An estimated 270 million hectares of irrigated land-about 18 percent of the world ' s cropland-generates about 40 percent of the world ' s food production. Among regions, there are great disparities in the distribution of irrigated land and its contribution to food security. Around 65 percent of the world ' s irrigated lands are in Asia, while Africa and South America have less than 5 percent each. Compared to other water uses, irrigation is a high volume, low quality, low cost use. Given the large demands placed on water resources by irrigation, the extent of irrigation development has major implications for other water uses, including water needs for cities, industries, and hydropower, as well as for national parks, wetlands, instream uses, and estuaries. Globally, about 150 to 200 million hectares are drained, including 100 to 150 million hectares of rainfed land and approximately 50 million hectares or irrigated land. This land contributes 10 to 15 percent to global food production (Smedema 2002). Drainage developments generally are intended to improve agricultural productivity, but can have significant environmental effects, including both benefits-such as control of salinization and waterlogging-and-costs-such as reduction in water quality downstream uses.
