OLI/Landsat-8 semi-analytical based model for retrieving Secchi disk depth in Nova Avanhandava Reservoir

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Water clarity can be considered as the key information to assess water quality. This information can be related to the vertical visibility, analogous to the Secchi disk depth (𝑍𝑆𝐷), which is measured manually with a Secchi disk or by remote sensing. The second one uses bio-optical models, however, most of them were calibrated for oceanic and coastal waters. The semi-analytical approach uses as input, the inherent optical properties (IOPs) retrieved from the quasi-analytical algorithm (QAA) designed for seawaters. Therefore, we hypothesized that error caused by the use of a semi- analytical designed for oceanic waters will prevent an accurately 𝑍𝑆𝐷 estimation in an oligo-to-mesotrophic reservoir. Aiming to test this hypothesis, we used in situ data collected in three different dates considering OLI/Landsat-8 bands. As result, we noticed that according to the reservoir optical water quality, the estimation of 𝑍𝑆𝐷 returned Mean Absolute Percentage Errors ranging between 12.23% and 28.10% when compared to in situ data. We also used a dataset (n = 6) collected two and three days before OLI’s overpass in order to validate the model using satellite data and the errors were 9.18% and 6.32% for the 2 and 3-day delay, respectively. These results showed that QAA designed for seawaters was able to retrieve 𝑍𝑆𝐷 in an oligo-to-mesotrophic reservoir, probably because the closed bio-optical properties from both environments, assuming to be applicable to water clarity monitoring.