Spectral measurements of underwater downwelling radiance of inland water bodies

摘要

The apparatus exploited in this work is composed of an optical cable linked to a portable FieldSpec UV/VNIRthat records the spectral downwelling radiance in underwater environment, allowing us to calculate theshortwave attenuation coefficient in water. Results for three inland water bodies are presented under differentatmospheric conditions (sun zenith angle and wind speed) and water composition (chlorophyll a concentrationand turbidity). We show that the spectral downwelling zenith radiance profiles under high sun elevationspresent a positive slope in the upper layers due to relatively high scattering of direct sunlight compared toattenuation. For deeper layers, attenuation overcomes the scattering of sunlight leading to a constant negativelogarithmic slope. For low sun elevations, a negative slope is observed in the entire water column since thescattering of direct sunlight is always lower than attenuation. Whenever a negative logarithmic constant slopeis observed, the attenuation coefficient was computed. A relation was observed between attenuation coefficientin the photosynthetically active radiation (PAR) spectral region and water turbidity, for the three water bodiesunder study.