Monitoring transparency in inland water bodies using multispectral images

Abstract

The aim of this research was to monitor transparency in inland water bodies from water reflectance field measurements and also from high and medium spatial resolution sensors. The most suitable band to measure transparency was determined by sampling irradiance, water reflectance and transparency at different depths. The results of this sampling show that the wavelength range 520–600 nm is the most suitable band for measuring transparency. Reflectance in this band was regressed against transparency field data to derive an empirical equation for transparency estimation. This equation was assessed using data from Landsat Thematic Mapper (TM) images from 1992 and Daedalus 1268 Airborne Thematic Mapper (ATM) data from 1997. The difference in the errors in the data observed between the two dates was attributed to the fact that in 1997 the sampling time was less than 3 h (1.5 h before and after the acquisition of the image), which allowed similar solar illumination conditions to be maintained, while in 1992 the sampling time was much longer. Once the equation was verified, water transparency monitoring was performed for a set of satellite images obtained from 1984 to 2000. The maps show that transparency in the water bodies responds to seasonal trends. Furthermore, this analysis enabled trophic state and Ecological Quality Ratio (EQR) transparency maps to be generated, which are very useful for the management of water bodies.