Comparative Research On The Application Of Different Base Flow Separation Methods In The Hanjiang River Basin

Base flow is an important source of basic river water,which can guarantee the basic water demand in the region's industry,production,residents' lives and natural ecological environment.In this study,base flow separation methods are used to estimate the base flow of Hanjiang River Basin,and the parameters required for the separation method are optimized.The basis of the rationality of the base flow process and the stability of the base flow index are evaluated to analyse the effect of the base flow separation methods.This study analyzed the temporal and spatial distribution characteristics of the base flow in the Hanjiang River Basin,and explored the hydrological factors effecting the base flow.The main research contents and conclusions were introduced as follows:(1)Parameter rules and optimization settings of different base flow separation methodsWhen digital filtering is applied,the base flow is decremented each time with the number of filtering increasing.The decreasing rates of F1,F2,F3 and F4 are different.The parameter setting has the phenomenon of “isoparametric same effect”.The forward and backward filtering is applied to correct the phase shift at the valley or peak,which offsets the effect of some phase distortion.In the application of the Hanjiang River Basin,the F1 method should use positive-reverse alternating filtering,that is,filtering once before,filtering from the end of the first filtering and then back again,and F2,F3,and F4 can be filtered once.When applying the sliding minimum method,for the selection of the N value,the trial calculation method should be considered first.If the transition point of the connection of base flow index and the N value is not obvious,the straight line oblique cutting method is used to modify the flow process.N is adjusted by the result of the cut and the fitting degree of the calculation result of the corresponding sliding period minimum value method.The inflection point test factor shows a gentle sensitivity of the base flow.The parameter N in the HYSEP method is only related to the watershed area,without considering the shape of the watershed,the slope of different basins,etc.(2)Applicability evaluation of different base flow segmentation methods in Hanjiang River BasinDifferent methods make different base flow process,the basic flow process divided by the digital filtering method is relatively smooth.Among them,the basic flow line of F1 fluctuates greatly,and the base flow process of F2 and F3 are more in line with the actual base flow recession process.The base flow process line divided by minimum smoothing method and the HYSEP method has more folding angles.The base flow divided by minimum smoothing method and the HYSEP method in the southern humid region is not necessarily smaller than the digital filtering method.The F3 method of digital filtering method is the most stable method,and the minimum smoothing method calculates the inter-annual variation of the base flow process,and has a good recognition for the base flow process in different years.Based on the comprehensive rationality of the basic flow process and the stability of the base flow index,the F3 method of the digital filtering method has better adaptability to the base flow separation in the Hanjiang River Basin.When the digital filtering method divides the low frequency signal,base flow is also partially attenuated with the superimposed signal.There is a lag time for the relative flow of the base flow.The average lag time of the maximum base flow relative to the maximum runoff in spring is 6d,and the lag time of the maximum base flow relative to the maximum runoff in summer is 4d.The base flow of the Hanjiang River Basin gradually increases from upstream to downstream.