Lake-water-temperature regulation under diurnal and annual cycles of environmental forcing, Agamon Hula, Israel

• Thermoregulation and heat fluxes of a shallow lake were explored over 2 years. • Balance of incoming and outgoing fluxes described the equilibrium temperature. • Annual temperature reached equilibrium, as evaporation followed radiation. • Diurnal temperature differed from equilibrium, as heat storage oscillated. • Expected water temperatures and heat fluxes under climate change were studied. Lake-water temperature regulation responds to annual and diurnal cycles of environmental forcing (radiation, wind speed, and air properties) through heat exchange, balancing stored heat with incoming and outgoing fluxes. Given the critical role of temperature response in influencing the ecological resilience and biogeochemical cycles of lakes, it requires precise mathematical expression based on direct observation. Here, this is addressed by accurately determining the equilibrium temperature, achieved when changes in stored heat are negligible, and its dependence on environmental forcing. Water temperature gradually approaches equilibrium, with a certain response time that depends on lake depth and wind velocities. The dynamics between water and equilibrium temperature and their corresponding surface heat fluxes are controlled by the ratio between the lake’s thermal response time to the environmental forcing timescale. The unique role of this ratio was examined using two years of continuous direct measurements from an eddy covariance tower at Agamon Hula, a shallow lake in northern Israel. Our results reveal that the lakes thermal response time is ∼1 day. Consequently, water temperature closely follows the equilibrium temperature at the intra-annual cycle, and deviates from equilibrium under the diurnal cycle. This agrees well with the measured heat fluxes, where under the intra-annual cycle, incoming radiation is balanced mainly by evaporation and changes in stored heat are negligible, as opposed to large oscillations in heat storage under the diurnal cycle. We leverage the results to study expected annual water temperatures under different environmental scenarios, and the role lake depth has on diurnal water temperatures.