LEVELnews: monitoring Great Lakes and St. Lawrence River water levels

LEVELnews is a newsletter that provides a monthly update on Great Lakes and St. Lawrence River water levels. The most recent editions of LEVELnews can be requested via the links below. Older editions of LEVELnews can be found in the Government of Canada Publications Catalogue, or contact ec.LEVELnews-infoNIVEAU.ec@canada.ca

A note on projections

Predicting future lake levels can be a benefit for many that live around and use the Great Lakes. In order to estimate where lake levels may be in a few months, water resources engineers look to measurements of the Great Lakes made in the past. Based on the record of Great Lakes levels (1918–Present), changes to lake levels can be predicted for a range of wet and dry conditions. If you look at the water levels graphs in the Canadian Hydrographic Service’s monthly water levels bulletin (The Canadian Hydrographic Service (waterlevels.gc.ca)) you will note a “Probable Range of Future Levels” depicted by dashed lines on the included figures. The red dashed line is a prediction of water levels if relatively wet conditions are encountered and lake levels are expected to be above these values 5% of the time.  The blue dashed line is a prediction of lake levels if relatively dry conditions are encountered and lake levels are expected to be below these values 5% of the time. The levels between these two lines is the “Probable Range of Future Levels”, which is based on the recorded history of the lakes.  This is the range that we would expect lake levels to be within 90% of the time.

What is net basin supply

The primary driver of water levels across the Great Lakes-St. Lawrence River basin is the amount of water coming into the system, referred to as water supplies. Total water supplies to the lakes is the combination of the water that is entering from the upstream lake as well as water entering from the lake’s basin itself, known as net basin supply (NBS). NBS is the total of the precipitation that falls directly on the lake surface and the runoff that enters the lake through the drainage basin, minus the evaporation that comes off the lake. NBS is calculated as follows and is a function of the water balance of the basins:

Long description  

The left side of the equation is called residual NBS and is the change in storage, minus inflow, plus outflow, plus or minus any diversions. The right side of the equation is the component NBS, which is precipitation, plus runoff, minus evaporation, plus an error term. The equation shows that the left side is equal to the right side, and therefore, residual NBS is equal to component NBS plus the error.

Residual NBS is a measure of the change in storage (ΔS) minus inflow (Qin) from an upstream lake, plus outflow (Qout) to a downstream lake, plus or minus any diversions (D) that existFootnote 1 . Component NBS is calculated by adding over lake precipitation (P) and runoff (R) and subtracting over lake evaporation (E). These are established through measurements and modelled estimates. A figure showing the components of residual and component NBS are provided below.

Residual NBS is computed with a great deal of accuracy due to extensive gauging on the connecting channels and lake level measurements. There is more uncertainty in the over lake precipitation and evaporation, due to the difficultly in establishing monitoring stations on the lakes and the large surface area of the lakes. Runoff from the land surface is not measured at every river entering the lakes and the inflow from the ungauged rivers must be estimated. Due to the higher level of uncertainty in the component NBS measurements, an error term (ε) is added to that side of the water balance. For these reasons, residual NBS is considered to have a higher degree of accuracy in its measurement.

In previous versions of LEVELnews, we have commonly referred to basin water supply conditions, which is in reference to NBS. Since it has such an important role in describing the basin scale water balance, we will include residual NBS for each basin, in addition to precipitation, in the current and future editions.

Long description  

The figure presents a lake level water balance, which shows a cross-sectional profile of a lake, a river flowing into the lake representing inflow from an upstream lake, and a river flowing out of the lake representing outflow to a downstream lake. Also included are light blue arrows pointing toward the lake indicating runoff into the lake from the watershed and over lake precipitation. Over lake evaporation is shown in the figure as red arrows pointing upward and leaving the lake surface. Diversions into and out of the lake are indicated with two black wavy arrows respectively.

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