Hysteresis Behavior of Surface Water Fluxes in a Hydrologic Transition of an Ephemeral Lake

Surface heat fluxes are crucial to the energy and water exchanges between surface and the atmosphere. Hysteresis effect poses a challenge in model simulation of evaporation (latent heat flux, LE) at hourly or intradaily scales. The hidden mechanisms behind the hysteresis remain unclear, given the diverse reports in various climates, geographical locations, and underlying surfaces. This study took advantage of eddy covariance flux observation over Poyang Lake of China, an ephemeral lake experiencing seasonal shifts from land to water. It depicted the hysteresis behavior of LE to environmental forces for the period from May to July in 2018, a hydrologic transition period from land to water surface. Our results showed obvious clockwise hysteresis loops between LE and water vapor pressure deficit (VPD), surface temperature (Ts), and air temperature (Ta). Counterclockwise loops exhibited between LE and surface net radiation (Rn), wind speed (WS), and temperature difference (Ts-Ta). Rn, WS, and Ts-Ta dominants control the hysteresis effects and regulate LE variations. The hysteresis loops changed in direction, shape, and area when the surface shifted from land to water. It was primarily related to the lengthened lag time between LE and other variables except for WS, which was attributable to enhanced impacts from WS and VPD, and weakened impacts from Rn and Ts-Ta. The findings should help to improve our understanding of the complicated water-atmosphere interactions at the intradaily scale.