The amount of precipitation falling on land is almost 110 000 km3 per year. Almost two-thirds of this amount evaporates from the ground or transpires from vegetation (forest, rangeland, cropland) (http://www.fao.org/nr/water/aquastat/water_use/index.stm). The remaining 40 000 km3 per year is converted to surface runoff (feeding rivers and lakes) and groundwater (feeding aquifers). These are called renewable freshwater resources. Part of this water is being removed from these rivers or aquifers by installing infrastructure. This removal of water is called water withdrawal. Most of the withdrawn water is returned to the environment some period of time later, after it has been used. The quality of the returned water may be less than the quality when it was originally removed.
At a global level, the withdrawal ratios are 70 percent agricultural, 11 percent municipal and 19 percent industrial. Irrigation water withdrawal (or water withdrawal for irrigation) largely exceeds irrigation water requirement due to significant losses in distribution and application. The demands for irrigation and better quality water are expected to grow in line with population growth, income growth and food consumption growth. Climate change may impact on supply. More efficient use of available water and improved infrastructure is required to meet the requirements of the next half century.