Lester R. Brown

Chapter 7. Raising Water Productivity: Raising Irrigation Water Productivity

Historically, farm productivity was measured in yield per hectare, since land was the constraining resource. But as the twenty-first century begins, policymakers are beginning to look at water as the limiting factor for food production. The common measure that is emerging to measure water productivity is kilograms of grain produced per ton of water. 

Since 1950, world irrigated area has nearly tripled. With this growth and with grain yields on irrigated land roughly double those on rainfed land, irrigated land now accounts for easily 40 percent of the world grain harvest. For China and India it is even higher. Four fifths of China's grain harvest and close to three fifths of India's comes from irrigated land. In the United States, one fifth of the grain harvest comes from irrigated land.12 

The relative contributions of surface water and groundwater irrigation vary widely among countries. Of China's 51 million hectares of irrigated land, 42 million depend on surface water and 9 million on underground water. For India, the breakdown is 44 million hectares and 42 million hectares, respectively, making groundwater even more important to India.13 

Although China has only 9 million hectares of land irrigated with groundwater, this land is disproportionately productive simply because groundwater is available precisely when the farmer needs it. By contrast, surface water is usually delivered by canal to farmers in local groups, usually on a rotational basis. This timing may or may not coincide with a farmer's needs. 

Although there are many ways of raising irrigation water productivity, a few stand out. For those using surface water irrigation, reducing seepage from the canals used to carry water from large reservoirs to farms cuts water use. It is not unusual, particularly where distances are long, for water seepage losses to reach 20-30 percent. This water can be saved if canals are lined with plastic sheeting or concrete—a more costly but more long-term solution.14 

A second approach is to use a more efficient technology, such as overhead sprinkler systems. Commonly used with center-pivot irrigation systems, their weakness is that some water is lost to evaporation even before it hits the ground, especially in hot, arid settings. Low-pressure sprinklers, which release water at a lower level, close to the soil surface, lose less water through evaporation and drift. These are now widely used in the Texas panhandle of the United States, where aquifer depletion is encouraging farmers to use water much more efficiently.15 

The gold standard for efficiency is drip irrigation, a method that supplies water directly to the root zone of plants. In addition to cutting water use by up to half, drip irrigation also raises yields because it offers a constant, carefully controlled supply of water. Israel, where water shortages are acute, is the world leader in developing drip technology. It is also now widely used in other countries, including Jordan and Tunisia.16 

In Jordan, for example, drip irrigation reduced water use an average of 35 percent. Crops such as tomatoes and cucumbers typically yielded 15 percent more. The combination of reduced water use and higher yields raised water productivity by more than half. Tunisia, where drip-irrigated area expanded from 2,000 hectares in 1987 to 36,000 hectares in 1999, has realized similar gains.17 

India in 1998 was irrigating 225,000 hectares with drip irrigation. Thirteen experiments at Indian research institutes on several different crops showed gains in water productivity ranging from a low of 46 percent to a high of 280 percent. (See Table 7-1.) On average, water productivity was raised by 152 percent, more than doubling.18 

Drip irrigation may be permanent—that is, with water delivered through pipes installed underground, as is often done for orchards, for example—or flexible, consisting of rubber hose or plastic tubing. The latter typically is moved by hand every hour or so across the field and is thus a labor-intensive system of irrigation. The traditionally high costs of both materials and labor used for drip irrigation are now dropping as new techniques and more flexible materials, including plastic tubing or pipe, become available. With these recent advances, the cost of drip irrigation systems has dropped from $1,200-2,500 per hectare to $425-625. Where water is costly, this is a financially attractive investment. And for countries where unemployment is high and water is scarce, the technology is ideal when it substitutes abundant labor for scarce water.19 

In recent years, the tiniest small-scale drip-irrigation systems—the size of a bucket—have been developed to irrigate a small vegetable garden with roughly 100 plants (25 square meters). Somewhat larger drum systems irrigate 125 square meters. In both cases, the containers are elevated slightly, so that gravity distributes the water. Small drip systems using plastic lines that can easily be moved are also becoming popular. These simple systems can pay for themselves in one year. By simultaneously reducing water costs and increasing yields, they can dramatically raise incomes of smallholders.20 

Sandra Postel believes that the combination of these drip technologies at various scales has the potential to profitably irrigate 10 million hectares of India's cropland, or nearly one tenth of the total. She sees a similar potential for China, which is now also expanding its drip irrigation area to save scarce water.21 

Another technique for raising water use efficiency in both flood- and furrow-irrigated fields is laser leveling of the land, a precise leveling that can reduce water use by 20 percent and increase crop yields by up to 30 percent, boosting water efficiency by half. This practice is widely used for field crops in the United States and for rice production in a number of countries.22

Raising crop yields is an often overlooked way of raising water productivity. In Zhanghe Reservoir in the Yangtze River basin, where water was becoming scarce, farmers had to share with urban and industrial users. As a result, they simultaneously reduced water use by using more-efficient irrigation practices and raised rice yields from 4 tons per hectare a year on average in 1966-78 to 7.8 tons per hectare in 1989-98. The combination of lower water use and higher crop yields almost quadrupled water productivity, raising it from 0.65 kilograms of rice per ton of water to 2.4 kilograms.23 

A comparison of wheat yields between countries also shows how higher crop yields boost water productivity. In California, where irrigated wheat produces some 6 tons per hectare, farmers produce 1.3 kilograms of wheat per ton of water used. But in Pakistan's Punjab, irrigated wheat yields averaged only 2 tons per hectare or 0.5 kilograms per ton of water—less than 40 percent the water productivity in California.24 

Yet another way of raising water productivity is to shift to more water-efficient grains, such as from rice to wheat. The municipal government of Beijing, concerned about acute water shortages, has decreed that production of rice, a water-thirsty crop, should be phased out in the region surrounding the city. Instead of planting the current 23,300 hectares of rice, farmers will shift to other, less water-demanding crops by 2007. Egypt, facing an essentially fixed water supply, also restricts rice production.25 

The economic efficiency of water use can also be raised by shifting to higher-value crops, a move that is often market-driven. As water tables fall and pumping becomes more costly, farmers in northern China are switching from wheat to higher-value crops simply because it is the only way they can survive economically.26 

Institutional shifts, specifically moving the responsibility for managing irrigation systems from government agencies to local water users' associations, can facilitate the more efficient use of water. Farmers in many countries are organizing locally so they can assume this responsibility. Since local people have an economic stake in good water management, they typically do a better job than a distant government agency. In some countries, membership includes representatives of municipal governments and other users in addition to farmers.27 

Mexico is a leader in this movement. As of 2002, more than 80 percent of Mexico's publicly irrigated land was managed by farmers' associations. One advantage of this shift for the government is that the cost of maintaining the irrigation system is assumed locally, reducing the drain on the treasury. This also means that associations need to charge more for irrigation water. Even so, for farmers the advantages of managing their water supply more than outweigh this additional expenditure.28 

In Tunisia, where water users' associations manage both irrigation and residential water, the number of associations increased from 340 in 1987 to 2,575 in 1999. Many other countries now have such bodies managing their water resources. Although the early groups were organized to deal with large publicly developed irrigation systems, some recent ones have been formed to manage local groundwater irrigation as well. They assume responsibility for stabilizing the water table, thus avoiding aquifer depletion and the economic disruption that it brings to the community.29 

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