Google Earth Engine-Based Multispectral Assessment of Spatiotemporal Riparian Ecological Quality in the Brantas River Basin, Indonesia

Background Riparian zones provide essential ecosystem services but face increasing degradation from urbanization and land conversion. This study evaluated the spatial-temporal dynamics of riparian ecological quality in the Brantas River Basin, Indonesia, over a six-year period (2021–2026). Methods Sentinel-2 satellite imagery was processed through Google Earth Engine to extract six spectral indices representing vegetation density, chlorophyll content, and surface moisture across the upstream, middle, and downstream zones of the basin. Principal Component Analysis (PCA) synthesized these indices into a single composite metric, the Riparian Ecological Quality Index (REQI). Spatial differences among zones were tested with the Kruskal-Wallis test and Dunn’s post-hoc comparisons; temporal trends from 2021 to 2026 were evaluated with the Mann-Kendall test and Sen’s slope estimator. Results Ecological quality declined consistently from the upstream to the downstream zone (Kruskal-Wallis p < 0.001 for all six indices). The first principal component explained 97.94% of total variance among the six indices, supporting REQI as a valid composite measure. Despite minor year-to-year fluctuation, no statistically significant temporal trend was detected in any zone over the six-year period (Mann-Kendall p > 0.05 throughout), indicating that ecological quality has remained largely stable rather than actively improving or declining. Conclusions The Brantas River riparian zone shows a persistent spatial degradation gradient without significant temporal change, consistent with a state of structural saturation in which upstream vegetation has reached its holding capacity and downstream areas remain constrained by built-up infrastructure. REQI offers watershed managers a reproducible tool for monitoring fragmented riparian landscapes. The findings support spatially targeted management: strict protection of upstream buffers combined with active rehabilitation in degraded downstream corridors.