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 4Â°C.
|Original language||English (US)|
|State||Published - Aug 1995|