Surface water constitutes the ordinary flow and underflow of rivers, streams, and lakes, as well as runoff from precipitation. There are over 6,687 square miles of inland water in Texas, ranking the state seventh among the contiguous forty-eight states. These inland waters are represented by fifteen major river basins, and eight coastal basins. The state also has over 665 miles of coastline. Over 11,000 named streams and tributaries with a combined length of over 80,000 miles provide drainage for rainfall runoff. The total annual discharge of water from these streams has historically varied from around 21 million acre-feet to 55 million acre-feet, depending on whether the state is in a wet or dry period. In 1990 surface water comprised 43 percent of the water used in Texas. That total was broken down for the following uses: municipal, 26 percent or 1.79 million acre-feet; agricultural, 48 percent or 3.25 million acre-feet; industrial, 19 percent or 1.3 million acre-feet; mining, 1 percent or .06 million acre-feet; and steam-electric power generation, 5.5 percent or .37 million acre-feet. Unlike underground water, which is subject to the rule of capture, surface water and its use is regulated by the state of Texas (seeWATER LAW). The Texas Natural Resource Conservation Commission (formerly the Texas Water Commission) was in 1995 the agency in Texas responsible for authorizing, enforcing, and protecting the use of surface water. A second state agency, the Texas Water Development Board, was responsible for making studies, plans, and providing financial support to public entities and leadership towards the responsible development and conservation of the state's water resources for current and future generations of Texans, their economy, and their environment. A third agency, the Texas Parks and Wildlife Department, deals with issues that impact the environment.
Surface water differs from other natural resources in that the supply is constantly being replenished, although the yield differs from year to year, and the distribution within a year varies from man-made or natural causes. Surface runoff is primarily dependent upon rainfall. It is, however, influenced considerably by evaporation, transpiration, surface topography, and the facilities for infiltration provided by surface soils and rocks. Based on weather records over an eighty-year period extending into the 1980s, precipitation supplies 413 million acre-feet of water a year; however, 90 percent of this is lost to evaporation and plant transpiration, leaving only 40 million acre-feet to recharge aquifers and flow to rivers and streams. Soil types, vegetation, surface slopes, and rainfall intensities are all factors of nature that vary the relation between rainfall and runoff. Generally speaking, however, the relative average quantity of water in Texas streams follows the rainfall pattern across the state. Precipitation across Texas is mainly dependent on the moisture content of the onshore flow of air from the Gulf of Mexico. Its proximity to the Gulf of Mexico puts all except the extreme western section of the state in the path of frequent tropical or semi-tropical storms. The general path of these storms is up or across the major streams. Rainfall events producing over twenty inches in a twenty-four-hour period have occurred throughout the state, causing significant floods, as runoff waters travel to the Gulf. Generally, moisture increases from the western to the eastern portions of the state. Less than one inch of the average annual rainfall of extreme West Texas appears as flow in the streams. The yield increases progressively to fifteen inches of average annual runoff along the Texas-Louisiana border, where the average annual rainfall is fifty to fifty-five inches.
The first gauging station of the United States Geological Survey in Texas was established on the Rio Grande at El Paso on May 10, 1889, and a few miscellaneous measurements of Central Texas streams were made in 1894, 1895, and 1896. In 1897 Thomas Ulvan Taylor began a systematic study of a few of the principal streams of Texas for the USGS. Beginning in 1915 the USGS, in cooperation with the Board of Water Engineers, the United States Army engineers, and various other agencies, established and maintained streamflow measurement stations at numerous sites where it was anticipated that future developments would be made. In 1945 there were 183 gauging stations maintained on the major rivers of Texas and their principal tributaries. In addition, stream-measurement stations on ten important canals were in operation, and gauges at eight reservoirs and lakes were maintained for determination of storage contents. Many miscellaneous measurements of springs and of streams at low and flood flows, as well as other special investigations of the surface-water resources, have been made by the Geological Survey. These data afford material for evaluating the availability of surface-water flows of the large Texas streams. Between 1941 and 1980 the average annual runoff of Texas streams was 52 million acre-feet. Fifty percent of the total Texas runoff originates from the eastern quarter of the state, principally from six of the smaller Texas streams—the Sulphur, Sabine, Neches, San Jacinto, and Trinity rivers and Cypress Creek in Northeast Texas. The total drainage area of these East Texas streams is 46,000 miles. Streams in the eastern portion of the state are not subject to as rapid variations in flow as are those of other sections of Texas. Usually they carry a considerable amount of flood runoff during the winter and spring months. The Brazos, Colorado, Guadalupe, Nueces, and Rio Grande basins are subject to long periods of low flow and short periods of flood flow; therefore, their runoff characteristics are quite different from those of streams in East Texas. From 70 to 90 percent of the total runoff from these basins occurs as flood flow extending over a few weeks of the year, except for streams such as the Devils and Guadalupe rivers, which derive a large portion of their flow from uniformly flowing springs. Historically, the Guadalupe River below New Braunfels has been the most dependable stream in the state from the standpoint of a well-sustained low flow. Comal Springs and San Marcos Springs are the principal sources for its abundant supply of water, though in the latter part of the twentieth century, increased population, pumping, and water use has threatened the stability of the springs. Surface streams in some areas also lose large quantities of water to underground formations. The stream network of the upper Nueces River Basin is a principal example, losing substantial flow as the water flows across certain sections of the Balcones Fault zone. The variations of flow of the Concho River near San Angelo during the 1936 water year are typical of the wide range in runoff that may be expected from average West Texas streams when floods of great magnitude occur. The total runoff during that year was 869,000 acre-feet; of this amount, 96 percent occurred during a sixteen-day period, and 30 percent occurred in one day. The average annual runoff for this station was 146,000 acre-feet. During the absence of triggering events, rainfall events decrease significantly resulting in drought conditions. During the 1950s drought, for example, the average annual runoff was only 23 million acre-feet for the years of 1950 to 1956. The annual yield of individual streams has an even wider variation.
Managing, tracking, and protecting the quality of existing water supplies has become increasingly important as population and industrial needs increase. Historically, because of the limited water supply that was available from the unregulated flow of all Texas streams, it was necessary that flood waters be stored and conserved for multiple uses. Over the last half of the twentieth century there was been a dramatic increase in the number of Texas reservoirs. There were only eight major reservoirs in Texas in 1913. By 1930 this had increased to thirty-two with a total capacity of 1,284,520 acre-feet. In 1960 there were 105 reservoirs amounting to a total capacity of 22,746,200 acre-feet. By 1993 Texas had 212 major reservoirs in existence or under construction with a conservation storage capacity totaling 40,825,072 acre-feet. This amounted to a total surface area of 1,695,647 acres. The extensive utilization of reservoirs across the state has led to improved flood control measures and increased conservation. There are over 5,700 reservoirs in Texas of at least ten surface acres in size. Of these reservoirs, 207 have a storage capacity of over 5,000 acre-feet and comprise 97 percent of the available surface water supplies in the state. The usable water, or conservation storage capacity of these reservoirs allocated to Texas, is 35 million acre-feet. Their total flood storage capacity is 17.4 million acre-feet, and their dependable water supply (firm yield) for the year 2000 is 8.1 million acre-feet annually. About 86 percent or seven million acre-feet of the total firm yield is currently being used for municipal, agricultural, mining, steam-electric power generation, and industrial purposes. Virtually all of the remaining water in the reservoirs has been committed, by the appropriation of water rights, to meet the growing municipal and industrial needs of the major metropolitan areas of the state. From 1930 to 1993 the state's population increased from six million to eighteen million. The development of additional supplies will become increasingly difficult since there are only a limited number of new reservoir sites available. Many of these sites are being developed, and environmental concerns regarding the changes a reservoir will cause to current ecosystems are increasing. Future plans regarding water use are more focused on conservation, reuse, regionalization, and the best available use of existing supplies.