How will this weekend’s rainfall impact Tucson’s groundwater supply?

This weekend’s record rainfall may give Tucson’s shallow groundwater a short term boost, especially near the mountain fronts and waterways where floodwaters moved across the landscape. It will not, however, quickly restore the deeper aquifer beneath central Tucson. Jennifer McIntosh, a University of Arizona hydrology professor, said the roughly 2 inches of rain that fell would take decades to hundreds of years to replace the groundwater removed from the central Tucson basin.

The storm produced record rainfall and historic flooding across southern Arizona, with rainfall records broken on three consecutive days. McIntosh said the strongest groundwater response is likely to occur near the mountain fronts, where rivers were running high and shallow groundwater can be replenished more readily. That recharge is different from restoring the deeper water tables that supply much of the region. In some areas, Tucson’s groundwater now sits more than 100 feet below the surface, largely because of decades of pumping for homes, farms and other human uses.

Only about 3% of Arizona’s annual rainfall reaches aquifers, according to the information provided by McIntosh. Much of the rest evaporates, runs off or is taken up by plants before it can move deep underground. Even when a storm produces unusually heavy rain, the water must travel through layers of soil and rock before reaching the water table. Flooding can therefore be dramatic at the surface while producing a relatively limited increase in the groundwater supply.

A University of Arizona study led by McIntosh helps explain why one wet weekend cannot reverse Tucson’s water losses. The research, published in Water Resources Research, reconstructed groundwater conditions over thousands of years, providing the region’s first long term record of water table changes. Researchers examined water from 12 wells at different depths, looking at chemical and isotopic markers that reveal when and under what conditions the water entered the aquifer. Computer models were used to separate water that arrived at different times, including fossil water that entered the ground more than 12,000 years ago.

The study found that natural climate variation has affected Tucson’s aquifer over time. Since the Last Glacial Maximum, about 20,000 years ago, precipitation has continued to recharge the basin, although dry periods reduced the amount of water entering the aquifer. During those naturally dry conditions, the water table fell as much as 105 feet compared with wetter periods. Modern pumping, which accelerated from the middle of the 20th century to the present, produced about twice as much drawdown as those natural climate fluctuations.

Heavy pumping began around the 1940s, as irrigated agriculture expanded and Tucson’s population grew. Signs of overdraft and well failures increased for decades while groundwater use faced few restrictions. Arizona’s 1980 Groundwater Management Act established formal controls and sustainability measures. The Central Arizona Project began delivering Colorado River water to Tucson in 1992, helping reduce pressure on local groundwater. Today, McIntosh said, about half of the water coming from Tucson taps is local groundwater and the other half comes from the Colorado River.

Tucson has also used conservation and recharge projects to help some wells recover from heavy use during the 20th century. McIntosh said the city’s strategy of treating wastewater and returning it to the aquifer is one of the most effective ways to add water where it can reach the water table. Tucson’s One Water 2100 plan calls for continuing and expanding the use of treated effluent, including releasing recycled wastewater into the Santa Cruz River. In locations where modern recharge already occurs, that water has a better chance of moving underground.

The storm’s benefits also must be measured against the condition of the Colorado River system, which supplies a major share of Tucson’s water. Nolie Templeton, who manages Colorado River policy and analysis for the Central Arizona Project, said the river basin is still dealing with a 27 year drought deficit. Recent storms raised Lake Powell by 14 feet, but the reservoir remains 15 feet below the level it needs to reach, according to Templeton. Recovering from the drought will require repeated wet periods and significant snow in the river’s watersheds, not one major storm or even one wet winter.

No additional rain was expected for the rest of the week, and Templeton said the Central Arizona Project is looking toward the possibility of a wet El Niño to bring more precipitation to the Colorado River basins. For Tucson residents, the weekend storm is encouraging because water is reaching places that can support shallow recharge and river flows. But the deeper lesson from the region’s groundwater history is that conservation, reduced pumping and deliberate recharge will matter far more than a single rainfall event. As McIntosh put it, even a return to the colder, wetter conditions of the last ice age would not restore all the groundwater removed by people.