Lake Water Clarity: Minnesota Statewide, 1985-2022
Water clarity, an indicator of water quality, is a loosely defined term generally related to how far one can see in a water body. Most commonly, this is measured using a Secchi disk, a 20-cm diameter white disk that is lowered into the water until it just disappears. That depth (in ft, cm, or m) is termed the “Secchi depth” (SD).
Using Landsat satellite imagery, we have been using procedures described on the methods page to classify the water clarity of all lakes 20 acres (8 hectares) or larger in the state of Minnesota. Using traditional image processing methods we completed eight mappings (or censuses) of Minnesota lake water clarity for the nominal years 1985, 1990, 1995, 2000, 2005, 2008, 2011, and 2015. For these early assessments, we used empirical methods and in situ Secchi calibration data which generally yielded R2 values in the range of 0.80-0.90. This work was published (Olmanson et al. 2008) in Remote Sensing of Environment.
More recently the automated high-performance computing methods described on the Methods – Near real-time monitoring page have been used to generate lake clarity classifications of 2017 to 2022 Landsat and Sentinel images. A map for 2022 is below and all of the years from 1985 to 2021 are in the LakeBrowser.
Temporal Trends and Geographic Patterns
We now have the capability to visualize and analyze temporal trends and spatial patterns of lake water quality over large geographic areas. A sequence of classification of lake clarity for the Upper Midwest from 2014 to 2019 is shown in the following animation.
We analyzed the 1985-2022 data for temporal and geographic patterns and trends, and relationships to land use and other factors that may cause changes in lakes. Key results from that analysis are shown in the figure below.
Considering mean water clarity at the lake level, a higher percentage of lakes had increased clarity than decreased clarity over the period 1985-2022, but large differences occurred in temporal trends among ecoregions. There also are strong geographic patterns in water clarity, with lower clarity in the south and higher clarity in the north. In addition, deeper 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. Areas dominated by forests and wetlands have higher water clarity than agricultural and developed areas and shallower lakes have lower clarity than deep lakes with similar land cover. Changes in water clarity were attributed to changes in both land use and climate. Clarity increases in many urban lakes were attributed to BMPs and the development of agricultural lands into urban/suburban lands. Clarity decreases were attributed to changes in precipitation, temperature, and crop types. Differences in CDOM were related to precipitation and predominant land cover, with wetland/forested areas associated with higher CDOM than agricultural areas.
Recent advances in satellite technology (improved spatial, spectral, radiometric, and temporal resolution) and atmospheric correction, along with cloud and supercomputing capabilities, have enabled the development of automated regional-scale measurements of water clarity. These new capabilities provide opportunities to improve lake and fisheries management by measuring more variables (chlorophyll, colored dissolved organic matter (CDOM), and total suspended matter, the main determinants of water clarity) more frequently. Combining these new capabilities with earlier assessments, we created a database of satellite-derived late summer water clarity spanning 37 years (1985-2022), enabling the analysis of long-term trends. If you are interested in more detailed information, including temporal trends in clarity for individual lakes, visit the Minnesota LakeBrowser.