Net basin supply (NBS) is the primary driver of Great Lakes water levels. NBS represents the total contribution of water to each lake, excluding inflows from upstream lakes, outflows to downstream lakes, and diversions into or out of the lakes, as shown in the graphic to the right. In other words, NBS represents the net influence of precipitation over the lake, runoff from a lake's watershed into the lake, and evaporation from the lake's surface.
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This page shows trends in precipitation, evaporation, and runoff from 1950 to 2018 for Lakes Superior, Michigan-Huron, Erie, and Ontario. There are four sections, one for each lake, that provide a table summarizing monthly and annual trends, a monthly graphic that displays values of precipitation, evaporation, and runoff by month from 1950 to 2018, and an annual graphic that shows the accumulated precipitation, evaporation, and runoff in each year from 1950 to 2018. A black line is plotted to help represent the patterns and trends in the data on a monthly and annual temporal scale. These trends are based on the data shown from 1950 to 2018 and may not be reflective of future trends. See the Data Description section for more information.
Use the links below to go directly to the graphs for each lake.
Lake Superior Lake Michigan-Huron Lake Erie Lake Ontario Data Information
Lake Superior
Precipitation |
Evaporation |
Runoff |
Precipitation during October shows an increasing trend from 1950 to the mid 2000s. |
Evaporation during the winter months has shown an increasing trend since the 1950s. |
Runoff in April has been lower in recent years, while in May a decreasing trend is seen from 1950-2018. |
Precipitation does not show signs of any trend on the annual timescale since 1950. |
Annual accumulated evaporation has been increasing over the last 7 decades. |
Over the last 7 decades there has been no clear trend in annual runoff, but in the last 15 years runoff rates have been lower. |
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Lake Michigan-Huron
Precipitation |
Evaporation |
Runoff |
In recent years, there have been more Octobers with higher precipitation. |
Evaporation during July and August has shown an increasing trend in the last 3 decades. |
Runoff in April has shown a decreasing trend since 1950. |
Precipitation does not show signs of any trend on the annual timescale since 1950. |
Annual accumulated evaporation has shown an increasing trend over the last 4 decades. |
In the last 10 years, runoff rates have been lower, but generally there is no clear trend. |
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Lake Erie
Precipitation |
Evaporation |
Runoff |
May and June precipitation show an increasing trend since 1950. |
Evaporation has a decreasing trend in the last 2 decades during September and in the last decade during October. |
Runoff shows a large amount of variability in the winter and early spring months, but no clear sign of a trend. |
Precipitation does not show signs of any trend on the annual timescale since 1950. |
Evaporation does not show signs of any trend on the annual timescale since 1950. |
Runoff does not show signs of any trend on the annual timescale since 1950. |
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Lake Ontario
Precipitation |
Evaporation |
Runoff |
June and October precipitation have an increasing trend since 1950, while September and November precipitation have a decreasing trend in the last 2 to 3 decades. |
Monthly evaporation shows no clear trend in any month. |
Runoff during March shows a decreasing trend in the last 20 years, while in May there has been an increasing trend. |
Since the 1970s, high annual rates of precipitation have been more frequent. |
Evaporation does not show signs of any trend on the annual timescale since 1950. |
After a few years with low runoff, recent years have had higher runoff rates, but generally there is no clear trend. |
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Data Information
NBS Component Data
Precipitation data is coordinated through 2016 and can be found on the Great Lakes Coordinating Committee website. Link: http://www.greatlakescc.org/wp36/home/coordinating-committee-products-and-datasets/ (not currently available)
Precipitation data for 2017 is from the USACE web reports. Link: https://www.lre.usace.army.mil/Missions/Great-Lakes-Information/Weather-Information/
Evaporation data is modeled using the Large Lake Thermodynamics Model (LLTM).
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Croley, T. E. (1989). Verifiable evaporation modeling on the Laurentian Great Lakes. Water Resources Research, 25(5), 781-792.
Runoff data is modeled using the Large Basin Runoff Model (LBRM).
Graphics inspired by Hunter et al. 2015.
- Hunter, T. S., Clites, A. H., Campbell, K. B., & Gronewold, A. D. (2015). Development and application of a North American Great Lakes hydrometeorological database—Part I: Precipitation, evaporation, runoff, and air temperature. Journal of Great Lakes Research, 41(1), 65-77.
**The black lines in the graphics are calculated using a locally weighted regression, more information located here: https://www.rdocumentation.org/packages/stats/versions/3.5.1/topics/loess
All NBS graphics will be updated in the spring of each year.
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