Retrieval and validation of the Secchi disk depth values (Z) from the Sentinel-3/OLCI satellite data in the Persian Gulf and the Gulf of Oman.

In this study, the Secchi disk depth (Z) values as an indicator of seawater clarity/transparency were estimated using the ESA (European Space Agency) Sentinel-3A and Sentinel-3B OLCI (S3/OLCI) satellite data in the Persian Gulf and the Gulf of Oman (PG&GO). To do so, two procedures were evaluated including an existing methodology developed by Doron et al. (J Geophys Res: Oceans 112(C6) 2007 and (Remote Sens Environ 115:2986-3001 2011) and an empirical model proposed in this research formed by employing the blue (B) and green (B) bands of S3/OLCI data. In this regard, a total number of 157 field-measured Z values (114 training points for calibration of the models and 43 control points for accuracy assessment of them) were observed during eight research cruises conducted by the research vessel, the Persian Gulf Explorer, in the PG&OS between 2018 and 2022. The optimum methodology was then selected based on the statistical indicators including R (coefficient of determination), RMSE (root mean square error), and MAPE (mean absolute percentage error). However, after the indication of the optimal model, the data of all 157 observations were utilized for the calculation of unknown parameters of the model. The final results demonstrated that compared to the existing empirical model proposed by Doron et al. (J Geophys Res: Oceans 112(C6) 2007 and (Remote Sens Environ 115:2986-3001 2011), the developed model in this study which was formed based on the linear and ratio terms of B and B bands, has more efficiency in the PG&GO. Consequently, a model in form of Z = e was suggested for the estimation of Z values from S3/OLCI in the PG&GO (R = 0.749, RMSE = 2.56 m, and MAPE = 22.47%). The results also showed that the annual oscillation of the Z values in the GO (5-18 m) is evidently higher compared with those in the PG (4-12 m) and the SH (7-10 m) regions.