Research On Instream Ecological Water Requirement Under The Hydrological Alteration Conditions

In China,because of the large population,uneven distribution of water resources,the uncoordinated allocation of soil and water resources,and the lack of ecological water requirement in many dispatching and allocation practices,ecological water consumption is crowding out,which is not conducive to the sustainable development of social economy.Protecting and improving the ecological environment is the basic policy that must be adhered to guarantee the sustainable development of social economy.In order to effectively prevent the water pollution to ensure the national waters security,in April 2015,State Council issued the "The Action Plan for Prevention and Treatment of Water Pollution" notice,which clearly proposed that the ecological flow should be scientifically determined,the water dispatching management of rivers and lakes should be strengthened,the ecological water requirement of river and lake should be maintained,and the ecological base flow should be ensured.In recent years,more and more researches on ecological water requirement have been focused on the interdisciplinary fields,such as the combination of ecological water requirement and water resources allocation,the ecological water requirement calculation based on physical mechanism or the combination with biology,and the evaluation of application effect of ecological water requirement calculation results.There are few studies on the calculation of ecological water requirement under the hydrological variation condition and the assessment of ecological water requirement.However,in the actual allocation and management of water resources planning,it is of practical significance to evaluate the ecological water requirement and the degree of satisfaction under the variation conditions.In this paper,according to the actual situation of the river basin,the water flow of the river is analyzed.Distributed Time Gain Variant Model(DTVGM)was carried out in the study area for runoff reduction,the ecological water requirement was calculated,and the ecological water requirement result was evaluated by using the water guarantee rate index.The main research work of the thesis includes the following aspects:(1)The results show that mutation of runoff sequence occurred in 1991,and the main cause is climate change.Most of the IHA index is higher than the RVA threshold limit.The average monthly flow rate has an upward trend,and the improvement of the overall runoff before and after hydrological change were 36.99%,which were moderate change.The most relevant ecological hydrological indices include Minimum seven days flow,cut-off days and so on.The number of low flow decreased after the mutation,the high pulse flow event and the large flood flow event value increased obviously,the flow pattern tended to be single mode.(2)The DTVGM model is used to reduce runoff,and the ecological water demand is calculated by hydrologic method.It is proved that the DTVGM model can simulate the natural distribution of water resources in the river basin.Then using five probability distributions to analyze the monthly runoff process,and to test the linearity according to the PPCC test,get the P-Ⅲ distribution,Gamma distribution and GEV distribution have the best fitting effect,the normal distribution is the worst.The optimal distribution function is chosen monthly to obtain the ecological water requirement,and the total ecological water requirement is 5.968 billion m3.The annual total ecological water requirement using Tennant method,NGPRP method,minimum monthly average flow method and RVA method was 6.898 billion m3,1.491 billion m3,542 million m3 and 7.288 billion m3,respectively.The ecological water requirement of the minimum monthly and NGPRP method can’t reflect the difference of ecological water requirement in different months of the year;the ecological water requirement process obtained by Tennant method only takes into account the difference between the general water period and the spawning period of fish during the year.The ecological water requirement obtained by the monthly frequency calculation method and RVA method takes into account the dynamic change of ecological water requirement during the year,and is more practical,which is suitable ecological water requirement and optimum ecological water requirement respectively.(3)Briefly describe the meaning of the guarantee rate of ecological water,the calculation method of the guarantee rate of ecological water which based on frequency analysis and satisfaction rate analysis was put forward.The optimum ecological water requiremen and the suitable ecological water requiremen were evaluated and analyzed for three kinds situation.The results show that the guarantee rate of ecological water in the flood season is low,which indicates that the ecological water use is low in flood season,and it can’t guarantee the demand of ecosystem.In the 60’s to 90’s,the guarantee rate of ecological water increased gradually.