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

2020 ◽  
Vol 12 (5) ◽  
pp. 768 ◽  
Author(s):  
Gonçalo Rodrigues ◽  
Miguel Potes ◽  
Maria João Costa ◽  
Maria Helena Novais ◽  
Alexandra Marchã Penha ◽  
...  
Keyword(s):  
Attenuation Coefficient ◽  
Secchi Depth ◽  
Atmospheric Stability ◽  
Temporal Analysis ◽  
Temporal Variations ◽  
Large Reservoir ◽  
Diffuse Attenuation Coefficient ◽  
Satellite Retrievals ◽  
Alqueva Reservoir ◽  
Sentinel 2

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.

scholarly journals Optical properties of Finnish lakes estimated with simple bio-optical models and water quality monitoring data

2006 ◽  
Vol 37 (2) ◽  
pp. 183-204 ◽  
Author(s):  
Kari Kallio
Keyword(s):  
Remote Sensing ◽  
Water Quality ◽  
Optical Properties ◽  
Chlorophyll A ◽  
Attenuation Coefficient ◽  
Secchi Depth ◽  
Reflectance Spectra ◽  
Optical Data ◽  
Diffuse Attenuation Coefficient ◽  
Sensing Applications

The aim of this study was to estimate the distributions of spectral diffuse attenuation coefficient, attenuation depth and subsurface reflectance of Finnish lakes. In addition, the optimum empirical water quality interpretation algorithms employing reflectance ratios were investigated for the needs of remote sensing. Estimations of the optical properties were based on simple optical models and measured concentrations of optically active substances (the sum of chlorophyll a and phaeophytin a, total suspended solids and coloured dissolved organic matter (CDOM)) at 1670 monitoring stations representing 1113 lakes. The models were parameterized using optical data from 10 lakes. The mean diffuse attenuation coefficient in PAR was 3.5 m−1 and the location of the maximum attenuation depth was in the range 564–714 nm. The simulated reflectance spectra showed a shift of the maximum value to longer wavelengths as trophic status changed from oligotrophic to hyper-eutrophic. High CDOM concentrations decrease the estimation accuracy of chlorophyll a from reflectance spectra using empirical algorithms, particularly in oligotrophic lakes. The models described can be used in studying light availability for photosynthesis at different depths, in the simulation of water temperatures, in estimating how different management alternatives affect light attenuation and Secchi depth, and in various remote sensing applications.

scholarly journals Diurnal Variation of the Diffuse Attenuation Coefficient for Downwelling Irradiance at 490 nm in Coastal East China Sea

2021 ◽  
Vol 13 (9) ◽  
pp. 1676
Author(s):  
Yu Zhang ◽  
Zhantang Xu ◽  
Yuezhong Yang ◽  
Guifen Wang ◽  
Wen Zhou ◽  
...  
Keyword(s):  
Diurnal Variation ◽  
Attenuation Coefficient ◽  
East China Sea ◽  
East China ◽  
Coefficient Of Determination ◽  
Target Area ◽  
Diffuse Attenuation Coefficient ◽  
China Sea ◽  
Mean Square Errors ◽  
Downwelling Irradiance

The diurnal variation of the diffuse attenuation coefficient for downwelling irradiance at 490 nm (Kd(490)) has complex characteristics in the coastal regions. However, owing to the scarcity of in situ data, our knowledge on the diurnal variation is inadequate. In this study, an optical-buoy dataset was used to investigate the diurnal variation of Kd(490) in the coastal East China Sea, and to evaluate the Kd(490) L2 products of geostationary ocean color imager (GOCI), as well as the performance of six empirical algorithms for Kd(490) estimation in the Case-2 water. The results of validation show that there was high uncertainty in GOCI L2 Kd(490), with mean absolute percentage errors (MAPEs) of 69.57% and 68.86% and root mean square errors (RMSEs) of 0.70 and 0.71 m−1 compared to buoy-measured Kd12(490) and Kd13(490), respectively. Meanwhile, with the coefficient of determination (R2) of 0.71, as well as the lowest MAPE of 27.31% and RMSE of 0.29 m−1, the new dual ratio algorithm (NDRA) performed the best in estimating Kd(490) in the target area, among the six algorithms. Further, four main types of Kd(490) diurnal variation were found from buoy data, showing different variabilities compared to the area closer to the shore. One typical diurnal variation pattern showed that Kd(490) decreased at flood tide and increased at ebb tide, which was confirmed by GOCI images through the use of NDRA. Hydrometeorological factors influencing the diurnal variations of Kd(490) were also studied. In addition to verifying the predominant impact of tide, we found that the dominant effect of tide and wind on the water column is intensifying sediment resuspension, and the change of sediment transport produced by them are secondary to it.

scholarly journals LIDAR BATHYMETRIC EVALUATION BASED ON SCATTERING CLASSIFICATION ALGORITHM

2020 ◽  
Vol XLII-3/W10 ◽  
pp. 97-103
Author(s):  
J. Gao ◽  
G. Q. Zhou ◽  
H. Y. Wang ◽  
X. Zhou ◽  
Y. X. Mu ◽  
...  
Keyword(s):  
Attenuation Coefficient ◽  
Classification Algorithm ◽  
Airborne Lidar ◽  
Discrimination Factor ◽  
Actual Situation ◽  
Particle Scattering ◽  
Lidar System ◽  
Scattering Intensity ◽  
Diffuse Attenuation Coefficient ◽  
532 Nm

Abstract. The evaluation of the bathymetric capability of traditional airborne lidar system is mostly based on the formula of bathymetric capability by evaluating the diffuse attenuation coefficient (Kd). This method is derived form the assumption that the reflectance of sediment is fixed. In this study ,however,the reflectance of sediment is not fixed. Therefore, this study improves the ability of bathymetric formula, and proposes a particle scattering classification algorithm to obtain the transmissivity value. The algorithm filters the scattering modes of particles by scattering discrimination factor (q), and obtains the transmissivity values by using the scattering intensity formulas. Experiments show that, when the transmissivity is in the range of 0–1 and the average values of Kd(532 nm) are 0.1150 m−1, 0.0894 m−1 and 0.0903 m−1 in January, June and October respectively, accordingly, the bathymetric capabilities are 0–44 m, 0–61.5 m and 0–52.5 m, respectively. Compared with the original bathymetric method, these results show that the maximum bathymetric value has measured by the improved bathymetric capability formula and scattering classification algorithm has decreased under the influence of the change of sediment reflectance, and the result is more consistent with the actual situation and more accurate.

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