Above-water spectral measurements are a critical component of ocean remote sensing and provide essential information for validating and improving remote sensing algorithms for water quality analysis. The downwelling irradiance (Es) is an essential component of above-water spectral measurements and it is particularly important for water constituent concentration, sea surface temperature, and atmospheric correction algorithms. The atmosphere particles scatter and absorb the direct and diffusive components of the downwelling irradiance, creating a complex and distorted spectrum. We analyzed 6 years of the above-water hyperspectral measurements collected every 15 minutes at the Royal Netherlands Institute for Sea Research (NIOZ) Jetty Station (JNS) installed in the Marsdiep tidal inlet of the Dutch Wadden Sea. The minimum level of solar irradiance and spectral shape of Es(λ) were simulated using Radiative Transfer (RT) to identify optimal Es spectra that could be favorable for remote sensing applications. The RT models were adjusted for the study area using in-situ bio-optical measurements. The spectrum of Es(λ) at the 3-optical depths were simulated to identify the minimum level of Es(λ). The red-shifted spectrum of Es(λ), caused by intense atmospheric scattering and the reddish hue of dusk or dawn radiations, was identified by Support Vector Machine applied to the simulated Es(λ) components. The results indicated that the maximum spectral value of Es(λ)max <= 25 mW m-2 nm-1 identifies the minimum level of above-water solar irradiance, and the ratios of Es(480)/Es(680) ≤ 1 mW m-2 nm-1 and Es(λ)max/Es(865) ≤ 1.25 mW m-2 nm-1 show the red-shifted spectra.
Long-term monitoring of Water Constituent Concentrations (WCCs) is essential for water quality assessment in coastal waters, a priority for various government agencies and environmental organizations. However, utilizing satellite observations to track the extended spatial and temporal variations of WCCs, including Chlorophyll-a (Chla), Suspended Particulate Matters (SPMs), and Coloured Dissolved Organic Matters (CDOMs), remains challenging in coastal waters due to their optical complexity and the need for accurate atmospheric correction. In this study, we examined two decades of spatial and temporal variations in Chla [mg m-3], SPM [g m-3] concentrations, and CDOM absorption at 440nm [m-1] using time series data from the Medium Resolution Imaging Spectrometer (MERIS) and the Ocean and Land Colour Instrument (OLCI) spanning from 2003 to 2023. Our research focused on the complex, shallow, and highly turbid waters of the Dutch Wadden Sea, the Netherlands. To achieve this, we employed a coupled atmospheric and water optical model known as MOD2SEA, enabling simultaneous atmospheric correction and WCC retrievals from MERIS images (2003-2012) and OLCI images (2018-2023) in the study area. Our findings reveal that SPM and CDOM variations followed a consistent seasonal pattern over the two-decade period, with SPM values ranging from 10 [g m-3] to 60 [g m-3] and CDOM absorption varying from 0 [m-1] to 0.8 [m-1]. Chla estimates, on the other hand, displayed a different trend, ranging from 5 [mg m-3] to 25 [mg m-3] until 2013. Subsequently, Chla concentrations increased, reaching 45 [mg m-3] by 2023.
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