Global critical soil moisture thresholds of plant water stress.

During extensive periods without rain, known as dry-downs, decreasing soil moisture (SM) induces plant water stress at the point when it limits evapotranspiration, defining a critical SM threshold (θ). Better quantification of θ is needed for improving future projections of climate and water resources, food production, and ecosystem vulnerability. Here, we combine systematic satellite observations of the diurnal amplitude of land surface temperature (dLST) and SM during dry-downs, corroborated by in-situ data from flux towers, to generate the observation-based global map of θ. We find an average global θ of 0.19 m/m, varying from 0.12 m/m in arid ecosystems to 0.26 m/m in humid ecosystems. θ simulated by Earth System Models is overestimated in dry areas and underestimated in wet areas. The global observed pattern of θ reflects plant adaptation to soil available water and atmospheric demand. Using explainable machine learning, we show that aridity index, leaf area and soil texture are the most influential drivers. Moreover, we show that the annual fraction of days with water stress, when SM stays below θ, has increased in the past four decades. Our results have important implications for understanding the inception of water stress in models and identifying SM tipping points.