Monsoon storms in the desert Southwest are vital for recharging groundwater – but it now appears likely this recharge effect may be compromised by climate change.
The major cities of the Southwest – Phoenix, Tucson, Albuquerque, Las Vegas – currently get most of their freshwater from the Colorado River or its tributaries. That river, however, is experiencing its 19th straight drought year, suggesting a new permanent dry state is gripping the giant watershed.
As a result, groundwater recharge from monsoons could become a much more important water supply in the long term.
Summertime monsoon storms are a distinguishing feature of the desert Southwest, marked by lightning and dramatic cloudbursts that offer a brief respite from baking heat. New research, however, suggests these storms will become less frequent but more intense as the climate warms due to greenhouse gas emissions. This could make monsoon rainfall more difficult to capture for reuse.
“If we accept as a general paradigm that monsoon precipitation is getting less frequent but more intense, how do we design capturing systems to recharge groundwater?” says Christopher Castro, an associate professor of hydrology and atmospheric sciences at University of Arizona. “I don’t precisely know. But it needs to be addressed.”
Castro is coauthor of a recent study that concludes that monsoon storms have become fewer but more extreme over the last 60 years, and that this trend is likely to continue. Monsoon storms are happening less often in summer, but they are much wetter and longer-lasting when they do arrive.
One reason, he said, is that the changing climate is more likely to create a high-pressure ridge over the western United States. This makes it less likely for monsoonal conditions to form over the Southwest. But when they do form, the warmer atmosphere holds more moisture, making the downpours heavier.
Rather than dozens of storms spread over a season, Castro said, “it’s maybe one or two big events and the rest of it is dry. Then it’s the same amount of water, but it will just run off right away and evaporate, and it won’t infiltrate. Certainly water resources are a big concern, and it could be potentially a dire future.”
Castro’s study is one of the few so far that has examined how climate change might affect the monsoons.
He found that these less frequent monsoons are also more likely to deliver severe downdraft winds and dust storms – the kind of conditions that make monsoons more destructive. And whereas monsoons were once more common during evening hours, they now happen earlier in the day when people are more likely to be outdoors.
Another new study is one of the first to explore how important monsoons are to groundwater recharge. It has long been assumed that rainfall from monsoons runs off too fast to have an effect on groundwater supplies. But the team from Arizona State University found that nearly 25 percent of monsoonal rainfall percolates into groundwater.
Their results are based on more than six years of research at a site in the Chihuahuan Desert of New Mexico. They learned that natural drainage channels as small as 2ft wide were important for groundwater recharge.
“Groundwater is much like a bank account,” says Enrique Vivoni, a hydrologist at ASU and a coauthor of the study. “This is an essential process that banks renewable surface water for future use as a groundwater resource in the arid Southwest and around the world.”
The study did not examine whether the same rate of recharge would be possible with fewer, wetter storms.
In Tucson, officials aren’t too concerned yet about how changes in the monsoon could affect groundwater. Early in its history, Tucson depleted its groundwater to fuel growth. Now it is considered a model for groundwater management. Thanks to the Central Arizona Project, completed by the U.S. Bureau of Reclamation in 1993, Tucson now relies mostly on water diverted from the Colorado River. Whatever it doesn’t consume from the river is pumped into the aquifer to recharge groundwater.
Today, 80 percent of Tucson’s water comes from the Colorado River, says James MacAdam, a spokesman for Tucson Water.
“We’ve got several decades worth of locally stored groundwater,” MacAdam said. “So local changes to the monsoon – it’s not something that we’re tracking extremely closely. It’s not at the top of the list, because that won’t mean anything, in reality, for our available water supply.”
Tucson is also a leader in rainwater capture. Since 2012, the city has spent more than $2 million on rebates to encourage its water customers to install rainwater capture systems to irrigate landscaping. Some of these projects are larger, neighborhood-scale projects designed to capture and filter street and parking-lot runoff for groundwater recharge.
Such systems largely target monsoon storms to ease pressure on the domestic water system during summer months, when demand is highest. MacAdam said the design of rainwater harvesting systems may need to change if monsoons become wetter and less frequent. Rainwater cisterns may be more likely to run dry between storms, forcing residents to go back on tap water.
“Obviously the heavier, flashier rain is more difficult to deal with, and less beneficial in terms of supply, than a longer, slower rainfall. If you get 2in of rain in half an hour, you lose a lot to overflow,” he said. “I think that’s a concern in terms of the whole Southwest.”
He says Tucson Water is starting work on a long-term water resource management plan, which will delve deeper into climate change projections.
Castro has undertaken additional research indicating that the trend of fewer and more extreme monsoon events is likely to worsen as climate change unfolds. He is now working on a new paper explaining those results.
“The best we know is that the mean monsoon precipitation here in the Southwest is likely to either remain the same or decrease,” he says. “And then the extremes, from our work, are likely to get worse.”
His results show the most likely places in Arizona for extreme monsoons are downwind of mountain ranges. This includes the Phoenix metro area, Bullhead City and Kingman, the Colorado River valley and Arizona’s low deserts, including Casa Grande and Yuma. Several military bases are also likely to see more extreme monsoons, including Luke Air Force Base, Barry Goldwater Air Force Range and the Yuma Proving Ground.
The next step, Castro said, involves translating the monsoon predictions into on-the-ground hydrology. How much will streamflow change? Are current flood protections adequate? Do urban drainage culverts need enlarging? Those are among the questions that need to be answered, if more research dollars can be found.
“It urgently needs to be done,” he says.