Global analysis of urban surface water supply vulnerability

This study presents a global analysis of urban water supply vulnerability in 70 surface water-supplied cities, with populations exceeding 750 000. Vulnerability represents the failure of an urban supply-basin to simultaneously meet demands from human, environmental and agricultural users. We assess a baseline (2010) condition and a future scenario (2040) that considers increased demand from urban population growth and projected agricultural demand under normal climate conditions. We do not account for climate change, which can potentially exacerbate or reduce urban supply vulnerability. In 2010, 36% of large cities are vulnerable as they compete with agricultural users. By 2040, without additional measures 44% of cities are vulnerable due to increased agricultural and urban demands. Of the vulnerable cities in 2040, the majority are river-supplied with mean flows so low that the cities experience 'chronic water scarcity' (1200 l p−1 d−1 ). Reservoirs supply the majority of cities facing individual threats to future freshwater supply, revealing that constructed storage potentially provides tenuous water security. In 2040, of the 31 vulnerable cities, 13 would reduce their vulnerability via reallocating water by reducing environmental flows, and 15 would similarly benefit by transferring water from irrigated agriculture. Approximately half remain vulnerable under either potential remedy. 2014 Environ. Res. Lett. 9 104004