The energy balance and vertical thermal structure of two small boreal lakes in summer

Two small lakes in Sweden were studied using one-dimensional approach. Their summer energy balance and temperature structure were considered based on measurements and on modelling on different scales. Tamnaren is a vertically-mixed lake because it is very shallow and has a large surface area. RÅksjo is small, sheltered and typically stratified during summer. Model adjustments and calibrations were made using detailed microm-eteorological input data, considering areal variations. Components of the heat balance were determined in order to solve for water temperature and heat exchange. Absorption of energy inside water was determined on the basis of measurements. The vertical temperature structure was solved using a ke-model. The results clearly show that sheltering caused by shores must be taken into account. Modifying the wind speed has a strong effect on temperature, especially at greater depths, and on heat balance. An isothermal box lake model, the SLAB model, was used with the same input. Surface temperature is strongly influenced by meteorological conditions, and both models solve it very similarly. Diurnal temperature variations, which can be as much as several degrees, are lost with the SLAB model. The PROBE model is able to solve these variations and the vertical temperature profile. However, if the important boundary effects are not taken into account, neither can be solved satisfactorily. Point measurements gave a residual of -53 W m -2 for Tamnaren, and -42 W m -2 for RÅksjo. When wind was reduced to 80% for Tamnaren, and 60% for RÅksjo to account for the net areal effect, the residuals were 0 and -1 W m -2 for the lakes. Temperature profiles were also satisfactorily solved.