Remote Sensing of Water Resources

Satellite Monitoring of Minnesota Lake Water Quality

Protecting the quality of lakes is a major concern of Minnesotans, but the expense and time required for ground-based sampling means that only a small fraction of this important resource can be monitored by conventional field methods. Satellite imagery now can measure key water quality constituents on virtually all Minnesota lakes at low cost and greatly supplement the information provided by ground monitoring programs. With support from a variety of state and federal agencies and the University of Minnesota we have measured lake clarity, a key indicator of water quality, on more than 10,000 lakes statewide using current and archived Landsat satellite imagery for seven time periods since 1975.  It has proven to be an accurate, economical method to monitor lakes over large geographic areas and time.

Landsat-measured lake water clarity, an indicator of water quality, for more than 10,500 Minnesota Lakes.

Landsat-measured lake water clarity, an indicator of water quality for more than 10,500 Minnesota Lakes.

Our past research documented a strong relationship between the spectral-radiometric responses of Landsat sensors and observations of water clarity by Secchi disk. The geographic coverage, spatial resolution and free availability of Landsat imagery make it particularly useful to assess inland lake clarity. Its coverage of 12,000 square miles per image allows simultaneous assessment of thousands of lakes in lake-rich areas. Its spatial resolution (30 meters) is suitable for lakes larger than ~10 acres, and it can be used to map in-lake variability. With an image archive dating to 1972, Landsat provides an unprecedented data repository. We have analyzed the water clarity data for temporal and geographic patterns/trends and relationships with land use and other factors that may affect lake quality. Mean water clarity at the state level remained relatively stable over the period 1975-2008, but strong geographic patterns are apparent, with lower clarity in the south and higher clarity in the north.  Deep lakes tend to have higher clarity and are more stable than shallow lakes, and agricultural and urban land use are generally associated with lower clarity.

Water clarity of Minnesota lakes can be explored in the Lake Browser, a web-based mapping tool that enables searches and display of results for individual lakes. The data are used by state and local agencies for lake management and by citizens and educators.

Our current research is focused on using new enhanced satellite imagery to measure other key metrics of water quality, including chlorophyll (a measure of algal abundance), suspended solids, and colored dissolved organic matter (CDOM). Together, these three parameters control many important biological, chemical and physical features of lakes. We recently mapped CDOM levels in Minnesota lakes using 2015 imagery; with NSF funding we are expanding this mapping across ecoregions that span Minnesota, Wisconsin, and Michigan. With Legislative-Citizen Commission on Minnesota Resources funding, we are mapping chlorophyll and suspended solids concentrations in lakes across Minnesota. Our current work uses Landsat 8 and the new Sentinel-2 satellites, which have more spectral bands and other improvements compared to previous Landsat sensors. With increasingly sophisticated sensors and expanded availability of imagery, remote sensing of water quality will continue to develop over the next decade.

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