Study On Ecological Flow Of Small Watershed Under Changing Runoff Condition

With the current population and economic and social development,human demand for water resources is increasing,and the development and utilization of rivers are increasing year by year,squeezing the normal water demand of river and lake ecosystems,resulting in more and more serious ecological problems in the basin.Because the ecosystem of small watershed is more fragile and sensitive than that of large watershed itself,once encountering extreme climate or excessive exploitation and utilization of water resources by human beings,the ecosystem of small watershed will suffer from a difficult recovery.China also pays more and more attention to the ecological management of small watersheds.In recent years,the Ministry of Water Resources has also put forward a series of requirements and opinions to speed up the clean construction of small watersheds,which is conducive to the development of China ’s environment-friendly society.Therefore,it is necessary to strengthen the research on the ecological problems of water resources in small watersheds.Based on the monthly average runoff data of two representative hydrological stations(Hongqi Reservoir Station and Puzhehei Station)in the Qingshui River Basin of Wenshan Prefecture in the past 60 years,this paper analyzes the runoff variation characteristics and future evolution rules of the Qingshui River Basin,and selects the ecological flow calculation method with good applicability.The ecological flow health evaluation system is established to dynamically test and analyze the ecological flow set in different runoff periods,so as to clarify the red line of water resources development and utilization in the basin and promote the sustainable and healthy development of the good social environment between man and nature.The main research contents and conclusions are as follows:(1)The uneven distribution coefficient,concentration degree and concentration period of runoff in two hydrological stations in Qingshui River Basin were calculated,and the trend analysis and hydrological variability test of annual runoff were carried out.The results show that the annual runoff distribution in the basin is extremely uneven,which is not conducive to the utilization and management of water resources.The Mann-Kendall method and Pettitt method were used to test the hydrological variation.The runoff variation year of Hongqi Reservoir Station was1990,and the runoff variation year of Puzhehei Station was 1983 and 1990.It can be seen that the hydrological situation of Qingshui River Basin changed significantly from 1983 to 1990.Using the 5a moving average method and linear trend fitting,the trend analysis of the interannual runoff of the two hydrological stations was carried out,and the interannual variation of runoff showed a downward trend.Especially since the 20 th century,the annual runoff decreased sharply due to the influence of arid climate until 2014.(2)By introducing Bayesian optimization algorithm into the long and short memory neural network to optimize the hyperparameters,the prediction accuracy of the model is improved,and the BOM-LSTM prediction model is created to predict and analyze the runoff sequence in the next 5 years and 60 months.Then six hydrological methods were selected to calculate the ecological flow,and the advantages and disadvantages of each method were compared to obtain the comprehensive determination method of the ecological flow in the Qingshui River Basin.The monthly ecological flow values before,after and under future runoff conditions were calculated respectively.The results show that the change trend of ecological flow value from Hongqi Reservoir Station to Puzhehei Lake section is basically unchanged,and the ecological flow from Puzhehei Station to downstream section shows an attenuation trend in different periods.The overall ecological flow of the basin showed a downward trend,and the protection and restoration of water ecology in the Qingshui River Basin should be strengthened in the future.(3)Based on the cloud model and D-S evidence theory,an ecological flow health evaluation system was established to analyze the adaptability of the set ecological flow to the periodic changes of the river and the sustainable and healthy development.The results showed that the ecological flow values in different periods were relatively healthy and reasonable.This evaluation mechanism can also be applied to the study of ecological flow in other small watersheds.