Energy Budget Processes of a Small Northern Lake

Abstract

There is a paucity of information on the energy budget of Canada's northern lakes. This research determines processes controlling the magnitude of energy fluxes between a small Canadian Shield lake and the atmosphere. Meteorological instruments were deployed on a floating platform in the middle of a 5-ha lake during the 1999 and 2000 open-water seasons. High attenuation of incoming radiation at shallow depths and the sheltered location of the lake allows a strong thermocline to develop during the summer months, which prevents deeper water from exchanging energy with the atmosphere. Only after the lake becomes isothermal in late August do deeper waters interact with the atmosphere. When the lake is warming, evaporation is controlled by net radiation, but when the lake is cooling, turbulent energy fluxes are mainly influenced by the vapor pressure deficit. An empirically derived logarithmic relationship was identified between the Bowen ratio and the vapor pressure deficit. The Canadian Global Energy and Water Cycle Experiment (GEWEX) Enhanced Study (CAGES) water year was characterized by a cool dry July that prevented the lake from warming to expected normal conditions. With less of the available energy directed to heating the lake, more was available for the turbulent fluxes, but evaporation rates did not increase. Because of the inability of radiation to penetrate to deep water in this lake, it is unlikely that even local extremes in air temperature and incoming solar radiation create the summer isothermal conditions observed in more southern Canadian Shield lakes, which allow more energy to be directed toward evaporation during the summer months.