东太湖CDOM吸收光谱的影响因素与参数确定

Influence Factors and Slope Coefficients of Spectral Absorption of Coloured Dissolved Organic Matter (CDOM) in East Taihu Lake, China

CDOM吸收特性是湖泊水色遥感的重要研究内容之一,影响着水体的遥感反射率;吸收光谱的形状一般符合波长的负指数关系,但不同水体的形态因子即S值是不同的.实地采集东太湖水样,实验室测量叶绿素、悬浮物质以及黄色物质的组份含量,室内测试计算水样的CDOM吸收光谱,根据光谱曲线形状,把样点分为三组,分别进行考察,结果发现对东太湖春季水体而言,浮游植物的降解对CDOM吸收具有重要甚至主导作用;水体中有机悬浮颗粒占有一定的比例,在测试或计算东太湖总的吸收或散射系数时,必须充分考虑有机悬浮颗粒的吸收与散射特性,否则会带来较大的误差;以500nm为分界点,把300—700nm的波段范围分为两个部分即300—500nm和500—700nm,分别定义CDOM吸收光谱的曲线斜率即S值,可以提高CDOM吸收光谱的估测精度,把S值定义为随波长线性变化的函数,可以进一步提高CDOM吸收光谱的估测精度,对东太湖春季水体而言,当:300≤λ<500nm时,S(μm-1)=-0.0193×λ+20.821,当500≤λ≤700nm时,S(μm-1)=-0.0121×λ+16.003.

Absorption property of CDOM(coloured dissolved organic matter) ,which influences remote sensing reflectance of water-body , is one of the important contents to remote sensing of water colour. Generally, the absorption spectra curves follow the minus-exponential function with wavelength. But the shapes have some difference in different water-body. Consequently, the spectral slope coefficients, viz. S-values,are different to different water-bodies. So,if we only use the S-value from some different literatures,the error for estimating absorption coefficient of CDOM will be great. Accordingly, we have to determine the S-value of absorption curves of CDOM in Taihu Lake,a large shallow lake with average depth 1.9 meters (max. 2.6m) ,68.5km length from south to north,average 34km width from east to west (max. 56km) ,covering an area of 2427.8 km2(including51 islands) ,one among the five fresh -water lakes of China and positioned in the Yangtze Delta (30°55'40'-31°32'58'N,119°52'32' - 120°36'10'E) ,east of China. One campaign was taken for 14 water-samples in East Taihu Lake,on April 4,2004. Surface water-samples were collected in situ in the vertical direction with a standard 2 liter polyethylene water-fetched instrument. Then they were brought from the deepfreeze to the laboratory using 2 liter polyethylene bottles for further analysis according to correlated investigation criterions about lakes of China. The parameters analyzed in the lab included concentrations of chlorophyll-a (CHL) ,suspended substance (SS)and yellow substance ( DOC). Then the absorption spectra of CDOM were also measured with the spectrophotometer in the lab according to some measurement criterion by NASA, and the final results were revised in scattering influences. According to the shapes of spectra curves measured,all the samples were divided into three groups for analyzing further,denoting the different water-quality environment. And the results show that influence factors of CDOM absorption are different in different water-quality environment. To the water-body of East Taihu Lake in spring, decomposition of phytoplankton plays an important role in absorption of CDOM. When phytoplankton is decomposed into organic suspended matter,its role to absorption of CDOM isnt evi- dent. However,when it continues decomposing and decays,the decayed matter plays a dominant role in absorption of CDOM. Accordingly, the organic suspended matter takes a relative stated proportion in total suspended matter. And the absorption and scatter properties of suspended matter should be taken fully into account when we measure or estimate the total absorption and scatter coefficients of the water-body in East Taihu Lake. Otherwise,great error will be caused. The wavelength range from 300 to 700nm is divided into two parts,one from 300 to 500nm and the other from 500 to 710nm, and then the different S-values are quantitated,which ran improve the estimation of CDOM absorption,proved by their comparative analysis results. In fact,S-value here is a linear function following with wavelength. So when we use the appropriate linear function to define S-value, the estimation precision of CDOM absorption can be improved farther. To the water-body of East Taihu Laihu Lake in Spring, when the wavelength λ is more than 300nm and less than 500nm, S (μm -1 ) = - 0. 0193 × λ + 20. 821. And when the wave-lengthλis more than 500 nm and less than 700 nm, S(μm-1) = -0.0121 × λ + 16.003.