Monsoon boosts aquifers
Study finds rainwater reaches water table
ASU researchers have determined that a significant portion of rainfall from monsoon storms soaks into the soil and percolates down to the water table.
A string of monsoon storms lit up the skies over Arizona this summer and sent torrents of water racing down desert washes and city streets.
You might think the fast-moving water from these downpours runs off the brittle desert landscape without much benefit to underground aquifers along the way. But scientists have found a significant portion of the rainfall from monsoon storms soaks into the soil and percolates down to the water table.
Researchers from Arizona State University spent eight years studying a watershed in the Chihuahuan Desert in New Mexico, and they found that nearly 25 percent of the rainfall from monsoon storms was absorbed into small streambeds and seeped into the groundwater.
“That was an unexpected outcome,” said Enrique Vivoni, an ASU hydrologist who co-authored the research. He said
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Track September’s highs and lows with the monthly weather chart. the findings show it makes sense to think of monsoon rains “not only as a flood hazard, but also as a resource that can be taken advantage of from a groundwater planning perspective.”
The researchers used a combination of instruments, drone photography and a hydrologic model to study a 12-acre area in the desert. They focused on a sloping watershed dotted with creosote and mesquite bushes.
Adam Schreiner-McGraw, the study’s lead author, conducted the research while he was a graduate student at ASU’s School of Earth and Space Exploration.
He visited the area regularly for more than six years to collect data and maintain the network of instruments while examining groundwater recharge on the hillslopes and channels.
“Soils on hillslopes are very different than those in the channels,” SchreinerMcGraw said. “They are compact and do not absorb water very quickly.”
He said the channels, on the other hand, have coarse and permeable sediments that can soak up water much more quickly.
Much of the water that doesn’t sink into the ground either evaporates from the soil directly or is absorbed by plants.
The research team included ASU students and collaborators from New Mexico State University and the U.S. Department of Agriculture.
They collected data from a watershed monitoring network at the Jornada Experimental Range in New Mexico, a landscape where there has been cattle grazing and desert shrubs have gradually replaced grasslands over the past century.
The researchers used sensors to measure the water content in the soil and determine how much rainfall was soaking into the ground.
They used the data to improve a computer model that Vivoni previous-
ly developed. The hydrologic model takes in variables including weather data, terrain, soil and plant types to estimate how much rainfall is recharging the groundwater in a given area.
“This is a type of model that can be used for watershed planning for large municipalities or counties,” Vivoni said. “The findings here would be applicable to other places where the monsoon is the major contributor to recharge.”
This summer’s monsoon storms have brought above-average rainfall in Arizona. The National Weather Service said this week that Phoenix has received 2.2 inches of rainfall this summer, above the 30-year average of 1.8 inches.
The new research indicates that an active season of summer rains like this year could be an opportunity to help boost desert water supplies in the future, Vivoni said, especially as climate change shrinks the average snowpack in the West and increases the strains on the Colorado River and other water supplies.
Monsoon rains, he said, are a resource “we might rely on more than we do currently, and so projects to promote the recharge of monsoon flooding would be a benefit to the overall groundwater budget.”
The study journal
Hoori Ajami, a groundwater hydrology professor at the University of California, Riverside, who wasn’t involved in the study, said the findings “highlight the role of intense rain events in recharging desert aquifers through stream channels located at the mountain front.”
She said it’s not clear how the intensity and frequency of monsoon storms might change in the future.
But monsoon storms play an important role in recharging groundwater in the desert, Ajami said, and “quantifying recharge from these events is important for sustainable groundwater management” and for calculating how much can be pumped sustainably from an aquifer.
“The study highlights the role of long-term observations in understanding was published in the of water balance in arid catchments,” Ajami said in an email. “These types of investigations require longterm investment and support from federal and state agencies to allow new discoveries and insights.”
This year, Schreiner-McGraw joined Ajami’s research group at UC Riverside as a postdoctoral fellow. He’s turned to studying how climate change affects groundwater recharge in California’s Central Valley.
At the same time, he’s also continuing his research in the desert in New Mexico.