Irrigation generally makes inefficient use of water. Traditional irrigation methods, which have been practiced for more than 5,000 years, involve flood­ing the [and or diverting water to fields through open channels. Water flow must be increased in order to guarantee that the far end of the field or higher elevations of the field receive water. Less than 50 percent of the water applied to the soil by such methods is absorbed by plants; the rest usually evaporates into the atmosphere.

One of the most important innovations in agri­cultural water conservation is micro irrigation, also called drip or trickle irrigation, in which pipes with tiny holes bored in them convey water directly to individual plants. This reduces the water needed to irrigate crops by a substantial amount, usually 40 to 60 percent. Micro irrigation also reduces the amount of salt left in the soil by irrigation water.

Another important water-saving measure in irrigation is the use of lasers to level fields. As a laser beam sweeps across a field, a field grader re­ceives the beam and scrapes the soil, leveling it. Because farmers must use extra water to ensure that plants on higher elevations of a no level field re­ceive enough, laser leveling of the field reduces the water required for irrigation.

The use of sound water management principles in agriculture reduces water consumption. Tradi­tionally, farmers have been allotted specific amounts of water at specific times, with a "use it or lose it" philosophy. This approach encourages waste. If, instead, water needs are carefully moni­tored {often through computer controls), water can be applied in small, regulated quantities, thereby reducing overall consumption.

Although advances in irrigation technology are improving the efficiency of water use, many chal­lenges remain. For one thing, sophisticated irriga­tion techniques are prohibitively expensive. Few farmers in developed countries, let alone subsistence farmers in developing nations, can afford to stall them.

Reducing Water Waste in Industry

Electric power generators and many industries re­quire water in order Co function (recall that power plants heat water to form steam, which turns the turbines). In the United States, five major indus­tries consume almost 90 percent of industrial water (not: including water used for cooling purposes): chemical products, paper and pulp, petroleum and coal, primary metals, and food processing.

Stricter pollution control laws in many coun­tries provide some incentive for industries to con­serve water. Industries usually reduce their water use, and therefore their water treatment costs, by recycling water. The National Steel Corporation plant in Granite City, Illinois, for example, recycles approximately two-thirds of the 62 million gallons of water it uses daily; the used water is cleaned up before being discharged into a lake that spilt into the Mississippi River.

It is likely that water scarcity, in addition to more stringent pollution control requirements, will encourage further industrial recycling.

Reducing Municipal Water Waste

Like industries, regions and cities can reduce their water consumption by recycling or reusing water before it is discharged. For example, individual homes and buildings can be modified to collect and store "gray water"—water that has already been used in sinks and showers. The "gray water" can then be reused to flush toilers, wash the car, or sprinkle the lawn.

Israel probably has the world's most highly de­veloped system of treating and reusing municipal wastewater. Israel does this out of necessity, be­cause all of its possible fresh water sources have al­ready been tapped. The reclaimed water is used for irrigation, which allows higher-quality fresh water to be channeled to cities. Used water contains pollutants, but most of these are nutrients from treated sewage and are therefore beneficial to crops.

Automated systems to purify and recycle waste-water have been developed and are cost-competi­tive with fresh water. In Tokyo, for example, the

wastewater in the Mitsubishi office building; is puri­fied and recycled.

In addition to recycling and reuse, cities can decrease water consumption through other conser­vation measures, including consumer education, the use of water-saving household fixtures, and the development of economic incentives to save (sec You Can Make a Difference: Conserving Water). These measures have been used successfully to pull cities through dry spells; they are effective because individuals are wilting to con­serve for the common good during water crisis peri­ods.

Increasingly, however, cities are examining ways to encourage individual water conservation methods all the time. The installation of water meters in residences in Boulder, Colorado, reduced water consumption by one-third. Before the instal­lation, homeowners were charged a flat fee, regard­less of their water use. In addition to installing water meters, a city might encourage water conser­vation by offering a rebate to any homeowner who installs a conserving device such as a water-saving); toilet. The city of Tucson, Arizona, for example, has broken its citizens' trend toward increased water consumption and now needs 25 percent less water per person than it did in the 1970s. In and towns such as Tucson, simply replacing grass lawns with native desert plants substantially reduce,* water use by making lawn irrigation unnecessary.