POSSIBLE IMPACTS OF CLIMATE CHANGE ON GROUNDWATER AND SURFACE WATER RESOURCES IN THE WESTERN U.S.A

Groundwater is a vital resource in the western USA. Groundwater accounts for ~26% of all western water consumption and 28% of irrigation. Western ecosystems also depend on groundwater, especially riparian zones. Groundwater is a major contributor to surface-water resources: groundwater inflows make up a significant portion western streamflow, sustaining flows during dry seasons and contributing significant fractions even during intense rainfall and snowmelt episodes.

Mountains are generally wetter and cooler than adjacent basins, so groundwater in the West is derived mainly from mountain precipitation. Because large infiltrations of water are required to break through the region's thick unsaturated zones, and snowpacks store and then release precipitation from several storms at once, snowmelt provides more recharge than does rain. Isotopic studies in western settings suggest that 50 to 90% of recharge is from snowmelt.

As the western climate warmed in recent decades, precipitation came more frequently as rain than as snow, snowpacks thinned, and melt-fed streams flowed earlier in the year. These trends would continue under projected 21st-century climates. Snowline elevations would rise, thus decreasing snow-covered areas in western mountains. If so, mountain-block recharge also may also decline, as recharge areas shrink and snow available for melt generation dwindles. Declines in mountain recharge triggered by loss of snowpack would have immediate impacts on mountain water resources, including low flows and stream temperatures, and may also have serious impacts on long-term ground water supplies in surrounding basins.

Another simulated result of warming is increased surface runoff. Although recharge that supplies mountain groundwater may decline, much of this unrecharged water may run off onto fans and basins, increasing recharge in those zones. However, if the unrecharged water is mostly evapotranspired from the mountain soils, the overall recharge (mountain plus basin) may decline.

Declines in recharge triggered by warming may seriously impact groundwater supplies as well as surface-water resources to which groundwater inflows contribute. Given groundwater's crucial role in western water, potential impacts of warming on recharge deserve more attention than they have received to date.