Actual evapotranspiration (ETa) is important since it is an important link to water, energy, and carbon cycles. Approximately 96% of the Qinghai-Tibet Plateau (QTP) is underlain by frozen ground, however, the ground observations of ETa are particularly sparse-which is especially true in the permafrost regions-leading to great challenge for the accurate estimation of ETa. Due to the impacts of freeze-thaw cycles and permafrost degradation on the regional ET process, it is therefore urgent and important to find a reasonable approach for ETa estimation in the regions. The complementary relationship (CR) approach is a potential method since it needs only routine meteorological variables to estimate ETa. The CR approach, including the modified advection-aridity model by Kahler (K2006), polynomial generalized complementary function by Brutsaert (B2015) and its improved versions by Szilagyi (S2017) and Crago (C2018), and sigmoid generalized complementary function by Han (H2018) in the present study, were assessed against in situ measured ETa at four observation sites in the frozen ground regions. The results indicate that five CR-based models are generally capable of simulating variations in ETa, whether default and calibrated parameter values are employed during the warm season compared with those of the cold season. On a daily basis, the C2018 model performed better than other CR-based models, as indicated by the highest Nash-Sutcliffe efficiency (NSE) and lowest root mean square error (RMSE) values at each site. On a monthly basis, no model uniformly performed best in a specific month. On an annual basis, CR-based models estimating ETa with biases ranging from -94.2 to 28.3 mm year(-1), and the H2018 model overall performed best with the smallest bias within 15 mm year(-1). Parameter sensitivity analysis demonstrated the relatively small influence of each parameter varying within regular fluctuation magnitude on the accuracy of the corresponding model.

The study of the diurnal response mechanism of the actual evapotranspiration (ETa) to the environment in the permafrost regions of the Qinghai-Tibet Plateau (QTP) using the LYS30 micro-evaporation instrument found that there are different feedbacks to the ETa under freezing and thawing cycles. The ETa process during the winter cooling period (WC) and the spring warming period (SW) is snow and ice sublimation and is mainly affected by the vapour pressure deficit (VPD). In the summer thawing period (ST), ETa can reach the maximum value when all meteorological elements reach a certain range of change at the same time, while ETa will decrease when the meteorological elements are not qualified. During the autumn freezing period (AF), the amount of condensate reached a maximum at 7:00, and due to the sudden change in meteorological elements at 9:00, the ETa increased rapidly at a rate higher than the condensation rate that occurred between 7:00 and 9:00. We also found that in different stages of freezing and thawing, the two physical processes of condensation and evaporation alternated in 1 day, with the process of evaporation occurring during the day and the condensation process occurring during the night. The diurnal response mechanism of the ETa to the environment in the permafrost regions of the QTP is expected to reveal the mechanism of soil hydrological processes and will provide a theoretical and scientific basis for water balance analysis and ecological environment protection in permafrost regions.

In this study, an assessment is made of the trends of actual evapotranspiration (AE) over China under global warming from an ensemble of the following data sets: National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis I, National Centers for Environmental Prediction-Department of Energy reanalysis II, Modern-Era Retrospective Analysis for Research and Applications (MERRA), MERRA Version 2, European Centre for Medium-Range Weather Forecasts Interim Re-Analysis, and Japanese 55-year Reanalysis. For China as a whole, annual AE exhibited a significant increasing trend from 1979 to 2015, and the rate of increase was highest in autumn and lowest in summer. By subdividing China into six climatic regions using the aridity index, significant increasing trends in AE are found over the hyperarid, arid, and humid regions, but slightly decrease over the dry subhumid and subhumid regions. For the hyperarid and arid regions, the increases in potential evapotranspiration and the water supply from melting glaciers due to both climate warming and human activities might be the main contributors to the AE increases. An increase in potential evapotranspiration driven by global warming is the main cause of the increased AE over the humid region. On a seasonal basis, most AE over China occurs in summer. However, increasing AE trends have been observed mainly in autumn and winter, and the increase is statistically significant over all of China's six climatic regions in winter. In view of data availability, reanalyses are compared with the results from Mezentsev-Choudhury-Yang equation (AE_Budyko). The correlations between individual reanalyses and AE_Budyko tend to be higher over the arid region.