Review of global agricultural water use per country

Water balance under natural conditions

The estimation of the water balance for an average year is based on three digital geo-referenced data sets for precipitation (Leemans and Cramer, 1991), reference evapotranspiration (Fischer et al., 2000) and soil moisture storage properties (FAO, 1995b). The computation of water balance is carried out by a model with a 10 km spatial resolution of grid-cells and in monthly time steps. The results consist of annual values by grid-cell for actual evapotranspiration, runoff and water stored as soil moisture.

For each grid cell, actual evapotranspiration (Eta) is assumed to be equal to the reference evapotranspiration (Eto), calculated for each cell with the Penman-Monteith method (FAO, 1998; New et al., 1999) in those periods of the year when precipitation exceeds reference evapotranspiration or when there is enough water stored in the soil to allow maximum evapotranspiration. In drier periods of the year, lack of water reduces actual evapotranspiration to an extent depending on the available soil moisture. Evapotranspiration in open water areas and wetlands is considered to be equal to reference evapotranspiration throughout the whole calculation period.

For each grid cell, runoff is calculated as the part of the precipitation that does not evaporate and cannot be stored in the soil. Runoff is always positive except for areas identified as open water or wetland, where actual evapotranspiration can exceed precipitation.

The method is calibrated by comparing calculated values for water resources per country (i.e. the difference between precipitation and actual evapotranspiration) with data on water resources for each country obtained from AQUASTAT country surveys (FAO, 1995a; 1997a; 1997b; 1999). In addition, the discharges of major rivers given in the literature have been compared with the calculated runoff for the drainage basin of these rivers. Where the calculated runoff values did not match the values available in the literature, correction factors have been applied to one or more of the basic input data on precipitation, reference evapotranspiration, soil moisture storage and open water.

Crop water requirements