Surface Inundation Gradients and Lagged Fractional Vegetation Cover Responses of Populus euphratica Oliv. Forests to Ecological Water Conveyance in the Lower Tarim River, China

To quantify the spatial and lagged associations between ecological water conveyance and fractional vegetation cover (FVC) in Populus euphratica Oliv. forests, we integrated Landsat imagery (1995–2025), conveyance records, groundwater observations, and multi-period random forest classifications in the lower Tarim River. Newly formed or expanded surface water was mapped within dynamic annual response windows and characterized by annual surface inundation expansion area, multi-year inundation frequency, and distance from forest pixels to recurrently inundated water surfaces. These metrics were related to FVC using Theil–Sen trends and Lag 0–Lag 3 correlations. After 2010, reservoir releases, annual surface inundation expansion area, and groundwater conditions generally improved. Mean FVC increased by 0.182 percentage points per year, with improvement and degradation interspersed spatially. Relatively low- to moderate-frequency inundation was associated with pronounced positive FVC trends, and FVC improvement was concentrated within 0–0.5 km of recurrently inundated water surfaces. Associations of FVC with reservoir releases, groundwater depth, and annual surface inundation expansion area were strongest after 2–3 years. Together, these surface inundation metrics complement reservoir-release and groundwater records and support spatially differentiated monitoring of FVC changes in P. euphratica forests.