Why Flooding Is Occurring More Often

ISU climatologist Elwynn Taylor explains why floods are occurring more often than they used to.

This year's excessive rainfall is a sign that the climate has changed. That's the view of Elwynn Taylor, Iowa State University Extension climatologist. It only takes a glance at this year's precipitation map of the United States to identify the region of abnormal wetness. The record-setting floods of 2008 in Iowa and the flooding in other Midwest states are no surprise, considering the amount of precipitation.

"If it seems that high water years in the Midwest are increasingly common since about 1970 - they are," says Taylor.

The 2008 floods arrived earlier than did the floods of 1993 and in many cases the record high flood levels of 1993 have been surpassed. Also, the 2008 flooding was more widespread in the Midwest. The 1993 floods became serious in July and continued into August, past the date when corn vigorously develops new and extensive roots systems. The 1993 flooding resulted in greatly reduced corn yield in every county in Iowa - only three reached 100 bushels per acre or greater.

The climate has indeed changed

The Midwest now receives about 10% more annual precipitation than was received before 1970, says Taylor. This increase has effectively doubled the annual stream flow in much of the region. Accordingly, rivers are more often over their banks. In the 40 years up to 1970 there were two "high" water years. In the subsequent 40 years there have been 12.

Accordingly, a flood event that might have been expected once every 200 years in the past would be expected every 33 years or so under current climate conditions. Rivers and streams across the western Corn Belt have responded to the changing climate.

Looking at what causes storm systems, Taylor says the Bermuda High is a consistent, persistent feature of summer. It is the primary force moving moisture into the Midwest and should it fail to develop, widespread drought is the result.

In 2008 an "early arriving" Bermuda High together with a Colorado Low resulted in a much stronger-than-usual flow of moist air into the Midwest. The jet stream associated with the Low system, also not a typical spring resident of the High Plains, provided the impetus for extensive storms to develop in the Midwest this year.

There has been a gradual migration of the Low toward a more typical summer location in Canada. So the extreme storms in the Midwest will likely diminish when the Low pressure system diminishes or moves north.

Similar conditions existed in 1947, a year with many record flood reports followed by severe drought in the Corn Belt. In Iowa, 1983 was also similar with a very wet spring and a harsh dry summer. The chance of changing to drought conditions this year appears to be about 25% and to the warm and dry side of usual (sufficient to reduce Corn Belt yields to below trend) is about 62%.

So what's the likely yield impact in 2008?

The sum of weather clues, including the long range forecasts from the National Weather Service, indicate that 2008 may be an extreme year, says Taylor. The U.S. corn yield is most likely to average 148 bushels per acre, and the chance of widespread drought remains higher than average at slightly less than one chance in three. However, extremes are likely during the coming years. A somewhat below-trend harvest in 2008 is expected.

As of June 20, Taylor's outlook is for Iowa to return to a normal weather pattern for summer. Normal weather will give people a chance to clean up in areas where there is disaster recovery work to do, he notes.

Taylor has a presentation "Floods 2008" that explains why Iowa is experiencing the current weather pattern, his forecast for the rest of the 2008 crop season and what impact weather is having on crop yield forecasts. You can listen to his presentation and get his latest forecast update by visiting his Web site at www.extension.iastate.edu/weather.

TAGS: Extension
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