Climate Influences on Groundwater Recharge: Implications for Western Groundwater and Surface Water Resources in the Face of Climate Change

Groundwater is a vital resource in the western USA, accounting for over a quarter of total supplies and irrigation uses. In addition, groundwater is a major contributor to surface-water resources, sustaining baseflows throughout the year, and contributes important fractions of streamflows even during high-flow periods following rainfall and snowmelt.

Because mountains are generally cooler and wetter than adjacent basins, groundwater in the West is derived mostly from mountain precipitation. Large infiltrations of water are required to break through the region's thick unsaturated zones. Because snowpacks store and then release precipitation from several storms at once, snowmelt contributes disproportionately more recharge than does rain. Warming temperatures have already caused declines in Western snowpacks and earlier flows in melt-fed streams. Current projections of future climate suggest that these trends will continue. Snowline elevations are expected to rise, reducing snow-covered areas in western mountains, and decreasing the amount of snow in areas where snowpacks remain. 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. Although recharge that supplies mountain groundwater may decline, much of this unrecharged water may run off onto fans and basins, increasing recharge beyond the mountains. However, if the water that is not recharged in the mountains is mostly evapotranspired from the mountain soils, the overall recharge (mountain plus basin) may decline. Changes in temperature will bring concomitant changes in water temperatures, and thus in streambed conductance and leakance; changes in dominant vegetation may also occur. All these factors can play a role in affecting groundwater recharge, and none are well understood or predicted at present. Declines in recharge triggered by warming could seriously impact ground-water supplies and surface-water resources to which ground water contributes. Given ground water's crucial role in western water, potential impacts of warming on recharge deserve more attention than they have received to date, preferably from a long-term monitoring system incorporating multidisciplinary observations.