USGS Study: Precipitation Increased for Great Lakes Basin

While more rain and snow has been falling recently in the Great Lakes Basin, streamflow in the area has experienced a modest increase, according to a report released recently by the U.S. Geological Survey (USGS).

During the past 90 years, total annual precipitation increased by 4.5 inches and much of that increase occurred during the most recent 20 to 30 years.

"We saw, as probably anyone living in the Great Lakes Basin has, substantial variability in precipitation from year to year and season to season," said USGS scientist Glenn Hodgkins, who led the study. "But clearly, the basin has been receiving more precipitation than it did in the early 1900s."

During the past 50 years, an average annual precipitation increase of 4.2 inches resulted in an average runoff increase of 2.6 inches as measured at 43 USGS streamflow-gauging stations. The difference may be partly caused by increased evapotranspiration, a condition in which water is lost to the atmosphere through evaporation from waterbodies and soil and transpiration from plants.

"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 United States and how those volumes have been changing over the last few decades."

This study is part of the USGS Water Availability and Use Initiative, which began in 2005 at the request of the Congress with a pilot study of the Great Lakes. The focus of the Great Lakes Basin study is on improving fundamental knowledge of the water balance of the basin, including the flow, storage and withdrawal of water by humans.

A report released earlier this year described historic, lake level changes and their ecological impacts. Reports on groundwater flow and storage and water use in the region also have been published.

The report, "Historical Changes in Precipitation and Streamflow in the U.S. Great Lakes Basin, 1915-2004," can be accessed at

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