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It is generally believed that evapotranspiration at night is too miniscule to be considered. Thus, few studies focus on the nocturnal evapotranspiration (ETN) in alpine region. In this study, based on the half-hour eddy and meteorological data of the growing season (from May to September) in 2019, we quantified the ETN of alpine desert (AD), alpine meadow (AM), alpine meadow steppe (AMS), and alpine steppe (AS) in the Qinghai Lake Basin and clarified the different response of evapotranspiration to climate variables in daytime and nighttime with the variation of elevation. The results show that: (1) ETN accounts for 9.88~15.08% of total daily evapotranspiration and is relatively higher in AMS (15.08%) and AD (12.13%); (2) in the daytime, net radiation (Rn), temperature difference (TD), vapor pressure difference (VPD), and soil moisture have remarkable influence on evapotranspiration, and Rn and VPD are more important at high altitudes, while TD is the main factor at low altitudes; (3) in the nighttime, VPD and wind speed (WS) control ETN at high altitudes, and TD and WS drive ETN at low altitudes. Our results are of great significance in understanding ETN in the alpine regions and provide reference for further improving in the evapotranspiration estimation model.

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Abstract. Saline lakes on the Qinghai–Tibet Plateau (QTP) profoundly affect the regional climate and water cycle through loss of water (E, evaporation under ice–free (IF) and sublimation under ice–covered (IC) conditions). Due to the observation difficulty over lakes, E and its underlying driving forces are seldom studied targeting saline lakes on the QTP, particularly during the IC. In this study, E of Qinghai Lake (QHL) and its influencing factors during the IF and IC were first quantified based on six years of observations. Subsequently, two models were chosen and applied in simulating E and its response to climate variation during the IF and IC from 2003 to 2017. The annual E sum of QHL is 768.58 ± 28.73 mm, and E sum during the IC reaches 175.22 ± 45.98 mm, accounting for 23 % of the annual E sum. The E is mainly controlled by the wind speed, vapor pressure difference, and air pressure during the IF, but driven by the net radiation, the difference between the air and lake surface temperatures, wind speed, and ice coverage during the IC. The mass transfer model simulates lake E well during the IF, and the model based on energy achieves a good simulation during the IC. Moreover, wind speed weakening results in an 11.14 % decrease in E during the IC of 2003–2017. Our results highlight the importance of E in IC, provide new insights into saline lake E in alpine regions, and can be used as a reference to further improve hydrological models of alpine lakes.

Author ContributionsXY Li conceived the idea, FZ Shi performed the analyses. XY Li, FZ Shi, DL Chen and YJ Ma led the manuscript writing. SJ Zhao, YJ Ma, JQ Wei and QW Liao provided analysis of datasets. All contributed to review and revise the manuscript.