Effects of freezing–thawing cycle on the daily evapotranspiration of alpine meadow soil in Qinghai–Tibet Plateau

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.