Evaluation of actual evapotranspiration measured by large‐scale weighing lysimeters in a humid alpine meadow, northeastern Qinghai‐Tibetan Plateau

The accurate estimation of evapotranspiration (ET) is essential for assessing water availability and requirements of regional‐scale terrestrial ecosystems, and for understanding the hydrological cycle in alpine ecosystems. In this study, two large‐scale weighing lysimeters were employed to estimate the magnitude and dynamics of actual evapotranspiration in a humid alpine Kobresia meadow from January 2018 to December 2019 on the northeastern Qinghai‐Tibetan Plateau (QTP). The results showed that daily ET a averaged 2.24 ± 0.10 mm day −1 throughout the study period, with values of 3.89 ± 0.14 and 0.81 ± 0.06 mm day −1 during the growing season and non‐growing season, respectively. The cumulative ET a during the study period was 937.39 mm, exceeding precipitation (684.20 mm) received at the site during the same period by 37%, suggesting that almost all precipitation in the lysimeters was returned to the atmosphere by evapotranspiration. Furthermore, the cumulative ET a (805.04 mm) was almost equal to the maximum potential evapotranspiration estimated by the FAO‐56 reference evapotranspiration (ET 0 ) (801.94 mm) during the growing season, but the cumulative ET a (132.25 mm) was 113.72% less than the minimum equilibrium ET eq ) (282.86 mm) during the non‐growing season due to the limited surface moisture in frozen soil. The crop coefficient (K c ) also showed a distinct seasonal pattern, with a monthly average of 1.01 during the growing season. Structural equation model (SEM) and boosted regression tree (BRT) show that net radiation and air temperature were the most important factors affecting daily ET a during the whole study period and growing season, but that non‐growing season ET a was dominated by soil water content and net radiation. The daily K c was dominated by net radiation. Furthermore, both ET a and K c were also affected by aboveground biomass.