Groundwater deficit growing in important western U.S. aquifers

Predictions at the major river basin or several-state level can be useful for developing water policy, the team wrote. However, the team found predictions from existing studies were either at a global scale or at the local level, not at the regional level.

UA notes that to create regional-scale predications, the scientists synthesized existing studies and applied current knowledge of recharge processes. The team studied eight economically important Western aquifers for which studies about their groundwater recharge budgets existed. In addition, models of how climate change would affect recharge were available for four of the aquifers.

To compare all eight aquifers, the team developed a uniform classification scheme for the components of groundwater recharge. The scientists identified four different components of groundwater recharge: diffuse, focused, irrigation and mountain system.

Some types of recharge are more easily affected by human behavior and water policy than others. Human decision-making can easily affect irrigation recharge (water that percolates deep into the soil from irrigating crops) and focused recharge (water that reaches the groundwater from streams or runoff).

In contrast, human behavior has a much smaller effect on diffuse and mountain-systems recharge. Diffuse recharge comes from the precipitation that falls on a specific spot and then percolates down into the groundwater.

Much of the mountain-systems recharge comes from snowpack, Meixner said. As the snow melts, the water fills mountain streams that end up in the flatlands below. Snowmelt also can percolate into the soil and eventually reach the valley below as the water moves downhill through the bedrock underlying the mountains.

The San Pedro aquifer in southeastern Arizona is one example of an aquifer where the human use of groundwater will increasingly outstrip recharge as the climate warms, the researchers report. Much of the San Pedro’s current recharge comes from mountain-system recharge, which the scientists expect will dwindle as more precipitation falls in the mountains as rain rather than snow and as the region dries.

When more groundwater is pumped than is replaced by recharge, rivers can be sucked dry, as happened to the Santa Cruz River in Tucson, Meixner said. Once the Santa Cruz flowed year-round; now in Tucson the river has water only after heavy rains.

“What you would expect to see is that climate change will exacerbate problems in the Southwest on the recharge end,” Meixner said.

“Our study reveals that the Western U.S. needs to redouble efforts to manage water resources to maximize benefits to individuals and society,” he said. “We can’t be wasting water.” 

— Read more in Thomas Meixner et al., “Implications of projected climate change for groundwater recharge in the western United States,” Journal of Hydrology 534 (March 2016): 124–38 (2016) (DOI: 10.1016/j.jhydrol.2015.12.027)