Spatial-temporal Dynamic Inversion Of Chaohu Lake Algal Blooms Based On Multi-source Remote Sensing Image Data Fusion

In the past 30 years,with the continuous development of Chaohu Lake basin economy and urban expansion,a large number of industrial,agricultural and domestic wastewater has flowed into the water body,seriously polluting the water quality of Chaohu Lake,and the eutrophication of the water body is becoming more and more serious.Every summer,cyanobacteria grow abnormally in Chaohu Lake,forming a large area of blooms.It directly affects the production and life of the surrounding residents.In this paper,we use Landsat,MODIS,HJ-1A/B and three different types of remote sensing data to pre-process remote sensing data such as radiation correction,atmospheric correction and geometric correction,and use Brovey transform to fuse remote sensing data.We use normalized vegetation index,enhanced vegetation index,cyanobacteria index and other methods to jointly retrieve the spatial distribution of Chaohu Lake algae bloom.The spatial distribution area of algal blooms in Chaohu Lake was calculated.Finally,the mechanism and temporal and spatial dynamics of cyanobacteria outbreak in Chaohu Lake are analyzed by selecting climate,human activities,water quality and other factors,combined with the distribution charts of algal blooms in Chaohu Lake.The main conclusions are as follows:(1)Based on the sensitivity of chlorophyll to different bands,the spatial distribution and temporal dynamic changes of water bloom in Chaohu Lake were retrieved by NDVI,GNDVI,EVI,CAI and other methods.Based on the different thresholds of algal blooms,the spatial distribution areas of algal blooms in Chaohu Lake were calculated.Finally,the distribution characteristics of algal blooms in Chaohu Lake were mapped by layered coloring based on red,yellow and blue colors.(2)According to the classification results of the algal blooms concentration in Chaohu Lake area by threshold method,the area and distribution of algal blooms in different periods were counted.The results showed that the algal blooms occurred every year in Chaohu Lake area from 2013 to 2018,with the total area of algal blooms reaching 588 square kilometers in 2015,accounting for 77.4%of the total area of Chaohu Lake,which was the most serious in the past years.Once.In July and August,the area of algal blooms in the western half of Chaohu Lake reached 91.4%.In the areas where algal blooms occurred,the calculated value of 83.1%of the regional algal concentration threshold was greater than 0.45,reaching the severe and moderate algal concentration,and the algal pollution in July was more serious than that in August.In September,the blooms in the western half of Chaohu Lake spread eastward.(3)Based on the analysis of different environmental impact factors such as climate conditions,geographical location,human activities and water quality in Chaohu Lake area,the results show that the generation of algal blooms in Chaohu Lake is the result of the interaction of many factors.Temperature and light are the key and prerequisite for the outbreak of algal blooms in Chaohu Lake.The unique geographical location and monsoon conditions play an important role in promoting the accumulation of algal blooms.The large amount of domestic wastewater and non-point pollution brought by agricultural production brought sufficient nutritional conditions for algae reproduction,such as cyanobacteria,which directly led to the deterioration of water quality and serious eutrophication of water body in Chaohu Lake,which was also the direct cause of the outbreak of algal blooms in Chaohu Lake.(4)Analyzing the spatial and temporal distribution characteristics of the algal blooms in Chaohu Lake,the results show that the algal blooms are widely distributed in the whole lake area of Chaohu Lake,especially in the western half of Chaohu Lake.The occurrence time of cyanobacterial blooms in Chaohu Lake area is mainly from April to October.From the end of March to the beginning of April,sporadic blooms began to appear in Chaohu Lake,especially in fine weather.The main occurrence area of algal blooms is the northwest part of Chaohu Lake,near the direction of Hefei city.As the weather gradually warms up,the occurrence area of algal blooms will gradually expand,and the distribution range will gradually spread.The distribution area of algal blooms and the content of algal blooms in Iake water will change in different years in summer.Usually from the end of June to July,almost all of Chaohu Lake area will have scattered distribution of algal blooms,but most of them will appear on the West Bank of Chaohu Lake.Along the coast,there will be a small scale of cyanobacterial blooms accumulation with the action of wind and waves.The accumulation area mainly distributes on the West Bank and North Bank of Chaohu Lake.It is found that the occurrence time of Chaohu Lake blooms is ahead of schedule.Generally speaking,the time of Chaohu Lake blooms begins in April every year and ends in December.Among them,July and August are the concentration time of Chaohu Lake cyanobacterial blooms,which spread from the western half of Chaohu Lake to the surrounding areas.