USGS Publications Warehouse

Abstract

The Southern High Plains of Texas occupies an area of about 22,000 square miles in n'Orthwest Texas, extending fr'Om the Canadian River southward. about 250 miles and fr'Om the New Mexico line eastward an average distance of about 120 miles. The economy of the area is dependent largely upon irrigated agriculture, and in 1958 about 44,000 irrigation wells were in operation. The economy of the area is also dependent upon the oil industry either in the f'Orm of oil and gas production or in the form of industries based on the producti'On of petroleum. The Southern High Plains of Tems is characterized. 'by a nearly flat land surface sloping gently toward. the southeast at an average of 8 to 10 feet per mile. Shallow undrained depressions or playas are characteristic of the plains surface, and during periods of heavy rainfall, runoff collects in the depressions to form temporary ponds or lakes. Stream drainage 'On the plains surface is poorly developed; water discharges over the eastern escarpment off the plains only during periods of excessive rainfall. The climate of the area is semiarid; the average annual precipitation is about 20 inches. About 70 percent of the precipitation falls during the growing season from April to September. Rocks of Permian age underlie the entire area and consist chiefly of red sandstone and shale containing nUmerous beds of gypsum and dolomite. The Permian rocks are not a source of water in the Southern High Plains, and any water in these rocks would probably be saline. The Triassic rocks underlying the 'S'Outhern Hi'gh Plains consist of three formations of the Dockum group: the Tecovas formation, the Santa Rosa sandstone. and the Chinle formation equivalent. The Tecovas and Chinle formation equivalent both consist chiefly of shale and sandy shale; however, the Santa Rosa sandstone consists mainly of medium to coarse conglomeratic sandstone containing some shale. Tbe formations of the Dockum group are capable of yielding small to moderate quantities of water in many parts of the Southern High Pl'ains; however, in practically all places the water is rather saline and pr<Ybalbly unsuitable f'Or most uses. The Cretaceous f'Ormati'Ons in the Southern High Plains consist of several formati'Ons of the Trinity, Fredericksburg, and Washita groups. The rocks underlie 'a large part of the southern part <Yf the Southern High Plains; they consist of sandstone, 'shale, and limestone, the sandstone and limestone being the principal water-bearing units. In a few pl'aces where the Cretaceous rocks appear to be in hydrauli'c coimection with the overlying Ogallala formation, moderate quantitie of water are obtained, particularly from the limestones. Locally the Cretaceous rocks may be important aquifers where other water is not available, but they generally do not constitute a large source of water for irrigation or municipal use. The Ogallala formation of Pliocene age is the principal aquifer in the Southern High Plains of Texas; it supplies practically all the water used for all purposes. The formation is continuous throughout most of the Texas part of the Southern High Plains and extends into New Mexico. The .formation consists chiefly of sediments deposited by streams that had their headwaters in the mountainous regions to the west and northwest. The Ogallala formation rests unconformably upon an erosional surface of the underlying Triassic and Cretaceous rocks. The Ogallala consists of beds and lenses of clay, silt, sand, and gravel; caliche occurs as a secondary deposit ,in many places in the formation. In general the Ogallala is thicker in the northern part of the area; the thickness ranges from 400 to 500 feet in central Parmer, west-central Castro, and southwestern Floyd Counties to a knife edge where the formation wedges out against outcrops of the older rocks. The Ogallala formation probably originally formed a continuous blanket of sediments extending from the Rocky Mountains on the west well into Texas. However, erosion has completely isolated the formation, and the segment in the Southern High Plains is cut off in all directions from any underground connection with water-bearing beds outside of the area except through the underlying older rocks which contain highly mineralized water entirely unlike the fresh water in the Ogallala. Consequently, the source of all the water in the Ogallala is precipitation that falls on the surface of the plains in Texas and New Mexico. Thin deposits of Pleistocene and Recent age overlie the Ogallala formation in many places; they consist of lake or pond, stream, and sand-dune deposits. These rocks are important hydrologically only where they form recharge facilities such as in the sand-dune areas and in the drainageways. The lake or pond deposits consist chiefly of clay and silt and, therefore, generally impede recharge rather than aid it. Caliche deposits underlie much of the surface of the Southern High Plains of Texas The caliche consists of beds, lenses, or nodules chiefly of calcareous and siliceous material. The caliche forms the conspicuous caprock of the eastern plains escarpment. In many places the caliche is almost completely indurated und is an impediment to recharge. In other areas the caliche is porous and in places may possibly be of such composition and permeability that it would not hinder the infiltration of recharge from the surface. Generally the water in the Ogallala occurs under water-table conditions; however, locally it may be under slight artesian pressure. The water in the Ogallala occupies the pore spaces and voids in the rocks and occurs between the water table and the underlying older rocks. The thickness of the zone of saturation in the Ogallala varies throughout the Southern High Plains chiefly because of the uneven nature of the bedrock surface. This thickness ranges from 0 to more than 300 feet. The coefficient of transmissibility of the Ogallala formation ranges rather widely. Tests at Amarillo indicate a coefficient of 6,000 to 7,000 gpd per ft (gallons per day per foot) and tests in the vicinity of Plainview indicate a transmissibility of about 34,000 gpd per ft. Numerous tests both in the laboratory and in the field indicate a specific yield of about 15 percent. The movement of water in the Ogallala formation is generally toward the southeast in the general direction of the slope of the water table. The water table slopes roughly parallel to. the slope of both the bedrock and land surface, the average slope of the water table being about 10 feet per mile. The rate of movement in the formation in the vicinity of Plainview has been estimated to be about 2 inches per day. The fluctuations of the water table in the Ogallala formation represent chiefly changes in the amount of water in storage. The trend of the fluctuations throughout the Southern High Plains has generally been a decline that reflects the large quantities of water withdrawn for irrigation. Locally, however, water-level rises have been recorded and indicate recharge at least in some areas. The depth to water in the Ogallala formation is affected by the topography of the land surface, the proximity to areas of recharge or natural discharge, the proximity to areas of withdrawal of water, and the configuration of the bedrock surface. In 1958, the depth to water in the formation ranged from less than 50 feet-to more than 250 feet. . The principal sources of ground-water recharge to the formation in the Southern High Plains of Texas are underflow from New Mexico and precipitation on the land surface in Texas. The amount of underflow is unknown; it is probably small but relatively constant from year to year. The amount of recharge from precipitation depends on many factors including the amount, distribution, and intensity of precipitation and type of soil and vegetative cover. Various estimates of the amount of recharge indicate that it is probably less than one-half an inch annually. Artificial recharge experiments have been conducted in many parts of the High Plains using the water that collects in the playa lakes during periods of high rainfall. The method of recharge has been by injection through wells drilled near the lakes. A study of the availability of lake water on the High Plains in a sample area consisting of parts of four counties showed that, in an area containing 1,348 lakes, nearly 200,000 acre-feet of water was caught in 1957, a year of above-normal precipitation. In 1958 when the rainfall was below normal, the lakes caught approximately 37,000 acre-feet of water. Experiments have shown that perhaps as much as 80 to 90 percent of the water caught in the lakes can be injected through wells to be stored in the ground-water reservoir for future use. By far the largest use of water from the Ogallala formation in the Southern High Plains is for irrigation, most of the irrigation development having been started since 1937. In January 1937, about 600 irrigation wells were in use in the entire Southern High Plains; during the succeeding 6 years, new wells were added at rates ranging from 120 to 480 wells per year. From 1943 to 1950 about 11,000 wells were drilled, and in 1951 about 14,000 wells were in use. The development has continued during the 1950's, and in 1958 about 44,000 wells irrigated a total of about 4.3 million acres. It is estimated that a total of about 36 million acre-feet of water has been pumped from the Ogallala formation in the Southern High Plains during the period 1938 through 1957. The estimated average yearly pumpage since 1954 has been about 5 million acre-feet. Most of the water has been pumped in what could be termed the heavily pumped areas including 11 counties where depletion of the ground-water reservoir amounted to about 26 million acre-feet from the time large-scale development of ground water began in the late 1930's to 1958. Although these large quantities of water have been removed from the Ogallala formation, it is estimated that about 160 million acre-feet of water remained in storage in 1958, most of this water being concentrated in the northern part of the area. The water in the Ogallala formation in the Southern High Plains of Texas can generally be said to be of good chemical quantity except that it is hard and has a high silica content. Most of the water is suitable for irrigation and meets the U.S. Public Health Service recommendations for public supplies. The water form some wells has an excessive fluoride content. The estimated amount of water withdrawn from .the ground-water reservoir each year so greatly exceeds even the most optimistic estimates of recharge that it is obvious that ground water is being 'mined,' that is, it is coming from storage. Without any other foreseeable source of large supplies of water except that which accumulates in the depressions, conservation and the most effective use of the remaining supplies are the only means of combating the depletion and extending the life expectancy of the ground-water supply.