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Global Water Clarity: Continuing a Century-Long Monitoring
Lee, Z., R. Arnone, D. Boyce, B. Franz, S. Greb, C. Hu, S. Lavender, M. Lewis, B. Schaeffer, S. Shang, M. Wang, M. Wernand, AND C. Wilson. Global Water Clarity: Continuing a Century-Long Monitoring. EOS. American Geophysical Union, Washington, DC, 99:1-10, (2018).
Here we provide an overview of the history of the ZSD measurement, its potential as a standard satellite data product, and its wide applications in addressing various environmental interests. The overarching objective is to advocate its continued measurement in the field and its production from satellite remote sensing.
Aquatic systems worldwide are changing due to increasing climate variability and human activities, yet it is difficult to capture such temporal changes without standardized long-term observations [Boyce et al. 2015, Barton et al. 2016]. Unlike the well-established Keeling curve that documents decadal-scale changes of atmospheric CO2 concentrations, there is not such a standardized product established yet to measure changes in water clarity in the world’s marine and inland water bodies. Water transparency or clarity, as commonly represented by Secchi depth (ZSD), is a measure of the optical properties of the water column. ZSD provides a first-order indicator of water quality and ecosystem health, as it depends on concentrations of dissolved and suspended matters in water. Especially, ZSD can be easily and cost-effectively measured in the field, as has been done for more than a century in global seas and lakes (see Fig. 1). Unlike many other climate variables, no changes in the calibration or methodologies exist to confound the evaluation of long-term trends. Thus, through a combination of historical ZSD records with continued field measurements and satellite products, a standardized global ZSD data product can be developed to form a unique, century-long, Earth system dataset that links the past with the future and fills a key gap in assessing changes in water clarity in global seas and lakes. In addition, such a product can be valuable in supporting positive economy as well as integrated water resources management.