Spatio-temporal variations of Secchi disk depth in coastal waters of the South China Sea from VIIRS observations
Water transparency is an important ecological indicator for evaluating the underwater light field,which determines the biological,physical and chemical processes in waters.Based on the field measurements from case Ⅰ and case Ⅱ waters,the effect of Secchi disk depth on remote sensing reflec-tance of water was performed to develop a Secchi disk depth estimation algorithm for coastal waters of the South China Sea.The optical model,validated by using local data in October 2015,was used with monthly VIIRS data from 2013 to 2016 to assess the temporal and spatial variability of the Secchi disk depth in coastal waters of the South China Sea.Furthermore,a comparison of VIIRS and MODIS for monitoring marine environment was performed in this study.Results demonstrated that the Secchi disk depth highly controlled the remote sensing reflectance of water at bands of 488 and 555 nm.A reflected peak was found at 555 nm when the Secchi disk depth higher than 5.3 m,while on the condition of the Secchi disk depth lower than 5.3 m,the peak moved to the waveband of 488 nm.The band ratio of VI-IRS,Rrs(488)/Rrs(555),was highly correlated to the Secchi disk depth in coastal waters of the South China Sea,which was then applied to the calibrated VIIRS images and found that the Secchi disk depth was higher in the offshore than nearshore and also in the east than west.Compared to MODIS data,the VIIRS instrument has higher spatial resolution and calibration activities,which will provide more ocean color products in future.
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