| The middle and lower reaches of the Han River are one of the regions in central China with the most active economy,concentrated population,and concentrated production factors.At the same time,as a national strategic water resource guarantee area,the Han River Basin is the water source area for major national water resource allocation projects such as the Middle Route Project of the South to North Water Diversion and the Water Diversion from Han to Wei.However,with the successive completion of cross basin and cross regional water transfer projects and cascade development projects,coupled with the continuous growth of economic and social water use,the contradiction between economic development and ecological environment protection in the middle and lower reaches of the Han River has become more prominent.The ecological environment water demand guarantee is insufficient,and the risk of water ecological environment is constantly increasing.Objectively understanding and reasonably determining the ecological flow of the Han River,It is urgent to implement strict ecological flow control and river ecological health protection.Therefore,this study selected the important control sections of the middle and lower reaches of the Han River(about 652km)as the research object.On the premise of revealing the biological environment change characteristics and main control factors of the important control sections in the middle and lower reaches of the Han River,fully considering the influence of various factors such as water quality,water bloom prevention and control,special habitats,hydrological conditions,etc.,mathematical models such as the Han River main flow dynamics model,water quality model,water ecology model,and water bloom model were constructed,Realize precise simulation of ecological water demand in the main stream of the Han River,and based on this,propose"five divisions"ecological environmental water demand results for important control sections in the middle and lower reaches of the Han River,including regions,types,time periods,frequencies,and stages.The main research conclusions are as follows:(1)Explored the characteristics and influencing factors of biological environment changes in the middle and lower reaches of the Han River after the South to North Water Diversion Project.The results indicate that the flow and water level in the middle and lower reaches of the Han River have decreased to varying degrees after the Danjiangkou Reservoir impoundment;The water quality of the main stream remains basically stable;The fish population and spawning volume in the separated river sections have been decreasing year by year;The outbreak period of water blooms is mainly concentrated in the dry season of spring(January to March),and the frequency of occurrence shows an increasing trend.Water blooms affect all downstream river sections about 400km below Zhongxiang,with a trend of spreading from the main stream of the Han River to its tributaries.This achievement provides a scientific basis for minimizing the impact of water diversion on the biological environment of the Han River Basin and formulating corresponding ecological compensation.(2)Explored the satisfaction level of ecological environmental water demand based on water quality standards in the main stream of the Han River after the South to North Water Diversion Project.The results indicate that the water volume in each section of the river before the South to North Water Diversion Project,except for some non flood seasons in dry years,can basically meet the ecological and environmental water demand requirements of the water function zone(with a compliance rate higher than 90%).After the opening of the middle route of the South to North Water Diversion Project,the satisfaction of ecological and environmental water demand at the Huangjiagang section decreased in January,December,and Huangzhuang section in January,September,and October.The ecological and environmental water demand results of each section showed a gradually increasing trend from the upstream Huangjiagang section to the downstream Shayang section.This achievement provides a method and means for scientific identification of ecological water demand based on water quality standards under the influence of large-scale water diversion projects.(3)Revealed the prevention and control cross-section,balance factors,and ecological flow of the Han River main stream where water blooms broke out.The results indicate that algal blooms in the middle and lower reaches of the Han River mainly occur during the dry season in spring.During this period,the main factors affecting algal growth include flow rate and total phosphorus,and there is a significant negative correlation between algal cell density and flow rate,as well as a negative correlation with TP.The main control sections are the Huangzhuang and Shayang sections.This achievement can provide scientific basis for suppressing the outbreak of water blooms in the main stream of the Han River and implementing ecological and environmental emergency dispatch.(4)We have obtained the regulated water volume of Danjiangkou Reservoir that meets the special habitat needs of the main stream of the Han River.The results indicate that the main control factor affecting fish spawning is flow velocity;In order to meet the spawning and reproduction conditions of drift spawning fishes in the the four famous domestic fishes Germplasm Resources Conservation Area in the middle and lower reaches of the Hanjiang River,Qianjiang Section and Hanchuan Section,ecological regulation is required in the Cuijiaying Xinglong section of the main stream of the Hanjiang River,and the annual average additional discharge of Danjiangkou Reservoir is581 million m~3.Research has shown that the implementation of large-scale water transfer processes should fully consider the natural laws and ecological rhythms of special habitat ecosystems,and reserve ecological water storage capacity or ecological water regulation capacity,which is of great significance for coordinating the contradiction between water diversion and special habitat water use.(5)Different hydrology methods and hydraulics methods were compared to calculate the eco-environmental water demand of typical sections in the middle and lower reaches of the Hanjiang River.The results show that the minimum ecological flow at each control section of the wet week method accounts for about 20%of the annual average flow at each hydrological station.The calculation results of the ecological hydraulic radius method are also within the optimal range of the Tennant method;The RVA results are closest to the natural variation pattern of rivers,and the fluctuation of the calculated results is smaller than the interannual variation of natural river runoff;The CV value of the coefficient of variation corresponding to the key month of the habituation method is the relatively minimum value in a year.This study provides technical support for understanding the hydrological,natural,and ecological laws of the Han River Basin,and for conducting research on the laws and characteristics of the Han River ecosystem in the later stage.(6)The"five point"ecological environmental water demand results were proposed for important control sections in the middle and lower reaches of the Han River,including regions,types,time periods,frequencies,and stages.The ecological and environmental water requirements of Huangjiagang,Xiangyang,Huangzhuang,and Shayang sections are 376m~3/s～474m~3/s,378m~3/s～517m~3/s,486m~3/s～1089m~3/s,and515m~3/s～850m~3/s.The results indicate that when calculating the ecological environment water demand,it is necessary to fully consider the variability characteristics of the main control factors of river ecological flow based on the coordination between upstream and downstream,main and tributaries,and comprehensively coordinate the ecological environment water demand results of different objectives in time,space,and process,laying the foundation for long-term protection of ecological environment water demand. |
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