Water for people.pdf - WHO Thailand Digital Repository

2 0 6 / C H A L L E N G E S T O L I F E A N D W E L L - B E I N GSecuring Food for a Growing World PopulationFigure 8.5: Irrigated area as proportion of irrigation potential in developing countries% of irrigated area1009080706050403020100Sub-SaharanAfricaLatin AmericaNear EastIndiaThis figure shows that a vast share of the irrigation potential is already being used in sub-Saharan Africa and Latin America, as well as in Asia, but there remains a largepotential still untapped in the Near East and India.Source: FAO, 2002.Future investments in irrigationIn many developing countries, investments in irrigated infrastructureshave represented a significant share of the overall agricultural budgetduring the second half of the twentieth century. The unit cost ofirrigation development varies with countries and types of irrigatedinfrastructures, ranging typically from US$1,000 to US$10,000 perhectare, with extreme cases reaching US$25,000 per hectare (thesecosts do not include the cost of water storage as the cost of damconstruction varies on a case-by-case basis). The lowest investmentcosts in irrigation are in Asia, which has the bulk of irrigation and wherescale economies are possible. The most expensive irrigation schemesare found in sub-Saharan Africa, where irrigation systems are usuallysmaller and the development of land and water resources is costly.In the future, the estimates of expansion in land under irrigationwill represent an annual investment of about US$5 billion, but mostinvestment in irrigation — between US$10 and 12 billion per year — willcertainly come from the needed rehabilitation and modernization ofageing irrigated schemes built during the period 1960–80. In the1990s, annual investment in storage for irrigation was estimated atabout US$12 billion (WCD, 2000). In the future, the contrasting effectsof reduced demand for irrigation expansion and increased unit cost ofwater storage will result in an annual investment estimated betweenUS$4 and 7 billion over the next thirty years.Typically, investment figures in irrigation do not include the portionof investment provided by the farmer as land improvement and onfarmirrigation; this can represent up to 50 percent of the overallinvestment. In total, it is estimated that annual investment in irrigatedSouth AsiaChinaEast AsiaWorld1997/992030agriculture will therefore range between US$25 and 30 billion, about15 percent of annual expected investments in the water sector.Water use efficiencyAssessing the impact of irrigation on available water resources requiresan estimate of total abstraction for the purpose of irrigation fromrivers, lakes and aquifers. The volume extracted is considerably greaterthan the consumptive use for irrigation because of conveyance lossesfrom the withdrawal site to the plant root zone. Water use efficiency isan indicator often used to express the level of performance ofirrigation systems from the source to the crop: it is the ratio betweenestimated plant requirements and the actual water withdrawal.On average, it is estimated that overall water use efficiency ofirrigation in developing countries is about 38 percent. Map 8.2shows the importance of agriculture in the countries’ water balance,and figure 8.6 shows the expected growth in water abstraction forirrigation from 1999 to 2030. The predictions are based onassumptions about possible improvements in irrigation efficiency ineach region. These assumptions take into account that, from thefarmer’s perspective, wherever water is abundant and its cost low,the incentives to save water are limited. If the cost of water isirrelevant in the financial equation, it may be left flowing unless it iscausing erosion or waterlogging. Conversely, if farmers canprofitably irrigate more land using their allocation in an optimumway, irrigation efficiency may reach high levels.Improving irrigation efficiency is a slow and difficult process thatdepends in large part on the local water scarcity situation. It may