Although years of drought and over-pumping have significantly depleted groundwater in Arizona and California, a new study shows the situation has an upside: It has created underground reservoirs where extra surface water can be stored during wet times so it is available during drought. The study, published in the journal Environmental Research Letters in March, also found that regions that actively store surface water in underground aquifers have increased their groundwater supply over time, even as surrounding areas depleted theirs. 

The findings are important because they show that techniques used to increase groundwater storage are working. With droughts and floods projected to increase due to climate change, and California investing $2.7 billion to expand water storage, these techniques could help drought-prone regions prepare for extremes, said lead author Bridget Scanlon of The University of Texas at Austin's Bureau of Economic Geology. The bureau is a research unit at The University of Texas Jackson School of Geosciences. 

"In many regions now we're dealing with these extremes of drought and then intense floods, and that's a real challenge for water resource managers," Scanlon said. "To try and resolve this disconnect between supply and demand, we can store water in depleted aquifers."

The study examined decades' worth of groundwater data from California's Central Valley and active management areas across central Arizona -- both regions that collect extra water from surface reservoirs and store it in underground aquifers using a technique called managed aquifer recharge. The method has been used in the Central Valley since the 1960s and in Arizona since the 1990s. 

As part of the storage strategy, the regions also managed their water resources through conjunctive use, relying primarily on surface aquifers during wet periods and on groundwater during dry periods. 

"When you do that, you stop pumping the groundwater so it can recover," Scanlon said. "But you are also adding recharge from surface water, so you have a double benefit."