Baseflow Separation And Characteristic Analysis Of The Loess Region

Under the condition of the increasingly prominent contradiction between supply and demand of water resources,the analysis of the basicflow characteristics of typical basins in the middle reaches of the Yellow River plays an important role in determining the mode of runoff production,the redistribution of water resources and the optimization of economic distribution in the region.Based on the hydrologic data,meteorological data and soil and water conservation measures of Huangfuchuan,Ganguyi and Wuqi hydrologic stations for more than 60 years,this paper adopted three types and nine automatic baseflow segmentation methods to segment the baseflow in Huangfuchuan,Yanhe and the upstream of Beiluo river basins.The suitability method of baseflow segmentation in this area was discussed.The variation of baseflow and its driving factors in Huangfuchuan,Yanhe and the upstream of Beiluo river basins were systematically analyzed by means of Mann-Kendall trend test,Pettitt mutation point test and double cumulative curve method.The main results obtained are as follows:(1)F4 method in digital filtering method was the optimal method to calculate the baseflow of Huangfuchuan and Yanhe River in loess area,while the optimal calculation method of baseflow in the upper reaches of Beiluo River was F1 method.On the baseflow calculated by three kinds of nine calculation methods,the values calculated by F2 and F3methods were small,and the values of baseflow index segmented by other methods were consistent.The average values of Huangfuchuan River Basin and Yanhe River Basin were0.08-0.16 and 0.27-0.45,respectively.The Beiluo River Basin was between 0.27 and 0.48.There were many inflection points in the baseflow process line of HYSEP method.The curve was serrated and highly synchronized with the river runoff curve,and the peak values were highly consistent.The hysteresis effect of runoff yield can not be reflected,and the baseflow process line obtained by digital filtering method was relatively smooth and not coincident with the runoff process line.Moreover,it can reflect the rising and retreating process of runoff,and the calculation result of baseflow segmentation was more reasonable.Among the F1 method and F4 method of digital filtering method,the variance and extreme value ratio of the three basins calculated by F4 method were the smallest.However,the baseflow process line of F1 method was more reasonable than that of F4 method in Beiluo River Basin.(2)The annual average baseflow in Yanhe River Basin was the largest,followed by Beiluo River,and Huangfuchuan was the smallest.The baseflow index of Yanhe River and Beiluo River showed a significant upward trend,and the abrupt change points were 1996 and2002,respectively.However,the baseflow index of Huangfuchuan showed a significant downward trend and the sudden change point was in 1978.The annual average baseflow of Huangfuchuan River Basin was 16.38 million m~3,and the annual baseflow showed a very significant downward trend.The annual average baseflow of Yanhe River Basin was 50.61million m~3,and that of the upper reaches of Beiluo River Basin was 25.77 million m~3.(3)High-frequency and low-frequency events of baseflow in Huangfuchuan River Basin from January to December showed a significant downward trend.Monthly baseflow decreased from July to November(wet season)in Yanhe River Basin,and increased significantly from December to February of the following year(dry season).Monthly baseflow decreased from July to October(wet season)in Beiluo River Basin,and increased significantly from November to February of the following year(dry season).(4)The relationship between rainfall and baseflow in Huangfuchuan and Yanhe river basins changed,and human activity was the main reason for the decrease of baseflow,while the relationship between rainfall and baseflow in the upper reaches of Beiluo River did not change.The rainfall in Huangfuchuan Basin and the upper reaches of Beiluo River Basin did not increase significantly,while that in Yanhe River Basin did not decrease significantly.There was no sudden change in precipitation series in Huangfuchuan,Yanhe River Basin and Beiluo River Basin from 1953 to 2015.The curve of the relationship between accumulated annual rainfall and accumulated annual baseflow in Huangfuchuan Basin appeared inflexion point in 1986.The contribution rates of rainfall and human activities to the decrease of baseflow were 5.88%and 94.12%respectively.The curve of cumulative annual rainfall and accumulated annual baseflow of Yanhe River Basin showed inflexion point in 2000.The contribution rates of rainfall and human activities to the decrease of baseflow were-94.09%and 194.09%respectively.There was no obvious inflection point in the double accumulation curve of precipitation baseflow in the upper reaches of Beiluo River Basin,which indicated that the relationship between rainfall and baseflow has not changed.