Evaporation represents the smallest loss of water from Mirror Lake. Evaporation from the lake is being determined by the energy-budget method, which accounts for all of the energy entering, leaving, and stored in the lake.

Pyranometer to measure solar radiation and radiometer to measure long wave radiation

To make this accounting, instruments are needed to measure the solar (short wave) radiation entering the lake and the atmospheric (long wave) radiation entering and leaving the lake. In addition, the energy brought into the lake by precipitation, surface water, and ground water, and leaving the lake by surface outflow and seepage to ground water needs to be determined. This is done by measuring the volume and temperature of each of these hydrologic components. To determine the energy stored in the lake, thermal surveys, which are measurements of the water temperature at different depths in the lake, need to be made at regular intervals (usually every week or two).

The energy balance of Mirror Lake is related to the distribution of energy within the lake over the course of a year. The main drivers of the energy balance of the lake are the radiation components. At the time the ice melts off the lake, the temperature of the lake is about the same from top to bottom, about 4°C. Immediately thereafter, solar radiation begins to warm the surface layer of the lake and the lake begins to stratify, with the warmer water on top and the colder water beneath it. Even though a large amount of stream flow from snowmelt usually enters the lake in the spring, the water is cold and it does not add much, if any, heat to the lake water. In addition, at the same time there is a large amount of inflow there is an equally large amount of surface outflow, so the residence time of the stream inflow is very short and it has little effect on the energy budget. Ground water inflow is a small part of the water budget of the lake, and ground-water temperatures are very low, about 9°C, so ground water inflow does not add much heat to the lake, either. The third input to the lake, precipitation, also adds little heat to the lake because it is intermittent and it usually is cold, especially in spring.

Because of the minor effect of heat input by streams, ground water, and precipitation, it is primarily the radiation input to the lake that causes the lake to heat up. As the summer progresses and the radiation input increases, the warmer upper part of the lake (termed the epilimnion) becomes thicker and thicker, and the lower colder part of the lake (termed the hypolimnion) becomes smaller. The lake surface temperature usually reaches a maximum, about 27°C, in late July and early August, and this is the time that the evaporation rates are also the greatest. By early fall nearly all of the lake consists of the warmer epilimnion. About this time the lake becomes very unstable thermally because of the small temperature contrasts from top to bottom. Under these conditions, on a windy day the lake will become pretty much the same temperature throughout, a process called “turnover”. Once ice forms on the lake, the water will remain close to 4°C throughout the winter.

Based on the interaction of climate and the thermal characteristics of the lake described above, Mirror Lake has a seasonal pattern of evaporation. Evaporation rates are small, generally less than 2 mm/day, in the spring when the air and the lake are cool. They increase to a maximum, generally more than 4 mm/day, in late July and early August when the lake’s surface is the warmest. The evaporation rates then decline to very low levels in the fall. However, this seasonal pattern can vary from year to year depending on how quickly the lake heats up from solar radiation.