Phase transition of surface energy exchange in China’s largest freshwater lake

Abstract

Energy exchanges between the water surface and the atmosphere over lakes play an important role in regulating regional climate. In shallow lakes, the lake bottom may shift from being inundated to being exposed, which substantially alters the energy exchange pattern. We measured the turbulent fluxes of sensible (H) and latent (LE) heat over Poyang Lake, the largest freshwater lake in China, using the eddy covariance method from January to December 2014. Our results show two different diurnal patterns of H and LE related to shifts in the lake’s surface type, from land-covered to water-covered. The diurnal variations in H and LE are coupled with net radiation (Rn) during the land-covered period and are out of phase with Rn during the water-covered period. The average Bowen ratio is 0.15 during the water-covered period and exceeds 0.27 during the land-covered period. The ratio of LE to Rn (LE/Rn) is approximately 0.6 on average and is not obviously different between periods. However, H/Rn and the Bowen ratio significantly decrease with increasing water depth, because more energy is stored in the water body during the water-covered period. The relative regulations of LE and H by different environmental variables largely depend on water surface coverage (WSC). Under low WSC conditions, daily LE and H variation are controlled primarily by Rn, whereas under high WSC conditions, daily LE and H are mainly determined by the water vapor deficit multiplied by the wind speed (UΔe) and the surface-air temperature difference multiplied by the wind speed (UΔT), respectively.