropline center pivot systems, used in conjunction with furrow dikes, are about 95 percent efficient, while buried subsurface drip irrigation lines approach 100 percent efficiency.

Producers are irrigating fewer acres. Land enrolled in the Conservation Reserve Program (CRP), rising energy costs, declines in well yields, and low farm prices also account for part of this reduction.  

The rates of decline in the water levels in the aquifer continue to be somewhat stable with noted annual decreases in decline resulting when annual precipitation is above average and less ground water needs to be pumped. Researchers continue to work on methods to increase natural recharge to the aquifer and to improve water-use efficiency.  The prospects for the future of the Ogallala aquifer ultimately depend upon its management by each of its water users. 

Additional information is available by contacting Technical Group Supervisor Don McReynolds at (806) 762-0181.

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