Generation of robust 10-m Sentinel-2/3 synthetic aquatic reflectance bands over inland waters

Inland waters comprise various aquatic systems, including rivers, lakes, lagoons, reservoirs, and others, and satellite data play a crucial role in providing holistic and dynamic observations of these complex ecosystems. However, available medium-spatial resolution satellite sensors, such as Sentinel-2 Multi-Spectral Instrument (MSI), are typically designed for land monitoring and lack suitable spectral bands and radiometric quality for water applications. This study developed a novel synthetic band generation method, called Sentinel-2/3 Synthetic Aquatic Reflectance Bands (S2/3Aqua), for computing eight 10-m synthetic spectral bands from multivariate regression analysis between Sentinel-2 MSI and Sentinel-3 OLCI image pair. Three multivariate regressor models, Multivariate Linear Regressor (MLR), Multivariate Quadratic Regressor (MQR), and Random Forest Regressor (RFR), were applied and assessed to replicate the Sentinel-3 spectral consistency on 10-m Sentinel-2 images. A cyanobacteria modeling was developed based on in-situ observations (n = 54), and we demonstrated, for the first time, the application of 10-m harmful algal bloom mapping over two eutrophic tropical urban reservoirs (Promissão and Billings, Brazil). Additionally, the generalization of S2/3Aqua was assessed by comparing its spectral signatures across different water optical types. Overall, the comparison between S2/3Aqua and Sentinel-3 bands achieved a mean absolute error of 6 % and a mean difference close to zero. We found that MLR exhibited a higher accuracy with in-situ observations (with a 28 % bias) and was more suitable than other tested models. S2/3Aqua also performed satisfactorily across all eight spectral bands, including at 620 and 681 nm, with a mean difference of less than 0.003 reflectance units. The cyanobacteria mapping showed a high level of agreement between S2/3Aqua and Sentinel-3 for low concentrations of Phycocyanin (less than 50 mg m⁻³), and S2/3Aqua effectively captured the spatial variability of narrower and smaller blooms. Finally, S2/3Aqua provides reliable synthetic spectral bands that can effectively be used in several aquatic system studies, including monitoring potentially harmful algal blooms.