Zheng Fu

During extensive periods without rain, known as dry-downs, decreasing soil moisture (SM) induces plant water stress that occurs at the point when SM limits transpiration, defining a critical SM threshold (crit). Mapping global ecrit is crucial for future projections of climate and water resources, food production, and ecosystems vulnerability, however, so far it has been lacking. Here we combine land surface temperature diurnal amplitude (dLST) and SM using measurements of in situ eddy covariance and multiple satellites during dry-downs to generate quantitative, spatially explicit global distribution of ecrit.
We find that the average écrit globally is 0.19 m3/m3, and low Acrit dominates in dryland ecosystems while high crit occurs in tropical rainforests. Global variation of crit in natural ecosystems is mostly explained by aridity index, leaf area index and soil texture, but écrit in croplands is also affected by crop varieties, irrigation and expanding areas. By documenting the environmental correlates of ecrit, our results can help towards a better representation of water stress in land surface models. Using this map, we further reveal the global distribution of fraction of time spent in water-limited stage each year and show that this fraction has been linearly increasing during the past four decades. Our results therefore demonstrate the global map of écrit and its drivers and applications, with important implications for identifying tipping points of water stress impairing ecosystem functioning and for future ecosystem services.