uses material that requires an average of 1,500 liters of water per day. This total includes the water that cools the turbines of power plants and the water that irrigates cotton fields, as well as the moisture that plumps the artichokes and provides the bulk of milk or muscatel. It is the fluid that showers bodies and washes cars. It drips from faucets and drowns the roots of suburban lawns. When it rains, this fluid runs in silken sheets along the slopes of parking lots and collects in open foundations at construction sites. It gets pumped into the sewer system and mixed with organic waste and then runs out—to someplace downstream. As the population expands, and as material expectations rise, the need for water increases at an exponential rate.

Over the past 300 years, the amount of water removed from freshwater reservoirs by humans has increased more than 35-fold. Most of the water used in North America does return to the hydrologic cycle after only a short interruption. The problem is that it frequently returns in a very different state and often to a very different reservoir. If pure water is taken from a river and returns to the river contaminated by fecal matter or toxic materials, it may take long reaches of that river out of the supply side. And if vast quantities of water are mined from a 100,000-year-old aquifer and run through an irrigation system, resulting in extensive evapotranspiration, the aquifer may not be replenished for another few hundred thousand years.

Because the hydrologic cycle transfers water from one reservoir to another at various rates and because it does not always transfer to the location most convenient for the schemes of civilized minds, humans must use available water resources very carefully. Water cannot be manufactured economically from its component elements and ocean water cannot be desalinated without incurring large, usually unacceptable, expenses. So humans must adapt to the natural limitations on available fresh water imposed by the hydrologic cycle.

In the United States the opportunity exists to deliberately formulate water distribution and wastewater treatment policies according to principles of conservation, wisdom, and justice. The United States uses 770 km3 of fresh water every year. About 340 km3 is consumed—exposed for evapotranspiration or consigned to uses sequestered from the hydrologic cycle. The rest is recycled into the system as wastewater. Irrigation uses 330 km3—41 percent of the total—and accounts for 215 km3 of the consumption. Domestic uses amount to 66 km3, consuming 20 km3; and industry uses 294 km3, but consumes only 29 km3.

Most of the river and lake water becomes available in the spring and early summer when the snow melts, ice breaks up, and rain falls to flush out—perhaps to flood—systems that run low in late summer. Mitigation of droughts and prevention of floods traditionally require water control projects. Management of water supplies—circulating 740 km3 through U.S. cities, suburbs, forests, and fields—necessitates planning that modifies seasonal supply to match independently fluctuating demands.

In the half of the United States west of the 100th meridian—which slices through Texas, Oklahoma, Kansas, Nebraska, South and North Dakota—stream runoff is less than 1 inch in an average year. Throughout that region runoff comes in early spring, largely as snowmelt from mountains hundreds of kilometers away. The water flows through the region months before the optimal time for watering crops. Storage of water—either in artificial surface reservoirs, lakes, or aquifers—dampens the lag in timing between supply and demand. But no reservoirs create water; they only allow a delay of the transfer within the hydrologic cycle. Artificial reservoirs and lakes can be refilled when the runoff returns. Underground aquifers—the groundwater—serve as vast and wonderful reservoirs; but they often cannot be refilled in the next season.

Several western states now depend on mining underground aquifers—taking water out faster than the rate of recharge. In the past, official policy toward groundwater use endorsed the idea of safe yield. Safe yield was a concept accepted by hydrologists as a maxim—an aquifer should not be pumped faster than it is naturally recharged. In the early 1960s this idea was replaced by one that treated underground water as a nonrenewable resource: depletion of groundwater is justifiable if it creates an economy that can afford to buy more expensive water when the well runs dry. Adoption of this maxim reflects the development of irrigated agriculture in the High Plains as well as population migration to the Sunbelt. Now, urban and energy developments are coveting the water available to agriculture—especially in the Southwest.

This competition will undoubtedly intensify, posing two major issues for society: how local, state, and regional communities can manage increased competition for water and to what extent the country can wean itself from irrigated agriculture in the West. The present domestic and industrial water requirements can be effectively met without serious impact on agriculture. Diverting 10 percent of current agricultural water consumption



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