Resource detail

Water demand projections (WDPs) are widely used for future water resource planning. Accurate WDPs can reduce waste or scarcity associated with overdevelopment or underdevelopment, respectively, of water resources. Considering that the projection period of some WDPs have now passed, this paper examines how closely such past projected withdrawals match current water withdrawals to identify lessons that can be learned and strengthen future studies on WDPs. Six WDPs conducted before 1990 and seven conducted after 1990 are analyzed in detail. The review shows that the pre-1990 WDPs, which considered population as the main driver of change, over predicted current water use by 20 to 130%. Unrealistic assumptions on the norms of water use in different sectors were the main reasons for large discrepancies. For India, the underestimation ranges from 20 to 90 billion cubic meters (Bm3) or 3 to 14% of the total water withdrawals. For China, they over-estimated the demand by 37-54 Bm3 or 7 to 11% of the total withdrawals. The projections for many small countries also differ substantially compared to their current water withdrawals. Moreover, there is no analysis that assesses the accuracy of projections. Overall, the value of long-term country-level projections in global WDPs is inadequate for local water resource planning. The accuracy and value of global WDPs could be increased, if past trends, spatial variation across and within countries, and influence of rapidly changing key exogenous and endogenous drivers of water demand in different sectors are taken into account. For individual countries, short-term projections and sensitivity analysis can be more useful.