How much water vapour does the Tibetan Plateau release into the atmosphere?

Abstract. Evapotranspiration (ET) is critical for understanding the earth climate system and the complex heat/water exchange mechanisms between the land surface and the atmosphere in the high-altitude Tibetan Plateau (TP) region. However, the performance of ET products over TP has not been adequately assessed, and there is still significant uncertainty regarding the amount of water vapour released by the TP into the atmosphere, as well as its variation. In this study, we evaluated 22 ET products over TP by validating with the in-situ observations and basin-scale water balance estimations. This study also inter-compared their spatiotemporal variations and components to clarify the ET magnitude and variability in TP. The results showed that the remote sensing high-resolution global ET data from ETMonitor and PMLV2 demonstrated high accuracy comparable to the regional MOD16STM ET product, with overall better accuracy than other global ET data with fine spatial resolution (~1 km), when comparing with in-situ observation. Their accuracy was also presented when compared with the water balance-based ET at basin scale, which further indicated the overall accuracy of GLEAM and TerraClimate for the coarse-resolution ET products. Different products showed different spatiotemporal variation patterns, with large discrepancy occurring in the middle to western TP. The multiple-year averaged ET over TP by these products was found to have an average value (standard deviation) of 350.34 (42.46) mm/yr. The different components (plant transpiration, soil evaporation, canopy rainfall interception evaporation, open water evaporation, and snow/ice sublimation) available from some products were also compared, and the separate contribution of these components varied substantially even in cases in which total ET agrees by different products. The response of annual ET to total precipitation, net radiation and leaf area index was explored to present their governing effect on ET, and the results indicated that precipitation effect mostly in the middle and northern TP and net radiation play significant role in the eastern TP.