Keywords:Euphotic zone depth, remote sensing, inland waters
Euphotic zone depth (Zeu) is defined as the depth where photosynthetic available radiation (PAR) is 1% of its surface value. It is of great importance in studying water biogeochemical processes. Satellite remote sensing is a powerful technique to monitor Zeu, as it can cover large areas at very frequent intervals. Several remote-sensing algorithms for estimating Zeu have been developed for oceanic water bodies; however, remote estimation of Zeu is still a challenging task for inland waters. In this study, an existing semianalytical algorithm was modified for remotely estimating Zeu in turbid inland waters by replacing the original quasi-analytical algorithm (QAA) by QAA_Turbid, an algorithm specially developed for remotely estimating total absorption and backscattering coefficients in turbid waters. Performance of the modified algorithm was evaluated using in situ radiometric data collected in Japan’s Lake Kasumigaura, known to be very turbid. Results showed that yielded acceptable estimation accuracy for Zeu (ranging from 1.15 to 2.26 m) with root-mean-square error (RMSE) of 0.12 m, normalized root-mean-square error (NRMS) of 8.01%, and mean normalized bias (MNB) of −1.84%, significantly outperforming the original version as well as three other Zeu retrieval algorithms. Application to the satellite images also yielded acceptable performances. These results indicate its great potential for operational estimation of Zeu over widespread turbid inland waters from satellite observations.