This paper presents a water quality study performed on five small domestic ponds and one bayou of the Ganga river in the State of West Bengal, India. The concept of eutrophication is used to describe water quality by using chlorophyll as an indicator for the presence of algae in the water.
During a field campaign in May 2003, reflectance spectra were measured 1m above the water surface using a handheld spectroradiometer. Concentration of Total Chlorophyll Content (TCHL) and other limnological standard parameters such as Secchi Disk Depth (SDD), nutrient content, Total Organic Content (TOC) and Dissolved Organic Content (DOC) were determined using laboratory analysis and a colorimetric testkit. A comparison of several existing semi-empirical algorithms to determine chlorophyll content was made by applying them to the spectra and chlorophyll measurements collected in-situ. Simple Reflectance Band Ratio and Continuum Interpolated Band Ratio (CIBR) were tested at several wavelengths. Another three tested algorithms rely on the reflectance peak at 700 nm whose shape and position depend strongly upon chlorophyll concentration: The Area and Magnitude of the Peak above a Baseline as well as the Position of the Peak show a linear relationship to CHL-a concentration from laboratory spectrophotometric measurements. All algorithms proved to be of value. Best results were obtained by using the algorithms Peak Magnitude above a Baseline and CIBR, which yielded R² around 0.97 and mean deviations around 20 μg/l.
The algorithms were further applied to spectra convolved to the bands of the hyperspectral sensors APEX, HyMap and Hyperion. A comparison of the spectral characteristics of the chosen sensors was made in regard of their suitability for water quality determination. The differences on the outcome of the sensors were small, but the results indicate that an intermediate band width of around 10 nm seems appropriate. All sensors showed an adequate band positioning for the algorithms based on the peak near 700 nm. Using Simple Band Ratios and CIBR, APEX and Hyperion perform slightly better than HyMap.