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 2021 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 2021, and an annual graphic that shows the accumulated precipitation, evaporation, and runoff in each year from 1950 to 2021. 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 2021 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 through the recent period. |
Evaporation during the winter months has shown an increasing trend since the 1950s, but has leveled off in recent years. |
After several years of high runoff in May and October, runoff has been lower in 2020 and 2021. |
After several years of higher annual precipitation, precipitation has been lower in the last two years. |
On average, annual accumulated evaporation has been increasing over the last 7 decades. |
After a period of higher annual runoff rates, runoff has been lower in the last two years. |
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Lake Michigan-Huron
Precipitation |
Evaporation |
Runoff |
In recent years, there have been more Aprils and Octobers with higher precipitation. |
Evaporation during July and August has shown an increasing trend in the last 3 decades. |
Runoff in May has shown an increasing trend over the last two decades. In recent years, higher runoff has also occurred during winter months with greater frequency. |
In 2021, annual precipitation was below the higher rates of the preceding 5 years. |
Annual accumulated evaporation has shown an increasing trend over the last 4 decades. |
Over the last two decades, annual runoff rates show an increasing trend. |
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Lake Erie
Precipitation |
Evaporation |
Runoff |
May and June precipitation show an increasing trend since 1950. Also, October precipitation shows an increasing trend over the last three decades. |
Evaporation rates during September show a decreasing trend in the last two decades, despite higher rates in recent years. |
Runoff during April and May have been higher in recent years and indicate a slight increasing trend. |
The last two decades have generally brought slightly higher annual precipitation. |
Evaporation does not show signs of any trend on the annual timescale since 1950. |
After several years of high runoff, rates were lower in 2020 and 2021. |
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Lake Ontario
Precipitation |
Evaporation |
Runoff |
June and October precipitation have seen higher precipitation totals in the last two to three decades, while September and November precipitation have a decreasing trend. |
Evaporation during the summer months (June. July, & August), has an increasing trend in the last two to three decades. |
Runoff during January shows an increasing trend over the last five decades. May and June runoff rates have also been higher in recent years. |
Since the 1970s, high annual rates of precipitation have been more frequent. |
Annual rates of evaporation in the last two decades have been generally higher and show a slight increasing trend. |
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 coming from the Great Lakes Seasonal Hydrological Forecasting System.
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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