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Chapter 7. Feeding Everyone Well: Raising Water Productivity
Over the last half-century, world irrigated area tripled, climbing from 90 million hectares in 1950 to nearly 270 million in 2000. Most of the growth occurred from 1950 to 1978, when irrigation expanded faster than population and boosted irrigated land per person from 0.037 hectares to 0.047 hectares, an increase of one fourth. After 1978, however, the growth in irrigation slowed, falling behind that of population and shrinking the irrigated land per person 8 percent. (See Figure 7-2.)28
In the years immediately ahead, the combination of aquifer depletion and the diversion of irrigation water to nonfarm uses may end the historical growth in irrigated area. If so, it will be more difficult to feed 3 billion more people.
In many countries, the competition for water between the countryside and cities is intensifying, underlining the value of raising water productivity. Although projections of the future diversion of irrigation water to residential and industrial uses do not exist for most countries, a World Bank forecast for South Korea—a relatively well watered country—gives some sense of what may lie ahead. Like many countries, Korea is now using virtually all available water. The Bank calculates that if the Korean economy grows 5.5 percent annually until 2025, growth in water withdrawals for residential and industrial use will reduce the yearly supply remaining for irrigation from 13 billion to 7 billion tons. Rising water prices and associated gains in water productivity will likely ameliorate the loss of water for irrigation, but this analysis nonetheless shows how difficult it may be for some countries even to maintain existing irrigated area.29
Farmers everywhere face an uphill battle in the competition for water since the economics of water use do not favor agriculture. Industry can often pay 50 to 100 times as much for water as farmers do. Wherever economic growth and the creation of jobs are a central preoccupation of political leaders, scarce water will likely go to industry.30
In addition, countries that are overpumping, including key food-producing ones such as China, India, and the United States, will lose irrigation water as aquifers are depleted. Once the rising demand for water surpasses the sustainable yield of an aquifer, the gap between demand and sustainable yield widens each year. As it does, the annual drop in the water table also increases each year, accelerating depletion of the aquifer and setting the stage for an abrupt fall in the food supply.31
The need for water in the Indian subcontinent is already outrunning the supply. Water tables are falling in much of India, including the Punjab, the country's breadbasket. (See Chapter 2.) The excessive use of water is encouraged by heavy electricity subsidies to farmers, who use electric pumps for irrigation.32
In sub-Saharan Africa, the potential for irrigation is limited simply because so much of the continent is arid or semiarid. The greater promise here may lie in water harvesting and systematically building soil organic matter so that soils can absorb and retain more of the low rainfall. The construction of earthen terraces supported by rocks retains water and reduces soil erosion. Leguminous trees planted as windbreaks reduce wind erosion and add nitrogen and organic matter to the soil.
The world water situation today is similar to that with cropland at the middle of the last century: the opportunities for developing new supplies are fast disappearing. By 1950, the frontiers of agricultural settlement had largely vanished, leaving little productive new land to plow. In response, governments launched a broad-based effort to raise land productivity, one that included price supports for farm commodities that encouraged farmers to invest in yield-raising inputs and land improvements, heavy public investment in agricultural research to raise crop yields, and the building of public institutions to support this effort—from agricultural extension services to farm credit banks. Societies mobilized a wide array of resources that doubled land productivity between 1950 and 1984.
The doubling of grainland productivity in little more than a generation is one of the remarkable scientific feats of the modern age. As the new century begins, a similar broad-based effort is needed to raise water productivity. There are several avenues to raising water productivity, but the key is pricing water at closer to market value, a step that leads to systemic advances in efficiency. China, facing acute water shortages, has recently announced a plan to raise water prices each year over the next five years. The attraction of market pricing is that it is systemic, promoting more-rational water use throughout the economy.
With 70 percent of the water that is diverted from rivers or pumped from underground being used for irrigation, any gains in irrigation water efficiency have benefits that extend far beyond agriculture. Indeed, getting enough water for cities and industry while maintaining food production may be possible only if irrigation productivity is systematically raised worldwide.33
The use of more water-efficient irrigation practices is the key. There are many ways to irrigate crops, including furrow, flood, overhead sprinkler, and drip irrigation. Furrow irrigation, probably the earliest form, is used with row crops, with a small trench being cut near each row of plants. Flood irrigation, traditionally used on rice, is now being reconsidered since recent research indicates that at least in some situations periodic flooding will produce the same yield as continuous flooding, but use much less water.34
Overhead sprinkler irrigation, which is widely used in the U.S. southern Great Plains, is often coupled with the use of underground water. The circles of green crops that can be seen when flying over this region during the summer are created with water from center-pivot overhead sprinklers that use well water to irrigate. (In this region, most of the water is drawn from the Ogallala aquifer—essentially a fossil aquifer since its recharge is limited.) Shifting from a high-pressure to a low-pressure overhead sprinkler system can boost irrigation efficiency from 65 percent to 80 percent. Shifting to a low-energy precision application sprinkler system can raise it to 90 percent or better.35
Drip irrigation technology, pioneered in Israel, is the most efficient of all irrigation systems. It typically uses a plastic hose with small holes or emitters, which either rests on the soil surface or is installed several inches below it. Sandra Postel and her colleagues report that studies in several countries show drip irrigation reducing water use by 30-70 percent. And because it provides a steady supply of water carefully geared to crop needs, it raises yields by 20-90 percent. The combination of reduced water use and higher yields can easily double water productivity, an attractive prospect.36
In the past, this high-cost, labor-intensive form of irrigation was used only on high-value crops such as fruits and vegetables. But this is now changing. New low-cost drip irrigation systems designed specifically for small farms, typically with a payback period of one year, are opening broad new horizons for expansion. Because they are more labor-intensive, these drip systems are well adapted to small holdings where labor is more plentiful. Postel reports that India has an estimated 10 million hectares that can profitably be irrigated with drip systems. There may be a similar potential in China.37
Another way to raise water productivity is to shift to more water-efficient crops. For example, wheat typically produces half again as much grain per unit of water as rice does. This is why Egypt restricts rice planting in favor of wheat.38
As a general matter, the higher the yield of a crop, the more productive the water use. For example, a rice crop that yields four tons per hectare uses little more water than one that yields two tons per hectare simply because so much of the water used to produce rice is lost through evaporation from the water surface. Simply put, raising land productivity also raises water productivity.
28. Figure 7-2 is an Earth Policy Institute estimate based on FAO, FAOSTAT, op. cit. note 14, and on USDA, Agricultural Resources and Environmental Indicators (Washington, DC: 1996-97).
29. Gershon Feder and Andrew Keck, Increasing Competition for Land and Water Resources: A Global Perspective (Washington, DC: World Bank, March 1995), pp. 28-29.
30. Estimates of water prices and priorities based on ratio of 1,000 tons of water for 1 ton of grain from FAO, Yield Response to Water (Rome: 1979), on global wheat prices from IMF, International Financial Statistics (Washington, DC: various years), and on industrial water intensity in Mark W. Rosegrant, Claudia Ringler, and Roberta V. Gerpacio, "Water and Land Resources and Global Food Supply," paper prepared for the 23rd International Conference of Agricultural Economists on Food Security, Diversification, and Resource Management: Refocusing the Role of Agriculture?, Sacramento, CA, 10-16 August 1997.
31. Postel, op. cit. note 11, pp. 56-57, 252.
32. Sandra Postel, Last Oasis, rev. ed. (New York: W.W. Norton & Company, 1997), p. 170.
33. Figure of 70 percent calculated from I.A. Shiklomanov, "World Fresh Water Resources," in Peter H. Gleick, ed., Water in Crisis: A Guide to the World's Fresh Water Resources (New York: Oxford University Press, 1993).
34. Postel, op. cit. note 11.
35. Postel, op. cit. note 32, p. 102.
36. Sandra Postel et al., "Drip Irrigation for Small Farmers: A New Initiative to Alleviate Hunger and Poverty," Water International, March 2001, pp. 3-13.
38. Water efficiency of wheat and rice production from Postel, op. cit. note 32, p. 71.
Copyright © 2001 Earth Policy Institute