Remote sensing research
Remote sensing research activities at the Wisconsin DNR include the
- Adaptation of image processing protocols for the retrieval of water clarity to new satellite sensors and technological advancements in GIS and remote sensing software packages,
- Systematic processing of Landsat 7 ETM+ and Landsat 8 OLI data for the retrieval of water clarity,
- Studies of the major drivers of lake water clarity, their interactions, and the potential impacts of land use and climate on water clarity
- Development of new interactive ways to present the satellite retrieved water clarity data for public use,
- Increase in Earth observation monitoring capabilities through the optical and biogeochemical characterization of lakes in support of algorithm calibration, refinement, and validation.
Remote sensing of water clarity
The satellite retrieval of water clarity requires several image processing steps. The image processing starts with the
identification of L7 ETM+ SLC-off and L8 OLI/TIRS data for the State of Wisconsin in the Landsat Archive of the
USGS EarthExplorer [exit DNR]. Level 1T GeoTIFF data
for a date range from 06/01 to 09/30 with less than 50% cloud cover are downloaded with the Bulk Download Application and
the quantized and calibrated scaled digital numbers are rescaled to top of atmosphere spectral radiance. The images are
reprojected to the Wisconsin Transverse Mercator coordinate system and cirrus clouds, land, and shallow waters and aquatic
vegetation are removed from the images through several image processing steps. Images acquired on the same day are combined
to a single image mosaic. The image processing concludes with the extraction of the spectral radiance values for stations
where on-the-ground Secchi depth data was collected within one week from the image acquisition date.
Image processing steps for the satellite retrieval of water clarity
(Source of Landsat 8 OLI data: U.S. Geological Survey)
The on-the-ground Secchi depth data are collected with the support of Citizen Lake Monitoring Network volunteers and are accessible through the Wisconsin DNR Surface Water Integrated Monitoring System (SWIMS). Stations in lakes at least five acres in size where on-the-ground Secchi depth data was collected within one week from the image acquisition date are identified for each image mosaic and the spectral radiance values for these stations are extracted and used to calibrate a model for the satellite retrieval of water clarity.
Distribution of stations with on-the-ground Secchi depth data collected in 2013.
Model calibration for the satellite retrieval of water clarity from Landsat 8 OLI bands 2 and 4 for 06/16/2013.
This graph is comprised of two panels. The panel to the left shows the model for the satellite retrieval of water clarity. This model relates the top of atmosphere spectral radiance in the Landsat 8 OLI bands 2 and 4 to the natural logarithm of the Secchi depth. The model is formulated a + b(x1) + c(x2), where a, b, and c are constants, x1 is the ratio of the radiance in band 2 to 4, and x2 is radiance in band 2. The panel to the right shows a plot from the multiple linear regression approach to calibrate this model for the satellite water clarity retrieval for Lake Winnebago on 06/16/2013. The x-axis of this scatter chart shows the ratio of the radiance in band 2 to 4 for values from 2 to 4 and the y-axis shows the natural logarithm of the Secchi depth for values from 0 to 4. The chart includes a data series of the natural logarithm of the on-the-ground Secchi depth and a data series of the natural logarithm of the modeled Secchi depth. The data points indicate a close match of measured and predicted values.
The calibrated model is used to produce summer water clarity maps for lakes across Wisconsin. These maps provide information to compare the variability in water clarity at the lake and regional level. Typical causes of the variability within a lake include the inflow of material from rivers and streams and changes in water temperature and wind direction. These effects are seen in the map for Lake Winnebago from 06/16/2013. The variability in water clarity within Lake Winnebago is the result of the inflow of material from the Fox River on the western side of the lake and a west wind with a maximum speed of 12mph.
Satellite water clarity retrieval for Lake Winnebago on 06/16/2013
Satellite retrieved water clarity values within individual lakes are averaged to provide a single water clarity value for
a lake for each image acquisition date. These values are submitted to SWIMS and accessible through the
Lakes and Aquatic Invasive Species (AIS) Mapping Tool.