Definition:
Annual quantity of water withdrawn for irrigation and livestock purposes. It includes renewable freshwater resources as well as potential over-abstraction of renewable groundwater or withdrawal of fossil groundwater, use of agricultural drainage water, desalinated water and treated wastewater. It includes water withdrawn for irrigation purposes and for livestock watering, although depending on the country this last category sometimes is included in municipal water withdrawal. As far as the water withdrawn for irrigation is concerned, the value far exceeds the consumptive use of irrigation because of water lost in its distribution from its source to the crops. The term «water requirement ratio» (sometimes also called «irrigation efficiency») is used to indicate the ratio between the net irrigation water requirements or crop water requirements, which is the volume of water needed to compensate for the deficit between potential evapotranspiration and effective precipitation over the growing period of the crop, and the amount of water withdrawn for irrigation including the losses. In the specific case of paddy rice irrigation, additional water is needed for flooding to facilitate land preparation and for plant protection. In that case, irrigation water requirements are the sum of rainfall deficit and the water needed to flood paddy fields. At scheme level, water requirement ratio values can vary from less than 20% to more than 95%. As far as livestock watering is concerned the ratio between net consumptive use and water withdrawn is estimated between 60% and 90%.
Source:
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Food and Agriculture Organization of the United Nations, AQUASTAT Web Site