Large-scale ecological restoration is a critical strategy for combating land degradation, however, its hydrological implications, especially on evapotranspiration (ET), remain uncertain due to complex spatial heterogeneity and non-linear feedback loops between vegetation and water. To address this, a novel attribution framework for identifying the driving mechanisms of ET and its components (transpiration, Ec; soil evaporation, Es) was developed on the Loess Plateau for the periods 2000-2020, which combines the Two-Source Energy Balance (TSEB) model with the Bayesian Ridge Regression. Methodologically, this zoning framework significantly outperformed traditional global modelling, reducing the prediction error for Ec by a median of 24.3% across the heterogeneous transition zones. The results indicate a fundamental shift in the regional water cycle: ET increased by 9.31 mm/yr, primarily driven by the increase in Ec (10.24 mm/yr). Crucially, the driving forces exhibited distinct spatiotemporal divergence: vegetation restoration dominated in the hilly-gully regions (Zones A and B), climatic factors controlled the arid sandy areas (Zone C). Furthermore, two universal non-linear regulation mechanisms have been identified: a "V-shaped" response for Es (shadow vs. interception) and an "Inverted Ushaped" response for Ec (saturation effect). Specifically, the optimal Leaf Area Index (LAI) threshold for transpiration in Zone A has been shifted from 0.47 (2000-2010) to 0.42 (2011-2020), indicating an intensified water stress. These findings challenge the "one-size-fits-all" greening policy and advocate for a paradigm shift towards a management based on a synergy between greening and water. In addition, actionable strategies have been proposed to ensure the sustainability of ecological engineering in water-limited regions globally, including thinning of dense plantations to maintain LAI close to optimal thresholds and prioritization of water-saving agriculture in arid areas. The results obtained in this study could provide scientific reference for the implementation of ecological engineering and effective utilization of water resource in arid areas.