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MODIS巢湖水体叶绿素a浓度反演模型
MODIS in Monitoring the Chlorophyll-a Concentrations of Chaohu Lake
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摘要: 对大面积水体进行水质遥感监测是比较典型的水体环境监测手段。该文利用地物光谱仪测定了巢湖水面的光谱反射率, 收集了相应时间的MODIS数据, 经过预处理之后, 首先分析了巢湖水面光谱特征, 并对实测水体反射率与实测叶绿素a之间的关系进行统计拟合计算。在经过MODIS大气校正后, 得到1~7通道的地表反射率。利用大气校正后的EOS/MODIS数据, 选择最佳通道组合, 定义了叶绿素a指数IChla, 建立了MODIS巢湖叶绿素a浓度的反演模型, 检验得到相关系数为0.5079。Abstract: Using remote sensing data for large area water quality survey is a typical water environment monitoring method. The Chaohu Lake reflectance spectra are measured using ASD field spectrometer and the MODIS data collected during the same period. After the MODIS data are preprocessed, the spectral characteristics of Chaohu Lake are analyzed and the spectral is compared with references. Then the relation between the measured water reflectance and chlorophyll a is fitted. The fitting formula is consistent with previous research results and the correction coefficient is 0.75. But after analyzing the apparent reflectance of the MODIS L1B and the measured concentration of the chlorophyll-a, the ideal inversion lake model can not be obtained because of the influence of the atmosphere and other factors. After a special atmospheric correction for MODIS data, the ground reflectance spectra are obtained in the channels 1—7. The relation between the ground reflectance and the chlorophyll a is fitted and the correction coefficient is improved. At last, the optimum bands combination are chosen after analyzing the relation between the reflectance spectra and the corresponding moment of the concentration of the chlorophyll a, using EOS/MODIS data after atmospheric correction. The chlorophyll a index IChla is defined. Therefore the inversion model for chlorophyll a concentration of Chaohu Lake is achieved, and the correction coefficient is 0.5079. This correction coefficient is not as good as the result(0.75)obtained by the hyperspectral data measured by the field spectrometer. There are 3 possibilities that may cause the errors. First, the bands selection of the MODIS data are not cover exactly the spectral sensitive of the concentration of the chlorophyll a(685—715 nm). Second, the narrowband of the field spectrometer and broadband of the MODIS data is not very exact. Third, the atmospheric correction errors above Chaohu Lake may be too big.
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Key words:
- inversion;
- chlorophyll-a;
- MODIS data;
- Chaohu Lake
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图 2 2003年1月10日巢湖原始(a)与归一化(b)光谱反射率
Fig. 2 The original(a)and normalized(b)spectral reflectance of light over Chaohu Lake on Jan 10, 2003
图 3 705 nm与680 nm光谱反射率比值与叶绿素a浓度的关系
Fig. 3 Relationship between chlorophyll-a concentrations and the reflectance ratios
图 4 叶绿素a浓度CChla与IChla关系图
Fig. 4 Relationships between chlorophyll-a concentrations and the IChla
表 1 2003年1月10日巢湖水质参数
Table 1 The water quality parameters of Chaohu Lake on Jan 10, 2003
表 2 2003年1月10日MODIS前11个通道表观反射率
Table 2 The apparent reflectance of MODIS CH1—CH11 on Jan 10, 2003
表 3 MODIS通道光谱范围
Table 3 Band spectral range of MODIS
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