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Application of the Quasi-Analytical Algorithm and Contrast Threshold Theory to Retrieve Secchi Depth from Optically Complex Lakes and Reservoirs of the Continental US using Landsat 8
Citation:
Keith, D., W. Salls, AND B. Schaeffer. Application of the Quasi-Analytical Algorithm and Contrast Threshold Theory to Retrieve Secchi Depth from Optically Complex Lakes and Reservoirs of the Continental US using Landsat 8. American Geophysical Union (AGU) Fall Meeting, Washington, DC, December 10 - 14, 2018.
Impact/Purpose:
The clarity of the water in lakes and reservoirs is a measure of how far down light can penetrate through the water column and is a visual indication of the condition of water. Because water clarity is closely related to light penetration, it has important implications for the diversity and productivity of aquatic life that a system can support. Clear waters are characterized by low concentrations of suspended soil particles and/or algae which allows more sunlight to reach submerged aquatic vegetation. Additionally, waters clarity is generally valued for aesthetic and recreational purposes. While the equipment (a secchi disk) and the process to collect this information are relatively inexpensive, extending this type of monitoring to the thousands of lakes in the conterminous US is costly and logistically prohibitive. Remote sensing using satellites creates many opportunities to provide high resolution (30 m) spatial measurements across large areas typically not possible with in situ sampling as well as examine changes over time. In this work, we have developed protocols using Landsat 8 spectral data to produce secchi depth information for more than 100 lakes and reservoirs within the conterminous U.S. that is scalable across any geo-political boundary and which can be prioritized for management actions.
Description:
Secchi depth (ZSD) is used by federal and state monitoring programs as an indicator of water clarity. Water clarity is affected by physical, chemical and biological factors that are connected to the natural geology and human use of the surrounding watershed. Water clarity is often monitored using an inexpensive Secchi disk, a metal or weighted plastic disk attached to a measured line and lowered into a lake until it can be no longer seen. The depth at which it disappears, the secchi depth, is inversely proportional to the amount of organic and inorganic substances along the line of site in the water column. Because of this relationship, ZSD is a visual measure of water clarity. Using the Quasi-Analytical Algorithm and the Landsat 8 surface reflectance product from 33 images, light attenuation (Kd) data were derived. The visibility theory of Lee et al. (2015) was used to transform the Kd data into secchi depths. The predicted ZSD values were compared with in situ data from the Water Quality Portal (WQP) which serves as a repository for environmental data collected by 400 state, federal, tribal and local agencies. Initial data screening included only scenes with less 25% cloud cover, and within ±3 days of the overpass. Validation regression results and statistical distributions for derived products are presented and evaluated for >100 sites across the conterminous US from 2014-2016. The intended outcome of this work is to use this satellite product to inform trophic status and eutrophication level assessments across the United States.