Physical limnology of Italian lakes. 1. Relationship between morphometry and heat content

The influence of morphometry on maximum and minimum heat content and on pluriannual heat budgets of 31 Italian lakes is analysed. The energy parameters are highly correlated with the morphometric parameters through power regressions. The lakes which markedly deviate from the regression curves are examined on the basis of their environmental conditions or their internal hydrodynamics. In deep lakes (i.e. depth >50 m) there is a high correlation between depth and maximum and minimum heat content, while the heat budget is correlated only with fetch and depths of 50 and 90% annual heat exchange. In shallow lakes there is no relationship between fetch and heat budget, while it is high with all the considered depths. This is due to the different thickness of the water layer, in relation to the maximum depth, which is involved in the annual heat exchanges. We evaluated the water thicknesses within which 40 to 90% of the total heat exchanges take place; the results showed that in deep lakes 90% of the energy exchange is confined to a layer comprising 14-16% of the maximum depth, while in shallow lakes it can reach 82% of the maximum depth. In Lago Maggiore the 90% heat exchange depth increased by 0.34 m per year from 1963 to 1999, thus contributing to heating the whole water mass of the lake.