Originally appeared in IowaNow, 2015.02.09. Story by GARY GALLUZZO.
The U.S. Midwest and surrounding states have endured increasingly more frequent flood episodes over the past half-century, according to a study from the University of Iowa.
The UI researchers based their findings on daily records collected by the U.S. Geological Survey at 774 stream gauges in 14 states from 1962-2011, a data-collection period in common for all the stations.
They found that 264 (34 percent) of the stations had an increase in frequency in the number of flood events, while only 66 stations (nine percent) showed a decrease.
“It’s not that big floods are getting bigger, but that we have been experiencing a larger number of big floods,” says Gabriele Villarini, UI assistant professor of civil and environmental engineering and corresponding author on the paper, published Feb. 9 in the advance online edition of the journal Nature Climate Change.
The findings likely come as no surprise to millions of people in the Midwest and bordering states. During the past several decades, large floods have plagued the region in 1993, 2008, 2011, 2013 and again in 2014. The floods have caused agricultural and economic losses in the billions of dollars, displaced people and led to loss of life.
“There is a pattern with increasing frequency of flood events from North Dakota south to Iowa and Missouri and east into Illinois, Indiana and Ohio,” says Iman Mallakpour, UI graduate student in civil and environmental engineering and lead author on the paper.
“We related this increasing number of big floods to changes in rainfall and temperature. There was an overall good match between the areas with increasing frequency of flood events and areas experiencing increasing frequency of heavy rainfall events,” adds Villarini, who is also assistant research engineer at IIHR – Hydroscience & Engineering.
He notes that seasonal analysis revealed that most of the flood peaks in the upper Midwest occur in the spring and stem primarily from snow melt, rain falling on frozen ground and rain-on-snow events. Interestingly, spring—in addition to being a season with increasing frequency of heavy rainfall—also has the strongest increase in temperature over most of the northern part of the region studied, he says.
The findings jibe well with current thinking among climate scientists about how the hydrological cycle is being affected by global warming. In general, as the atmosphere becomes warmer, it can hold more moisture. One consequence of higher water vapor concentrations is more frequent, intense precipitation.
Villarini says the current study did not attempt to link the increase in the number of episodes to climate change.
“What causes the observed changes in precipitation and temperature is not something we have addressed because of the difficulties in doing so just based on observational records,” Villarini says.
The study region included: Iowa, Illinois, Indiana, Wisconsin, Ohio, Michigan, Minnesota, Kansas, Nebraska, Missouri, West Virginia, Kentucky, North Dakota and South Dakota.
The methodology used in the study involved establishing a threshold level of two flood events per year, on average, for each of the 774 stream gauges in the study, so as to focus on the larger flood events. In order to avoid counting the same event twice, the researchers allowed for the recording of only one event within a 15-day period.
The paper titled, “The Changing Nature of Flooding across the Central United States,” can be found on Nature’s website.
The U.S. Army Corps of Engineers Institute for Water Resources, the Iowa Flood Center and IIHR-Hydroscience & Engineering funded the work. Funding also came from the National Science Foundation, under NSF CAREER Grant #AGS-1349827.
In 2010, Scott Rowe spent his spring break sandbagging in Fargo, N.D., to help protect the city from floodwaters. At the time, he was an undergraduate studying atmospheric sciences at the University of North Dakota. The sandbagging effort was successful, and Rowe says he was intrigued to see how the community came together to respond to the flood threat. “It was fun. We were overfed, almost,” Rowe says, remembering the outpouring of gratitude and food for the volunteers.
He also came away with a serious interest in hydrology, which brought him to IIHR and the Iowa Flood Center for graduate studies. But Rowe has been interested in weather for a long time. He spent the first 12 years of his life in Tornado Alley. Since Texas homes rarely have basements, Rowe took shelter in the bathtub when tornado warnings were issued, sometimes with his bike helmet on for extra protection.
From then on, Rowe was fascinated by weather. He is now working on a master’s degree at IIHR under the supervision of his advisor, Gabriele Villarini. Together, they are developing a statewide climate forecast for Iowa to offer a basic idea of what precipitation can be expected in the season ahead. Their forecasts will be basic by design, Rowe explains, and limited to predicting whether precipitation will be average, above average, or below average. “It’s difficult to even make an exact precipitation forecast for 24 hours for now,” he says. Compound that over an entire season, Rowe says, and that error can become “ginormous.” With that said, depending on the success of the new system, Rowe and Villarini may attempt more quantitative statements in the future.
Rowe is analyzing Iowa’s climate data to rank the average monthly temperature and precipitation back to 1950. By correlating the atmospheric anomalies associated with the wettest and driest months, Rowe says, they’re able to use a number of existing climate models to develop a long-lead climate forecast.
Once Rowe and Villarini have a few forecasts under their belt, they’ll evaluate the accuracy by comparing their prediction to what actually happened. That information will be vital to improving the predictions, Rowe explains. “A bad forecast model is of no use to us.”
Although Rowe has no interest in broadcasting, he enjoys weather forecasting and hopes to find a job with the National Weather Service or some other forecasting agency after graduation. He may even find his way back to North Dakota, where he first got his feet wet in weather forecasting.
Iowa Flood Center
The University of Iowa
100 Stanley Hydraulics Laboratory
Iowa City, IA 52242
Contact: Breanna Shea