Water Ecology Monitoring And Restoration Evaluation Of Wangyu River Basin Based On Measured Hyperspectral Data

The water quality of Taihu Lake Basin is closely concerned by the public and relevant departments.As one of the main flood discharge channels in Taihu Lake,the water quality of Wangyu River is also very important.Due to the lack of primary producers in Wangyu River Basin,it is necessary to diversify the primary producers to restore the ecological status of the basin.The restoration status needs to be monitored and evaluated.However,the traditional manual detection method is costly and inefficient,so it is necessary to use hyperspectral technology to monitor the growth status of primary producers in river basins.In this study,taking the Wangyu River Basin as the study area,the hyperspectral inversion technology for lower plants and algae,and hyperspectral identification and classification technology for higher aquatic vegetation were constructed.The two technologies were used to monitor the basin before,during and after restoration based on the measured hyperspectral data,and the results were compared and evaluated.The main conclusions are as follows:(1)A hyperspectral retrieval technology for algae in lower plants was constructed.Xiedaidang Basin in Wangyu River was selected as the sampling point,and two field samples were taken in the study area on April 19,2019 and June 11,2019.The reflectance was obtained by using the measured spectral data of the water area.After the normalization of the reflectance,the chlorophyll a concentration was estimated by using the ratio and three-band model.Among them,the coefficient R2 estimated by the band ratio model was 0.7237,and the coefficient R2 estimated by three band model was 0.8538,which indicates that the accuracy of three-band model is significantly higher than that of ratio model.The three-band model was selected as the chlorophyll a concentration inversion model in Xiedaidang Basin.The algae density obtained by microscopic examination of algae samples and chlorophyll a concentration determined by chlorophyll a in the laboratory were regressed and fitted,and the model determination coefficient R2 was 0.9214.The experiment proved that the inversion accuracy of algae density in Xiedaidang waters was higher based on the measured hyperspectral data,which made the monitoring of algae density in Xiedaidang waters more convenient and rapid,and had practical significance for water quality monitoring.(2)The technology of identifying and classifying aquatic higher vegetation by hyperspectral was constructed.Changguangxi Wetland was selected as the study area,and the samples of aquatic higher vegetation were collected on June 12,2019.The measured vegetation included 8 kinds of emergent and floating-leaf vegetation,namely Nelumbinis folium,Salvinia natans,Nymphaea,Nymphoides,Eichhornia crassipes,Pontederia cordata,Lemna and Trapa natans.Based on the measured spectral data of aquatic higher vegetation,the decision tree classification file was established according to the node threshold set by the spectral characteristics of each vegetation.The kappa coefficient was calculated by using the confusion matrix to measure the classification accuracy.The calculated kappa coefficient was 0.8502,that is,the total recognition accuracy was 85.02%,and a higher recognition accuracy was obtained.The identification method of aquatic higher vegetation could identify the aquatic higher vegetation automatically,accurately,quickly and effectively.“Automation” was reflected in the fact that without visual interpretation or other manual intervention,the program can automatically complete the whole recognition process by writing the recognition method into the program,writing the set node threshold into the database and establishing the decision file.(3)The water ecological environment and water quality restoration in Wangyu River Basin were monitored by the constructed hyperspectral technology.Ten watershed sections of Wangyu River Basin were selected as the study area,and nine ground experiments were conducted in May 2018,August 2018,November 2018,may2019,August 2019,November 2019,may 2020,August 2020 and November 2020 respectively.The 10 sites are Xintang Bridge,Xiuxia Bridge,Jiulihe River Bridge,Dacheng Bridge,Xuchong Bridge,Xiedaidang Bridge,Diaozhu Bridge,Huochebang Bridge,Chengzekan Bridge and Yangjiali River Bridge.Based on the measured hyperspectral data,the established mathematical model of algae density inversion was used to invert the algae density of 10 monitoring sections.The results show that the algae density in Wangyu River Basin was closely related to the chlorophyll a concentration.Among them,the algae density of Jiulihe Bridge,Diaozhu Bridge,Huochebang Bridge,Chengzekan Bridge and Yangjiali River Bridge was higher than that before restoration.There is an obvious downward trend,showing a good repair effect.The algae density of Xiuxia Bridge,Dacheng Bridge and Xiedaidang Bridge decreased significantly in the middle stage of restoration compared with that before restoration.However,the algae density increased in summer in the later stage of restoration,showing instability.The algae density of Xintang Bridge and Xuchong Bridge did not show a downward trend before and after remediation,and the remediation effect of water quality was poor.On August 30,2019 in Changguangxi Wetland Park,September 3,2020 in Xiedaidang Bridge and September 16,2020 in Huochebang Bridge,Airborne Hyperspectral technology was used to identify and classify the aquatic higher vegetation in three monitoring sections.The growth status of aquatic higher vegetation in Xiedaidang Basin and Changguangxi water basin is diverse,low pigment water accounted for a large proportion,and the aquatic ecological environment was good.The vegetation in the riparian zone of Huochebang Bridge basin is in good condition,and the water quality has been significantly improved after restoration.However,a large number of water hyacinth grows on the river surface,and there are hidden dangers in the water quality,which need to be treated.In conclusion,the application of hyperspectral technology to monitor the water ecological environment in Wangyu River Basin had proved the advantages of real-time,macro-and-long-time series monitoring.Its advantages of wide monitoring range,highspeed and low-cost make it an important monitoring means.