Retrieval of Water Constituents from Hyperspectral In-Situ Measurements under Variable Cloud Cover—A Case Study at Lake Stechlin (Germany)

Abstract

Abstract:Remote sensing and field spectroscopy of natural waters is typically performed under clear skies, low wind speeds and low solar zenith angles. Such measurements can also be made, in principle, under clouds and mixed skies using airborne or in-situ measurements; however, variable illumination conditions pose a challenge to data analysis. In the present case study, we evaluated the inversion of hyperspectral in-situ measurements for water constituent retrieval acquired under variable cloud cover. First, we studied the retrieval of Chlorophyll-a (Chl-a) concentration and colored dissolved organic matter (CDOM) absorption from in-water irradiance measurements. Then, we evaluated the errors in the retrievals of the concentration of total suspended matter (TSM), Chl-a and the absorption coefficient of CDOM from above-water reflectance measurements due to highly variable reflections at the water surface. In order to approximate cloud reflections, we extended a recent three-component surface reflectance model for cloudless atmospheres by a constant offset and compared different surface reflectance correction procedures. Our findings suggest that in-water irradiance measurements may be used for the analysis of absorbing compounds even under highly variable weather conditions. The extended surface reflectance model proved to contribute to the analysis of above-water reflectance measurements with respect to Chl-a and TSM. Results indicate the potential of this approach for all-weather monitoring.

Design and implementation of an illumination system to mimic skyglow at ecosystem level in a large-scale lake enclosure facility

“…The modeling parameters were set as follows: the anisotropy factor was g = 0.9 to simulate forward scattering, the scattering coefficient was and the absorption coefficient was to represent a wavelength of 500 nm. These values are on the order of previously measured values for the lake 47 , while our own measurements for the diffuse downwelling attenuation coefficient ranged between 0.3/m and 1.5/m during summer months. Since the focus was on the spatial distribution of light in the enclosures, the modeling was performed for monochromatic light only.…”

Mimicking skyglow at ecosystem level in a large-scale lake enclosure facility

“…the anisotropy factor was g = 0.9 to simulate forward scattering, the scattering coefficient was μ s = 1/m and the absorption coefficient was μ a = 0.55/m, which are on the order of previously measured values for the lake (Göritz et al 2018) while own measurements for the diffuse downwelling attenuation coefficient k d ranged between 0.3/m and 1.5/m during summer months. Since the focus was on the spatial distribution of light in the enclosures, the modeling was performed for monochromatic light only.…”

Determination of the remote-sensing reflectance from above-water measurements with the “3C model”: a further assessment

“…The term δ aims at facilitating retrievals with measurements affected by broken clouds or by high sea states causing bubbles and whitecaps. Specifically, it could help to model the spectral shape of glint contributions for overcast and scattered cloud conditions [10].…”