Abstract
This study proposed a green–red band quasianalytical algorithm, QAA-GRI, and evaluates its performance using an in situ dataset from Lake Qiandaohu, a drinking water source, in China. First, by shifting the reference wavelength from 555 to 510 nm, a green–red index (GRI) can be calculated from remote sensing reflectance at 510, 560, and 620 nm, and the index was then used to retrieve the total absorption coefficients at the reference band, a ( 510 ) . Second, a semianalytical model based on a ( 510 ) and the GRI was deduced to establish the QAA-GRI, replacing the empirical model in quasianalytical algorithm version 5 (QAA-v5). The QAA-GRI was applied to retrieve absorption coefficients from an in situ dataset of Lake Qiandaohu, and the QAA-GRI’s performance was compared with that of QAA-v5 and another red–green QAA-based approach (QAA-RGR). The results demonstrate that, for this dataset, the QAA-GRI exhibited better performance (R2 = 0.81, mean absolute percentage error, MAPE = 15.7 % ), which indicated a clear improvement in the accuracy of absorption coefficient retrieval, compared with QAA-v5 (R2 = 0.56, MAPE = 21.2 % ) and QAA-RGR (R2 = 0.67, MAPE = 22.6 % ). In addition, to illustrate the QAA-GRI’s assumptions and its applicability, the QAA-GRI was also applied to an in situ dataset from Taihu Lake, a highly turbid and eutrophic water source. As predicted, when applied to Taihu Lake, the QAA-GRI did not perform as well as it did when applied to Lake Qiandaohu because of the former’s extremely high turbidity, which can cause greater uncertainty with regard to backscattering coefficients. This study suggests that the QAA-GRI is better suited to the retrieval of absorption coefficients from drinking water resources. Our algorithm will also have potential applications to satellite data from the Medium Resolution Imaging Spectrometer or Ocean and Land Color Instrument.
Highlights
Absorption coefficients are important inherent optical properties (IOPs) of a water body
This study aims to remedy this deficiency with an improved algorithm for drinking water sources, in which a semianalytical model based on a green–red index is established to estimate
With a semianalytical model that establishes a relationship between að510Þ and the green–red index (GRI), the proposed quasianalytical algorithm (QAA)-GRI was developed to derive the total absorption coefficients for drinking water sources analytically
Summary
Absorption coefficients are important inherent optical properties (IOPs) of a water body. Water color remote sensing is based on the relationships between remote sensing reflectance and IOPs, emphasizing total absorption and backscattering coefficients.[6] Total absorption and backscattering coefficients determine the upwelling light field and can be retrieved using water-leaving reflectance determined from satellite observations.[7] The variability of absorption coefficients in open ocean waters (“case 1”) has been thoroughly documented over the last several decades.[8,9,10] In recent years, methods for the accurate retrieval of total absorption in coastal or inland waters (“case 2”) have been under investigation, with proposed-models including empirical,[11] spectral optimization,[12] artificial neural network,[13] linear matrix inversion,[14] and semianalytical[15] approaches
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