Remote sensing identification of urban black-odor water bodies in Shenyang city based on GF-2 image

Abstract

The situation of urban black and odorous water in China is serious, but the monitoring of this water is just starting from remote sensing and many problems need to be solved. The research provides technical support for the effective regulation of urban black and odorous river. Taking the main rivers in the urban built-up area of Shenyang city as the research object, we carried out ground surveys from 2015-2016 years, and obtained 46 general water samples in Hun river and Pu river, and 50 black and odorous water samples in Huishanming channel, Mantang river, Xi river, the river near the Weishanhu road, and the river of north of Dingxiang Lake, including the water spectrum and water quality parameters. Through the analysis of spectral characteristics of the black-odor water and general water, we found that the urban black and odorous water reflectance spectra in green - red band is gentler than the general water, then we put forward an index of BOI (Black and Odorous water Index) based on the reflectivity spectrum of black and odorous water recognition model. By comparing the BOI index with the ratio index of red and green bands (Wen, et al., 2018 ), BOI index has better recognition accuracy. The results show that: (1) When BOI based on the remote sensing reflectance (Rrs) is less than the threshold of 0.065, which can be awarded as black and odorous water. (2) Due to the difficulties of accurate atmospheric correction in GF2 image, Rayleigh scattering correction reflectance (Rrc) can be used to replace Rrs. When BOI is less than 0.05, it can be discriminated as black and odorous water. And the simulation prove that when the aerosol optical thickness increases gradually, the spectral differences of black and odorous water and general water will decrease, so this method is applicable to the clear image with small aerosol optical thickness (such as AOT (550) ≤0.5). (3) BOI based on Rrc can be applied to GF2 image better, and has better recognition accuracy. The results that are extracted from the three images from 2015 to 2016 are showed that the black and odorous phenomenon of Mantang river and Xinkai river have been gradually improved, but Huishanming channel is still serious. In this paper, the identification algorithm of black and odorous water is mainly based on the spectral characteristics of Shenyang city, it has been validated only in Shenyang, and further verification in other cities. And it is necessary to consider the influence of various factors on the reflectivity of water body in the future.