An Improved QAA-Based Method for Monitoring Water Clarity of Honghu Lake Using Landsat TM, ETM+ and OLI Data

Secchi disk depth (ZSD) is used to quantify water clarity as an important water-quality parameter, and one of the most mechanistic models for ZSD quasi-analytical algorithm (QAA), which latest version QAA_v6. There are two in QAA clear turbid waters (referred QAA_clear QAA_turbid). QAA_v6 switches between by setting a threshold value remote sensing reflectance (Rrs, sr−1) at selected reference band 656 nm. However, some researchers found that this or does not apply many inland lakes. In Honghu Lake, Rrs (656) (Rrs nm) whole lake less than 0.0015 sr−1; therefore, only QAA_turbid can be applied. Moreover, we resulted overestimation while significant underestimations. The lakes usually continuously vary water. We proposed hypothesis transition evenly, rather being distinguished value, developed model combined according our assumption. This simulated process continuous change clarity. results showed had better performance with RMSE reduced from 0.5 0.28, MAE 0.43 0.21, bias −0.4 −0.05 m compared applied QAA_Honghu Landsat TM, ETM+, OLI data obtained 205 maps high spatial resolution Lake. were consistent existing situ measurements. From 1987–2020, Lake overall downward trend distinct seasonal pattern.