Terrace sidewalls represent a critical yet frequently overlooked pathway for lateral vapor loss in semi-arid landscapes. While traditional land surface models primarily focus on vertical evapotranspiration, they often neglect the three-dimensional (3D) nature of topographic evaporative interfaces. To address this gap in understanding, we conducted a three-year (2023-2025) in-situ monitoring campaign in typical terraced fields on the Chinese Loess Plateau, utilizing spatial neutron probe transects to capture deep-profile (0-300 cm) soil moisture dynamics. We employed a coupled statistical framework, integrating Random Forest (RF) to rank non-linear environmental drivers and Structural Equation Modeling (SEM) to disentangle causal mechanisms. We demonstrate that sidewall evaporation drives a persistent moisture gradient extending 450 cm, causing a 30 % cumulative water deficit compared to the unaffected reference area (D600); notably, >70 % of this loss occurs within 150 cm of the edge. RF analysis identified horizontal distance (26.9 %) as the primary driver, surpassing soil depth. SEM further revealed the mechanism: the sidewall functions as an "evaporative pump," where synergistic solar radiation and topography-induced aerodynamic turbulence maintain a steep vapor pressure gradient, extracting deep soil moisture. This process establishes a distinct geomorphologically induced permanent moisture deficit zone characterized by low temporal stability. Consequently, we propose a paradigm shift from a 2D planar view to a 3D interface-based framework to accurately capture land-atmosphere water exchanges in complex terraced terrains.