![]() ![]() Measuring Ed values at different water depths (i.e., Ed profiling) in shallow waterbodies is a challenging task with the available profiling equipment. Remote sensing approaches offer a unique opportunity to frequently monitor Zeu, although its estimation from diffuse attenuation coefficient (Kd) requires depth profiles of the down-welling irradiance (Ed). ARTICLE HISTORYĮuphotic zone depth (Zeu) is a key indicator of underwater light environment that regulates different biogeochemical and physical processes in aquatic ecosystem. Matching fish landing trends with SPM concentrations was also observed, which may open new opportunities to manage inland water bodies using remote-sensing observations. The mean SPM trend from 90 different S2A/S2B MSI acquisition dates from 2017 to 2021 indicated that SPM concentration in the lagoon is closely related to wind velocity. Better root-mean-squared error (RMSE) value of 10.91 mg L −1 for SPM (ranging from 11.54 to 99.00 mg L −1) and 0.21 m for Z SD (ranging from 0.15 to 1.2 m) was observed when compared with in situ measurements (n = 77). We further used MIA to retrieve b bp (665) and K d (490) values from the Sentinel 2A/2B (S2A/S2B) multispectral instrument (MSI) R rs data for each pixel over the Chilika lagoon to estimate SPM and Z SD respectively. The bias for b bp and K d was significantly reduced using MIA, which improved the accuracy of SPM and Z SD estimation. Values of b bp at 665 nm and K d at 490 nm were then used to estimate suspended particulate matter (SPM) and Secchi disk depth (Z SD), respectively. ![]() Therefore, a modified inversion algorithm (MIA) using the combined features of the sixth version of QAA (QAA v6) and the IOPs inversion model of inland waters (IIMIW) is proposed and validated. However, existing QAAs underestimated the observed b bp and K d values in the Chilika lagoon. We evaluated exiting quasi-analytical algorithms (QAAs) to retrieve particle back-scattering coefficients (b bp) and diffuse attenuation coefficient (K d) from surface remote-sensing reflectance (R rs) data. Therefore, we measured optical properties along with water quality parameters in the Chilika lagoon, the second-largest brackish water lagoon in the world. The uncertainty associated with the bio-optical complexity of these water bodies limits remote-sensing approaches to monitor such fragile ecosystems. Increasing sediment load and deteriorating water clarity are the key challenges for many inland water bodies. The insight gained allows a more informed interpretation of Secchi disk measurements in turbid water. The modified theory corrects the under‐estimate of Secchi depths in turbid waters and gives good agreement with observations over a wide range of turbidity. We have modified the theory of the Secchi disk in turbid water to allow for a mixture of beamed and diffuse light contributing to disk visibility. In these conditions, the disk appears blurry, and it seems likely that some of the light reflected by the disk returns to the eye as diffuse light, photons being scattered one or more times on their journey from the disk surface to the observer. Observations presented here show that, in the most turbid waters sampled, the Secchi disk is visible at greater depths (by a factor of up to 4) than predicted by this theory. The disappearance depth, ZSD, of the disk is then expected to vary inversely with the sum of the beam and diffuse attenuation coefficients: c + KD. In the classical theory of the Secchi disk depth, diffuse sunlight falling on the disk is reflected back to the observer's eye along the most direct route, as a beam. ![]()
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