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Chapter 7. Raising Water Productivity: Rainwater Harvesting
For many countries, particularly those with monsoonal climates and long dry seasons, water shortages result not from a lack of rainfall but from a seasonally uneven supply. When annual rainfall is concentrated in a few months, storage is difficult. To illustrate, India has 2.1 trillion cubic meters of fresh water available each year, and the United States has 2.5 trillion cubic meters. While rain falls in the United States throughout the year, in India—which is geographically only one third as large—most of the rainfall comes between mid-June and mid-September. As a result, most of this deluge runs off and is quickly carried back to the sea by the country's rivers. Although there are thousands of dams in India, they can collectively store only a fraction of the rainfall.30
The focus on building large dams to capture and store surface water before it runs off dominated most of the last century. But because sites were becoming scarce and because the construction of large dams often inundates large areas, displacing local populations and irreversibly altering local ecosystems, this era has now largely run its course. More and more countries are turning to local water harvesting to ensure adequate supply.
In India, Rajendhra Singh is a leader of this movement. Some 20 years ago, when he was visiting semiarid Rajasthan province, he realized that water shortages were constraining development, preventing people from escaping poverty. As he surveyed the area and talked with villagers, he saw that local earthen dams to collect and store rainwater would help satisfy the need for water, both for residential use and for irrigation.31
Singh began working with the villagers, helping them design local water storage facilities. Once villagers helped select a site, they would organize to build an earthen dam. All the materials, the stone and the earth, were local. So, too, was the labor—sweat equity provided by the villagers. Singh would help with the engineering and design. He told villagers that in addition to meeting their daily needs for water, the seepage from the small reservoir would gradually raise the water table, restoring wells that had been abandoned. He also told them this would take time. It worked exactly as he said it would.32
Singh's initial success led him to create a local nongovernmental organization with 45 full-time employees and 230 part-timers. Funded by the Ford Foundation and other groups, it has not only helped build 4,500 local water storage structures in Rajasthan, it has also raised villagers' incomes and improved their lives.33
When the local topography is favorable for building successful small water storage structures, this can be a boon for local communities. This approach works not only in monsoonal climates, but also in arid regions where low rainfall is retained for local use. With a modest amount of engineering guidance, hundreds of thousands of communities worldwide can build water storage works.
Another technique to retain rainfall is the construction of ridge terraces on hillsides to trap rainfall near where it falls, letting it soak into the soil rather than run off. Using a plow to establish the ridges, local farmers can build these terraces on their own, but they are more successful if they are guided by a surveyor who helps establish the ridgelines and determines how far apart the ridges or terraces should be on the hill. Once the terraces are established, the moisture that accumulates behind them can help support vegetation, including trees that can both stabilize the ridges and produce fruit and nuts or fuelwood. The terraces, which are particularly well adapted to the hilly agricultural regions of semiarid Africa, can markedly raise land productivity because they conserve both water and soil.
The water storage capacity of aquifers can also be exploited. In some ways, they are preferable to dams because water underground does not evaporate. As indicated, percolation from locally constructed water storage facilities often helps recharge aquifers. Similarly, land that is covered with vegetation retains rainfall, reducing runoff and enabling water to percolate downward and recharge aquifers. Without vegetative cover, rainfall runs off immediately, simultaneously causing flooding and reducing aquifer recharge, thus contributing to water shortages. In effect, floods and water shortages are often opposite sides of the same coin. Reforestation, particularly in the upper reaches of a watershed, not only helps recharge aquifers but also conserves soil that if washed away might end up behind dams downstream, reducing the storage capacity of reservoirs.
In summary, water harvesting and local water storage behind dams and in aquifers expands the supply and strengthens the local economy. These same initiatives also help conserve soil, since any action that reduces runoff reduces soil erosion. The net effect is conservation of both water and soil: a classic win-win situation.
30. Gardner-Outlaw and Engelman, op. cit. note 7, pp. 14-18.
31. Fen Montaigne, "Water Pressure," National Geographic, September 2002, pp. 2-34.
Copyright © 2003 Earth Policy Institute