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Hydrological Budgets

This page is part of the project: Great Lakes Climate Change and Lake-Levels

Click on the arrows to learn more about each variable.

For more information on Lake-Level terms, see the project's Glossary of Terms and Acronyms.


Runoff Over-lake Precipitation Over-land Precipitation Land Evapotranspiration Evaporation from Lake St. Marys River Flow Straits of Mackinac Flow Huron-Erie Corridor Flow Niagara River Flow St Lawrence River Flow Chicago Diversion Ogoki Diversion Long Lac Diversion

Overview of Lake Levels

The above figure represents the hydrologic inputs to and outputs from the Great Lakes basin that contribute to changes in lake levels. Precipitation that falls over the lakes and evaporation from the lake surfaces contribute directly to the water levels, and precipitation over land and evapotranspiration from the land contribute indirectly through their effects on runoff into the lakes. Other factors, such as dams and other man-made control systems also contribute to the water levels. These natural processes and man-made control systems are modeled by the Net Basin Supply (NBS) Equation, shown below. When the NBS is positive, there is an increase in lake levels, and when it is negative, there is a decrease.

Net Basin Supply Equation:

NBS = I + P + R - O - E - CU +/- D

NBS = Net Basin Supply
I = inflow from an upper lake
P = over-lake precipitation
R = runoff into the lake (includes over-land precipitation and evapotranspiration)
O = outflow from the lake
E = evaporation from the lake surface
CU = consumptive use
D = diversion

This equation can apply to the entire Great Lakes basin, as well as each individual lake. Changes in lake levels can be explained by a whole-basin approach, but not all of the lakes behave in exactly the same way. Because of the different properties of each of the lakes, including management and control systems, as well as differences in climatology over the area of the basin, the level of each lake is affected differently by the effects of changing climate. For example, Lakes Superior and Ontario have man-made control structures at their outlets (St. Marys River and St. Lawrence River, respectively), providing more stable lake levels compared to the rest of the lakes.

For more information and data regarding Great Lakes water levels, visit the Great Lakes Water Level Dashboard.