A semi-analytical approach to estimate euphotic depth for optically complex coastal and inland waters

The accurate estimation of in -situ euphotic depth $(Z_{eu})$ remains a greater challenge for optically complex water types and turbid productive waters owing to competitive interplay of optically active in-water constituents namely, chlorophyll concentration and non-algal particles in conjugation with yellow glebstoff matter (Colored Dissolved Organic Matter, CDOM). The present study intends to reveal a new semi-analytical approach to estimate in-situ euphotic depth that essentially relies on two-fold integral factors, one being the contrast transmittance factor represented by Length Attenuation Coefficient (LAC) and the second being the correction factor represented by Del. The LAC factor has been accounted in the newly proposed Z
eu
algorithm to closely follow the underlying principle of underwater visibility based on contrast transmittance theory for optically complex underwater structure for a given geometric illumination condition. These parameters were further investigated systematically which revealed that the inverse ratio of LAC factor in the green band (530nm) to the chlorophyll concentration (Chl) have a potential capability of predicting the variation in the light penetration for the optically complex waters. Furthermore, the correction factor Del was derived as a power function of Inherent Optical Properties (IOPs) and represents the decay in the light penetration in the upper surface mixed layer of the euphotic depth due to the presence of in-water particulate matter The newly derived in-situ Z
eu
algorithm reveals that the variability in Z
eu
values can be accurately addressed provided that the LAC factor and spectral absorption-based correction factor are meticulously accounted in estimating Z
eu
. Subsequently, the newly developed Z
eu
model was validated using an independent dataset which were extracted from SeaBASS and its performance was further tested based on certain meaningful statistical measures to gain a keen insight into deviation in the modelled Z
eu
values with respect to measured Z
eu
values. The modeled Z
eu
values revealed nearly good agreement when compared to in-situ Z
eu
values with a considerably low Mean Relative Error (MRE) of ±21% and a high correlation coefficient of 0.89 $(R^{2}=0.89)$. The comparative assessment of proposed Z
eu
model with existing standard algorithms revealed appreciably lower deviations when tested for Case 1 and Case 2 optically complex water types.