Baseflow Characteristics And The Correction Of Baseflow Separation Methods In A Forested Watershed In Permafrost Region Of The Da Hinggan Mountains

Baseflow is usually defined as runoff from groundwater or other delayed parts,which is a stable component of river runoff and the main source of runoff in dry season.In order to understand the characteristics of baseflow and its relationship with precipitation,soil moisture and extreme dry event in the forested watershed in permafrost region of Da Hinggan Mountains,the conductivity mass balance method(CMB)was used to separate the baseflow from the total streamflow,the double cumulative curve method was used to analyze the influence of precipitation on runoff components.Meanwhile,the causal relationship and time delay between soil moisture and runoff components,as well as the influence of extreme dry event on runoff components were studied.The empirical parameters of numerical simulation method were corrected by the conductivity mass balance method.The results showed that:(1)The annual(May to September)baseflow,the surface runoff and the total runoff was 123.93 mm,67.57 mm and 191.50 mm,respectively.The baseflow was the main source of river runoff,accounting for 65%of the total runoff.(2)During the extreme dry period from April to July in 2020,the baseflow index increased significantly,but the response time of runoff to rainfall also increased.The extreme dry period reduced the runoff coefficient and increased the threshold value of accumulated rainfall.In the extreme dry period,the fluctuation coefficient of runoff components decreased significantly.There were significant seasonal variations in runoff components.In the snowmelt runoff period(May and June),the contribution of baseflow to the total runoff decreased with the increase of rainfall duration.In the growing season(July to September),the proportion of baseflow in each month was greater than 65%,and the baseflow index was the largest in August,reaching 0.91.The contribution of baseflow to the total runoff also increased with the increase of rainfall duration in this season.There was a significant(P<0.05)correlation between basalflow and soil water content in the growing season,during which there was a significant time lag effect between baseflow and soil moisture.Such effect was more significant more pronounced in the rainfall-runoff process with greater amount and rainfall peak.(3)The empirical parameters of numerical simulation method were modified based on the baseflow data obtained by conductivity mass balance method.The parameters of digital filtering method F1 and F4 were 0.87 and 0.92,respectively;the unit time(n)of HYSEP method was 7;the unit time(n)of BFI method was 3.Compared with CMB method,the results of optimized digital filtering method(F4)was the closest to that of the CMB method in terms of baseflow index.Meanwhile,the root mean square error and correlation coefficient of evaluation index proved that F4 method was the most suitable numerical simulation baseflow separation method in the small forested watersheds without conductivity data in Da Hinggan Mountains..