Satellite remote sensing of chlorophyll and Secchi depth for monitoring lake water quality: A validation study

Satellites that capture large areas with high spatial and temporal resolution allow extensive analyses of water bodies and thus represent an ideal supplement to existing in situ point measurements. In the joint project WasMon-CT (Water Monitoring of Chlorophyll and Turbidity) the usability of satellite data for official monitoring of flowing waters and lakes was examined. The subproject at the Institute for Lake Research of the LUBW focused on satellite-based monitoring of chlorophyll a, an important indicator for water quality, in lakes. Freely available data from spatially reasonable high-resolution satellites, e.g. Sentinel-2, open up new possibilities for monitoring the water quality of a larger number of small lakes. The aim of the comprehensive validation study presented here was to get information about applicability and potential limitations of remote sensing techniques for different types of lakes. EOMAP processed the satellite data used in the validation (Sentinel-2/3, Landsat 7/8 and MODIS) by applying its Modular Inversion and Processing System MIP. Results extracted from satellite data between 2000 and 2017 were compared with in situ measurement data of about 20 lakes in Baden-Wuerttemberg, including Lake Constance, for water quality parameters such as chlorophyll a and Secchi depth. First results of the validation study show that in general the statistical values, e.g. annual mean values of in situ and remote sensing retrieved chlorophyll a and Secchi depth data, agree well, but some systematic differences occur. Further validation and data interpretation steps take into account methodical differences as well as time differences between in situ and satellite measurements.