Risk Assessment Of Cyanobacteria In Chaohu Lake Based On MODIS Image And Vine Copula Model

Eutrophication and rising global temperatures have led to rapid growth of cyanobacteria.Lake cyanobacteria bloom leads to black and smelly water,which not only seriously affects coastal landscape and drinking water safety,but also leads to pollution of various derivatives and degradation of ecological environment.The distribution of cyanobacteria in lakes is highly variable and unbalanced,which brings great challenges to the management of lakes.Based on the Vine Copula "quantity-distribution" joint distribution model,the joint distribution function was used to evaluate the risk of cyanobacteria,and the spatial cooccurrence of cyanobacteria was analyzed.This thesis mainly carries out research from the following three aspects:(1)Risk assessment of cyanobacteria based on Vine Copula model.The Vine Copula model was established with the "quantity" of cyanobacteria in Chaohu Lake as the marginal distribution and the "spatial distribution" of Chaohu lake as the dimension from May to November of 2000 to 2021.The model comprehensively considered the quantity and distribution of cyanobacteria.The "quantity-distribution" comprehensive risk of cyanobacteria in Chaohu Lake in 22 years was calculated,and the risk of cyanobacteria in Chaohu Lake from 2000 to 2021 was ranked.The ranking results from high to low were as follows: 2015>2019>2018>2012>2014>2010>2007>2009>2020>2008>2006>2016>2011>2013>2017>2005>2021>2000>2003>2002>2001>2004.Finally,the FAI mean value of each year was compared as a conventional evaluation index.The results showed that compared with the single FAI mean value evaluation method,Vine Copula model could take into account the quantity and spatial distribution characteristics of cyanobacteria,and the evaluation results were more objective and accurate.(2)Study on spatial cooccurrence of cyanobacteria based on Vine Copula model.The variation of Copula function,Kendall correlation coefficient and tail correlation coefficient of Vine Copula model were analyzed month by month from May to November,and the spatial co-occurrence of cyanobacteria between different periods was identified.The results showed that,from the results of spatial cooccurrence,the cyanobacteria outbreak frequency was high and the diffusion was serious in northwest China.The cyanobacteria were active in the northwest region,while the cyanobacteria activity in the central region was relatively low.Therefore,the correlation between the cyanobacteria in the two regions was relatively weak,but they had strong spatial co-occurrence.The correlation of cyanobacteria was strong in the central and northeastern regions,but the cyanobacteria outbreak frequency was low,and the spatial co-occurrence was weak.(3)Study on the influence of spatial zoning on risk assessment and spatial co-occurrence.Spatial partitioning plays an important role in risk assessment.In order to illustrate the impact of spatial zoning on risk assessment,two groups of zoning schemes with 6 zoning and 3 zoning were further set as controls,and comprehensive risk assessment was carried out respectively.The results showed that due to the over-fine zoning of the 6-zone scheme,the risk of slight cyanobacteria in some areas may be higher,or the risk of slight cyanobacteria in some areas may be serious,but the risk of slight cyanobacteria in other areas may be lower.At the same time,the change of cyanobacteria in a small range may lead to huge changes in the Copula function and correlation coefficient,which is not conducive to the analysis of the spatial co-occurrence of cyanobacteria in Chaohu Lake.Zone 3 is too rough,especially zone 1 includes all areas with high cyanobacteria outbreak frequency,which is not conducive to risk assessment.In small areas with severe cyanobacteria,FAI value is pulled down due to the large zoning space,which results in the phenomenon of multi-cyanobacteria low risk.In addition,due to the rough partition,the obtained spatial cooccurrence path is too limited,which is also not conducive to the observation of the spatial co-occurrence process of cyanobacteria.In conclusion,the Vine Copula model based cyanobacteria risk assessment method proposed in this study takes into account the quantity and spatial distribution characteristics of cyanobacteria,which can provide methodological basis for improving the risk assessment of cyanobacteria in lakes.At the same time,a Vine Copula model was proposed to analyze the spatial co-occurrence of cyanobacteria in Chaohu Lake.The activity,correlation and spatial co-occurrence probability of cyanobacteria in each region during the outbreak period were analyzed.The results of this study can provide scientific basis for the comprehensive prevention and control of cyanobacteria pollution in lakes,and provide decision-making support for the risk warning and control of cyanobacteria.