Long-term energy flux measurements and energy balance over a small boreal lake using eddy covariance technique

[1] Measurements of the energy balance components of a small boreal lake (area 0.041 km2, mean depth 2.5 m) in southern Finland were performed during four open water periods (April–October) in 2005–2008. Turbulent fluxes of sensible and latent heat acquired using the eddy covariance technique were accompanied by net radiation and water heat storage measurements. In April the lake was near isothermal, whereas in May the development of a thermocline was enabled by dark water color and a sheltered location. The thermocline continued to deepen until September down to the depth of 3.5 m and prevented the deeper water from interacting with the atmosphere. The sensible heat flux was governed by the air-water temperature difference and had its minimum in the afternoon (values down to −45 W m−2) and peaked in the early morning (values up to 32 W m−2). The monthly means ranged from −9 W m−2 (April 2005) to 24 W m−2 (July 2008). The diurnal variation of the latent heat flux, controlled by vapor pressure deficit, had an opposite diurnal phase with a maximum in the afternoon (values up to 120 W m−2) and minimum in the morning (values down to ∼2 W m−2). The monthly averages ranged from 4 W m−2 (October 2008) to 77 W m−2 (July 2006). Furthermore, the lake acted as a heat sink until July and August when the maximum heat content was about 230 MJ m−2. The monthly energy balance closure varied from 57% to 112%, and the average closure was 82% (residual 16 W m−2) and 72% (residual 23 W m−2) for 2006 and 2007, respectively.