Underground water includes all water that occurs below the earth's surface, occupying interstices or voids of pervious rocks and soil; like surface water, it is derived principally from precipitation that falls upon the earth's surface and percolates downward under gravity. Underground water in the zone of saturation may occur in either water table (unconfined) aquifers or artesian (confined) aquifers. Confined water is generally under pressure greater than atmospheric pressure, and wells penetrating a confined aquifer will permit water to rise above the confining strata. If sufficient pressure exists flowing wells may result. In the case of water table aquifers, water is derived from local precipitation; but in the case of artesian wells, water may enter the permeable strata ten or even hundreds of miles from the point where it is intercepted by wells; hence, water taken from a well is no more a part of the land than is the water of a surface stream that crosses it. The city of Houston, for example, derives water from the Beaumont clay, from the Goliad-Willis-Lissie formations, and in smaller quantities from the Logarto clay, which outcrops approximately 100 miles to the west. Most geologic formations in Texas contain water, but only a relatively few yield abundant supplies. Those formations yielding abundant supplies which have been classified as major aquifers include: the Gulf Coast aquifer, which corresponds to the Gulf Coast; Carrizo-Wilcox aquifer, just north of the Gulf Coast aquifer; Trinity Group aquifer, mainly North-Central Texas; Edwards-Trinity aquifer, identified with the Edwards Plateau area; Edwards aquifer, corresponding to the Balcones Escarpment zone; Ogallala aquifer, roughly the High Plains area; and Alluvium aquifer, in widespread areas in Texas but occurring principally in West Texas. In all, Texas has seven major and sixteen minor aquifers. They underlie approximately 76 percent of the area of Texas. The massive Ogallala aquifer accounts for 90 percent of the total water in all the Texas aquifers. In the mid-to-late 1980s 11 million acre-feet a year was being withdrawn from aquifers in the state of Texas. However, only 5.3 million acre-feet a year replenished the aquifers.
Underground water is a principal water resource in Texas, and its importance as a source of supply for municipal, industrial, and irrigation uses, as well as domestic and livestock purposes, is immeasurable. In 1990 more than half of the water used in Texas was underground water; about 71 percent was for agricultural use and 21 percent for municipal use, with the remainder going to industrial needs. Many large cities and most of the smaller cities and communities in Texas supply their water needs from municipally-owned wells or from a combination of surface and underground water sources. More than half of all municipal water in Texas comes from underground water. The largest single use of underground water in Texas is for irrigation. In 1990, 6 million acres in Texas was irrigated, and about 70 percent of all irrigation water came from underground sources. More underground water is used for irrigation in Texas than for all other uses combined. Heavily irrigated areas using underground water are the High Plains region, the Winter Garden Region, Pecos-Coyanosa, and the Salt Basin areas in the Trans-Pecos. Of all water pumped for irrigation purposes in Texas, 68 percent is produced in the Southern High Plains.
In addition to municipal and irrigation uses, many industries have developed underground water supplies for cooling and industrial processes. Characteristics of underground water that make it particularly desirable for industry are its uniform temperature and uniform quality at a given source. Although all underground water contains mineral matter derived principally from soil and rocks through which it moves, its quality and temperature will generally remain constant at a given locality unless it is contaminated by human activities. Several factors have contributed to the development and widespread use of underground water as a supply source in Texas. Nearly all of the geologic formations in the state yield some water, generally in sufficient quantities for domestic and livestock purposes. Where it is available in sufficient quantities, underground water is generally cheapest to develop, since it does not need the extensive pipelines and treatment facilities required for surface supplies. Unlike surface water, which flows in definite and limited channels, underground water can often be developed at the point of use, requiring little or no transportation. Large areas of Texas deficient in rainfall and having few perennial streams available for water supply contain vast quantities of underground water in storage.
Underground water, unlike surface water, which is the property of the state of Texas, is the exclusive property of the owner of the land surface and subject to the rule of capture. It is subject to barter, sale, or lease and not to the complicated and often conflicting riparian and appropriative doctrines governing surface-water rights. Except for a few statutes pertaining to conservation, protection, and waste, the state of Texas does not regulate the production or use of underground water. Some local control is exercised by underground water conservation districts, which are empowered to promulgate rules and regulations for conserving, preserving, protecting, and recharging underground water reservoirs within their boundaries, including well spacing and well permitting. In the latter part of the twentieth century many areas in the state were grappling with the problems of overpumping. Land subsidence, especially in the Houston-Galveston area, has been a major concern and resulted in the creation of a subsidence district in 1975. Also, too much pumping may result in saltwater intrusion. Increasingly, many municipalities and agribusinesses looked to more efficient conservation methods and the use of surface water. See also WATER LAW.