Assessing how much water evaporated from the Tibetan Plateau using multiple datasets

The Tibetan Plateau (TP) is also known as the ‘Water Tower of Asia’ as it is the source of 10 major rivers. Significant changes in the natural and social environment of the TP have occurred over the last 50 years (e.g., temperatures have warmed twice as much as the global average over the same period), and there is considerable uncertainty about future environmental change. Water vapor flux, expressed as evapotranspiration (ET), is crucial for understanding the water balance over the TP. The TP is rich in land cover types, including grasslands, deserts, lakes, forests, glaciers, snow, etc. The dynamics and thermodynamics of the subsurface vary greatly between different climate types, making it a major challenge to conduct large-scale studies of ET processes over the TP and to explore the governing mechanisms and feedbacks to the climate system and hydrological processes. However, a single ET dataset cannot provide a reliable answer on how much water is evaporated from the TP due to model limitations in describing complete processes and uncertainties in different datasets. In this study, we first evaluated 22 ET products in the TP against in-situ observations and basin-scale water balance estimates. The spatiotemporal variability of the total vapor flux was also evaluated to clarify the vapor flux magnitude and variability over the TP. The results showed that the high-resolution (~1km) global ET data based on observations from ETMonitor and PMLV2 were more accurate than than other global and regional ET data with fine spatial resolution (~1km), when comparing with in-situ observations. When compared with basin scale water balance estimates of ET, ETMonitor and PMLV2 at finer spatial resolution and GLEAM and TerraClimate at the coarse spatial resolution showed good agreement. Different products showed different patterns of spatiotemporal variability, with large differences in the central to western TP. The mean water vapor flux over multi-year and multi-product over the TP was 333.1 mm/yr with a standard deviation of 38.3 mm/yr. Soil evaporation accounts for most of the total water vapor flux over the TP, followed by plant transpiration and canopy rainfall interception evaporation, while the contributions from open water evaporation and snow/ice sublimation are not negligible.