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
_e
u retrieval algorithms. These results indicate its great potential for accurately estimating Z
_eu
over widespread turbid inland waters from satellite images.