Development of clarity model for Caspian Sea using MERIS data

Abstract

Secchi disc is often used to measure the clarity of water in oceans and lakes. In this study, an empirical model is developed for the monitoring of secchi disc depth (SD) in the Caspian Sea using satellite images. In situ measurements of secchi disc depth have been performed in the Caspian Sea between July and October 2005. The in situ water samples were taken during 25 one-day campaigns at different distances to the coast. Totally 37 secchi depth data of Caspian Sea were gathered. The database was divided to 25 and 12 data for training and testing of empirical models, respectively. A total of 25 Level 1B and 25 Level 2 MERIS images acquired over the Caspian Sea between July and October 2005 were used in this study. The level 1b and level 2 data are containing of remote sensing reflectance (Rrs) data of TOA (Top Of Atmosphere) and BOA (Bottom Of Atmosphere), respectively. The relation between secchi depth and Rrs, logarithm of Rrs, ratio of Rrs data and logarithm of ratio of Rrs data in different wavelengths for TOA and BOA data were investigated and then the highest correlation coefficient between secchi depth and mentioned Rrs parameters were selected and some empirical models were developed using them. The results showed that the modeling using BOA data can lead to better results than TOA. The best developed model using BOA data is as 1/SD=-0.138 +2.08*[Rrs(681nm)/Rrs(560nm)] with 28% error and with correlation coefficient equal to 0.86.