IES publication on Sentinel satellite data use for lake water monitoring

The team of  researchers from the Institute for Environmental Solutions (IES) has developed internationally recognized research paper “Validation and Comparison of Water Quality Products in Baltic Lakes using Sentinel-2 MSI and Sentinel-3 OLCI Data”, which is available on “Remote Sensing, An Open Access Journal from MDPI”

The importance of inland water bodies including lakes has never been greater because they help to regulate the carbon cycle, thus impacting local and regional climate. Moreover, lakes are economically important – they provide drinking water, fishing and recreation opportunities. Therefore, the monitoring of lakes and water quality has become a global concern. Responding to it, EU implements the Water Framework Directive to reach good water quality status in all lakes in Europe that are larger than 0.5 km2. To be able to contribute Latvia has to improve the methods and fill the gaps of lake monitoring.

IES foresee that ICT-based remote sensing tool for lake monitoring can help to fill the gaps in Latvia’s national water monitoring program, thus supporting smart and scientifically based decisions for lake management. So far, the water monitoring program has been fully based on field sampling methods. They are time consuming and expensive. Satellite data based remote sensing offers to add spatial and temporal dimension, thus complementing the standard field sampling methods, creating more holistic approach of lake research and testing options for automatized lake monitoring.

“Advantages of this remote sensing based lake quality monitoring tool is the frequency of data. It is not possible to assess the quality of all the lakes with fieldwork campaigns. With this tool you will gain information about over-all situation in the lakes. You may not have precision of milligrams, but it will tell you if the lake is clear and have a very good water quality or it has blooming water and needs a closer inspection,” highlights post-doctoral researcher Tuuli Soomets.

Lake water quality can be measured by its optical properties, but as lakes are known to be optically complex and more diverse than marine or ocean waters. Optical properties can have larger temporal and spatial variation, depending on the changes in weather, biological composition, and physical parameters. In the research paper “Validation and Comparison of Water Quality Products in Baltic Lakes Using Sentinel-2 MSI and Sentinel-3 OLCI Data” IES’s researchers describes the process of satellite data accuracy validation.

Water quality in lakes were measured in 3 levels to validate the outcomes:

• In‐situ Data – 49 samples gathered and analysed from field sampling points in 4 optically different lakes (lake Rāzna, Lubāns, Burtnieks in Latvia and lake Võrtsjärv in Estonia).

• Satellite Data. Up to three Sentinel-2 and Sentinel-3 cloud‐free match‐up images (satellite-derived images retrieved almost simultaneous to the in-situ data gathering campaigns) were used with the researched lakes. Researchers used algorithms to retrieve the water quality products (chlorophyll‐a, total suspended matter etc.).
• Classification of optical water type (OWT). This method classifies inland and coastal waters of Northern region into five OWTs, each with specific bio‐optical conditions The OWT classification was applied to each Sentinel-3 Ocean and Land Color Instrument (OLCI) and Sentinel-2 multispectral instrument (MSI) satellite image pixel reflectance spectrum.

“This research shows that we can evaluate water quality parameters quite precisely. For example, transparency of the water from Sentinel-2 data can be estimated with great precision. Sentinel-3 precision tends to be always lower but it’s due to lower resolution of the satellite images, pixel is 300 x 300 m. It is very big area that we are looking on and it can be affected by vegetation, coast line and also errors of atmospheric correction” researcher T. Soomets about precision of ICT-based remote sensing data and tools for water quality management in lakes.

Synergizing remote sensing products with in‐situ data is not exploited for national inland water monitoring programs yet, but there is a big potential for the future. “In my opinion the National Water monitoring would gain a lot from this method. Because they can have information about all the lake quality status at least 8 times a year. This information can be used for EU Water Framework Directive reporting” T. Soomets highlighting possibilities of satellite data-based water quality monitoring.

As the end of this research is approaching the team of IES’s researchers continues the work on development on the carbon fixation modelling tool for lakes using combination of modelling and remote sensing data.

The research is a part of the project “ICT-based remote sensing tool for retrieving water quality products for lake research and monitoring” (FLUID), No. 1.1.1.2/VIAA/1/16/137. Agreement with State Education Development Agency of the Republic of Latvia No. 1.1.1.2/16/I/001. The project is funded by the European Regional Development Fund and partly funded by State budget and the Institute for Environmental Solutions.

Find more about the FLUID project here.