Warmer atmosphere means more moisture, more rain
Missouri River flooding in July, 2011, via NASA’s Earth Observatory program.
Staff Report
FRISCO — After carefully reviewing data from hundreds of stream gauges, University of Iowa scientists say they’ve identified a clear trend of increasing floods during the past 50 years.
“It’s not that big floods are getting bigger, but that we have been experiencing a larger number of big floods,” said Gabriele Villarini, a civil and environmental engineer and corresponding author on the paper, published Feb. 9 in the advance online edition of the journal Nature Climate Change.
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 (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.
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,” said Villarini.
A seasonal analysis showed 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 mesh with other research showing how global warming is affecting the global hydrological cycle. 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 said 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.
