Flood threat levels around the United States are changing, according to a new study from the University of Iowa.
Across the northern half of the country, the risk of floods is rising, while in the southern half it is declining.
This is due in part to the amount of groundwater in those regions. The Midwest, for example, has more groundwater stored up than the Southwest. But this distribution isn’t even, which is causing some confusion among scientists.
While the northern half of the country in general is retaining more groundwater, the flood threat is not evenly distributed, and some parts of the north are seeing more risk than others. This is most likely due to overall rainfall, with the Midwest seeing increased heavy rains over the last half a century, leading to what the researchers call a “higher water base line,” while the land west of the Rocky Mountains has been in a drought for the past several years.
“The river basins have a memory,” says post-doctoral researcher and paper co-author Louise Slater. “So, if a river basin is getting wetter, in the Midwest for example, your flood risk is also probably increasing because there’s more water in the system.”
What the researchers hope to achieve is to align the results of flood risk assessments done by the geological community with the National Weather Service terminology that most people understand. In the past, flood risk has been communicated using stream flow, or the amount of water flowing per unit time.
“The concept is simple,” says study co-author Gabriele Villarini. “We’re measuring what people really care about.”
Ultimately, the differences in flood threat across the country are due to regional differences in rainfall and stored groundwater, the researchers say. Both of these things have been changing considerably in recent decades thanks to global climate change.
As we get a better handle on the effects of changing weather patterns, we may be able to respond to them and find ways to prevent flooding in wetter areas and offset droughts in drier areas. But that’s going to require time, effort, funding, and most importantly support from federal, state, and local governments.