Spectral measurements of underwater downwelling radiance of inland water bodies

Abstract

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

Temporal and Spatial Variations of Secchi Depth and Diffuse Attenuation Coefficient from Sentinel-2 MSI over a Large Reservoir

“…Lower KD and higher SD values were estimated in colder months, with lower water temperature, and no significant rainfall events. An excellent relationship between the diffuse attenuation coefficient and turbidity has been identified in several studies (e.g., [60][61][62][63]) and also in the Alqueva reservoir with r 2 = 0.95 [39]. Since SD is directly related to turbidity, a strong indication of the quality of the estimations of SD and KD for the Alqueva reservoir is that the increase in one of the parameters is related to the decrease in the other.…”

“…In order to ensure adequate illumination, these measurements were limited to the daylight period between 10:00 a.m. and 4:00 p.m. UTC. In this study, the measurements of KD in water were made using the method described in Potes et al [39]. According to this method, the measurements of underwater downwelling spectral irradiance were performed with the use of a portable FieldSpec (Analytical Spectral Devices; Boulder, CO, USA) together with an optical fiber linked to a hemispherical tip (180 • ).…”

Concept for a hyperspectral remote sensing algorithm for floating marine macro plastics

“…(A3)), so if we have enough measurements of R t over a range of f values we could find c 1 , c 2 (and thus ε), and ρ p,RS (λ) through curve fitting. It would be possible to examine the magnitude of ε(f,λ) from measurements of L ds /L ds,0 for the range of f values, using an underwater spectroradiometer (e.g., Potes et al, 2013). Experimentally derived ε(f, λ) and ρ p,RS (λ) represent the plastic objects shape and not just apply for a single flat layer of plastic.…”

Lake–atmosphere interactions at Alqueva reservoir: a case study in the summer of 2014

“…The aquatic photosynthesis processes occur in the euphotic zone, the layer between the surface and the depth of 1% of the subsurface irradiance, and this layer can be thicker in clear waters and thinner in turbid ones (Bukata et al, 1995;Wozniak et al, 2003). Some instrumentation has been developed to measure the underwater radiance field in lakes, fjords and oceans (Sasaki et al, 1958;Jerlov and Fukuda, 1960;Tyler, 1960;Lewis et al, 2011;Antoine et al, 2013;Potes et al, 2013). In particular, Potes et al (2013) developed a new apparatus to be coupled with a portable spectroradiometer, allowing for spectral radiance measurements in underwater environments and to subsequently obtain estimates of the spectral and broadband light attenuation coefficients in water.…”