HIGH PLAINS AQUIFER
The High Plains aquifer in Oklahoma and Texas is part of a regional aquifer that extends into parts of Colorado, Kansas, Nebraska, New Mexico, South Dakota, and Wyoming (fig. 38). Only that part of the aquifer in Oklahoma and Texas is described in this chapter; descriptions for other States are in other chapters of this Atlas. The aquifer consists predominantly of the Ogallala Formation of late Tertiary age; locally, unconsolidated deposits of Quaternary age are included in the aquifer. In places, the High Plains aquifer is in hydraulic connection with permeable parts of the underlying bedrock, which ranges in age from Permian to Cretaceous.
The High Plains geographic area is in the Great Plains Physiographic Province and consists of an elevated plain that is relatively undissected. The population of the High Plains geographic area is sparse, but the combination of level topography, excellent soils, and an abundant supply of ground water for irrigation makes this an important agricultural region.
Average annual precipitation ranges from about 12 inches in the southwest to 24 inches in the northeast. Average annual runoff ranges from about 0.2 inch in the west to 0.5 inch in the east. The High Plains aquifer in Segment 4 underlies an area of about 43,000 square miles mostly in the panhandle parts of Oklahoma and Texas. About 4.5 billion gallons of water per day was withdrawn from the High Plains aquifer in Oklahoma and Texas during 1985. The aquifer is by far the most intensively developed aquifer in the two-State area.
The High Plains aquifer described in this chapter has been called the Ogallala aquifer in many published reports. The age of the Ogallala Formation is considered to be Miocene in this chapter, but is listed as Pliocene or Pliocene and Miocene in many published reports.
At the close of deposition of the Ogallala Formation several million years ago, the Great Plains was a vast, gently sloping plain that extended from the edge of the Rocky Mountains eastward for hundreds of miles. Regional uplift and erosion stripped away the plain in many places, but a large central area was little affected by eroding streams and is preserved. This preserved remnant of the uplifted Ogallala Formation is known as the High Plains. Although the surface of the High Plains has been modified little by streams, it has been pitted by carbonate dissolution and deflation, thus forming many playas, or shallow depressions, that collect and store water during periods of precipitation and runoff.
The Canadian River has cut through much of the Ogallala Formation in the Texas Panhandle. The High Plains south of the Canadian River is referred to locally and regionally as the Southern High Plains. This area also is known as the Llano Estacado (Staked Plain).
During Miocene time, the uplifted and tectonically active Rocky Mountains provided source material for deposition of the Ogallala Formation. Valleys and basins that developed by erosion on the surface of Permian, Triassic, Jurassic, and Cretaceous rocks (fig. 39) became filled with Ogallala sediments. In northern Texas, some collapse structures in Permian rocks are filled with Mesozoic (Triassic, Jurassic, or Cretaceous) rocks, as well as with Ogallala deposits. Where the Mesozoic rocks have secondary permeability, they are considered to be part of the High Plains aquifer; however, they are a very minor component. The Ogallala sediments were deposited by braided streams that spread across a generally level plain. The eastward-flowing streams deposited a heterogeneous mixture of gravel, sand, silt, and clay.
Upwarping and climatic change in Pliocene time caused deposition of alluvium to cease and erosion to begin. Preservation of the remnant of uplifted Ogallala Formation that composes most of the High Plains aquifer is due largely to the presence of resistant caliche cap rock that formed over much of the surface of the Ogallala. The cap rock consists of zones that are cemented with calcium carbonate; these zones are resistant to weathering and cause the formation of ledges and escarpments.
The thickness of the Ogallala Formation is as much as 650 feet. The overlying Quaternary alluvium and windblown sand, which are locally as much as 150 feet thick, are part of the High Plains aquifer in some places (fig. 40). The base of the aquifer generally slopes to the east and southeast. The altitude of the base ranges from about 2,000 to 4,000 feet above sea level (fig. 41). The altitude of the water table before development ranged from about 2,400 to more than 4,000 feet above sea level (fig. 42). The regional movement of ground water is from west to east toward the cap-rock escarpment that forms the eastern margin of the High Plains geographic area.
The High Plains aquifer is recharged by the infiltration of precipitation that falls directly on the aquifer. This recharge is estimated to range from 0.024 inch per year in the Southern High Plains of Texas to 2.2 inches per year in Texas County, Okla. and is about 0.1 percent and 12 percent of average annual precipitation, respectively. Additional recharge may occur when a part of the water that is pumped for irrigation infiltrates the soil and returns to the water table. As much as 54 percent of irrigation pumpage might be reentering the aquifer in Castro and Parmer Counties, Tex., whereas only 20 percent of irrigation water applied in the Oklahoma Panhandle might be returned to the High Plains aquifer.
Ground water discharges naturally through seeps and springs, primarily along the eastern escarpment and the Canadian River. Most ground water is discharged artificially through wells.
Hydraulic conductivity and specific yield of the sediments that compose the High Plains aquifer are important properties that control well yields and resulting water-level depths and rates of water-level declines. The areal distribution of hydraulic conductivity, as shown in figure 43, was estimated from records collected by water well drillers. Values range from less than 1 to 200 feet per day, and the range is 25 to 100 feet per day for most of the aquifer. The average hydraulic conductivity for the 35,450 square miles of High Plains aquifer in Texas is estimated to be 65 feet per day; the average for the 7,350 square miles of the aquifer in Oklahoma is estimated to be 61 feet per day.
Specific yield also was estimated from lithologic descriptions made by drillers during the construction of water wells. The areal distribution of specific yield is shown in figure 44. Values range from less than 1 to 30 percent; most of the area is in the 10 to 20 percent range. The estimated average specific yield for the High Plains aquifer in Texas and Oklahoma is 15.6 percent and 18.5 percent, respectively.
Small concentrations of dissolved solids in ground water in the High Plains aquifer indicate that the water either has had a short residence time in the aquifer or has been in contact with relatively insoluble minerals, or both. Larger concentrations indicate longer residence time, contact with soluble minerals such as gypsum, anhydrite, and halite, or mixing with more mineralized water from bedrock.
Water from the High Plains aquifer is used mostly for crop irrigation. If leaching or drainage is adequate, then concentrations of dissolved solids between 500 and 1,500 milligrams per liter in irrigation water are not likely to be harmful to crops. Concentrations of individual chemical constituents, such as sodium, also are important in determining the suitability of the water for most uses. Excessive sodium concentrations, for example, can cause chemical imbalances and can interfere with normal plant growth.
Most of the water in the High Plains aquifer has a dissolved-solids concentration of less than 500 milligrams per liter (fig. 45). Concentrations exceed 500 milligrams per liter in water from a large part of the Southern High Plains in Texas. In water from the southernmost part of the aquifer in Texas, concentrations of dissolved solids exceed 1,000 milligrams per liter but are generally less than 3,000 milligrams per liter. In this area, highly mineralized water in underlying Mesozoic rocks of marine origin probably moves into the High Plains aquifer in response to hydraulic-head differences. Locally, the more mineralized water seems to be associated with several alkali lake basins in areas underlain by Cretaceous rocks in Lamb, Hockley, Terry, Lynn, eastern Gaines, and Martin Counties. Sodium and increased dissolved-solids concentrations may increase locally because of industrial activities and irrigation practices.
Pumpage of ground water for irrigation on the High Plains began in the early 1900's and increased slowly until the mid-1940's. In Texas, the acreage irrigated by ground water increased rapidly between the mid-1940's and 1959 but increased little between 1959 and 1980. The irrigated acreage in 1980 on the High Plains of Texas was 3.9 million acres, which was about the 1959 level. This leveling off after 1959 is primarily the result of declining water availability in the Southern High Plains. Acreage irrigated by ground water in the Oklahoma part of the High Plains in 1980 was about 389,000 acres. During the 1980 growing season, an estimated 5,169,000 acre-feet of water was pumped from the High Plains aquifer for irrigation in Texas, and an estimated 540,000 acre-feet was pumped in Oklahoma.
The density of acreage that was irrigated by ground water from the High Plains aquifer during 1978 is shown in figure 46. Most of the irrigated acreage was in the northern one-half of the Southern High Plains of Texas. In Texas alone, the High Plains aquifer supplied water to about 75,000 irrigation wells.
Because pumpage to satisfy the large demand for crop irrigation has been considerably in excess of recharge, water levels in the High Plains aquifer have declined substantially. The altitude of the water table in the High Plains aquifer in 1980 is shown in figure 47. When compared with the predevelopment water table (fig. 42), the general westward shift of the contours indicates water-level declines.
The change between the predevelopment and the 1980 water tables is shown in figure 48. Water-level declines of 50 to more than 100 feet have been measured in a large area in the northern part of the Southern High Plains of Texas where the irrigated acreage is most dense. Water levels in most areas declined between 10 and 50 feet but rose in some areas. Water-level rises in Texas probably resulted from the clearing of native vegetation for cultivation, which increased the rate of recharge from precipitation by reducing transpiration. Water-level rises in Oklahoma probably represent a recovery from abnormally low water levels during the drought of 1933-40. These low water levels were among the earliest data available in Oklahoma and were used to construct the predevelopment water-table map.
The general decline of the water table has resulted in a considerable loss of water from storage and a decreased saturated thickness of the High Plains aquifer. The total volume of drainable water in storage is a product of specific yield, saturated thickness, and area. In 1980, the estimated total volume of drainable water in storage in the High Plains aquifer was 390 million acre-feet in Texas and 114 million acre-feet in Oklahoma. The saturated thickness of the aquifer ranged from 0 to 600 feet in 1980 (fig. 49). The saturated deposits generally thicken from south to north. Most of the aquifer south of the Canadian River had a saturated thickness of less than 100 feet.
Changes in saturated thickness and in well yields are directly related. The saturated thickness of the High Plains aquifer in Texas reportedly decreased by more than 50 percent in large parts of Castro, Crosby, Floyd, Hale, Lubbock, Parmer, and Swisher Counties, south of the Canadian River. From 1958 to 1980, irrigated land in the seven counties decreased from 2.5 million to 1.9 million acres, while the number of irrigation wells increased from about 21,000 to 30,000. The average number of acres irrigated per well decreased from 118 in 1958 to 62 in 1980. Decreased well yields are one result of water-level declines.
Another result of water-level declines and decreased saturated thickness is an increase in the depth to water. The generalized depth to water in the High Plains aquifer in 1980 is shown in figure 50. Depths ranged from 0 to 400 feet and exceeded 100 feet for most of the area. Greatest depths to water are in the vicinity of the Canadian River. Increased depths to water equate to increased pumping lifts which, together with decreased well yields, add substantially to the cost of withdrawing water from the High Plains aquifer.
FRESH GROUND-WATER WITHDRAWALS
Withdrawals of freshwater from the High Plains aquifer in Texas and Oklahoma totaled 4,508 million gallons per day during 1985 (fig. 51). Agricultural purposes, the principal water use, required about 4,343 million gallons per day. About 93 million gallons per day was withdrawn for public supply and about 9 million gallons per day was pumped for domestic and commercial uses. Withdrawals for industrial, mining, and thermoelectric-power uses were 63 million gallons per day.
POTENTIAL FOR DEVELOPMENT
The map of potential yields of wells completed in the High Plains aquifer shown in figure 52 is based on hydraulic conductivity and the 1980 saturated thickness. In a large part of the area, especially north of the Canadian River, well yields in excess of 750 gallons per minute can be expected. One well capable of yielding 750 gallons per minute can irrigate 160 acres and effectively operate a quarter-section (0.25 square-mile) center-pivot irrigation system. Irrigation development is less favorable in areas, such as a large part of the Southern High Plains of Texas, where well yields are less than 250 gallons per minute. Areas with further declines in water level (therefore declining well yields) may experience a decline in irrigated acreage, as noted in the "Ground-Water Development" section above for the seven-county area in Texas. In some areas, particularly the southernmost area, irrigation development may be limited because of large sodium or dissolved-solids concentrations (fig. 45).
Because the High Plains aquifer is being pumped far in excess of recharge, the ground water is a limited resource. Questions of major concern are: How long will the ground-water resource last?; and How can the remaining water be managed and used most efficiently? Among the factors that influence further development of the High Plains aquifer are crop prices, energy and other farm costs, droughts and surplus precipitation, conservation practices, regulatory policies, and water-use technology improvements.
The Texas Department of Water Resources projects an increasing shortage of water from the High Plains aquifer for future irrigation needs. Unless an effective conservation program is implemented, it is estimated that the irrigated acreage on the High Plains of Texas will be decreased by slightly more than one-half of the present acreage by 2030. Water conservation methods and secondary recovery of capillary water are among some of the alternatives that are being explored to solve the water-supply problems in the High Plains of Texas. To assist in that exploration, digital computer simulations have been used to predict the possible effects of future ground-water pumpage on the High Plains aquifer under various pumpage estimates and management strategies.