Most irrigation systems waste water. Typically, only between 35% and 50% of water withdrawn for irrigated agriculture ever reaches the crops. Most soaks into unlined canals, leaks out of pipes, or evaporates before reaching the fields (139). Although some of the water "lost" in inefficient irrigation systems returns to streams or aquifers, where it can be tapped again, water quality is invariably degraded by pesticides, fertilizers, and salts that run off the land. Poorly planned and poorly built irrigation systems have limited the yields on one-half of all irrigated land (185).

Paradoxically, even when sufficient irrigation water reaches agricultural fields, it can spoil much of the land unless drained properly. Particularly in many arid areas, salts that occur naturally in the soil must be drained away with irrigation runoff. If left to accumulate in the soil, they eventually work their way to the surface, poisoning the land (102). Also, poorly drained irrigation water can raise the groundwater table until it reaches the root zone, waterlogging and drowning crops. Globally, some 80 million hectares of farmland have been degraded by a combination of salinization and waterlogging (185).

Improving irrigation efficiency. A number of countries are working to improve irrigation efficiencies, thus saving water and protecting the land. Drip irrigation is one technique. Drip irrigation consists of a network of porous or perforated piping, usually installed on the surface or below ground, which delivers water directly to the root zones of the crops. This technique keeps evaporation losses low, at an efficiency rate of 95%. Drip irrigation systems cut water use by an estimated 40% to 60% compared with gravity systems (139, 142).

In the 1970s drip irrigation systems were used on only 56,000 hectares worldwide, mostly to supply water for vegetables and fruit orchards in Australia, Israel, Mexico, New Zealand, South Africa, and the US. By 1991 this figure had grown to 1.6 million hectares. Although this area constitutes less than 1% of all irrigated land worldwide, drip irrigation is widely used in some countries. Israel, for instance, uses drip irrigation on 50% of its total irrigated area (142).

Another promising conservation method—low-energy precision application (LEPA)—offers substantial improvements over conventional sprinkler systems that spray water into the air. Instead, the LEPA method delivers water to the crops from drop tubes that extend from the sprinkler's arm. When applied together with appropriate water-saving farming techniques, this method also can achieve efficiencies as high as 95%. Since this method operates at low pressure, energy costs drop by 20% to 50% compared with conventional systems. Farmers in Texas who have retrofitted conventional sprinkler systems with LEPA have reported that their yields have increased by as much as 20% and that their investment costs have been recouped within one or two years (139, 142).

Irrigation options for the rural poor. Many developing countries cannot afford to invest in such techniques as drip irrigation and LEPA. Yet pressures to feed rapidly growing rural populations require making better use of scarce freshwater resources. In many regions that face acute seasonal water shortages, conservation projects are working with rural farmers to build small reservoirs that collect and store water from the rainy season for use in the dry season (35).

Also, in parts of East Africa subsistence farmers use "water harvesting"—ancient techniques that consist mainly of digging deep holes near each plant to collect and store water from the wet season for use during the dry season (35, 186). Another traditional method involves placing long lines of stones along the contours of gently sloping ground to slow runoff and spread the water across a wider area. Developed in the Yatenga region of Burkina Faso, this method is now being used on over 8,000 hectares in 400 villages throughout the country. It is also used in Kenya and Niger. Combined with the practice of deep-hole water harvesting, this practice has increased crop production by about 50% (30, 35).

Reusing urban wastewater. A number of countries channel treated urban wastewater from towns and cities onto nearby farms growing vegetables and fruits. Today at least half a million hectares in 15 countries are being irrigated with treated urban wastewater, often referred to as "brown water." Israel has the most ambitious brown-water program of any country. Most of Israel's sewage is purified and reused to irrigate 20,000 hectares of farm land (139).

Some developing countries also use this technique (177):

• In Mexico City treated urban wastewater irrigates and fertilizes alfalfa fields. The alfalfa in turn is sold as feed to small-scale farmers who raise guinea pigs and rabbits.
• One-third of the vegetables grown in Asmara, Eritrea, are irrigated with treated urban wastewater.
• In Lusaka, Zambia, one of the city's biggest squatter settlements irrigates its vegetable crops with sewage water from nearby settling ponds.

Such natural water treatment technologies as using wetlands often can be an alternative to modern water treatment systems that are too costly for poor urban areas of developing countries. Recycling waste for agricultural purposes by using oxidation ponds and aerated lagoons does not require as much land as is often assumed. Moreover, it decreases pollution, reduces the need for fertilizers, and often can be accomplished with "small-scale, low-cost technology that is based in local traditions, decentralized, and ecologically sound," according to water resource engineer Janus Niemczynowicz (124).