the demand for nonagricultural use of water is calculated at the world region level and downscaled to gridcell level using the spatial distribution of people and gdp as a measure of human activity together with demands for irrigation per gridcell see below the runoff is then corrected for the total withdrawals in the cell the resulting runoff is passed on to the next downstream cell and so on until the entire water basin is covered the gap is filled by irrigation using surface water and groundwater available in the same or neighbouring grid cells