De Liz Arcari, A.

Retsios, V.

Salama, M.S.

Tavora, J.

van der Wal, D.

English

Water clarity relates to the combined effects of water constituents that attenuate light, such as suspended material, phytoplankton, and coloured dissolved organic matter. Besides being strongly related to primary productivity and trophic state, this ensemble indicator also influences water-atmosphere heat transfer. Water clarity, expressed as the diffuse attenuation coefficient of photosynthetically active radiation (kd(PAR)), is therefore a key input for lake models in numerical weather prediction. Current satellite sensors offer the possibility of capturing kd(PAR) spatial-temporal variability in waterbodies, which is critical to advance understanding of the nexus between water quality and climate. In this study, we introduce a fully analytical model to derive kd(PAR) from Sentinel-2 imagery based on the solution of the two-stream radiative transfer equations. The analytical solution implies the model does not rely on empirical relationships and regional tuning. It thus provides a general approach to derive kd(PAR) in diverse optical complexities. The applicability of the model to Case-2 waters is demonstrated in Conceição Lagoon, Southern Brazil. A time series of kd(PAR) maps was constructed from Sentinel-2 images captured between 2019 and 2021. Analysis allowed the identification of a consistent spatial pattern and the detection of temporal anomalies. Hotspots of reduced water clarity are seen in the lagoon’s margins and where a cyclonic gyre favours resuspension. A trend of decreasing water clarity is also seen in areas of intensifying anthropic pressure. The model can therefore provide valuable information to further investigate the feedback mechanism between water quality and the thermal balance of aquatic systems

University of Twente Research Information

Analytical Derivation of Water Clarity Time-Series from Sentinel-2 MSI ImageryAmanda de Liz Arcari1, Bas Retsios1, Juliana Tavora1, Daphne van der Wal1,2 and Mhd. Suhyb Salama11 Department of Water Resources, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Enschede, Netherlands 2 Department of Estuarine and Delta Systems, NIOZ Royal Netherlands Institute for Sea Research, Yerseke, Netherlands I. BackgroundOperational implementation of a physically-based model to derive water clarity (kd(PAR))  from Sentinel-2 MSI imagery. Highlights of the method include:• Automatic image retrieval from Copernicus Hub• Masking of land, clouds, and glint correction• Application for multiple water management purposesII. MethodologyOperational implementation of a two-stream radiative transfer model -  2SeaColor (Salama & Verhoef, 2015):Scan to access contact details and referencesEmail: a.delizarcari@utwente.nl III. Preliminary Results• Water clarity depends on the combined effects of all light-attenuating constituents• Key parameter in aquatic studies of primary productivity, eutrophication and heat transfer• Secchi depth is a traditional but spatially limited measurement of water clarity• The diffuse attenuation coefficient of photosynthetically active radiation (kd(PAR)) describes how visible light decreases with depth – water clarity proxy• kd(PAR) can be derived from satellite imagery• High spatial resolution kd(PAR) products are needed for coastal and inland waters studiesInversion by spectral optimization𝒌𝒅 𝟒𝟒𝟑 , 𝒌𝒅 𝟒𝟗𝟎 , 𝒌𝒅 𝟓𝟔𝟎 , 𝒌𝒅 𝟔𝟔𝟓 , 𝒌𝒅 𝟕𝟎𝟓• Copernicus Credentials• Area of Interest (GeoJSON)• Acquisition period• Cloud coverUser Defined:• Water temperature• SalinityRetrieval from Copernicus Open Access HubCalculation of water absorption and scatteringSurface roughness / glint correction• IOPs ParametrizationEstimation of IOPs initial valuesDefault or retrieved:• Wind speed and direction• Scene Classification Map• Illumination Geom.• Spectral Info.Sentinel-2 Level 2A ImagesLand and cloud pixels masking𝒌𝒅 𝑷𝑨𝑹Spectral IntegrationForward 2SeaColor modelConceição Lagoon, Southern  BrazilJan 2019- Dec 2021Max. 30%139 scenes (42-49/year)CASE STUDY:IV. Summary• Overall lower water clarity in South Lagoon• Significant (p < 0.05) decreasing water clarity trend in most of the Lagoon, especially in the SouthSouthCentralNorth• Low water clarity hotspot in Central Lagoon consistent with edge of 2 cyclonic gyresMAIN SPATIAL PATTERNS:TREND ANALYSIS:organic matter + poor circulation• High kd(PAR) values at the marginsadjacency effects suspectednutrient resuspension + increase in primary productivityincreasing anthropic pressure + poor circulation• One match-up with 11 in-situ data points • General inverse relationship in line with other studies• Limitations due to:• No. of matchups• Small range of valuesDERIVED kd(PAR) VS. IN-SITU SECCHI DEPTH:Modeled mean circulation pattern in Central Lagoon (from from Silva (2013))11 Secchi measurements* w/ Sentinel-2 overpass*Credits to Guimarães (2017) for data collection and sharing 

Analytical derivation of water clarity time-series from Sentinel-2 MSI imagery

https://ris.utwente.nl/ws/files/319408716/Poster_TPSF_A0_V2_HighRes.pdf

Analytical derivation of water clarity time-series from Sentinel-2 MSI imagery

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