The best way to measure the Sylvan Lake evaporation rate is to calculate the slope of the lake level over time during the months of July to November when precipitation decreases and warm dry air causes loss of water. In the period 2000 to 2010 the years of 2000 and 2008 provide two good examples of steady lake level decrease:
In 2000 the net rate of lake level decrease was 2.5 mm per day during the August 1-November 1 period. In 2008 the net lake level decrease occurred at a rate of 2.3 mm per day between July 1 and November 1. That’s enough water to full a cube that is 47 metres on each side, or to cover a football field 8 metres deep. So natural evaporation can remove large quantities of water from Sylvan Lake each day.
Rates of change were never observed to be greater than those values in that decade and in some years no significant evaporation occurred. Later in the year as the lake cools, some of the lake level decrease is also caused by an increase of water density and decrease of water volume with the seasonal temperature drop.