Abstract
Several reports have assessed water scarcity globally using a widely accepted index on an annual basis, namely, withdrawal-to-water resources ratio (WWR). Here, we ask whether it is appropriate to use the WWR to assess the impact of climate change. Global warming is projected to increase the temporal and spatial variability of precipitation, decrease snowfall, and change the timing of snowmelt. To assess the impact of climate change on global water resources incorporating sub-annual time-scale phenomena, this study applies a new water scarcity index on a daily basis termed the cumulative withdrawal-to-demand ratio (CWD) . Our results indicated that global warming increased the mean annual runoff in 61% of the total land area globally. However, in 22% of the area where runoff increased, the CWD showed increased water stress. Those regions included India, northern China, and Europe. For India, the increase in water stress was attributed to the seasonal gap between runoff and water demand. The increased runoff was concentrated in a few months, while the high irrigation water demand months differed and were much longer. For Europe, the change was attributed to the shift in the timing of snowmelt, which occurred a few months earlier than at present, causing water shortages in early summer.
