Decadal surge of water-surface solar in China's Yangtze Delta: A high-fidelity SAR-optical fusion inventory (2015–2024)

Abstract. China hosts approximately 97 % of the world's water-surface photovoltaics (WPV), with nearly two-thirds of its national capacity concentrated in the Yangtze River Delta (YRD), a densely populated economic powerhouse facing intense land-energy trade-offs. Despite this dominance, no high-resolution, decade-long inventory has existed to track this rapid expansion. WPV detection using optical RS imagery is severely limited by persistent cloud cover, water surface reflections, and spectral confusion, compromising long-term consistency over aquatic environments. Here, we developed a multi-sensor fusion framework integrating all-weather Sentinel-1 Synthetic Aperture Radar (SAR) and annual composite Sentinel-2 optical imagery. Key features include six Sentinel-2 bands, spectral indices (NDVI, MNDWI, NDBI, NDPI, and SAVI), texture metrics, and dual-polarization SAR backscatter. We trained a Random Forest classifier on 55,849 verified samples to generate annual WPV maps for 2015–2024. Afterwards, we applied post-processing procedures, including noise removal, patch merging, and area thresholding, and further validated installation years and eliminated errors through manual inspection of Google Earth time-series imagery. The well-constructed dataset of the first 10 m-resolution WPV atlas for the YRD maps 401 validated projects with a cumulative area of 145.4 km² by 2024. It outperforms existing global PV inventories with an overall accuracy of 97.3 % and a Kappa coefficient of 0.94. The results reveal rapid expansion from 17.4 km² in 2015 to 145.4 km² in 2024, with 87 % deployed on natural lakes, with a marked shift in leadership from Jiangsu to Anhui, and clear spatial clustering near grid infrastructure and stable water bodies. This high-fidelity inventory provides a robust foundation for monitoring WPV evolution, assessing environmental impacts, and informing sustainable energy planning in the world's leading floating solar region.