The Evolution Process And Driving Factors Of The Runoff In The Upper Reaches Of Beiluo River

This paper takes the upper reaches of Beiluo River(above the Wuqi hydrological station)as the research area,based on the measured daily runoff data and meteorological data from 1971 to 2014,the F1 in the digital filtering method was used to separate the baseflow from runoff;M-K trend analysis,Pettitt abrupt point test and R/S method were used to analyze the variation trend,abrupt point test and variation persistence of runoff,baseflow and climatic factors in the future.The Morlet wavelet was used to analyze the multi-scale variation period of runoff;the characteristics of LUCC before and after the policy of returning farmland to forest and grass were analyzed based on three periods of land use data(1990,2005 and 2010).At the same time,the effects of climatic and non-climatic factors,as well as precipitation and human activities on runoff variation,were also quantitatively analyzed by Budyko assumption and double mass curve.The main conclusions are as follows:(1)The annual distribution process of runoff in the upper reaches of Beiluo River from 1971 to 2014 showed a“double peak”,with the peak values appearing in March and August respectively.The annual runoff variation coefficient and the complete adjustment coefficient both showed a significant decreasing trend(p=0.05),and the trend will have strong continuity in the future.The skewness coefficient showed a trend of no significant decrease,and the trend was relatively weak in the future.The annual distribution of runoff in the upper reaches of Beiluo River gradually changes from non-uniform to uniform.Both the annual and quarterly average runoff showed a significant decreasing trend,the abrupt point of the former was 2002,and the latter were between1979 and 2002.The variation processes of annual runoff and runoff in flood season were highly synchronized and consistent,with similar characteristics of“peak value”and“valley value”,and both have similar main variation periods of 3 a,11 a,19 a.(2)The annual baseflow of the upper reaches of Beiluo River from 1971 to 2014was mainly concentrated in August-October,also showed a“double peak”curve,with the peak values appearing in March and October respectively,slightly lagging behind the spring flood season and summer flood season of runoff.The annual baseflow variation coefficient and complete adjustment coefficient both showed a significant decreasing trend(p=0.01),the annual distribution of baseflow showed a gradual change from non-uniform to uniform,and the variation trend in the future will have a strong long-term memory;The skewness coefficient showed an insignificant increase trend(p=0.1),the distribution alternated to the left or the right,and variation trend in the future will be random.The variation trend of the annual and different quarters baseflow showed no significant and no abrupt point had been detected,the BFI showed a significant upward trend(p=0.01)in both annual records and different quarterly records,moreover,the variation trends in the future will be highly persistent,and the same abrupt point(2002)was detected in the baseflow of annual and July-September.The variation trend of the baseflow sequence in the annual and flood season showed a high synchronization and gradual increasing trend.There were 5 obvious wet seasons and 6 obvious dry seasons in the annual baseflow.(3)From 1990 to 2005,in the upper reaches of Beiluo River,the area of farmland decreased by 37.33%,mainly converted into grassland and forestland,with an area of382.78 km~2;the area of grassland decreased by 21.93%;the area of forestland increased significantly,with a total area increase of 950.49 km~2.From 2005 to 2010,the area of grassland decreased by 3.14%,and the area of forestland increased by 15.37%;51.31km~2 and 4.78 km~2 of farmland were converted into grassland and forestland,accounting for 55.06%and 0.51%of the original area,respectively.(4)The annual precipitation in the upper reaches of the Beiluo River showed an insignificant upward trend(p=0.1)as a whole,no abrupt point was detected,and variation trend in the future will present a weak anti-sustainability trend.The overall annual average temperature showed a significant upward trend(p=0.01),with the abrupt point year in 1993;the annual potential evapotranspiration showed an insignificant upward trend as a whole,and no abrupt point year was detected.Based on the Budyko assumption,the contribution rate of precipitation and potential evapotranspiration to runoff variation was 40.4%,of which the precipitation accounted for 36.4%;the potential evapotranspiration accounted for 4%and all non-climatic factors accounted for 59.6%.Based on the double mass curve,the contribution rate of precipitation was only 2.33%,while the contribution rate of human activities was as high as 97.67%.