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

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 absorption 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 (R (λ)) collected on October 26, 2013 over Guanting Reservoir and aCDOM (λ) measured in the laboratory. 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 σ of 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 and σ of 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.