Remote sensing monitoring of chlorophyll concentration—an important basis for water resource protection and restoration

Inland water bodies are an important component of the natural ecosystems upon which humankind depends for survival and development, playing an irreplaceable role in water supply, drought and flood control, transportation and navigation, climate regulation, providing habitats for organisms, and maintaining biodiversity. In recent years, under the influence of human activities and climate change, inland water resources, the water environment, and aquatic ecosystems have faced varying degrees of problems and threats. Eutrophication and the resulting algal blooms are prevalent pollution problems in inland water bodies, and the steady-state balance between watershed development and ecological coordination has gradually received attention from relevant departments.

Chlorophyll Concentration Remote Sensing Monitoring

Currently, numerous scholars both domestically and internationally are utilizing remote sensing data to monitor chlorophyll a concentrations in inland rivers, large lakes, reservoir areas, and small urban water bodies. This remote sensing-based method for monitoring chlorophyll a concentrations in inland water bodies is of significant practical importance for strengthening watershed water environment management, pollution source control, and ecological civilization construction.

Chlorophyll-a is a core element for photosynthesis in cyanobacteria, and its content is relatively stable in plankton, making it easy to purify and analyze under laboratory conditions.Measuring the chlorophyll a content in water bodies can provide a preliminary assessment of the degree of eutrophication, thereby allowing for a better understanding of the water quality.Therefore, dynamic monitoring of chlorophyll a concentration in inland water bodies over a long period of time is an important prerequisite for the protection and restoration of terrestrial water resources.

CAS Xiguang Aerospace - Chlorophyll Concentration Inversion

When sunlight enters the surface of inland water bodies from the atmosphere, optical phenomena such as reflection, refraction, and absorption occur at the water vapor interface. The spectral characteristics of the water body ultimately captured by the sensor are mainly determined by its own optical properties, the composition of substances in the water, meteorological conditions, and the sensor's imaging angle. During the process of sunlight passing through the atmospheric medium to reach the water surface, a small portion is scattered by the atmosphere, while the majority reaches the water surface. Most of the light transmitted to the water surface is reflected, and the remaining portion enters the water body.

The functional relationship between the visible light band and spectral reflectance of water with high purity is almost linear. When other substances are present in pure water, the reflectance curve of the water will change.As a substance with high optical sensitivity, chlorophyll a will show corresponding peaks or valleys in the spectral curve of water when its concentration changes. Based on the study of the characteristic spectrum of chlorophyll a, it is possible to realize the inversion monitoring and prediction of chlorophyll a concentration in water.  

CAS Xiguang Aerospace - Chlorophyll Concentration Inversion

The image above shows the chlorophyll a concentration results for the Bohai Bay obtained by the Xi'an Aerospace Team of the Chinese Academy of Sciences through inversion. As can be seen from the image,The concentration of chlorophyll a near the shore is much higher than that in waters further offshore.  

Currently, data used for remote sensing monitoring of chlorophyll a concentration in inland water bodies mainly comes from traditional image sources such as multispectral and hyperspectral images. Compared with traditional multispectral data, hyperspectral sensors have more bands, narrower band widths, and significantly improved spectral resolution, making them an ideal data source for inland water quality inversion. With the increasing maturity of spectral theory and inversion methods, hyperspectral imagery is gradually gaining importance and being widely used in inland water quality positioning remote sensing monitoring.