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Variability of Surface Heat Fluxes and Its Driving Forces at Different Time Scales Over a Large Ephemeral Lake in China

Poyang Lake is the largest freshwater lake in China. It is considered as an ephemeral lake—experiencing dynamic seasonal transitions ranging from a lake surface to wetland conditions. The influence of the seasonal water level transition on lake‐atmosphere energy exchange can have important consequences for local climate and water resource management. Here we study the dynamics of surface‐atmosphere energy exchange and examine the hydrological and meteorological controls on its partitioning for the period August 2013 to July 2016. Change in the water depth (WD) directly impacts water surface contribution and the inundated duration. The latent heat (LE) and sensible heat (H) fluxes showed an unimodal diurnal pattern in the wetland‐covered period and changed to an irregular pattern during the lake‐covered period when water surface contribution exceeded a value of 62%. The Bowen ratio (H/LE) was large/small in the wetland‐/lake‐covered period and decreased significantly with increasing WD. On a diurnal and daily timescale, the LE and H showed less dependence on wind speed (U) and water vapor or temperature gradient and greater dependence on net radiation when transitioning from the lake‐covered period to wetland‐covered period, respectively. On a monthly timescale, both the LE and H were mainly controlled by the net radiation. However, the interannual variations in LE and H were significantly correlated with the inundated duration and WD over Poyang Lake. The decreasing trend in Poyang Lake level, therefore, has potential to reduce lake evaporation and local precipitation and further amplify regional hydrometeorological droughts.