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

Abstract

The Alqueva reservoir (South of Portugal) in the Guadiana river basin constitutes the most important water resource in southern Portugal for domestic and agricultural consumption. We present a methodology developed to characterize spatial and temporal variations of Secchi depth and diffuse attenuation coefficient (both related to dissolved/suspended particles and to water transparency), using high spatial resolution satellite images from Sentinel-2 Multi-Spectral Instrument (MSI). Empirical relations between satellite retrievals of surface reflectances and in situ measurements of water parameters were defined and applied to the entire reservoir for spatial and temporal analysis in the period July 2017-June 2019, useful in the identification of microalgae blooms and rapid variations in water characteristics, which allowed us to differentiate five zones. Water estimates with lower transparency and higher attenuation of radiation were found in the northern area of Alqueva reservoir during the months characterized by higher water temperatures, with Secchi depth monthly averages near 1.0 m and diffuse attenuation coefficient near or above 1.5 m −1 . Satellite retrievals of water with greater transparency in the reservoir were obtained in the southern area in months with low water temperature and atmospheric stability, presenting some monthly Secchi depth averages above 3 m, and diffuse attenuation coefficient below 0.8 m −1 . January 2018 presented great transparency of water with a Secchi depth of 7.5 m for pixels representing the 95th percentile and diffuse attenuation coefficient of 0.36 m for pixels representing the 5th percentile in the Southern region.

Quantitative Inversion Model Design of Mine Water Characteristic Ions Based on Hyperspectral Technology

“…− , and pH ion information, wherein the potassium ion and the chloride ion are indicated by KCl standard solution for a set of data (see Table 1 for details) [31][32][33]. Before measurement, the mother liquor is diluted with deionized water, and according to the test requirements, the sodium ion, potassium ion, chloride ion, and calcium ion dilution levels are 10, 50, 100, 500, 1000, and 10000 mg/ L, the carbonate dilution levels are 0.44, 2.2, 4.4, 22, 44, and 440 mg/L, and the pH dilution levels are 4, 6.86, and 9.18.…”

The Use of Sentinel-3/OLCI for Monitoring the Water Quality and Optical Water Types in the Largest Portuguese Reservoir

“…Cyanobacterial blooms are currently a great threat to the ecological integrity and sustainability for some of the lakes and reservoirs in the world. Episodes of microalgae bloom emergence in the Alqueva reservoir were identified through satellite data [30,31]. The present study analyzes the appearance of these microalgae blooms, also identifying and quantifying the presence of the pigment that signals the presence of cyanobacteria, the phycocyanin.…”

“…The same authors applied the semi-empirical biooptical models from the MERIS sensor to the Sentinel-2 Multi-Spectral Instrument (MSI), estimating the turbidity and Chl-a/cyanobacteria concentrations during a microalgae bloom that occurred in late summer/early autumn of 2017 [31]. Empirical algorithms have recently been established for Secchi depth and diffuse attenuation coefficient using the instrument MSI [30].…”

Remote Detection of Cyanobacterial Blooms and Chlorophyll-a Analysis in a Eutrophic Reservoir Using Sentinel-2

“…Most studies of this type focus on finding the bands that correlate most closely with the presence of cyanobacteria, while others analyze photosynthetic pigments. Previous studies [32,33] of other areas with similar environmental problems reveal that bands 3 and 4 (wavelength 500-650) are the most commonly used to detect cyanobacteria. For photosynthetic compounds, band 5 (705 nm) seems to be the most suitable for chlorophyll detection.…”