基于水面光谱数据的官厅水库有色可溶性有机物反演

Retrieving Chromophoric Dissolved Organic Matter in Guanting Reservoir Based on in-situ Measured Reflectance Data

有色可溶解性有机物(简称CDOM,又称为黄色物质),存在于所有自然水体中,可以用于指示水体有机物污染的状况。相比于常规采样监测方法,基于遥感数据反演CDOM具有重要优势。但是遥感数据反演CDOM的方法通常具有区域局限性,因此需要对不同区域水体进行反演方法的检验和完善。我国北方水体CDOM遥感反演的相关研究较少,选择位于河北省张家口市和北京市延庆区交界的官厅水库为研究区,利用2013年10月26日获取的水面遥感反射率光谱(Rrs(λ))和实验室测得的CDOM吸收系数(aCDOM(λ))数据,首次进行了CDOM浓度(以440nm处CDOM的吸收系数(aCDOM(440))表示)反演。对半解析方法即QAA-CDOM方法进行了检验和改进,并建立了CDOM反演的经验模型。QAA-CDOM方法反演结果的均方根误差RMSE为0.10,平均相对误差σ为10.8%。通过实测数据计算了每个波段的水面以下上行辐照度与辐亮度的比值Q,代替了QAA-CDOM方法中的固定Q值,对QAA-CDOM方法进行改进,反演结果精度仅略有提升,RMSE=0.09,σ=10.2%。同时,用四个遥感反射率的比值与aCDOM(440)进行回归分析,建立了CDOM反演的经验模型。结果显示Rrs(531)/Rrs(551)与CDOM浓度的相关性最大,决定系数为0.63;基于该波段比值建立的CDOM反演经验模型的反演结果的均方根误差RMSE为0.08,平均相对误差σ为8.8%。经验方法反演结果的精度更高,但需要同步实测数据进行所选波长和模型系数的标定;半解析方法不需要标定,更易于推广。

In the present paper ,Guanting Reservoir located in Zhangjiakou City ,Hebei province and Yanqing District ,Beijing , was selected as the study area ,and Chromophoric dissolved organic matter （CDOM ） content （represented by the CDOM absorp‐tion coefficient at 440 nm ,aCDOM （440）） was inverted through semi‐analytical method and empirical method .The data used in this paper include the spectral data （Rrs （λ）） collected on October 26 ,2013 over Guanting Reservoir and aCDOM （λ） measured in the la‐boratory .A semi‐analytical method （QAA‐CDOM ） was validated and improved accordingly .The inversion result accuracy of QAA‐CDOM was not bad ,the root‐mean‐square error （RMSE） was 0.10 and the mean relative error （σ） was 10.8% .The QAA‐CDOM method was improved by recalculating the value of Q （the ratio of upwelling irradiance to upwelling radiance just below the water surface） by the field spectral data to replace the fixed value .After this improvement ,the inversion accuracy was slightly improved ,with RMSE of 0.09 and σof 10.2% .After that ,four band ratios were used in linear regression with aCDOM （440） to build empirical inversion models .The results showed that the performance of the model based on Rrs （531）/Rrs （551） was the best ,with R2 of 0.63 .The RMSE andσof the aCDOM （440） result inverted by the empirical method were 0.08 and 8.8%respectively .The empirical method was of higher precision than that of the semi‐analytical method ,but the bands and coeffi‐cients used in the empirical method need to be calibrated by concurrently measured data in each study area .On the contrary , semi‐analytical method does not need calibration ,and is easier to be applied .