Determination of chlorophyll content of small water bodies (kettle holes) using hyperspectral airborne data

This study presents an approach for chlorophyll content determination of small shallow water bodies (kettle holes) from hyperspectral airborne ROSIS and HyMap data (acquired on 15 May and 29 July 2008 respectively). Investigated field and airborne spectra for almost all kettle holes do not correspond to each other due to differences in ground sampling distance. Field spectra were collected from the height of 30–35 cm (i.e. area of 0.01–0.015 m2). Airborne pixels of ROSIS and HyMap imageries cover an area of 4 m2 and 16 m2 respectively and their spectra are highly influenced by algae or bottom properties of the kettle holes. Analysis of airborne spectra revealed that chlorophyll absorption near 677 nm is the same for both datasets. In order to enhance absorption properties, both airborne hyperspectral datasets were normalized by the continuum removal approach. Linear regression algorithms for ROSIS and HyMap datasets were derived using normalized average chlorophyll absorption spectra for each kettle hole. Overall accuracy of biomass mapping for ROSIS data was 71%, and for HyMap 64%. Biomass mapping results showed that, depending on the type of kettle hole, algae distribution, the ‘packaging effect’ and bottom reflection lead to miscalculations of the chlorophyll content using hyperspectral airborne data.

► The main topic of this research is a biomass mapping of small shallow waters (less than 1 ha in size) using airborne hyperspectral data. All kettle holes, except K4, are around 0.5–1.5 m in depth and ≈40 m in diameter.
► Water sample collection was realized by submerging a sampler (with 0.5 l volume) 10–15 cm below the surface 4–5 times in the centre of the kettle hole in order to fill two 1-l flasks. Therefore, the collected samples are mixed samples representing the area of the pond where spectrometer measurements (using a hand-held device) were conducted (in addition to the acquisition of the airborne data).
► Chlorophyll mapping was performed using ROSIS and HyMap airborne datasets. Existing methods of biomass mapping in water bodies are mostly based on the chlorophyll reflection peak near 700 nm. The proposed method of biomass mapping is based on enhancing of the chlorophyll absorption peak (near 677 nm) using continuum removal approach.