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Water quality monitoring of Hongze Lake based on Xiguang-1 05 satellite (Tianxianpei).
Background Introduction
In lake and wetland monitoring supported by hyperspectral remote sensing technology, total suspended sediment (TSM) is a key indicator for evaluating water transparency and lake siltation. As a transient lake, Hongze Lake has a large fluctuation in water area with water level and an average water depth of only 1.5 m. The complex sources of sediment and the distribution of aquatic vegetation constitute a unique spectral environment.
The physical basis of hyperspectral remote sensing monitoring of suspended sediment lies in the strong scattering effect of suspended particles on incident light. As the concentration of suspended sediment in water increases, the reflectance spectral curve changes significantly in the visible to near-infrared bands, especially in the 600 nm to 800 nm range, where reflectance increases dramatically with increasing concentration. Compared to conventional multispectral imagery, hyperspectral imagery captures these subtle spectral features through continuous nanometer-scale bands, effectively distinguishing the contributions of sediment scattering, algal absorption, and bottom sediment reflection.
Methods and Principles
This case study used hyperspectral images taken by Xiguang-1 05 satellite (Tianxianpei) on July 14, 2025, to conduct suspended sediment monitoring, and compared and analyzed different methods based on the extraction results.
Table 1. Data Introduction
In this case, the index calculation uses atmospherically corrected L2C level reflectance data. The specific operation procedure is as follows:
1. Data Preprocessing
(1) Radiometric calibration: The original DN value data is radiated by using the radiometric calibration coefficients provided by the radiance to obtain the apparent radiance data. This can be done using the Radiometric Calibration tool in ENVI.
(2) Atmospheric correction: Atmospheric correction is performed using atmospheric correction tools based on 6S or MODTRAN models to obtain surface reflectance data, such as the FLAASH atmospheric correction tool in ENVI.
(3) Spectral smoothing: In ENVI, the Savitzky-Golay (SG) algorithm is used to smooth the surface reflectance data, which reduces spectral noise and improves the reliability of subsequent classification. You can use the Savitzky-Golay Filter extension tool in the App Store of ENVI or the built-in THOR Spectral Smoothing tool. When using Savitzky-Golay Filter, the parameters are set as follows:
(4) Image Registration (Optional): Georegister the processed data against the base map. Registration can be performed in ENVI, QGIS, or ArcGIS. Google Imagery is recommended as the reference map.
2. Normalized Difference Water Index (NDWI)
Water body extent was extracted using the spectral index method. Specifically, the Normalized Difference Water Index (NDWI) was calculated based on the surface reflectance image processed by FLASH. Then, based on the histogram of the NDWI image, a threshold was set to complete the water body extraction. The calculation formula is as follows:
3. Normalized Difference Suspended Sediment Index (NDSSI)
The Normalized Difference Suspended Sediment Index (NDSSI) is constructed by fully utilizing the penetrating power of blue light and the strong absorption of near-infrared light into water. Since pure water absorbs almost all incident energy in the near-infrared band, and suspended particulate matter exhibits significant scattering characteristics in this band, the normalized difference between blue and near-infrared light can effectively enhance the contrast between suspended matter and the water background.
4. Band Ratio Method (TSM Ratio)
The TSM ratio method eliminates environmental background noise and highlights target features through nonlinear combinations of different spectral channels. The physical basis of this method is the significant "redshift" of the water's reflectance spectral peak as the concentration of suspended solids increases. In practical applications, the ratio of red to green light (Red/Green) is often used as a sensitivity indicator. This is because the red light band (around 665 nm) is extremely sensitive to backscattering from low to medium concentrations of suspended particles, while the green light band (around 560 nm) is less affected by pigments such as chlorophyll. This ratio calculation is mathematically simple and efficient, and can partially eliminate radiation errors caused by sensor viewing angle and terrain shading. It is a common method for water quality monitoring in complex-colored lakes such as Hongze Lake.
Results Analysis
Figure 1. True-color composite image of Xiguang-1 05 satellite (Tianxianpei) 20250714
Figure 2. Distribution of NDSSI results for Xiguang-1 05 satellite (Tianxianpei) 20250714
Figure 3. Distribution of TSM Ratio results for Xiguang-1 05 satellite (Tianxianpei number 20250714)
According to the calculation results of NDSSI and TSM Ratio, the suspended sediment in Hongze Lake exhibits a highly consistent spatial distribution pattern, characterized by "high values in the northwest and low values in the lake center and southeast." High-value areas are significantly concentrated along the northwestern shoreline, directly reflecting the initial depositional state of large amounts of sediment carried into the lake by inflowing runoff. Furthermore, concentrated summer rainfall, coupled with the monsoon climate of the Hongze Lake basin and its large surface runoff, enhances the river's erosion and sediment transport capacity, leading to a significant increase in sediment inflow. Hongze Lake is a typical "suspended lake" with a low-lying terrain; the lakebed is higher than the surrounding land. Rivers flowing into the lake (such as the Huai River) experience a sharp drop in elevation upon entering the lake area, slowing their flow velocity and reducing their transport capacity, resulting in substantial sediment deposition. In contrast, the open waters in the lake center and the deep waters in the southeast are both in the low-value range in both maps, indicating that these areas are less affected by land-based inputs and have relatively high water transparency.
In summary, these two methods not only confirm the geographical fact that the Huai River is the main source of sediment for Hongze Lake, but also provide a scientific quantitative basis for subsequent removal of emergent vegetation interference and precise monitoring of wetland evolution.