Ecohydrological Changes Associated with Long-Term Ecological Water Conveyance in the Lower Tarim River: Linking Groundwater, Relative Surface Salinity, and Vegetation Productivity

Long-term ecological water conveyance has been implemented to reconnect hyper-arid inland-river corridors, but ecological effectiveness cannot be inferred from delivered water volume alone. We assessed ecohydrological changes in the lower Tarim River from 2000 to 2023 using annual water conveyance records, groundwater observations, growing season Carnegie–Ames–Stanford Approach (CASA) net primary productivity (NPP), Landsat-derived Normalized Difference Salinity Index (NDSI), climate data, and four operational NPP-NDSI states. Annual conveyance averaged 4.08 × 108 m3 and peaked in 2017 at 12.09 × 108 m3. Mean groundwater depth decreased from 7.49 m in 2010 to 4.25 m in 2023. Current-inclusive multi-year conveyance windows were more strongly negatively associated with groundwater depth than same-year conveyance (W3: rho = −0.743; W4: rho = −0.782); Newey–West tests remained significant at lags 1–3, and moving-block bootstrap intervals excluded zero for W3 and W4, whereas the strictly antecedent three-year metric was less robust. The high-NDSI area decreased from 1795.04 to 42.19 km2, whereas the higher-productivity area increased from 91.81 to 1127.81 km2. A paired Enhanced Thematic Mapper Plus/Operational Land Imager (ETM+/OLI) analysis showed high NDSI correspondence (r = 0.980) but a mean OLI-minus-ETM+ bias of −0.0122; the corresponding 2010 cross-sensor adjustment was 63.9% of the absolute raw 2010–2015 mean-NDSI shift. Cross-era NDSI class areas and sampled-state trajectories are therefore treated as sensor-sensitive descriptive quantities. Spatial support sensitivity was also important: the functionally limited low-salinity state (FLLS) was the largest 2023 state on the fine-alignment grid (53.93%), whereas the higher-productivity low/moderate-NDSI state (HPLN) became dominant after aggregation to 500 m (66.57%); so, no scale-invariant claim is made about the dominant 2023 state. A fixed-2000-land-cover CASA run produced no material change in NPP because the land-cover transitions represented in the common mask occurred among classes sharing the same NDVI endmembers and εmax. Overall, the results support asynchronous indicator change and a state-based monitoring framework, while temporal dependence, irregular groundwater sampling, residual spatial autocorrelation, cross-sensor discontinuity, and spatial support constrain causal and fine-scale inference.