NOEL KING, HOST:
It's monsoon season in the American Southwest. For Arizona, New Mexico and Colorado, the seasonal rains are very important for rivers and pastures and for keeping wildfires in check. But climate change has made the monsoon less regular. Here's Michael Elizabeth Sakas from Colorado Public Radio.
(SOUNDBITE OF RAIN FALLING)
MICHAEL ELIZABETH SAKAS, BYLINE: This is a welcome sound in the city of Gunnison, where Colorado's governor has declared a drought emergency. This rain has brought some much-needed relief after three years of almost no summer monsoon rains for the Southwest.
BILL TRAMPE: We call it the no-soon because we just didn't get anything.
SAKAS: That's Bill Trampe, a third-generation Gunnison rancher. During a break from the rain, he sits in a plastic lawn chair in his backyard, which overlooks 150 acres where Trampe grows grass and hay.
TRAMPE: Our hay crop was terrible a year ago. It just wouldn't grow. It just turned off horribly dry in June, July, August and September - no monsoon at all.
SAKAS: Trampe was worried it was too dry for the number of cows he had, so he killed more cows in the fall than he usually would. He says he's running out of water on the federal land where they graze.
TRAMPE: Got one ditch. It's got the whole crick (ph) in it. And we're trying to irrigate 300, 400 acres out of that one ditch. And we've got probably 600 acres that are nothing. And so these rains are super important.
SAKAS: But these rains are becoming less predictable and less effective. Rosemary Carroll is an associate professor of hydrology at the Desert Research Institute in Nevada, but she lives in Crested Butte near Gunnison. We meet on a high dirt road overlooking a tributary that feeds the Colorado River. The two major reservoirs on this crucial water supply are at record lows. And Carroll's research shows that with climate change, less monsoon rain makes it into this system. She says the problem starts with the snowpack, what collects in the mountains over the winter.
ROSEMARY CARROLL: If you have a big snow year and that snowpack lasts late into the spring, early summer, then your soil moisture storage is high.
SAKAS: Cool temperatures and wet soil mean rainfall is more likely to reach a stream. But if it's hot with less snowpack, the soils are dried out and will soak in more of the water.
CARROLL: That then will buy us, for the same amount of rainfall, less monsoon rain making it into the stream.
SAKAS: Another benefit at risk - normally, the arrival of monsoon rains has ended the worst of the fire season. But as temperatures get hotter and the atmosphere can hold more moisture, that's changing how the monsoon behaves. A study out of the University of Arizona found that now rainstorms happen less often. When the rain does fall, there's more of it all at once. Joel Biederman is the co-senior author and a research hydrologist with the U.S. Department of Agriculture.
JOEL BIEDERMAN: Even if the total amount of rainfall is the same during the growing season, a few really large rainstorms isn't that beneficial.
SAKAS: This summer, the monsoon rain has caused extreme flash floods in Arizona, with big, sporadic storms instead of consistent moisture. Vegetation and soils dry out. Don Falk, a forest ecologist at the University of Arizona, says a weak or sporadic monsoon means the fire season extends into the hottest months of the year.
DON FALK: It really plays a huge role in setting up these gigantic fires that have been happening in Colorado and throughout the Southwest because the monsoon isn't playing that role of ending the major part of the fire season.
SAKAS: The rain that has shown up this summer has helped dampen some active wildfires. Now the hope is that short, frequent monsoon storms stick around through August. That could help parts of the Southwest avoid another year of dangerous late-season wildfires.
For NPR News, I'm Michael Elizabeth Sakas in Denver.
(SOUNDBITE OF MUSIC) Transcript provided by NPR, Copyright NPR.