Evapotranspiration saturation amplifies climate sensitivity of terrestrial water yield

Predicted climate change over land indicates decreasing precipitation in many regions and increased flooding in others. Globally, over 60% of land precipitation is consumed by evapotranspiration (ET); the remainder, available as runoff, recharge, and for consumption, is termed water yield (WY). Using a global dataset, we show that ET from ecosystems reaches a 'saturation' limit of about 480 ± 210 mm yr⁻¹ across climates and biomes, well below the energy-based limit predicted by the Budyko equation. This inflexibility in ET increases the sensitivity of WY to precipitation variability, implying enhanced vulnerability to flooding in wet regions and an accelerated approach to the limits of ecosystem and societal sustainability in dry regions. Both effects are also supported by model-based projections. WY thus provides a more sensitive and integrative indicator of climate impacts on terrestrial water resources and associated risks for ecosystems and society.