Review of global agricultural water use per country

Methodology

Renewable water resources available to irrigation and other uses are commonly defined as the part of precipitation which is not evaporated or transpired by plants, including grass and trees, which flows into rivers and lakes or infiltrates into aquifers. The annual water balance for a given area in natural conditions, i.e. without irrigation, can be defined as the sum of the annual precipitation and incoming flows (transfers through rivers from one area to another) minus evapotranspiration. Recently FAO compiled its best estimates of the main elements of the water balance for each country. The results of this exercise are presented in the section Review of water resource statistics by country.

To estimate the pressure of irrigation on the available water resources, an assessment has to be made both of irrigation water requirements and of water withdrawal for agriculture. Precipitation provides part of the water crops need to satisfy their transpiration requirements. The soil, acting as a buffer, stores part of the precipitation water and returns it to the crops in times of deficit. In humid climates, this mechanism is sufficient to ensure satisfactory growth in rainfed agriculture. In arid climates or during extended dry seasons, irrigation is necessary to compensate for the evaporation deficit due to insufficient or erratic precipitation. Net irrigation water requirements in irrigation are therefore defined as the volume of water needed to compensate for the deficit between potential evapotranspiration and effective precipitation over the growing period of the crop. It varies considerably with climatic conditions, seasons, crops and soil types. For a given month, the crop water balance can be expressed as follows:

IWR = Kc * ET0 - P - DS

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. In this study, irrigation water requirement has been computed for each country on the basis of the irrigated and harvested areas by crop for the reference year of 2000. The methodology is described in detail below and is presented schematically in Figure 1.

Figure 1: Schematic description of the method used to compute agricultural water use:

Water balance under natural conditions