Comparison of NIR/RED ratio and first derivative of reflectance in estimating algal-chlorophyll concentration: A case study in a turbid reservoir

The objective of this research was to test the use of the well-documented ratio of near-infrared (NIR; 705 nm) to red (670 nm) reflectance in estimating chlorophyll content in a relatively turbid midwestern reservoir. The results were compared with those obtained by analyzing the first derivative of reflectivity near 690 nm. Data were collected in three different months over a 2-year period, and the characteristic spectral features of surface reflectance associated with algal chlorophyll were analyzed. Results indicated that the NIR/red ratio was not an effective predictor of algal-chlorophyll concentration in our case, except on one occasion. The first derivative of reflectance, on the other hand, was better correlated with chlorophyll concentration than the 705/670 band ratio. The highest correlation between the first derivative and chlorophyll was at 690.7 nm.