From San Francisco to Denver a north-south line will increasingly divide the West into two distinct regions in coming decades: one that’s wetter, and the other drier.

You can guess which one will be on the losing end as far as water is concerned: the American Southwest. Now, a new study led by the University of Arizona details projected groundwater deficits in four economically important aquifers.

The paper, “Implications of projected climate change for groundwater recharge in the western United States,” was published in the March edition of the Journal of Hydrology.

By 2050, researchers of the study conclude, climate change will further worsen current groundwater deficits in the four aquifers. These aquifers are in California’s Central Valley, a critically important breadbasket for the nation; central and southern parts of the High Plains; and Arizona’s San Pedro. The four aquifers are already seeing groundwater tapped faster than it is being recharged by rain and snow.

Warmer climate means reduced aquifer recharge

Unlike in the rest of the nation, 40 percent of the water in the West comes directly from groundwater. And aquifers in the Southwest are all expected to experience decreases in recharge as the climate continues to warm.

The projections for those four aquifers contrast with those for western aquifers about the latitude of Boulder, Colorado and north. These are projected to recharge faster than they’re drained, the study found. Earlier this year, the Sierra in Northern California had been on track to have a good winter, but recent news points to disappointing snowpack levels and renewed calls for water conservation.

The University of Arizona study reflects the work of 17 co-authors who attended a workshop, sponsored by the National Science Foundation and the U.S. Geological Survey, to synthesize what experts know about how climate change is expected to affect western groundwater resources.

Impacts for regional water management plans

The study is expected to be used in developing regional water management policies. Existing studies had previously focused either on water issues at a global scale or at local scales.

In their study, the authors synthesized existing studies and applied knowledge of the processes that recharge aquifers.

They considered four types of aquifer recharge: “diffuse,” which refers to rain that falls on a particular spot and percolates into the ground; “focused,” which refers to water from streams and runoff; “irrigation,” which refers to water that percolates deep into the ground from irrigating crops; and “mountain system,” which refers to water that comes mostly from snowpack.

Image of 4 aquifer types
Conceptual illustration of four different ways aquifers are recharged under 20th century climate (a) and future climate (b).

Irrigation and focused recharge can be easily influenced by human actions, but people obviously have much less effect on diffuse and mountain systems recharge.

The team studied eight economically important western aquifers, although models for how climate change is expected to change aquifers was available for only four of them (those described above).

Aquifers studied
Projected changes in recharge for the aquifers studied. For each aquifer, the change in each recharge mechanism is represented by an arrow with a width proportional to its fraction of total recharge based on the current climate and understanding. Total recharge is represented by dark grey arrow on far right. Arrow direction (up, down, or bidirectional) indicates the direction of change expected based on the analysis. Two arrows pointing opposite directions indicate an equal likelihood of positive and negative changes. The letters L (low), M (medium), and H (high) refer to confidence in the change indicated. Arrow lengths are all equal, and do not represent change magnitudes. Quantitative estimates of change magnitude are shown as percentages when available from existing studies.

The San Pedro in Arizona is an example of an aquifer projected to drain faster than it recharges as the climate warms. Much of the recharge now comes from mountain-system recharge – and that contribution is expected to decrease as more precipitation comes down as rain and not snow.

The Santa Cruz River in Tucson, Az., illustrates what can happen when aquifers are emptied faster than they are naturally recharged. Once filled year-round, the Santa Cruz River now has water only after heavy rains.

“Our study reveals that the western U.S. needs to redouble efforts to manage water resources to maximize benefits to individuals and society,” Thomas Meixner, lead author of the study, said in a U.A. press statement. “We can’t be wasting water.”

News coverage of the study:
Steller: Arizona’s self-satisfaction on water must end
Climate change will reduce San Pedro recharge, study says

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