As a crucial variable in hydrological and climate modeling, a lot of efforts have been made to estimate the evaporation (E, including soil evaporation, canopy interception, and plant transpiration) on the Tibetan Plateau (TP). Most studies showed that the evaporation on the TP stops increasing or even decreases in the recent two decades. However, the spatio‐temporal variability of this phenomenon and its underlying mechanism remains uncertain. Here, we utilized a state‐of‐the‐art version of the sigmoid generalized complementary equation to estimate the monthly evaporation over the TP for the period of 1979–2018. The results show that the annual E increases significantly during 1979–1998 with a rate of 1.94 mm yr−1 (based on the total E over the TP), however, it decreases during 1999–2018 with a rate of −1.06 mm yr−1. The most significant decreasing trend of E occurs in the spring. The areas with the most significant decrement of E include the central and western Qiangtang Plateau, the middle reaches of Yarlung Zangbo River, and the Yangtze River basin. By introducing a new aridity index Rwe, we identified that the decrement of E mainly occurs in the water‐limited regions, which contribute to 60% of the total decreasing trend of the whole region; at the same time, the energy‐limited regions also contribute to 40% of the decreasing trend. This study is important for the hydrological researches and decision making because the trend of E can influence the water resources and agriculture.

Previous projections show consistent increases in river flows of Asian Water Towers under future climate change. Here we find non-monotonic changes in river flows for seven major rivers originating from the Tibetan Plateau at the warming levels of 1.5 °C, 2.0 °C, and 3.0 °C based on an observation-constrained hydrological model. The annual mean streamflow for seven rivers at 1.5 °C warming level decreases by 0.1–3.2% relative to the present-day climate condition, and increases by 1.5–12% at 3.0 °C warming level. The shifting river flows for the Yellow, Yangtze, Brahmaputra, and Ganges are mostly influenced by projected increases in rainfall, but those for the Mekong, Salween, and Indus are dictated by the relative changes in rainfall, snowmelt and glacier melt. Reduced river flows in a moderately warmed climate threaten water security in riparian countries, while elevated flood risks are expected with further temperature increases over the Tibetan Plateau.