GLOBAL CROPLANDS AND THEIR WATER USE: REMOTE SENSING AND NON-REMOTE SENSING APPROACHES

Global croplands use overwhelming proportion (60-90 percent) of the water used by humans. Much of the water is used for food production, making global croplands and their water use important to to understand and manage global food security. There are two types of cropland water use: (a) green water use (from moisture in unsaturated soil zone) by rainfed croplands; and (b) blue water use (from rivers, reservoirs, lakes, and saturated zone or ground water aquifers) by irrigated croplands. Irrigated cropland water use should also account for precipitation (green water) falling directly over irrigated croplands. However, alternative demands for water use are increasing steeply from urbanization, industrialization, environmental flow requirements, ecosystem services, and recreational needs. In this paper, we first discuss the four major cropland area maps and statistics at the global level. The total global cropland areas estimated by these four studies range between 1.3-1.54 billion hectares for the nominal year 2000. Second, we assess global crop water use which varied between 6,685 to 7,500 km³ yr^-1; of this about 70% by rainfed croplands (green water use) and the rest 30 percent by irrigated croplands (blue water use). However, irrigated croplands use blue water (water in rivers, reservoirs, lakes, and pumped ground water from the saturated zone). Nearly 80 percent of all blue water currently used by humans goes for growing food in irrigated croplands. This highlights the need for continued focus on irrigated croplands and their water use for enhancing global food security. Uncertainties in global and regional cropland areas, their water use, and the precise geographic location of these parameters are quite high at present. The issues pertaining to this will be discussed.