MODIS-Based Mapping of Secchi Disk Depth Using a Qualitative Algorithm in the Shallow Arabian Gulf

Abstract

Abstract:Regionally calibrated algorithms for water quality are strongly needed, especially for optically complex waters such as coastal areas in the Arabian Gulf. In this study, a regional qualitative algorithm was proposed to retrieve seawater transparency, with Secchi disk depth (SDD) as a surrogate, in the Arabian Gulf. A two-step process was carried out, first estimating the diffuse attenuation coefficient of downwelling irradiance at 490 nm (K d _490) from MODIS/Aqua imagery and then SDD based on empirical correlations with K d _490. Three satellite derived K d products were tested and assessed against a set of in situ measurements, and one from a semi-analytical algorithm based on inherent optical properties gave the best performance with a R 2 of 0.62. Comparisons between the performances of SDD models developed in this study and those established in other regions indicated higher accuracy of our proposed model for the Gulf region. The potential factors causing uncertainties of the proposed algorithm were also discussed. Seasonal and inter-annual variations of SDD over the entire Gulf were demonstrated using a 14-year time series of MODIS/Aqua data from 2002 to 2015. High SDD values were generally observed in summer while low values were found in winter. Inter-annual variations of SDD did not shown any significant trend with exceptions during algal bloom outbreaks that resulted in low SDD.

Evaluation of Satellite Retrievals of Chlorophyll-a in the Arabian Gulf

“…Due to the shallowness and turbulence found in the sampling area, the waters are likely to be turbid, which means that the bottom reflectance issue is likely minimum. Similar patterns have been reported by Al Kaabi et al [8], were the authors demonstrated seasonal variations of Secchi disk depth (SDD) over the entire Gulf using a set of in situ SDD measurements versus 14-year time-series of MODIS/Aqua K d data. Generally, higher SSD values were observed in summer, while lower values were found in winter [8] (Al Kaabi et al, 2016).…”

“…Similar patterns have been reported by Al Kaabi et al [8], were the authors demonstrated seasonal variations of Secchi disk depth (SDD) over the entire Gulf using a set of in situ SDD measurements versus 14-year time-series of MODIS/Aqua K d data. Generally, higher SSD values were observed in summer, while lower values were found in winter [8] (Al Kaabi et al, 2016). The authors developed a locally-adapted algorithm to estimate SSD from satellite-derived K d over the Gulf using Lee's algorithm [32].…”

“…Based on a set of SDD in situ measurements from the Arabian Gulf, the developed algorithm performed well in comparison with other SDD models established in other regions [8]. We compared our first optical depth, calculated based on OC-CCI K d data, with the SDD calculated using Al Kaabi et al [8] proposed algorithm for the Arabian Gulf, and the result appeared to be exactly the same (r = 1, p < 0.001) indicating that our findings well captured the first optical depth acquired by the satellite sensors in the Arabian Gulf ( Figure S1). Moreover, our findings suggest that the satellite sensors do not capture accurately the DCM in most of the seasons (Figure 4).…”

“…The authors developed a locally-adapted algorithm to estimate SSD from satellite-derived K d over the Gulf using Lee's algorithm [32]. Based on a set of SDD in situ measurements from the Arabian Gulf, the developed algorithm performed well in comparison with other SDD models established in other regions [8]. We compared our first optical depth, calculated based on OC-CCI K d data, with the SDD calculated using Al Kaabi et al [8] proposed algorithm for the Arabian Gulf, and the result appeared to be exactly the same (r = 1, p < 0.001) indicating that our findings well captured the first optical depth acquired by the satellite sensors in the Arabian Gulf ( Figure S1).…”

“…The Arabian Gulf (Figure 1a) is the most active oil production and trade area in the world [8]; it is also considered among the highest anthropogenically impacted regions on Earth [9]. The Gulf is witnessing a continuous and unprecedented development and growth in human population in its coastal zone, which is accompanied by increasing exploitation of resources and demands of services from the marine environment.…”