Critical regions: A model-based estimation of world water resources sensitive to global changes

This paper presents a top-down approach for identifying regions whose water resources have higher sensitivity to global change than other regions. The aim of this approach is to provide an overview of regions that may justify special attention from the research and development assistance community, under particular global change scenarios. As a 'top-down' method it is best seen as a type of sensitivity analysis that can complement rather than replace other 'bottom-up' studies of the vulnerability of particular watersheds. An increase in 'water stress' is used as a measure of increasing sensitivity of watersheds to global change, and this stress is computed with the global water model, WaterGAP. Stress increases when either water withdrawals increase or water availability decreases. Since the criteria for determining critical regions is uncertain, they are calculated and compared for four different sets of criteria. To examine the difference in critical regions under different socio-economic and climate scenarios, they were also calculated for four distinctive scenarios. Under the scenario showing the largest increase in water stresses, the estimated area of critical regions (in 2032) ranges from 7.4 to 13.0 percent of total land area, depending on the criteria for identifying critical regions. As expected, the estimate of critical regions is very scenario-dependent, showing smaller areas under scenarios having smaller increases in water stress. However, some regions always appear as critical regions regardless of the scenario. These include parts of central Mexico, the Middle East, large parts of the Indian sub-continent, and stretches of the North African coast.