Taihu Lake Analysis Case Based on Hyperspectral Chlorophyll Index

Background Introduction

Satellite remote sensing technology plays a crucial role in inland water quality monitoring, enabling the retrieval of key water quality parameters over a large area, quickly, and at low cost. However, inland water quality monitoring also places high demands on satellite remote sensing technology: small water bodies require high spatial resolution; rapidly changing water quality necessitates high temporal resolution; and complex and variable optical characteristics of water bodies require high spectral resolution. Therefore, satellite data that simultaneously meets high spatial, high temporal, and high spectral resolution has significant advantages in inland water monitoring.

Chlorophyll a is an important water quality parameter and a key pigment in algae. It can reflect the total biomass and distribution of different algae and measure the degree of eutrophication in water bodies.

During an algal bloom, the chlorophyll concentration in most areas of the water body generally exceeds 100 mg/m³.^-3At this point, chlorophyll a concentration is no longer sufficient as an early warning indicator for algal blooms. Monitoring chlorophyll a concentration in water bodies before or at the beginning of an algal bloom can provide information on algal biomass and water quality, which is of great significance for early warning and targeted prevention of algal blooms.

Methods and Principles

This case study used hyperspectral images of the Taihu Lake region taken by Xiguang-1 05 satellite (Tianxianpei) on June 18, 2025, to calculate various chlorophyll indices, and analyzed the water quality of the Taihu Lake region based on the corresponding index 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

Radiometric calibration: Radiometric correction is performed on the raw DN value data using the radiometric calibration coefficients provided by radiance to obtain the apparent radiance data. This can be done using the Radiometric Calibration tool in ENVI.

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.

Spectral smoothing: The Savitzky-Golay (SG) algorithm is used to smooth the surface reflectance data in ENVI, which reduces spectral noise and improves the reliability of subsequent related index calculations.

Water body mask: By normalizing the surface reflectance number to calculate the water body index, and selecting an appropriate threshold based on the histogram of the calculation results, the Taihu Lake area is extracted.

Index Calculation: Calculate the spectral indices.

2. Spectral index calculation

• SEVEN

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:

• Two-band ratio index (BR)

Remote sensing reflectance R in the 705nm and 671nm bands_rs​The ratio highlights the differences in chlorophyll spectrum, and the calculation formula is as follows:

• Normalized Dichlorophyll Index (NDCI)

The normalized difference in reflectance between the 705nm and 671nm bands can be used to extract information on chlorophyll-related ground features (such as algae and vegetation). The calculation formula is as follows:

• Three-Band Index (TBI)

Combining the reciprocal difference of reflectance in the 671nm and 705nm bands, and then multiplying it by the reflectance in the 731nm band, can potentially be used to highlight changes or differences in turbidity in turbid water bodies. The calculation formula is as follows:

Results Display

Figure 1. True-color composite image (Xiguang-1 05 satellite (Tianxianpei), 20250618)

Comparison of spectral curves in the Taihu Lake region before and after SG filtering (reflectivity increased by 10,000 times, Xiguang-1 05 satellite (Tianxianpei), 20250406)

Figure 2 Comparison of spectral curves before and after SG filtering

The above-mentioned water extraction and index calculation were performed on Xiguang-1 05 satellite (Tianxianpei), and the following results were obtained:

Figure 3. Distribution of two-band ratio index (BR) (Xiguang-1 05 satellite (Tianxianpei), 20250618)

Figure 4. Distribution of Normalized Dichlorophyll Index (NDCI) (Xiguang No. 1 05 satellite (Tianxianpei), 20250618)

Figure 5. Distribution of Three-Band Index (TBI) (Xiguang-1-05 satellite (Tianxianpei), 20250618)

The results shown in the above figures are relatively consistent in terms of the overall distribution of BR and TCDI indices, while the distribution of TBI shows some differences. Overall, algal blooms are severe in the western Taihu Lake area, with higher chlorophyll a concentrations near the blooms. This is because the western part of Taihu Lake is where rivers flow into the lake, bringing in large amounts of nutrients, leading to vigorous algal growth and thus causing cyanobacterial blooms and increased chlorophyll a concentrations. The central and eastern Taihu Lake areas are less affected, resulting in better water quality than the western Taihu Lake. Aquatic plants grow in the eastern part of the lake, and the chlorophyll a concentration is lower.

Analysis of historical data shows that the northwestern bays of Taihu Lake, including Zhushan Bay and Meiliang Bay, are the areas with the highest frequency of cyanobacterial blooms, and these three areas experience the most severe blooms. Surface algae are affected by wind and waves. On one hand, they move with the current, accumulating in the bay areas and forming blooms. On the other hand, because the prevailing winds in the Taihu Lake region during summer are easterly and southeasterly, when cyanobacteria rise to the surface, they drift north and west of the lake under the influence of wind, combining with existing cyanobacteria in these areas to create severe blooms.

The central lake area is a large-scale algal bloom distribution area. Once an algal bloom occurs in the central lake area during the summer, it covers a large area. Historically, the central lake area has experienced large-scale algal blooms that have exceeded the range of algal blooms in other water areas. In contrast, algal blooms rarely occur in the southwest lake area, and algal blooms have almost never occurred in the eastern Taihu Lake area, indicating that the environment of the eastern Taihu Lake area is relatively good.