Evaporation 

Evaporation plays an important role in the hydrology of Pelican Lake. Typically evaporation exceeds rainfall by about 10 to 12 inches during open water. The rate of evaporation in July and August can be up to a quarter of an inch per day. The table below shows the range of daily evaporation for 12 drier years. 

The key drivers of evaporation from a lake are solar radiation, wind, humidity and surface water temperature. Pelican is protected from some winds by the high valley walls so evaporation might be a little less than some similar lakes. Air temperature is not very important except how it affects humidity. Surface water temperature is primarily a function of the solar radiation. 

As this chart shows the evaporation does not change with air temperature. 

In 2023 the rate of drop in the level was fairly steady from July 1 to September 17 (78 days). The the average evaporation was about 0.17 inches per day. That's about 1.44 cubic meters per second or the same as having one gate half open with the lake at 1351.7.

Here is a chart showing the water temperature for the water at the south end for May 2023. It was warmer than usual but still a good example of how quickly the lake warms up.

It's interesting to compare consecutive days. For example the chart below shows the solar radiation for May 15 and 16, 2023.  On the 15th the total solar heat was 6.75 kilowat-hours per square meter compared to 4.75 on the 16th. The 15th was 42% more than the 16th yet the average air temperature for both days was 18.8 with highs of 26.8. On the other hand water temperature peaked at 18.5 on the 15th and 16.9 the 16th both at 4 pm.

Below is a chart showing the sun's radiation, water and air temperatures for July 17, 2022, a calm hot day. The grey line for radiation peaks at noon, at 780 watts per square meter (right axis), air temperature, the blue line, peaks at 9 p.m. at 33 celcius and the surface  water temperature, orange line, at 7 p.m. at 28.5 c.

On that day the lake received about 6.8  kilowatt-hours per square meter. This is enough, in theory, to raise the temperature of the lake about 1.5 degrees Celsius. However things are  complicated, some of the energy goes into warming the water, some is reflected by the water and  some goes into evaporation. The split depends primarily on how clean and warm the water is. For example in the spring the lake water is clear and cold therefore the sunlight penetrates deep into the lake and the water absorbs the heat with little evaporation. In July the radiation can't penetrate as  far into the lake so heats the top of the water quickly and more of the heat goes into evaporation and warming the surface water. This is further complicated by wind and humidity. 

The amount of energy gained from solar radiation also depends on the angle of the sun. When directly overhead as much as 90% of the heat will warm the water but at sunrise almost all the energy is reflected. 

Typically during the summer roughly 6 kw-hr per square meter per day  reaches the water surface. Let's assume half is reflected. It takes about 4.5 kw-hr per square meter to raise 3.9 meters depth of water (the average depth of Pelican Lake) one degree Celsius. So on a typical day through the summer the water warms about 0.67 C per day. If the lake was a foot deeper it would be about 0.62 C. It would take 26 days, almost two days longer, to raise the lake 16 degrees as in May, 2023. 

Pelican Lake is a shallow lake (less than 10 meters) so for most of the summer the water temperature is the same at the bottom as the surface. As weather warms up the lake generally maintains a temperature in the low 20's. From then on through July and August the energy from the sun and air is balanced by evaporation and some cooling at night. 

It takes 125 million kw-hr to raise the temperature of the lake one degree Celsius. This is 3.3 times the energy Manitoba Hydro generated on average per day in 2022.

For some further background here are averages for Baldur 1981 to 2010.