A Modified Semianalytical Algorithm for Remotely Estimating Euphotic Zone Depth in Turbid Inland Waters

Euphotic zone depth (Z_eu), defined as the depth where photosynthetic available radiation (PAR) is 1% of its surface value, is of great importance in studies of water biogeochemical processes. Satellite remote sensing is a powerful technique to obtain Z_eu, as it can cover large areas at very frequent intervals. Several remote-sensing algorithms for estimating Z_eu have been developed for oceanic water bodies; however, remote estimation of Z_eu is still a challenging task for inland waters. In this study, an existing semianalytical algorithm (named as Lee07 in this study) was modified for remotely estimating Z_eu 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 turbid. Results showed that yielded acceptable estimation accuracy for Z_eu (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 Z_eu retrieval algorithms. These results indicate its great potential for accurately estimating Z_eu over widespread turbid inland waters from satellite images.