Remote Sensing: Frequently Asked Questions

What is water clarity?

Water clarity or Secchi disk depth is probably the most common limnological parameter measured in lakes today. The Secchi disk was invented by Father Pietro Angelo Secchi, an astronomer and scientific adviser to the Pope who was asked by Commander Cialdi, head of the Papal Navy, to develop a transparency instrument. The devised disk was first lowered from the papal steam corvette Immacolata Concezione (Immaculate Conception) in the Mediterranean Sea on April 20, 1865 and has seen many changes in terms of size and color in its history of use. The disk Citizen Lake Monitoring Network volunteers use today to measure the water clarity of lakes across Wisconsin is an 8-inch black-and-white Secchi disk. Secchi disk depth is related to the absorption and scattering of sunlight by the constituents in the water column. It is an integrative measurement of a number of constituents and provides an easy and economical assessment of lake health.

How much does the water clarity vary from lake to lake?

The Secchi disk visibility world record of 262 ft was set in the eastern Weddell Sea, Antarctica in 1986 (Gieskes et al., 1987). Even the clearest lakes in Wisconsin only have maximum water clarities of 25 to 55 ft. These lakes include the aptly named Crystal Lake in Sheboygan County with a water clarity of 32.5 ft in March 2012, Maiden Lake in Oconto County with a water clarity of 47.5 ft in May 2009, and Black Oak Lake in Vilas County with a water clarity of 53.5 ft in June 2012. These lakes stand in stark contrast to some of the more turbid lakes we find in Wisconsin. Some of these lakes include Lake Wissota in Chippewa County with a water clarity of 0.25 ft in June 2012 and Lake Winnebago in Winnebago County with a water clarity of 0 to 0.25 ft in July 2012.

Why is my lake not included?

The satellite retrieval of water clarity is limited to lakes at least 5 acres in size (51 % of Wisconsin’s lakes). Satellite measurements from lakes smaller than 5 acres can be influenced by adjacency effects, a term used to describe light scattered from shoreline vegetation, soils, and developments. Lakes in which the Secchi depth is more than half of the maximum depth of the lake are excluded from the analysis to remove possible interference from the lake bottom. The widespread cloud cover typical for the summer months in Wisconsin might prevent the satellite retrieval of water clarity for your lake in some years. Researchers from the University of Wisconsin-Madison Environmental Remote Sensing Center (ERSC) used satellite data from three years to provide a complete picture of the water clarity in Wisconsin when they started the remote sensing program in 1999. The latest satellite retrieved water clarity data for your lake is accessible through the Lakes and Aquatic Invasive Species (AIS) Mapping Tool.

Why is the satellite retrieved water clarity different from the water clarity I measured directly on my lake?

The satellite retrieved water clarity represents the average water clarity for your lake on the image acquisition date. The inflow of material from rivers and streams and changes in water temperature and wind direction result in dynamic changes in the water quality of lakes. The water clarity you measured directly on your lake represents the water clarity for a specific station in this lake on the field data collection date. It might not represent the average water clarity the satellite sees on the image acquisition date. The dynamic changes in the water quality of lakes are apparent if we look at Lake Winnebago where the Fox River provides a localized inflow of suspended solids and nutrients and contributes to the development of algal blooms in different parts of the lake dependent on the wind direction. The satellite retrieved water clarity for Lake Winnebago reflects these differences in water clarity within a lake.

Why is it important to continue with the collection of water clarity data if it is possible to retrieve water clarity from satellite images?

The collection of water clarity data is required to calibrate the model for the retrieval of water clarity from satellite images for a sub-sample of the lakes in an image. Therefore, the on-the-ground Secchi depth data is related to the spectral reflectance values of the satellite image for every station where data was collected within one week from the image acquisition date. The established relationship makes it possible to extrapolate the Secchi depth for all the lakes in the same image. This relationship is influenced by changes in the atmospheric and light conditions and we have to calibrate the model for every image acquisition date. This means it is critical to have volunteers collect on-the-ground Secchi depth data so the satellite model can be continually updated. Satellites provide an important complementary screening tool to identify lakes with rapidly changing conditions even though they will probably never replace field measurements to make management decisions for a particular lake.

How can I participate in the remote sensing of water clarity?

Volunteers typically participate in field data collection efforts on image acquisition dates in the summer months. Look up the satellite schedule on the Remote Sensing – Satellite Schedule page and collect your data on any of the listed dates you can. Even if you collect the data within a couple of days from the image acquisition date we might still find a relationship of the on-the-ground Secchi depth data and the spectral reflectance values of the satellite image we can use to calibrate the model for the retrieval of water clarity. Sample your lake and submit the data to the DNR Surface Water Integrated Monitoring System (SWIMS) as you normally would. We will retrieve the data from SWIMS and match it with the satellite data. Contact your local Citizen Lake Monitoring coordinator if you have any questions!

Which satellite path is my lake in?

You can find the satellite path for your lake on the Remote Sensing – Satellite Paths page.

References

Gieskes, W.W.C., Veth, C., Woehrmann, A., and M. Graefe (1987) Secchi visibility world record shattered. Eos, Transactions American Geophysical Union, 68 (9), 123