Effects of water salinity on the multi-angular polarimetric properties of light reflected from smooth water surfaces

Salinity is an important environmental factor regulating the aquatic system structure of lakes and other water bodies. Changes in salinity, which can be caused by human activities, can adversely impact the life of water organisms. The refractive index, which can be directly related to water salinity, also controls the polarimetric properties of light reflected from the water surface. In this study, polarimetric measurements of smooth water surfaces with different salinity content were performed at different viewing zenith angles in the wavelength range of 450-1000 nm in the specular reflection directions. The results show that the light reflected from the water surface (defined as reflectance factor) in one measurement direction can be replaced by the reflectance factor derived from polarimetric measurements, and if the polarizer absorptance is considered, the average relative difference is less than 3%. The degree of linear polarization (DOLP) was used to retrieve the refractive indices of water with different salinities based on the Fresnel reflection coefficient. The inverted refractive indices not only have high accuracy (uncertainty from 0.9% to 1.8%) but also have a very strong relationship with the water salinity content. Our study shows the possibility of estimating the variation in water salinity using multi-angular polarimetric measurements.