ASD | Retrieval of Suspended Sediment Concentrations in the Pearl River Estuary Using Multi-Source

Retrieval of Suspended Sediment Concentrations in the Pearl River Estuary Using Multi-Source Satellite Imagery

Monitoring and quantifying suspended sediment concentrations (SSC) in estuaries such as the Pearl River Estuary (PRE) provide crucial information for environmental processes, hydrological infrastructure, and navigation. Traditional SSC mapping based on in situ investigations lacks the spatial coverage necessitated by detailed analysis. Whereas many previous studies have shown that SSC monitoring based on satellite imagery is an effective way to investigate the patterns and changes of SSC across a large estuarine area at a suitable scale. However, as satellite images from individual sensors often cannot be guaranteed for large spatial scale or long-term studies, using multi source satellite images for SSC retrieval is becoming increasingly popular among the academic community. Regarding the retrieval approaches, nowadays, empirical analysis based on linear regression and multi-factor statistics are still widely used, while principal component analysis and artificial neural networks are also effective alternatives for prediction accuracy improvement. However, the prediction of low SSC in small waterbodies is still a challenging task for remote sensing.

Based on this, a group Chinese scientists took the PRE as the study area, based on in situ spectral data (ASD FieldSpec 4 Spectroradiometer) and SSC measurements, assisted by environmental information, such as longitude, latitude, wind speed, and other ambient environmental factors, and then developed models to quantify SSC based on Landsat TM/OLI and Sentinel-2 imagery. The RMSE root mean square error and the relative error (RE) were used to test the model performance, Finally, the models were then used to map the SSC distribution in the PRE for the period 1986-2020. The main objectives of this study are: (1) investigate the spatial patterns of SSC distribution across the PRE; (2) explore the spatio-temporal variation in SSC over the past 25 years; (3) analyze the influencing factors of these changes in SSC and their relationship with human activities.

Results:

Field-measured spectral series of Rrs collected on 22 July and 20 December 2020.

Spatial distribution of the multi-year average of SSC retrieved from Landsat-8 OLI. (A–H) are eight major monitoring locations selected according to SSC distribution.

Long-term changes in SSC for the period 1986 to 2020 based on Landsat TM/OLI and Sentinel images (left); Z-statistic distribution of the long-term SSC change based on the M-K test (right).

Temporal trend of SSC at sites A, B, G, and H.

Conclusions:

The results indicated that SSC in the PRE exhibited typical periodic changes in the spatiotemporal distribution. Its spatial distribution represented a trend of high along the west coast and low along the east coast. Over the past 35 years, the SSC showed a relatively evident decreasing trend in most PRE regions, although the degree of reduction varies in different regions and needs further investigation. More attention should be paid to the decrease in SSC regarding estuary management, coastal protection, and the economic and environmental significance of estuarine sediment sustainability. The decreased sediment output from the Pearl River Basin is the major cause of the changes in SSC. However, natural factors such as wind are possible factors regulating the spatial patterns of SSC in the PRE. Human activities, such as channel dredging and the construction of artificial facilities can also disturb the spatial distribution of SSC. In the future, regions with a significant decrease in SSC, such as the region between Qi’ao Island and Inner Lingding Island and the areas along the west coasts with high SSC distribution, should receive more attention for sustainable sediment management.