Wetter climate, earlier snowmelt runoff noted in Great Lakes study

According to a report from the U.S. Geological Survey (USGS), more precipitation has been falling recently in the Great Lakes Basin than in the more distant past. During the past 90 years, total annual precipitation increased by 4.5 inches and much of that increase occurred during the most recent third of that time span. "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...

RESTON, VA, Oct. 29, 2007 -- According to a report from the U.S. Geological Survey (USGS), more precipitation has been falling recently in the Great Lakes Basin than in the more distant past. During the past 90 years, total annual precipitation increased by 4.5 inches and much of that increase occurred during the most recent third of that time span.

"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."

Despite the increase in precipitation, the scientists determined that streamflow in the basin increased more modestly. 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-gaging stations. The difference may be partly caused by increased evapotranspiration -- water lost to the atmosphere through evaporation from water bodies 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 U.S. and how those volumes have been changing over the last few decades."

The Great Lakes Basin, which encompasses Lakes Superior, Michigan, Huron, Erie, and Ontario, contains 95 percent of the fresh surface water in North America and 18 percent of the fresh surface water in the world. Ground water underlying the basin constitutes another large volume of freshwater. It has been estimated that streams contribute 46 percent of the water that goes into the Great Lakes. Direct precipitation into the lakes makes up about 53 percent. The remaining 1 percent of water comes to the Great Lakes by diverting water from outside of the basin.

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.

The report, Historical Changes in Precipitation and Streamflow in the U.S. Great Lakes Basin, 1915-2004, can be found at: http://pubs.usgs.gov/sir/2007/5118/

This report and others on the Great Lakes are also available on USGS's new website: National Water Availability and Use Program - Great Lakes Basin Pilot

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