Hysteresis Responses of Evapotranspiration to Meteorological Factors at a Diel Timescale: Patterns and Causes

Evapotranspiration (ET) is an important component of the water cycle in terrestrial ecosystems. Understanding the ways in which ET changes with meteorological factors is central to a better understanding of ecological and hydrological processes. In this study, we used eddy covariance measurements of ET from a typical alpine shrubland meadow ecosystem in China to investigate the hysteresis response of ET to environmental variables including air temperature (T (a)), vapor pressure deficit (VPD) and net radiation (R (n)) at a diel timescale. Meanwhile, the simulated ET by Priestly-Taylor equation was used to interpret the measured ET under well-watered conditions. Pronounced hysteresis was observed in both T (a) and VPD response curves of ET. At a similar T (a) and VPD, ET was always significantly depressed in the afternoon compared with the morning. But the hysteresis response of ET to R (n) was not evident. Similar hysteresis patterns were also observed in the T (a)/VPD response curves of simulated ET. The magnitudes of the measured and simulated hysteresis loops showed similar seasonal variation, with relatively smaller values occurring from May to September, which agreed well with the lifetime of plants and the period of rainy season at this site. About 62% and 23% of changes in the strength of measured ET-T (a) and ET-VPD loops could be explained by the changes in the strength of simulated loops, respectively. Thus, the time lag between R (n) and T (a)/VPD is the most important factor generating and modulating the ET-T (a)/VPD hysteresis, but plants and water status also contribute to the hysteresis response of ET. Our research confirmed the different hysteresis in the responses of ET to meteorological factors and proved the vital role of R (n) in driving the diel course of ET.