Abstract
Owing to the significant societal value of inland water resources, there is a need for cost-effective monitoring of water quality on large scales. We tested the suitability of the recently launched Sentinel-2A to monitor a key water quality parameter, coloured dissolved organic matter (CDOM), in various types of lakes in northern Sweden. Values of a(420)CDOM (CDOM absorption at 420 nm wavelength) were obtained by analyzing water samples from 46 lakes in five districts across Sweden within an area of approximately 800 km2. We evaluated the relationships between a(420)CDOM and band ratios derived from Sentinel-2A Level-1C and Level-2A products. The band ratios B2/B3 (460 nm/560 nm) and B3/B5 (560 nm/705 nm) showed poor relationships with a(420)CDOM in Level-1C and 2A data both before and after the removal of outliers. However, there was a slightly stronger power relationship between the atmospherically-corrected B3/B4 ratio and a(420)CDOM (R2 = 0.28, n = 46), and this relationship was further improved (R2 = 0.65, n = 41) by removing observations affected by light haze and cirrus clouds. This study covered a wide range of lakes in different landscape settings and demonstrates the broad applicability of a(420)CDOM retrieval algorithms based on the B3/B4 ratio derived from Sentinel-2A.
Highlights
Long-term water quality information is important for securing and restoring ecosystem services provided by lakes [1]
We evaluated the relationships between a(420)coloured dissolved organic matter (CDOM) and band ratios derived from Sentinel-2A Level-1C and Level-2A products
The clearest lakes were located in Västerbotten W, Jämtland, and Norrbotten, with observed a(420)CDOM between 0.4 m−1 and 2.4 m−1
Summary
Long-term water quality information is important for securing and restoring ecosystem services provided by lakes [1]. Lakes are generally small in size and widely distributed in remote locations, which make regular monitoring difficult [2]. Similar challenges are faced by countries in northern boreal and Arctic regions that have an abundance of lakes scattered over large areas with low accessibility. Water monitoring programs in Europe and North America have observed increases in dissolved organic matter (DOM) over the past decades [4], but knowledge of the geographic extent of these changes remains poor. DOM increases the cost of drinking water production and decreases recreational value of lakes, and impacts the structure and function of aquatic ecosystems [5]
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