Estimation of the Secchi disk depth from the NASA MODIS-Aqua diffuse attenuation coefficient data in the northern Persian Gulf and the Gulf of Oman: A spatiotemporal assessment

Abstract

This study aimed to propose a method for the estimation of the Secchi disk depth ( Zsd) values in the northern Persian Gulf, the Strait of Hormuz, and the Gulf of Oman (N-PG-SH-GO), based on MODIS-Kd(490) data. The field-observed Zsd values were collected during seven research cruises conducted by a research vessel between 2018 and 2021. A total number of 114 field-observed Zsd values were observed in 68 stations. The daily Kd(490) of MODIS-Aqua/L2 data were downloaded for all days (± three), wherein the field-observed data were available. Different fitting models were examined to estimate the Zsd values from the Kd(490). A total number of 73 Zsd values observed in the first six research cruises were utilized to propose the optimum model, and the data of 41 stations, observed in the last research cruise were used for the validation of the results. Upon designating the optimum method, all the field-observed Zsd values were then employed to create the final model. The results demonstrated that the power model is optimal (R2=0.81 and RMSE = 2.04 m), so the model in the form of Zsd=0.34Kd(490)−1.42 might be utilized to compute the Zsd values in the N-PG-SH-GO. This model was then employed to determine the monthly and seasonal Zsd values to assess the spatiotemporal variation of water clarity in the N-PG-SH-GO. The results showed that the annual variation domain of the Zsd values in the N-GO was extremely higher than that in the N-PG and N-SH so that the maximum and minimum water clarity respectively occurred in warm months and colder ones. Compared with the N-PG and the N-GO, the average annual water clarity of the N-SH was lower where the mean annual Zsd values for the N-PG, N-SH, and N-GO were determined as 8.91, 6.27, and 10.27 m, respectively.