Review of global agricultural water use per country

Crop water requirements

The calibrated water balance under natural conditions, the global irrigation map and the irrigated crops database produced in the framework of the study "Agriculture - Towards 2015 and 2030" (FAO, 2002) have been used as inputs for the computation of the amount of water withdrawn for crop production. Like the computation of the water balance under natural conditions, the calculation of consumptive water use in irrigation (or net irrigation water requirements) is carried out by grid cells of 10 by 10 km in monthly time steps and can be presented in statistical tables or maps at different levels of spatial aggregation.

Consumptive use of water in irrigated agriculture is defined as the water required in addition to water from precipitation (soil moisture) for optimal plant growth during the growing season. Optimal plant growth occurs when actual evapotranspiration of a crop is equal to its potential evapotranspiration.

Potential evapotranspiration of irrigated agriculture is calculated by converting data of irrigated area by crop (at the national level) into a cropping calendar with monthly occupation rates of the land equipped for irrigation. The cropping calendars for the 90 countries of this study are presented in this table and an example is provided below.

The evapotranspiration (ETc in mm) of a crop under irrigation is obtained by multiplying the reference evapotranspiration (ETo) with a crop-specific coefficient (ETc = Kc * ETo). This coefficient has been derived for four different growing stages: the initial phase (just after sowing), the development phase, the mid-phase and the late phase (when the crop is ripening to be harvested) (FAO, 1998a). In general, these coefficients are low during the initial phase, after which they increase during the development phase to high values in the mid-phase and again lower in the late phase. It is assumed that the initial phase, the development phase and the late phase each take one month for each crop, while the duration of the mid-phase varies according to the type of crop. For example, the growing season for wheat in Morocco starts in October and ends in April, as follows: initial phase: October (Kc = 0.4), development phase: November (Kc = 0.8), mid-phase: December - March (Kc = 1.15), and late phase: April (Kc = 0.3). The crop coefficients for the different crops and growing stages are presented in this table.

The rate of evapotranspiration coming from the irrigated area per month and per grid cell is calculated by multiplying the area equipped for irrigation with cropping intensity and crop evapotranspiration for each crop.

ET = IA * S_c(CI_c * Kc * ETo)

The difference between the calculated evapotranspiration of the irrigated area and actual evapotranspiration under non-irrigated conditions is equal to the consumptive use of water in irrigated agriculture in the grid cell, i.e the net irrigation water requirement. In the case of paddy rice, an additional amount of water is needed for flooding. In this study it was computed by by multiplying the area under irrigated rice by a water layer of 25 centimetres.

Irrigation water withdrawal

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