Maintenance of the instream flow is becoming an important constraint on water availability as the sensitivity of individual ecosystems is recognized. A familiar example is the case of Mono Lake in eastern California. Mono Lake is the last puddle left of an inland sea that has occupied the basin for more than 1 million years. During the most recent glaciation 12,000 years ago, the lake reached depths of 250 m and extended over 870 km2. In the course of postglacial warming, the lake level fell, but it has managed to maintain a rich variety of biotic systems thriving in its salty waters. In 1941 the city of Los Angeles—540 km to the south—began to divert the water that feeds Mono Lake. The lake's area decreased nearly 25 percent and its surface fell almost 20 m. If the trend continues, there will be no ecosystem left in the lake: its instream flow requirement will not be met, and the lake will be too saline and too alkaline to support any life. A 1991 court decision has forced Los Angeles to stop diverting much of the water that normally would flow to Mono Lake, which may be sufficient to save the lake. If not, it will share the fate of Owens Lake, which once glistened 150 km to the south. All Los Angeles has left is the lake bed—250 km2 of alkaline salt flats.
The withdrawals from Owens and Mono lakes are only a fraction of the water exported to urban centers. In 1990 approximately 50 percent of the U.S. population depended on groundwater for domestic use. Efforts to eliminate the overdrafts of groundwater in Arizona have led to increasing dependence on surface supplies from remote locations. The Central Arizona Project provides surface water to the Phoenix area from the Colorado River, over 200 km to the west. Arizona has also implemented a new law to eliminate groundwater mining by the year 2020.
California has eliminated relative depletion in the Central Valley by increasing surface water supplies. Southern California continues to import 6.0 billion m3 of water every year. The city of Los Angeles alone extends its water delivery system over hundreds of miles throughout the Southwest and draws the power for the system from projects reaching across six states. It has been involved in disputes over water with the state of Arizona and most recently with Mexico. But in California the cities use only 15 percent of all the water consumed. The rest goes to irrigation. In 1990, 16 percent of the state's water supply irrigated fields of alfalfa, which is used to feed horses and cattle. That adds up to more water than is needed to supply 30 million people for a year. Difficult choices lie ahead for urban and rural Californians as droughts continue.
The southern High Plains represent an area of the United States that will return, sooner or later, to dry-land farming. The transition will come sooner and with fewer ecological and economic crises if the agricultural industry is weaned gradually from its dependence on groundwater irrigation. If nothing is done until all the accessible water in the Ogallala aquifer has been removed, the transition will be ecologically dangerous and economically dreadful. Approximately two-thirds of this water is in Nebraska, which has an enormous reserve of groundwater and is nearest to the most productive recharge area. In Texas and New Mexico more than 10 percent of the water initially in storage has been depleted. However, the depletion statistics for an aquifer such as the Ogallala are deceptively optimistic simply because it has not been economically feasible to remove all the water in underground storage because of the high cost of pumping from great depths. It is estimated that 50 to 60 percent of the water might be removed under favorable economic conditions. Some areas in Texas and New Mexico have already reached the point at which irrigation from the Ogallala aquifer is no longer practical. In such areas of severe groundwater depletion, episodic surface runoff might serve for aquifer recharge if appropriate capture strategies were devised.
Humanity plays an integral role in the hydrologic cycle. While consuming less than 3 percent of the rainfall and returning large portions of that small percentage to the system, humans still profoundly disrupt the natural cycle. Human influence on our water resources is disproportionate to actual use because of the tendency to concentrate efforts in areas where the natural water supply is sparse. The earliest irrigation was practiced in the deserts of the Middle East and was characterized by the same aims as today: to redistribute water geographically and seasonally—in space and time. Mesopotamians built canals and dams that would divert and delay the water flowing through the Tigris and Euphrates rivers. Early Egyptians worked to maintain the ponds and sluices that would trap the flood waters of the Nile and its rich silt. Both the water and the silt would be redistributed over fields at the proper time.
Water may have been a truly renewable resource in early history. Although monsoons and even the annual Nile flood might occasionally fail to take place, they always returned in a year or in seven.