The demand for non-agricultural use of water is calculated at the world region level and down-scaled to grid-cell level using the spatial distribution of people and GDP as a measure of human activity. Together with demands for irrigation per grid-cell (see below), the run-off is then corrected for the total withdrawals in the cell. The resulting run-off 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.