Retrieval and Validation of the Secchi Disk Depth Values (Zsd) from the Sentinel-3 satellite data in the Persian Gulf and the Gulf of Oman

AbstractIn this study, the Secchi disk depth (Zsd) values as an indicator of seawater clarity/transparency were estimated using the ESA (European Space Agency) Sentinel-3A and − 3A 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 empirical methodology developed by Doron et al., 2007 and 2011 and a novel model proposed in this research formed by employing the blue (B4) and green (B6) bands of S3/OLCI data. In this regard, a total number of 157 field-measuredZsdvalues (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,R2(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., 2007 and 2011, the developed model in this study which was formed based on the linear and ratio terms of B4and B6bands, has more efficiency in the PG&GO. Consequently, a model in form ofZsd= e1.638B4/B6− 8.241B4− 12.876B6+ 1.26was suggested for the estimation ofZsdvalues from S3/OLCI in the PG&GO (R2 = 0.749, RMSE = 2.56 m, and MAPE = 22.47%). The results also showed that the annual oscillation of theZsdvalues 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.