The U.S. Midwest region and surrounding states have been through increasingly more frequent floods during the last half-century, according to results of a new study prepared by researchers affiliated with the University of Iowa.
The 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, or 34%, of the stations had an increase in frequency in the number of flood events, while only 66 stations, 9%, showed a decrease.
"We have been experiencing a larger number of big floods," says Gabriele Villarini, UI civil and environmental engineer and corresponding author of a paper reporting the results published this week in the journal Nature Climate Change.
During the past several decades, large floods have been recorded in the Midwest and surrounding states in 1993, 2008, 2011, 2013 and again in 2014. The floods caused agricultural and other economic losses in the billions of dollars.
"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 civil and environmental engineer and lead author of the paper.
"We related this increasing number of big floods to changes in rainfall and temperature," Mallakpour said. "There was an overall good match between the areas with increasing frequency of floods and areas experiencing increasing frequency of heavy rainfall."
Seasonal analysis revealed that most of the flood peaks in the upper Midwest occur in the spring and stem primarily from snowmelt, rain falling on frozen ground, and rain-on-snow events. Spring also has the strongest increase in temperature over most of the northern part of the region studied.
Villarini says the current study did not attempt to link the increase in the number of episodes with climate change.
"What causes the observed changes in precipitation and temperature is not something we have addressed because of the difficulties in doing so based on observational records," he says.
The study region included Iowa, Illinois, Indiana, Wisconsin, Ohio, Michigan, Minnesota, Kansas, Nebraska, Missouri, West Virginia, Kentucky, North Dakota and South Dakota.
The method used involved establishing a threshold level of two flood events per year, on average, for each of the 774 stream gauges in the study.
To avoid counting the same event twice, the researchers allowed for the recording of only one event within a 15-day period.
The research was also funded by the National Science Foundation, the U.S. Army Corps of Engineers Institute for Water Resources, the Iowa Flood Center and IIHR-Hydroscience & Engineering.