Evapotranspiration and its dominant controls along an elevation gradient in the Qinghai Lake watershed, northeast Qinghai-Tibet Plateau

Evapotranspiration (ET) and its dominant controls show obvious elevational pattern on the Qinghai-Tibet Plateau, due to heterogeneous environmental conditions regulated by altitude. To understand better how and why ET varies at different altitudes in the Qinghai Lake watershed, this study measured and retrieved soil moisture, solar radiation, and ET data between 3300 m and 4700 m, and further investigated the influence of water and energy conditions on ET. Results showed that, the mean annual terrestrial ET was 343 and 360 mm in this watershed in 2014 and 2015, respectively. The higher ET mainly appeared in the midstream of Shaliu River and on the southern region of Qinghai Lake, while the sand land on the east of Qinghai Lake and the bare rock in the northwest of the watershed had the lowest values. The ET increased firstly and then decreased with the increase of altitude, and its peak value appeared at 3600-3700 m. The dominant controls for ET switched from water conditions to energy conditions with the increase of elevation. The maximum limited fraction [(predicted value - actual value)/predicted value x 100] of ET by water condition (soil moisture content) achieved 11.2% and 10.1% at 3300-3350 m in 2014 and 2015, and the corresponding mean area-weighted values were 7.3% and 5.4% at 3300-3650 m, respectively. The maximum limited fraction of ET by energy condition (shortwave radiation and air temperature) achieved 24.4% and 29.8% at 4650-4700 m in 2014 and 2015, and the corresponding mean area-weighted values were 10.8% and 11.9% at 3650-4700 m, respectively. Overall, the energy conditions were more influential factors limiting the ET in the entire watershed, and this conclusion is critical for the management of regional terrestrial ecosystem with future climate change.