Percentage of watersheds for which estimated annual surface flows are the minimum (green) or maximum (blue) over the period 1999–2007 in the western (a) and eastern (b) United States
Figure 6. Percentage of watersheds for which estimated annual surface flows are the minimum (green) or maximum (blue) over the period 1999–2007 in the western (a) and eastern (b) United States. East is defined by 2-digit HUCs 01–09, west by 10–18. Both drought and flood events punctuated the baseline period used to represent average surface flows in figure 1 (1999–2007). As reflected by the years of high and low flows for each basin shown in figure 5, a widespread drought event occurred in 2002 in the west, while 1999 was a particularly wet year in the region. In the east, high surface flows occurred during 2003, while 2007 saw significant drought and associated low flows.
Here, we assess current stress in the freshwater system based on the best available data in order to understand possible risks and vulnerabilities to regional water resources and the sectors dependent on freshwater. We present watershed-scale measures of surface water supply stress for the coterminous United States (US) using the water supply stress index (WaSSI) model which considers regional trends in both water supply and demand. A snapshot of contemporary annual water demand is compared against different water supply regimes, including current average supplies, current extreme-year supplies, and projected future average surface water flows under a changing climate. In addition, we investigate the contributions of different water demand sectors to current water stress. On average, water supplies are stressed, meaning that demands for water outstrip natural supplies in over 9% of the 2103 watersheds examined. These watersheds rely on reservoir storage, conveyance systems, and groundwater to meet current water demands. Overall, agriculture is the major demand-side driver of water stress in the US, whereas municipal stress is isolated to southern California. Water stress introduced by cooling water demands for power plants is punctuated across the US, indicating that a single power plant has the potential to stress water supplies at the watershed scale. On the supply side, watersheds in the western US are particularly sensitive to low flow events and projected long-term shifts in flow driven by climate change. The WaSSI results imply that not only are water resources in the southwest in particular at risk, but that there are also potential vulnerabilities to specific sectors, even in the 'water-rich' southeast.