The study quantifies the critical soil moisture threshold (θcrit) for plant water stress using satellite observations of diurnal land surface temperature (dLST) and soil moisture (SM) during dry-down periods, complemented by in-situ data from flux towers. The average global θcrit is 0.19 m³/m³, varying from 0.12 m³/m³ in arid ecosystems to 0.26 m³/m³ in humid ecosystems. Earth System Models (ESMs) overestimate θcrit in dry areas and underestimate it in wet areas. Machine learning models reveal that aridity index, leaf area index, and soil texture are the most influential factors. The annual fraction of stressed days has increased over the past four decades, highlighting the growing vulnerability of ecosystems to water stress. The study provides insights into the spatial and temporal variations of θcrit, which are crucial for understanding and projecting future climate and water resource impacts on ecosystems and food production.The study quantifies the critical soil moisture threshold (θcrit) for plant water stress using satellite observations of diurnal land surface temperature (dLST) and soil moisture (SM) during dry-down periods, complemented by in-situ data from flux towers. The average global θcrit is 0.19 m³/m³, varying from 0.12 m³/m³ in arid ecosystems to 0.26 m³/m³ in humid ecosystems. Earth System Models (ESMs) overestimate θcrit in dry areas and underestimate it in wet areas. Machine learning models reveal that aridity index, leaf area index, and soil texture are the most influential factors. The annual fraction of stressed days has increased over the past four decades, highlighting the growing vulnerability of ecosystems to water stress. The study provides insights into the spatial and temporal variations of θcrit, which are crucial for understanding and projecting future climate and water resource impacts on ecosystems and food production.