Study On The Risk Of Algal Blooms Outbreak In The Mid-Down Stream Of The Han River Driven By Hydrological Factors

River algal blooms is a deterioration of water environment caused by regional climate,hydrology,water quality and other factors.As the important water source of the middle route of the South-to-North Water Diversion Project in China,the mid-down stream of the Han River(MSHR)has occurred frequently algal blooms events in the past 20 years since the first algal blooms event was reported in the 1990s in its middle and lower reaches.The algal blooms events caused serious water environmental pollution and seriously affected the social and economic development along the river as well as the production and life of the residents.Therefore,the algal blooms event has attracted wide attention both at home and abroad.Exploring the identification of multiple influencing factors and algal blooms risk simulation as well as prediction in the MSHR is not only a major demand for regional and national development,but also a hot topic in the interdisciplinary research of hydrology,ecology and other fields.Based on the collected data of hydrology,meteorology,water quality,chlorophyll a,and algal cell density in the Han River Basin,this study analyzed and identified the key driving factors of algal blooms outbreak in the MSHR by constructing a hierarchical and complementary multi-factor identification statistical analysis system.Besides,based on MIKE 11 model,the eutrophication model of Xiantao to Hankou section of the Han River was constructed.Based on the statistical analysis of hydrological driving factors in the prone period of algal blooms,different scenarios of hydrological driving factors for the algal blooms outbreak were set up to simulate chlorophyll a(Chl-a)conditions,so as to explore the relationship between key hydrological driving factors and algal density in the MSHR,and determine the threshold range of hydrological driving factors that induce algal blooms.What’s more,the Variation diagnosis and Indictors of hydrology alteration were used to analyze the characteristics and rules of hydrological driving factors.On these basis,the Copula function was used to construct the joint distribution model of different hydrological encounter combinations between the Han River and the Yangtze River estuary to calculate the risk probability of different hydrological encounter combinations and different severity of algal blooms outbreak.The main contents and conclusions of the study are as follows:In this paper,based on the investigation and analysis of previous blooms events,hydrology,meteorology,water quality,algae and other data in the Han River basin were collected.The multiple influencing factors of algal blooms in the MSHR were analyzed based on statistical methods,and the key driving factors of the blooms were quantitatively studied.Besides,the MIKE 11 model was used to construct the eutrophication model in the lower reaches of the Han River to explore the relationship between hydrodynamic conditions and the algal density in the bloom outbreak,and to identify the threshold range of hydrological driving factors that induced algal blooms.In addition,Variation diagnosis and Indictors of hydrology alteration were used to systematically study the variation characteristics of key hydrological factors that induced the algal blooms in the MSHR.The joint distribution model of hydrological encountering of Han River and Yangtze River was established by Copula function to study the hydrological encountering risk of blooms outbreak in different hydrological periods and different severity.The main contents and conclusions of the study are as follows:(1)On the basis of systematically sorting out the influencing factors of the algal blooms outbreak in the MSHR,a progressive and complementary multi-influencing factor identification system using Pearson correlation analysis,Spearman correlation analysis,K-S distance analysis and redundancy analysis was established to quantitatively identify the key driving factors of algal blooms in the MSHR.The results show that the key driving factors of algal blooms in the MSHR are the flow of the Han River,the water level into the Yangtze River Estuary and the total phosphorus.And the contribution of three factors to algal blooms were 39.5%,24.6%and 27.2%,respectively.The algal blooms outbreak in the MSHR was closely related to the flow of the Han River and the water level into the Yangtze River estuary.(2)The MIKE 11 eutrophication model of the Xiantao to Hankou section in the Han River was constructed.Based on the statistical analysis of the relationship between the discharge in the MSHR,the water level into the Yangtze River estuary and the algal cell density during the algal blooms prone period from 1992 to 2017,different scenarios are set for the flow of the Han River and the water level into the Yangtze Estuary.The eutrophication conditions under the set scenario were simulated to identify the thresholds of key hydrological factors for algal blooms outbreak.The result show that the MIKE 11 model has a relatively ideal simulation effect,and the relative error of the hydrodynamic model is 5%and 4.9%in the periodic and verification period,respectively.The average relative error of the eutrophication model for Chl-a is 17.1%,which meets the simulation requirements.And the result of hydrological factor threshold studies based on different hydrological scenarios show that when the flow of the MSHR exceeds 900m3/s,the water level of the Yangtze River is below 14m,it is considered that there are no algal blooms.The flow is between 650～900m3/s,and the water level is between 14～16m,it will occur light algal blooms.The flow is between 500～650m3/s,and the water level is between 16～17m,it will occur moderate algal blooms.When the flow is less than 500m3/s and the water level is higher than 17m,severe bloom appears.(3)Based on the data of flow of the Han River and the water level of the entrance to the Yangtze River during 1960—2017,hydrological variation diagnosis analysis was used to identify the abrupt change points and test the trend of the discharge of the MSHR and the water level of the entrance to the Yangtze River,as well as the IHA method was used to analyze the related IHA indicators.The results show that the flow of the MSHR and the water level into the Yangtze River estuary have abrupt changes in 1990 and 2006,respectively.The analysis of the selected IHA indexes showed that the minimum 1,3,7 and 30-day discharge of the Han River decreases continuously from period I to III,while the lowest water level of the Yangtze River entrance changes in the opposite way.The minimum discharge mainly appeared in spring dry season,which was consistent with the time of water bloom outbreak.The occurrence frequency of minimum discharge and minimum water level increased with the passage of time.(4)Based on the Copula function,the joint probability distribution model of the flow of the Han River and the water level of the Yangtze River estuary was constructed to calculate the risk probability of algal blooms outbreak under various combined conditions under the actual incoming water and different water transfer schemes.The results showed that the risk probability of the bloom outbreak increased successively from period I to III under the actual incoming water regime.Under different water transfer schemes,the probability of algal blooms outbreak increased with the increase of water transfer,and the probability of algal blooms after the construction of Three Gorges Reservoir was higher than that before.It is worth noting that although the risk of severe bloom outbreak is low,its growth rate is significant,which should be paid great attention to.