The Middle Rio Grande Basin, as defined for this study, is the area within the Rio Grande Valley extending from about Cochiti Lake downstream to about San Acacia. It covers approximately 3,060 square miles in central New Mexico, encompassing parts of Santa Fe, Sandoval, Bernalillo, Valencia, Socorro, Torrance, and Cibola Counties and includes a ground-water basin composed of the Santa Fe Group aquifer system. (It is equivalent to the Albuquerque Basin referred to by other authors.) The climate over most of the basin is semiarid. In 2000, the population of the Middle Rio Grande Basin was about 690,000 or about 38 percent of the population of New Mexico. Currently (2002), the source of water for municipal and domestic supply is almost exclusively from ground water.