Simulating the palaeorecord of northern European lakes using a coupled lake-catchment model

Abstract: A coupled lake-catchment model was developed to examine the controls on lake-level changes in currently overflowing lakes during Holocene. The lake energy-balance is simulated as the one-dimensional vertical heat transfer by eddy diffusion and convective mixing. The accumulation and ablation of lake ice, and snow cover on the ice, is simulated thermodynamically. The lake water-balance is given by the balance between precipitation over the lake, evaporation from the water surface, catchment runoff, and lake outflow. Runoff is calculated using a one-dimensional, two layer soil covered by vegetation and a snowpack. Snow accumulation and ablation are controlled by air temperature. Outflow is controlled by the outlet size and outflow velocity, using the Manning equation. The coupled model was validated for Lake Bysjön (southern Sweden) and Lakes Karujärv and Viljandi (Estonia). The simulated monthly lake level matched observations of lake-level changes between 1944-1956 at Lake Viljandi (r=0.78), between 1976-1987 at Lake Karujärv (r=0.78) and between 1973-1977 at Lake Bysjön (r=0.7). The model was used to examine lake-level sensitivity to changes in individual climatic parameters. Changes in radiation, temperature, vapour pressure and wind strength produce lake-level changes of <0.5 m. Changes in mean annual and winter precipitation produce changes an order of magnitude larger. The lake-level sensitivity to precipitation changes is greatest when winter temperatures were higher than present. The magnitude of the response to a specific climatic change is strongly affected by the ratio of the lake area to the catchment area.