Distribution of Secchi depth and chlorophyll content as retrieved from casi data. (blue: high Secchi depth and low chlorophyll content, magenta: low Secchi depth and high chlorophyll content)
The analysis of the hyperspectral data follows semi-empirical approaches. Secchi depth is being determined using the integrated area between the spectrum and a baseline that is fitted to the two minima of the spectrum minimizing the included area. The result is being called spectral coefficient and is inversely, exponentially correlated to the Secchi depth as measured in the field with a Secchi disk. Chlorophyll is being quantified following an approach after Dekker (1993) using the ratio of the reflectance maximum at 705 nm and the chlorophyll-a absorption at 678 nm. This ratio correlates linearly with the chlorophyll-a content as analyzed from in situ sampling.
Secchi depth was retrieved with a mean standard error of 1.20 m and chlorophyll content with a mean standard error of 10.2 µg/l. Highest Secchi depth and lowest chlorophyll concentration within the test area is reached by lake Wumm. This is the lake in the northwest of the test area; it has a maximum depth of 36 m. Lowest Secchi depth (0.25 m) and highest chlorophyll concentrations (more than 70 µg/l) could be determined in lake Bramin (southwest of the test area). It is a shallow lake (2 to 3 m deep) that is in a natural stage of filling-up. The high chlorophyll concentrations can be explained by re-suspension of nutrients from the sediment. But still, the ground does not mix with the reflectance signal because of the very low Secchi depth. All other lakes vary between those two extremes. The spatial resolution of the remote sensing data shows some mixing zones where two lakes of different water quality are interconnected by a river or canal.