Lake Simcoe Water Balance

Quantification of the components of the hydrologic cycle described in the previous section is undertaken regularly to yield a water budget for Lake Simcoe.  A mass balance approach is followed that requires knowledge of inputs, outputs and changes in storage over time.  From the water balance values, nutrient loadings to the lake are estimated from the various pathways based on measured concentrations where possible, and interpolation from known concentrations where not.

INPUTS

Precipitation

Direct precipitation on the lake is estimated based upon interpretation of measurements made at inland locations until 2002 when a gauge was added on Georgina Island.  The mean lake precipitation between 1990 and 1998 was 5.882 x 10^8 cubic metres per year.  

Surface Water Inflow

Stream discharge flowing into Lake Simcoe is monitored in 8 of the lake’s subwatersheds at various points on the stream (some near the headwaters, some near the outlets).  Based upon the measured discharge, the contributing area to the gauge location, and the land use in that contributing area, measured discharge values are extrapolated to the lake outlets in each gauged subwatershed (see LSEMS technical Report A4).  For those areas not monitored, discharge values are estimated based upon the average behaviour of gauged areas. 

Point source inputs of water in ungauged areas from sewage treatment plants (STP) (discharge records are maintained by the municipalities) are included in the gauged discharge term.  Some STPs that discharge directly to the lake in ungauged areas, however, were not included in the inputs term because Lake Simcoe serves as the potable water source and takings were assumed to balance STP discharge.  Mean gauged discharge into Lake Simcoe between 1990 and 1998 was 3.61 x 10^8 cubic metres per year, and mean ungauged runoff was estimated at 4.85 x 10^8 cubic metres per year.

Groundwater Flow

Direct groundwater discharge to the lake has not historically been considered because of the lack of information available on groundwater flux. 

OUTPUTS

Evaporation

Evaporation from the surface of Lake Simcoe is estimated based upon meteorological measurements made at two stations in the watershed (see LSEMS technical Report A4).  From these data, mean lake evaporation was estimated to be 5.372 x 10^8 cubic metres per year for the period 1990-1998.

Surface Water Outflow

Lake Simcoe ultimately drains to Georgian Bay through Lake Couchiching and the Black River via the Atherly Narrows.  Unfortunately, no gauge exists at the Atherly Narrows.  Lake Simcoe outflow is therefore calculated based upon measured discharge in the Black River at Washago, which is pro-rated to Lake Couchiching and then to the Atherly Narrows (see LSEMS technical Report A4).  From this process, mean lake outflows were calculated to be 9.429 x 10^8 cubic metres per year for the period 1990-1998.

Groundwater Flow

Like groundwater discharge to the lake, any loss of water from the lake via deep recharging gradients has not historically been considered.   

STORAGE

Lake Levels

Water storage in Lake Simcoe is estimated based upon fluctuation in lake level as measured at two Trent Severn Waterway stations.  From these data, mean lake storage was estimated to be –1.0702 x 10^8 cubic metres per year (indicating a small net loss of water from the lake) for the period 1990-1998.

Groundwater Storage

Fluctuation in groundwater levels in the watershed has not historically been considered in the water balance exercise.   

WATER BALANCE

In order to evaluate the accuracy of the estimation of water balance components, the difference between inputs and outputs was compared as a percentage.  Using 8-year mean data, it is apparent that there is either a consistent underestimation of inputs or overestimation of outputs.  The difference over this time frame, however, was only 3.22% indicating acceptable margins of error given the assumptions necessary in such a large-scale investigation and the omission of groundwater influences (see LSEMS technical Report A4).

From each of the water balance components evaluated, the relative contributions to and losses from Lake Simcoe can be illustrated.  This pie chart demonstrates that surface water runoff (both gauged and ungauged) is the most significant source of water to the lake, which makes sense in light of the catchment area (3,634 square kilometres) versus lake area (722 square kilometres).  The largest loss of water from the lake is outflow while the change in lake storage, which was negative over the period 1990-1998, is too small to be discerned on the chart.