Biophysical factors control the interannual variability of evapotranspiration in an alpine meadow on the eastern Tibetan Plateau

Abstract

Evapotranspiration (ET) is a key parameter regulating land–atmosphere interaction processes and the water cycle. The seasonal and interannual variability of ET and its environmental controls over an alpine meadow in a subfrigid humid zone of the Tibetan Plateau (TP) are reported on. Direct measurements were made using the eddy covariance method over a 10-year period with a significant increase in growing season length (GSL). The results showed that annual ET for the alpine meadow site was 492 ± 66 mm in comparison with 635 ± 88 mm of precipitation (P), with a ratio of ET/P ranging from 0.63 (2017) to 1.04 (2010). The path analysis and Priestley–Taylor coefficient (ET/ETeq) revealed that the daily ET experienced energy-limited and water-limited seasons within the year. During the water-limited non-growing season, the daily surface conductance (Gs) and ET/ETeq increased positively with soil water content (SWC). During the energy-limited growing season, the 16-day average ET, ET/ETeq, and Gs scaled positively with the normalized difference vegetation index (NDVI). ET/ETeq increased nonlinearly with an increase in Gs, but was insensitive to increases in Gs greater than the threshold Gs*. The mean Gs and Gs* were strongly regulated by the maximum NDVI. The maximum NDVI, Gs*, and annual mean Gs explained 46%, 80%, and 68%, respectively, of interannual variation in annual ET. Thus, we concluded that biophysical factors, rather than P and GSL, mainly controlled the interannual variability in annual ET. These findings are critical for understanding the response mechanism of ET to the changing biotic and abiotic conditions in the TP.