Abstract
The objective of this paper was to compare the limits of three image-based atmospheric correction models (top of the atmosphere (ToA), dark object subtraction (DOS), and cosine of the sun zenith angle (COST)), and three physical models (atmospheric correction for flat terrain (ATCOR), fast line-of-sight atmospheric analysis of spectral hypercubes (FLAASH)), and ACOLITE) for retrieving suspended particulate matter (SPM) concentrations in inland water bodies using Landsat imagery. For SPM concentration estimates, all possible combinations of 2-band normalized ratios (2bNR) were computed, and a stepwise regression was applied. The correlation analysis allowed highlighting that the red/blue 2bNR was the best spectral index to retrieve SPM concentrations in the case of image-based models, while the red/green 2bNR was the best in the case of physical models. Contrary to expectations, image-based atmospheric models outperformed the accuracy of physical models. The cross-validation results underlined the good performance of the DOS and COST models, with R2 > 0.83, NASH-criterion (Nash) > 0.83, bias = −0.01 mg/L, and RMSE < 0.27 mg/L. This outperformance was confirmed using blind test validation data, with an R2 > 0.86 and Nash > 0.58 for the DOS and COST models. The challenges and limitations involved in the remote monitoring of SPM spatial distribution in turbid productive waters using satellite data are discussed at the end of the paper.
Highlights
Suspended particulate matter (SPM) is an important element for water quality evaluations [1] and plays a major role in the ecological regulation of aquatic systems [2]
Based on the median results, the spectra extracted from the Fast Line-of-Sight Atmospheric Analysis of Spectral Hypercubes (FLAASH), Atmospheric Correction for Flat Terrain (ATCOR), and ACOLITE models have the same shape, typical of case-2 waters; a reflectance peak at the green wavelength, and almost total absorption of backscattered water in longer wavelengths, especially in the short wave of infrared (SWIR) portions
Shorter wavelengths are most affected by the Rayleigh scattering and the reflectance at shorter for the Dark Object Subtraction (DOS), Cosine of the Sun Zenith Angle (COST), and ACOLITE models were narrower for the blue band compared to the rest of the models
Summary
Suspended particulate matter (SPM) is an important element for water quality evaluations [1] and plays a major role in the ecological regulation of aquatic systems [2]. A part of the sunlight’s ultraviolet wavelengths are backscattered by SPM, which both favors the increase of bacteria and decreases their mortality [5]. Such conditions are ideal for the development of cyanobacteria, and once established, they are very difficult to control and to mitigate [6]. The SPM in lakes causes distinct changes in the water color by absorbing and scattering the sunlight at specific spectral wavelengths. This physical behavior can be used to retrieve SPM concentrations based on the reflectance of remotely sensed data, by relating optical changes observed in these specific spectral wavelengths to in situ measurements of SPM concentration
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