Environmental and biophysical controls on the evapotranspiration over the highest alpine steppe

- In-situ observation of evapotranspiration (ET) in a data scarce region.
- ET from semi-arid alpine steppe was mainly controlled by precipitation.
- Different drivers for evapotranspiration for different seasons.
- ET and precipitation are nearly balanced in the closed basin of central TP.

SummaryCharacterizing the water and energy flux in the alpine steppe ecosystem in Tibetan Plateau (TP) is of particular importance for elucidating hydrological cycle mechanisms in high altitude areas. In the present study, two years of actual evapotranspiration (ET) values from a semi-arid alpine steppe region (4947 m above sea level) and their environmental and biophysical controls were investigated using the energy balance Bowen ratio energy balance (BREB) method. Seasonally, ET was much lower in frozen soil period and transition period mainly because of low soil water availability. However, ample soil water supplied by rainfall during the rainy period substantially increased ET. The available energy played an important role in controlling ET in the rainy period. Also, the leaf-level stomata closure and plant leaf development could impact the ET through changing bulk surface conductance (Gs) in rainy period. Similarly, the land-atmosphere energy exchange was dominated by latent heat flux (λE) in July, but was dominated by sensible heat flux (H) in December and May. Annual ET (plus sublimation) were 362.9 mm and 353.4 mm in the first and second observation year, respectively, which were close to the annual precipitation. On annual scale, the low Gs (3.30-3.62 mm s−1), decoupling factor (Ω, 0.25-0.27) and the ratio of ET to equilibrium evapotranspiration (ET/ETeq, 0.34-0.35) corroborated the overall water-limited conditions for the high-altitude alpine steppe. This research provides not only the ground truth data for future hydrological modeling in the data scarce region of TP but also the insights for elucidating how the environmental and biophysical stress factors control the land surface ET in high-altitude region.