For the Secchi Disk Depth, Z SD (m), estimation of the semi-analytical model from Lee et al. (2013, 2015) was performed based on the diffuse attenuation coefficient of downwelling irradiance (K d) considering its applicability in a wide range of environments (Eq. (1)).

where Min(Kdtr) is the minimum value within the visible spectral domain (443–665 nm) of the attenuation coefficient of downwelling irradiance of the transparent window, and Rrstr is the remote sensing reflectance at the same wavelength chosen for Kdtr (Lee et al., 2015). Kd(λ) can be estimated by (Eq. (2)):

where θs is the solar zenith angle, here considered to be 30° and bbw is the backscattering coefficient for water molecules based on Smith and Baker (1981); m03 and γ are equal to 0.005, 4.26, 0.52, 10.8 and 0.265, respectively; a is the total absorption coefficient and bb is the backscattering coefficient, obtained by the quasi-analytical algorithm (QAA) and here based on the version 5 (QAA_v5, Lee et al., 2009). The reference wavelength used in QAA_v5 was 561.41 nm.

The QAA_v5 was selected based on Rodrigues et al. (2017) results; the authors used data from Barra Bonita reservoir, which is near our study area. They tested the QAA_v5 and adjusted their own QAA (QAAM14) and find a better result using the QAA_v5.

A boxplot of ZSD was used to compare the images through Two Sample t-Tests and confidence level of 0.05. The boxplot was based on 40 pixels selected manually in the reservoir taking care of avoiding the shallows areas; this is an important step to avoid the adjacency effects.

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