A lake is exposed to meteorological forcing through the lake surface and
hydrologic inputs from the lake basin. Solar radiation and atmospheric long wave
radiation heat the water column, while evaporation and back radiation cool the water
column. Inflows may heat or cool the water depending on the relative thermal state of
the water column at the time of concern. In addition, convective heat transfer driven by
the temperature difference between the water temperature and air temperature can also
warm or cool a lake. The differential radiative heat absorption throughout the lake depth
causes thermal stratification of the water body. The stronger the stratification, the more
quiescent i.e. the more stable the water body. The external forcing i.e. wind exerts a
drag force on the surface of the lake which, through a variety of external and internal
wave motions tends to vertically mix the stratified water column (partially or completely).
The external mechanical energy input from the wind is opposed by the potential (buoyant)
energy "locked" in the stratification. The stronger the stratification, the more mechanical
energy is needed to mix the water column.
A schematic representation of a seasonal temperature stratification in a dimictic
lake is given in Fig. 1a. The open-water season usually starts some time in April or May
in Minnesota lakes depending on the geographical location and the size of the lake. Most
lakes are well mixed throughout the entire lake depth in spring. The onset of stratification
occurs with the increase of solar radiation intensity and some decrease in the wind
activity. The thermal stratification increases in strength from May through July or
August. Further water temperature increase in summer is limited by the evaporative heat
losses, and by back radiation. In September, solar radiation and air temperature are
significantly lower, and wind is often higher, resulting in strong surface cooling, natural
convection, and wind-induced mixing. A three layer structure is well defined throughout
the summer in many lakes. The surface mixed layer is called 'epilimnion', underneath
is a zone of temperature gradient, the 'metalimnion'; and below is the 'hypolimnion'.
Surface mixed layer depth increases in the fall until the lake becomes isothermal at a
temperature above or equal to 4°C.
Environmental Research Laboratory, US Environmental Protection Agency
Stefan, Heinz G.; Hondzo, M.; Fang, Xing.
Simulated Long-Term Temperature and Dissolved Oxygen Characteristics of Minnesota Lakes and Resulting Habitat Limits for Fishes.
St. Anthony Falls Hydraulic Laboratory.
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