When we consider a schematic water balance in an irrigated area, there are three inflows of water: rainfall (P), groundwater from upstream and diverted river water (V_c). Often groundwater inflow is in the same order as groundwater outflow. Part of the precipitation and diverted river water (P+V_c) leaves the area as actual evapotranspiration (ETa). The remaining part is stored in the irrigated area or drained. The ratio ET_a/(P+V_c) is called the depleted fraction DF (Molden 1997, Bastiaanssen et al 2001). The depleted fraction can be used as an indicator to assess irrigation water use. When ET_a and rainfall are known, the depleted fraction can be influenced by means of the diverted irrigation water V_c.

The innovative aspect about the use of the depleted fraction is that the spatial variation of the actual evapotranspiration (ET_a) of an area can be calculated from satellite images. Low resolution images (NOAA or MODIS) are adequate to calculate monthly values. In practice ET_a and rainfall are not calculated for the irrigated area only, but for the gross command area.

The value of the depleted fraction influences the volume of water stored within the irrigated area. When the depleted fraction is low, water is stored in the area and the water table will rise. When the depleted fraction is high, water leaves the area and the water table will drop. For a certain depleted fraction, the volume of water stored in the area is stable (DF_sustainable). This value depends on the natural drainage of the irrigated area and often is about 0.6. The depleted fraction can be managed by changing the volume of diverted water.

The volume of water diverted into the irrigated area can be reduced during months with a low depleted fraction (for instance if rainfall is high). The strategy is to manage this depleted fraction in such a way that the depleted fraction remains stable over the year and thus the water table. During part of the year accumulated salts need to be leached. This will take place when the depleted fraction is less than DF_sustainable, but also depends on agricultural practices as crops and crop stages show various sensitivities to salts. More water should be released then. When the depleted fraction then is less than DF_sustainable, the evaporative fraction ET_actual/ET_potential will remain about 1. For values of the depleted fraction of more than DF_sustainable, ET_actual/ET_potential will decrease by 10 to 20 percent. To sustain agriculture on one side (leaching) and attain a high productivity (yield per cubic meter of water) on the other side, monthly values of the depleted fraction should range between 0.5 and 0.9 (Bos, 2004).

To apply this method, the relationship between the depleted fraction and monthly fluctuation of the groundwater table must be established and the relation between the ratio ET_a/ET_p and the depleted fraction. This information needed must come from field measurements (change of the groundwater table, volumes of diverted water, the data from CRIWAR part 1, soil data and rooting depth for capillary rise, irrigation application methods for efficiencies) and satellite images.

Outputs of the model are thus monthly volumes of irrigation water fitting the objectives for sustainability and productivity. The user can formulate strategies that better match the objectives through, for instance, the use of other irrigation methods, improving conveyance efficiencies and irrigation scheduling, defining other cropping patterns, etc.

End-users (farmers) are especially focused on productivity or crop yield per hectare. Using historical series of surveyed yields, average relations are derived with (satellite based) actual evapotranspiration and total biomass. Such relations form then the basis for evaluation of scenarios through a direct link between water use and crop yields, and, hence, provide essential inputs for an economic analysis.