Assessment of Water Surface Reflectance and Optical Water Types Over Two Decades in Europe's Largest Artificial Lake: An Intercomparison of ESA and NASA Satellite Data

This study focuses on comparing surface reflectances and optical water types (OWTs) obtained in Alqueva reservoir, located in the Alentejo region (Portugal), over a period of two decades (2003–2022) using four datasets: the moderate resolution imaging spectroradiometer (MODIS), the medium-resolution imaging spectrometer, the ocean and land color instrument aboard Sentinel-3, and the multispectral instrument aboard Sentinel-2. The MODIS instrument covers the entire study period and acts as the benchmark for intercomparing the surface reflectances obtained with the other three sensors. The classification of OWTs is based on differences in reflectance spectra, facilitating a qualitative assessment of water quality. This approach identified four distinct clusters, with two OWTs representing water with higher turbidity, facilitating the differentiation of reflectances associated with microalgae blooms and other phenomena such as runoff. However, when using MODIS, which covers only the central area of the reservoir, only three clusters were identified as the ideal number. Using 300 or 1000 m of spatial resolution, the Alqueva reservoir mostly exhibits high water transparency, associated with low surface reflectances for the majority of the time. Seasonal analysis revealed periods with the presence of microalgae in summer and autumn (SON), with a noticeable increase in the intensity and duration of these blooms in the SON period over the last ten years. This methodology enables the identification of advantages and disadvantages associated with the utilization of each sensor in large reservoirs and across extensive datasets.