Surface energy budget with water level fluctuating over the largest freshwater lake in China

Energy exchanges between water and atmosphere over lakes differ from those over vegetated surfaces due to high heat capacity of water. For a shallow lake, the underlying surface generally changes between water covered and land covered with water level fluctuation, which significantly influences energy fluxes. Turbulent fluxes of sensible (H) and latent heat (LE) over the Poyang Lake, a large shallow lake in china, were measured using the eddy covariance method during January to May in 2014. Our results show two kinds of diurnal variation patterns of H and LE when source area transits from the land-covered to the water-covered. The diurnal variations of H and LE are coupled with and controlled by net radiation (Rn) during the land-covered period, while that are out of phase with Rn during the water-covered period. The energy fluxes partition also shows different patterns for the two periods. Bowen ratio was 0.38 on average during the land-covered period, larger than 0.15 on average during the water-covered period. The ratio of LE to Rn (LE/Rn) remained close to 0.6 for both periods. However, the H/Rn decreased significantly for an increasing water depth, since more Rn was stored in water body during the water-covered period. The fluctuation of diurnal LE was mainly controlled by friction velocity (u*), and amplitude of LE was determined by vapor pressure deficit (VPD). However, regulations of H by environmental variables depended largely on atmospheric condition. Under atmospheric stable condition, diurnal H was negative with low magnitude, and not sensitive to the changes in environmental variables. Under atmospheric unstable condition, diurnal H variations were mainly determined by changes in water and air temperature difference (Tw-Ta) and the friction velocity. Based on unique characteristics of land cover transition over the Poyang Lake, totally different patterns of energy fluxes were observed under the same geographical and climate conditions. These results strongly improved our understanding on response of energy fluxes to different land cover and water depth. In light of these findings, continued long-term monitoring of the Poyang Lake energy fluxes and related hydrologic processes is important for understanding and predicting the future impacts of climate variability and change on lake water levels.