Great Lakes Seeing More Precipitation

While water levels in the Great Lakes are dropping, more precipitation is actually falling on the Great Lakes Basin now than in the past, according to a new study from the U.S. Geological Survey (USGS).

The study, "Historical Changes in Precipitation and Streamflow in the U.S. Great Lakes Basin, 1915-2004," finds that the area is getting 4.5 inches more rain or snow per year now than it did 90 years ago. Most of that increase has occurred over the past 30 years, the study added.

"This study is a key component in assessing water availability and use and in building a water census for the nation," said Robert Hirsch, USGS Associate Director for Water. "We now have a much clearer picture of the volume of water being added to the largest fresh-water system in the U.S. and how those volumes have been changing over the last few decades."

The state of the nation's water supply has received increasing attention lately, thanks to ongoing extreme drought in much of the Southeast, wildfires in the West and shrinking lake levels in the Great Lakes. According to the National Oceanic and Atmospheric Administration (NOAA), levels in all five Great Lakes have been declining since the late 1990s.

This past September, both Lakes Michigan and Huron were about two feet below their long-term normal levels, while Lake Superior was about 20 inches below normal. Lakes Ontario and Erie were also lower than normal, by about seven and three inches, respectively.

Precipitation in the Great Lakes Basin accounts for a little over half (53 percent) of the freshwater entering the lakes, while streamflow into the basin adds 46 percent. Another 1 percent comes from water being diverted from outside the basin.

The USGS study found that, while precipitation over the basin has increased, streamflow into the lakes hasn't grown by nearly as much. While annual rainfall and snowfall have increased by 4.2 inches over the past 50 years, average runoff into the basin has grown by only 2.6 inches per year. USGS researchers speculate the difference might be caused in part by increased evaporation and transpiration (evaporation through the leaves and other parts of plants).

The study also reported that average precipitation has dropped in February and March, while streamflow during those months increased. The situation becomes reversed in April, when precipitation has increased but streamflow has declined. Those findings indicate that snow is melting and running off earlier in the year now than in the past.

The USGS began its Water Availability and Use Initiative in 2005, with the Great Lakes study being its first pilot study. The agency hopes to expand its research to include other water-resource regions as funding becomes available. It says the reports are aimed at helping resource managers, communities and citizens better understand how much water is available now and how much will be available in the future.

U.S. Geological Survey, "Wetter Climate and Earlier Snowmelt Runoff Noted in Great Lakes Basin Study." URL: (http://www.usgs.gov/newsroom/article.asp?ID=1813)