Uncovering critical thresholds of root-zone soil moisture for plant water stress in terrestrial ecosystems

The critical root-zone soil moisture (SM) threshold is a fundamental parameter that marks the transition from energy-limited to soil-moisture-limited evapotranspiration (ET) regimes, yet regional and global studies often rely on near-surface SM and its associated threshold as a proxy. This study presents a global, measurement-based evaluation of critical root-zone SM threshold by analyzing 666 dry-down events across 34 eddy covariance flux tower sites equipped with multi-layered SM sensors reaching depths of at least 1 meter. The results demonstrate that critical thresholds derived from near-surface and root-zone SM are significantly inconsistent, with an overall root mean square error (RMSE) of 0.11 m³ m⁻³. This discrepancy is primarily driven by the vertical SM gradient and the decoupling of near-surface and root-zone layers during drydown periods, which leads to substantial errors in identifying the onset and duration of plant water stress. For instance, at a forest site (US-Me2), using the critical threshold derived from near-surface SM delayed the detected onset of moisture stress by 27 days and underestimated the duration of the moisture-limited regime by 36 days. Across the diverse biomes and climate types studied, the global mean  was 0.12 ± 0.11 m³ m⁻³. These findings provide a critical observational benchmark for the evaporative fraction-root zone soil moisture relationship, highlighting that transitioning from near-surface to root-zone-based assessments is essential for accurate land-surface model evaluation and the quantification of ecosystem vulnerability to drought.