Texas Water Atlas

Glossary

acre-foot: The amount of water required to cover an area of 1 acre (43,560 square feet) to a depth of 1 foot that totals 43,560 cubic feet or 325,851 gallons. aquiclude: A rock or sedimentary unit of low transmissivity that precludes the flow of groundwater. aquifer: A body of rock or sediment that contains water that can be withdrawn. artesian well: A well in which the water is under hydrostatic pressure and may flow without pumping. bay: A body of water that is partially enclosed by land with a wide opening to the ocean. bayou: A term given to a slow-moving stream that meanders across a flat coastal plain in parts of the American South. brackish water: Water that has greater than 1,000 parts per million (ppm) dissolved salts but less than the 34,000 ppm salinity of ocean water. climate: The long-term weather characteristics and phenomena for a location. The U.S. National Weather Service uses decadal 30-year averages, such as 1971–2000, to describe the climate for sites in the United States. cubic feet per second (cfs or ft3 /sec or ft3 s–1 ): The rate of flow of a stream that is equal to the width times the averaged depth times the average velocity of a stream (W ⫻ D ⫻ V). One cubic foot of water is equal to 7.48 gallons, which means that a stream flowing at the rate of 1 cfs is flowing at the rate of 7.48 gallons per second. Many Texas streams with a long-term average flow of 200 to 300 cfs or less have record floods of greater than 100,000 cfs or 748,000 gallons per second. desalination: The process of removing dissolved materials, “salts,” from brackish or oceanic water. The two major methods are “flash distillation,” which is essentially boiling the salty water and capturing the condensing steam, and “reverse osmosis,” which entails forcing the salty water through a semipermeable membrane under great pressure. ecosystem: The assemblage of plants and animals (biota) in a particular habitat. endangered species: Species whose survival is at risk of extinction . Federal and state governments prescribe procedures for characterizing a species as “endangered,” thereby giving the species and its critical habitat legal protection. escarpment: A steep drop-off from an elevated landscape. One of the most famous escarpments in Texas is the Balcones Escarpment in Central Texas, which breaks the Edwards Aquifer to form the Texas “spring line” from Waco to Del Rio. Another famous Texas escarpment is the Caprock Escarpment, which forms the eastern edge between the High Plains and the Llano Estacado in the Panhandle. estuary: To geographers and geologists, an estuary is the flooded lower reaches of a river valley caused by the rise in sea level since the last glacial maximum 18,000 years ago. To a biologist, an estuary is a zone of mixing of fresh and marine waters so that the water has less than ocean salinity —critical for the survival of the juvenile forms of many economically and environmentally significant species, such as blue crabs, shrimp, and redfish. Most geographic/geologic estuaries are estuaries to biologists, but many biological estuaries are not geographic/geologic estuaries and are other water bodies that are isolated from the open ocean, such as lagoons and bays, but not necessarily drowned river valleys. eustasy: The uniform worldwide change in sea level caused by a change in the volume of the oceans. The major cause of eustasy over the past 3 million years has been the uptake of the ocean’s waters to form glaciers during the Pleistocene glaciations (“ice ages”), resulting in a drop of sea level as much as 400 feet below the present-day level. Global warming today is melting the world’s glaciers and causing thermal expansion of the ocean’s waters that produces a rise in sea level. evapotranspiration: The transfer of liquid water from the soil to water vapor in the atmosphere through the process of evaporation from the soil or transpiration through plants. Generally, the hotter and drier the climate, the greater will be the potential for evapotranspiration. fault line: A break in the earth’s surface caused by seismic forces within the earth. floodplain: The region adjacent to the channel of a stream that is periodically flooded during high flow conditions. A “100-year” floodplain is a floodplain that has a 1 percent chance of being flooded in any given year. A “500-year...