While dry events in California aren’t getting drier, extreme wet weather events have steadily increased in magnitude since the middle of the last century, new research shows.
These increased extreme wet events can lead to more dangerous flooding and also fuel wildfires.
“Most research after 2015 has focused on this climate variability and how it will worsen in the future,” says Diana Zamora-Reyes, graduate student in the Department of Hydrology and Atmospheric Sciences at the Institute. University of Arizona and lead author. paper in the International Journal of Climatology.
“But, in this article, the main takeaway is that it’s happening right now and the variability has increased over the last 50 or so years.”
California fires and floods
The team has been collecting data on seasonal precipitation and stream flow — the runoff resulting from rainfall — across the state of California for 80 years, when enough data began to become available.
There is usually a lag between precipitation and stream flow, as it takes time for snow to melt and water to move across the landscape. Zamora-Reyes and her team focused on fall and winter data because most precipitation in California occurs during those seasons.
“Going into this research, I thought that wet events would get wetter and dry events would get drier, and that was what was causing the increased variability,” Zamora-Reyes says. “I thought it would be both, but it wasn’t. My hypothesis was based on previous research which found that there will be an increase in extreme precipitation events and that they occur in a shorter window of time.
“Droughts in California do happen, our research just shows they’re not caused by less precipitation,” says co-author Valérie Trouet, professor of dendrochronology at the Tree Ring Research Laboratory. “We focused on rainfall and river flow, which are not the only variables contributing to drought. Temperature also plays an important role, and with global warming, droughts are intensifying.
The team, which also includes Bryan Black, an associate professor in the Dark Circles Research Laboratory, compared their findings from northern and southern California. These comparisons are important because Southern California is home to most of the state’s population, but Northern California is where most of the water resources are. Both regions depend on the water supply, but in different ways.
“With this increased variability and warming, it’s also important to consider that much of the precipitation is going to start falling as rain rather than snow,” Zamora-Reyes says.
This could mean more dramatic and destructive flooding, but it could also cause increased plant growth. If a dry year follows, it could lead to more fuel for wildfires. Less snow also means less snowmelt, which generally distributes melted water at a less destructive rate.
“These patterns will continue to cause billions in damage and are something we need to consider, especially for infrastructure planning,” Zamora-Reyes says.
Tree rings and precipitation
The researchers also found that precipitation variability decreases in the northern part of California in the fall.
“That’s where you get most of the water resources and that’s also where you have a lot of forested areas,” says Zamora-Reyes. “We’re also seeing fewer of these really wet events happening in the fall, and instead we’re getting most of the concentrated precipitation in the winter. We see this pattern across California. If you look statewide, you see the same trend in rainfall and stream flow. »
The paper is unique in that it highlights the relationship between increasing variability in river flow and precipitation. “They’re completely independent records, but they show the same thing,” says Zamora-Reyes.
“Precipitation follows complex processes before it turns into stream flow,” says Trouet. “Stream flow is a much slower process, so we didn’t expect to see a pattern of variability in stream flow so clearly, but we do.”
Zamora-Reyes was surprised by the team’s results.
“I expected to see an increase in variability over the last 20 years at most,” she says, “but we’ve been seeing a steady increase for much longer. People notice. I’ve talked to strangers about my research, and they say they’ve noticed what I’m talking about. It has been interesting and eye-opening to watch my research unfold in real time. »
The researchers looked only at data collected by sensors and instruments installed decades ago, but in the future, Zamora-Reyes and Trouet want to use tree rings to study rainfall and river flow variability. water dating back hundreds of years.
Zamora-Reyes says it will help them answer questions like, “How rare is this variability? Are there periods in the past comparable to what we are experiencing now? »
Source: University of Arizona