Transition to transpiration-dominated evapotranspiration on the Loess Plateau: spatially divergent driving mechanism and threshold effect after two decades of reforestation

Large-scale ecological restoration is a critical strategy for combating land degradation, yet its hydrological consequences, particularly regarding evapotranspiration (ET) partitioning, remain uncertain due to complex spatial heterogeneity and non-linear vegetation-water feedbacks. To address this, we developed a novel "zonal and screened" attribution framework that integrates the Two-Source Energy Balance (TSEB) model with Bayesian Ridge Regression to disentangle the driving mechanisms of ET and its components (transpiration, Ec; soil evaporation, Es) on the Loess Plateau (2000–2020). Methodologically, this zoned framework significantly outperformed traditional global modeling, reducing the prediction error for Ec by a median of 24.3% in heterogeneous transition zones. Results indicate a fundamental shift in the regional water cycle: ET increased by 9.31 mm/yr, primarily driven by a surge in Ec (10.24 mm/yr). Crucially, the driving mechanisms exhibited distinct spatiotemporal divergence: while vegetation restoration dominated Ec in the hilly-gully regions (Zones A and B), climatic factors controlled the arid sandy areas (Zone C). Furthermore, we identified two universal non-linear regulation mechanisms: a "V-shaped" response for Es (shading vs. interception) and an "Inverted U-shaped" response for Ec (saturation effect). Specifically, the optimal Leaf Area Index (LAI) threshold for transpiration in Zone A shifted from 0.47 (2000–2010) to 0.42 (2011–2020), signaling intensified water stress. These findings challenge the "one-size-fits-all" greening policy and advocate for a paradigm shift towards "green-water synergy" management. We propose actionable strategies, including thinning dense plantations to maintain LAI near optimal thresholds and prioritizing water-saving agriculture in arid zones, to ensure the sustainability of ecological engineering in water-limited regions globally.