Decadal dynamics of surface water extent and quality in Fujian Province, China: An integrated optical-SAR remote sensing analysis using Landsat and Sentinel-1 time series (2015–2024)

Surface water resources are essential for ecosystem services, human welfare and climate regulation, yet face increasing pressure from climate variability and human activity. This study examined surface water extent, relative water-quality proxies and thermal regulation in Fujian Province, China, during 2015–2024 using Landsat 8/9 and Sentinel-1 SAR. Optical water was mapped with a majority-voting ensemble of NDWI, MNDWI and AWEI (threshold >0) and fused with Sentinel-1 detections constrained by MNDWI > −0.15. A − 0.85 dB shift in Sentinel-1 VV backscatter between 2015 and 2016, coinciding with the start of Sentinel-1B acquisitions, was identified; it produced a spurious +577 km 2 increase in SAR-only water area, so SAR was used only as an MNDWI-constrained complement to the optical mask. Against the JRC Global Surface Water occurrence layer (>80%), used as a Landsat-derived consistency benchmark, the optical ensemble achieved F1 = 0.70, Kappa = 0.70 and IoU = 0.54. Fused water area ranged from 1476.76 to 1729.26 km 2 (mean 1573.95 km 2 , 1.27% of the land area). After a two-year persistence filter, net change was −106.63 ± 32.80 km 2 in 2015–2020, +65.05 ± 23.40 km 2 in 2020–2024 and − 41.58 ± 35.72 km 2 over the decade. On pixels that were water in all ten years, turbidity and total suspended matter proxies declined by 5.64% and 15.84% (Mann–Kendall p < 0.05), whereas the CDOM proxy showed no significant trend. In summer, water bodies were 2.45 ± 0.44 °C cooler than land within 1 km and 1.60 ± 0.52 °C cooler than land within 2 km ( p < 0.001), whereas a province-wide comparison reversed the sign because of elevation effects. These results provide a baseline for water-resource management and heat mitigation in rapidly developing coastal regions.