Water use begins with data on total water withdrawals from FAO Aquastat. These are divided by the size of the population to get an estimate of water use per capita.
In future years, water use per capita is forecast to increase in parallel with crop production per capita. Specifically, an expected level of water use per capita as a function of crop production per capita (see figure below) is calculated for crop production in the current year (CropPC) and crop production in the first year (CropPCI). The ratio of these values is multiplied by the water use per capita in the first year (WatUsePCI) to get water use per capita in the current year (WatUsePC). This is multiplied by population (POP) to get total water use (WATUSE).