Research on the Ecological Flow of the Qujiang River Using the Monthly Frequency Calculation Method Improved by the Copula Function

Under the combined influence of climate change and highly intensive anthropogenic activities,the natural hydrological regimes of rivers have changed to some extent,interfering the structural stability and the function of riverine ecosystems. Ecological water requirement assessment is very important to the maintenance of river ecosystem health and is one of the important research contents in water science. The improved monthly frequency calculation method based on the Copula function is proposed to calculate the ecological flow and applied in a case study of the Qujiang River. The joint distribution between the monthly average flow and the annual average flow is constructed by using the Copula function. Based on the correlation between the conditional probability of the monthly average flow and the joint probability,the conditional probability target of the monthly average flow in the dry year is set up,and the joint probability is calculated. The design frequency of the monthly average flow is calculated by the joint probability function,and the design value of the monthly average flow is analyzed according to the marginal distribution function and checked for reasonableness. The results show that the Clayton Copula function has a good fitting effect on the joint probability distribution between the monthly average flow and the annual average flow of the Qujiang River. The basic ecological water requirement of the Qujiang River is 2.12 billion m3,accounting for 36% of the multi-year mean annual runoff. The analysis results of the improved monthly frequency method based on the Copula function pass the rationality test of“Specification for the Calculation of Environmental Flow in Rivers and Lakes”,and are relatively safe in riverine ecological security. This method considers the frequency difference between annual average flow and monthly average flow and is an effective method for ecological flow assessment. The research results can provide a scientific basis and technical support for the development and utilization of water resources and the protection of river ecological environment in the Qujiang River,and have an important reference value for the evaluation of ecological water demand and the construction of water ecological civilization in other river basins.