Analysis Of The Baseflow Variation Characteristics And Its Impacting Factors In Chaohe Watershed

Due to the strong influence of human activities and climate change,the formation and composition of watershed streamflow are undergoing significant changes,and a large number of watershed change attribution analyses have been conducted at home and abroad.As an important component of streamflow in a watershed,there are few studies on the characteristics of baseflow changes and attribution analysis.Therefore,studying variation characteristics and driving factors of the baseflow in Chaohe watershed is of great significance to the maintenance of healthy river ecosystems.In the meantime,it also provides a scientific basis for assessing the variations in water resources,and formulating rational scientific water resources management and watershed management plans Therefore,well-established baseflow separation methods of Chapman-Maxwell filtering method and SWAT model were employed to separate the basesflow from daily streamflow data for Chaohe watershed located in the upstream of the Miyun Reservoir from 1963 to 2015.In addition,Mann-Kendall test and double mass curve approaches were applied to track the change trend and change points of annual baseflow and to quantify the contributing percentage by clinate change and human activities during the studied period.The results indicated(1)By comparing the baseflow in study area from 1963 to 2015 separated by the Chapman-Maxwell filtering method and the SWAT model,it was found that the two methods were relatively close.Compared with the results of the Chapman-Maxwell filter,the baseflow simulated by SWAT model was on the high side,and the simulated value of the model was higher in the high baseflow months and lower in the low baseflow months.(2)The main characteristics about baseflow variation in Chaohe watershed from 1963 to 2015showed that:the baseflow was mainly distributed from June to September during the year,with the baseflow index(BFI)the smallest in July while the largest in May.The baseflow in the Chaohe watershed significantly declined in the whole year,both wet and dry seasons.Mainly affected by the wet season,the BFI significantly increased during the whole year and the wet season.The interannual distribution of baseflow in the study area was not uniform,which varied greatly in different years,but the interannual variation of the summer baseflow was the most significant.(3)From 1963 to 2015,the results of Mann-Kendall test about precipitation,potential evapotranspiration(PET),and baseflow in the Chaohe watershed indicated that the precipitation(p=0.02)and baseflow(P=0.001)during wet season in the Chaohe watershed significantly declined with the change points in 1995 and 1997,respectively.PET significantly increased(p=0.001)without change points during the wet season.In dry season,the precipitation did not significantly increase(p=0.61),while the PET and baseflow significantly declined(p=0.005)with the change points in 2013 and 1999.The annual precipitation did not significantly decline(p=0.24)with change point in 2008,and the baseflow significantly declined(p<0.001)with change point in 1999,while the PET significantly increased(p=0.02)with change point in 2007.(4)As the double mass curve from 1963 to 2015 revealed,the main reason that led to the change point of baseflow in 1999 was human activities,with a contribution rate of 81.1%.Such activities included land use/land cover changes,water conservancy structure construction,and water withdrawal in the watershed.From the change of land use,the land conversion programs(Grain for Green)in the Chaohe watershed caused significant increase in forestland and decrease in grassland and cropland,leading to more evapotranspiration and the reduction of baseflow.