The word "irrigation" refers to a large variety of activities. Many think of irrigation as a storage dam on a majestic river with off-take structures and a network of canals and drains. For others, it is a well equipped with a pump that is linked by water channels to nearby fields, or a tool to lift stream water to a field by hand, or stone weirs to trap seasonal water in a wadi, or bunds, sluices, and pumps to control water levels in a delta. These varieties of irrigation have little in common except that they all involve moving water to a place and time where it benefits crops. They all result in farming that uses more labor and produces more output per hectare than before.
This diversity makes it difficult to analyze irrigation. A study of Pakistan's Indus system, with its immense dams, thousands of kilometers of canals and drains, and millions of farms requires different categories than a study of a tubewell in Bangladesh that permits a handful of farmers to extend cropping in winter months, or a study of a several-hundred hectare Brazilian farm served by well, pump, and central pivot sprinkler system.
The engineering and agronomy are totally different; so are the underlying elements of social organization.
Irrigation is associated with “high levels of cropping intensity, and organizationally advanced civilizations." Initially, farming depends on rain and takes place where the water regime and other factors (such as temperature and soils) are favorable. Population growth then pushes farming into marginal areas, raising the marginal costs of expanding production. At some point, it pays to invest in irrigation to expand production instead of expanding the land frontier. Initially, irrigation occurs where the unit investment costs to increase production are lowest and then expands to more marginal, more expensive sites. The rising costs of developing ever more marginal sites for irrigation are compensated by the development of cost-saving techniques unique to irrigation. As the economic and agronomic milieu changes, so does the selection of appropriate irrigation structures and system.
Characteristically, irrigation started in the best farming areas. As population density rose, the costs of expanding the farming frontier became prohibitive. The Nile Valley is a good example; the drying of the Sahara desert forced people into the Nile flood plain. The river valleys of China, south India, the Indo-Gangetic Plain, Japan, Korea, Mesopotamia, and Sri Lanka offer less dramatic examples. The fertile islands of Indonesia (Bali, Java, and Madura) early reached high population densities; the sea then became a natural barrier to crop expansion. Sometimes, the losers of wars have established agricultural frontiers in their refuge areas to preserve their identity.
In the last 50 years, irrigation development has continued to be concentrated in areas of high population density and high population growth, most notably in parts of China, India, and certain other Asian countries. Where population density was lower but growth was high (and especially where there is enough rainfall for rainfed farming to be practical), agriculture expanded mainly by bringing new lands into cultivation. This was the case in most of subSaharan Africa, Latin America, “except the dry west coast, and pockets of Asia.
Yet this horizontal expansion was minor compared to the vertical expansion brought about by technological improvements. For the 1962-90 period, 8 percent of the increase in world cereal production is attributable to horizontal expansion and 92 percent to "vertical" increases produced by irrigation and other intensification measures. The same proportion applies for developing countries as a group. Even in the remaining areas with greatest land availability, subSaharan Africa and Latin America, "vertical" expansion accounted for 52 percent and 71 percent respectively of the increase.
The "vertical" intensification, mainly in wheat and rice production, that began in the late 1960s has been called the Green Revolution. This is not the place to recount the history or predict the future of the Green Revolution. Suffice it to say that plant-breeding innovations have gone hand-in-hand with increased use of fertilizers and the expansion of irrigation. New, high-yielding varieties outperform their predecessors over a wide range of conditions, but they outperform them by much more when the plants get nutrients and water where and when and in the amounts they prefer.
By 1970, with world farm prices still high, technological optimism which had risen to the point that development planners sometimes asserted that agriculture could be the leading growth sector had inspired increased agricultural investment. A large share of public agricultural investment, and a significant part of private investment, was in irrigation.
Private investors respond to market forces, while public-sector planners typically share the popular perception that agricultural intensification will take care of itself. World Bank lending to irrigation projects became significant in 1960, rose dramatically from 1973 to a peak in 1978, and has fallen since then. When agricultural prices started to rise and there was pessimism about food supplies, the Bank began to invest heavily in irrigation, a trend that intensified during the Green Revolution period. The continuous and substantial fall in world staple food prices since then has prompted the Bank to lessen its emphasis on irrigation. Whatever the likelihood of this future scenario may be, the stakes for the world's poor are high.
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