Mechanisms of Long-Term Variations of the Thermal Structure in a Warm Lake

Abstract:

Analysis of a 39 year record (1969-2008) of temperature profiles from Lake Kinneret, Israel, revealed that average epilimnion thickness decreased by ~1.2 meters (~3 cm yr^-1), metalimnion thickness decreased by between ~1.0 m (December) to ~2.0 m (April), and average temperature of the epilimnion increased by ~1^oC (~0.028^oC yr^-1). Average hypolimnetic temperature remained constant (~15^oC), so that the thermal gradient across the metalimnion increased. The average duration of the stratification period (~286 days) remained unchanged. We show that the stratification changes are the result of lake level decrease, a slight increase in air temperature during the spring and summer, and a reduction of inflows to the lake. These changes in time affect the stratification pattern in the same direction, i.e., an increase in the lake epilimnetic temperature and an increase in the thermal gradient across the metalimnion. Analysis of the data indicates that the effect of over pumping, which leads to a reduction in lake level is larger than the effect of changes due to air temperature increase and the reduction in inflow volumes.