Spatially bounded riparian greening under long-term ecological water transfer: Evidence from individual-tree greenness and groundwater recovery.

In arid regulated rivers worldwide, ecological water transfer (EWT) is increasingly used to restore degraded river corridors, yet its effectiveness is commonly evaluated using water-delivery volume, downstream channel reconnection, groundwater recovery, or average vegetation improvement. Whether these indicators adequately represent the spatial extent of vegetation recovery remains insufficiently quantified, particularly at the individual-tree scale. This study aimed to quantify the effective restoration footprint of long-term EWT in the lower Tarim River and to determine whether groundwater recovery alone adequately represents ecological restoration success. We combined field survey data for more than 5000 Populus euphratica trees, very high-resolution satellite imagery from 2005 to 2022, and groundwater records from six monitoring wells located approximately 50-1050 m from the river channel. Vegetation change was quantified as ΔNDVI at mapped tree locations and analyzed using generalized additive models and distance-class summaries, while groundwater trends and lagged associations with annual EWT volumes were assessed using the Mann-Kendall test, Sen's slope estimator, and Spearman's rank correlation. Groundwater depth decreased significantly at all six wells during 2001-2022 (Sen's slopes: -0.127 to -0.230 m yr-1; p ≤ 0.002), indicating a coherent rise in the groundwater table across the riparian corridor. The zone-level association between NDVI and groundwater depth weakened markedly between 2005 and 2022. However, vegetation recovery was spatially bounded: mean ΔNDVI was high within 0-300 m of the channel (0.170-0.196), declined sharply in the 300-500 m transition band (0.051), and approached near-zero values beyond 500 m (0.013-0.038). Moreover, the zone-level NDVI-groundwater association weakened from strong and significant in 2005 (Spearman's ρ = -0.94, p = 0.005) to weak and non-significant in 2022 (ρ = -0.20, p = 0.704). These findings highlight the joint importance of water-table recovery and the spatial accessibility of EWT in shaping riparian greening. Incorporating the effective restoration footprint into EWT assessment may improve the spatial efficiency of water allocation and support more targeted riparian forest restoration in arid regulated rivers.