Exploring Advances in water quality monitoring with satellite imagery

5-minute read

Water quality is a critical aspect of environmental monitoring and influences the way a water resource can be used. Water quality encompasses parameters such as chlorophyll concentrations, turbidity, water colour and transparency, vegetation cover and algal matter concentrations. These parameters are directly or indirectly associated with optical water properties, which can be measured or inferred from satellite imagery.

In this blog, we explore the intricacies of using satellite-based water quality monitoring. Various light pathways travelling into and exiting from water features contributes to the complexities involved in capturing accurate water quality data from space.

Water quality conditions are subject to significant temporal and spatial variation due to changes in weather, biological composition, and physical attributes. This creates a high dependency on the timing of satellite image acquisition, making it essential to use the most suitable satellite image formats to generate reliable water quality data.

There are advantages and limitations with the various sensors used in water quality modelling. MERIS and S3-OLCI can provide near-daily data at coarse resolutions, resulting in high frequency observations at low resolution.  Landsat and S2-MSI offer higher spatial detail but with less frequent updates. The choice of sensor impacts the accuracy and applicability of water quality information, particularly for smaller water features or specific spectral characteristics such as chlorophyll absorption peaks.

Additionally, accurate water quality modelling requires sensors with a high signal-to-noise ratio (SNR) to differentiate the small water-leaving signal from other sources of radiance. Historical sensors like Landsat 4 & 5 were unsuitable for this purpose, but newer sensors such as Landsat 8 & 9, S2-MSI, and S3-OLCI have significantly improved SNR, enhancing their capability for water-only applications.

Complementary advancements in Atmospheric Correction algorithms further enhance optical image data for water-specific applications, enabling more precise water quality monitoring from satellite imagery. These technological advancements have tremendous implications for water quality management.

Image-based Water quality algorithms, available in various formats, use spectral band data to measure key water quality indicators such as chlorophyll-A concentration, turbidity, floating algal concentration, CDOM levels, and cyanobacteria algal concentrations.

EkoSource Insight, in collaboration with GEOTERRAIMAGE, has generated a test-case for weekly-based water quality data over several dams from 2017 to 2023 using various water quality algorithms. This dataset, consisting of approximately 400 observations, is being calibrated against in-situ water quality reference data collected over the same period. The final selection of satellite-based water quality indicators will be deployed for monitoring, with a 5-day reporting and update cycle, across all identified dams.

The data is stored in a database accessible to the client. The information is categorized into indicators such as chlorophyll-a, near-surface vegetation cover, coloured dissolved organic matter, total suspended matter, and algal concentrations. The mapped results for each indicator are provided with user-friendly colour gradients for easier end-user interpretation and inter-date comparison.

The results show that some Water quality constituents can be measured reliably from satellite data, while others provide good relative indicators.

The final analysis supports the expansion of water quality monitoring to all dams in the system, aiding hydrodynamic and limnological assessments by tracking the movement and mixing dynamics of various water quality constituents. This will enhance understanding of water quality and eutrophication in the reservoirs, allowing for the development of an automated monitoring process across the system. The results of this water quality service development capability confirms that EkoSource Insights and GEOTERRAIMAGE can now offer a fully automated water quality operational service, with weekly updated intelligence generation on individual dam water quality conditions.