Analysis of runoff variations and impact factors in Xilinhe River Basin in recent 50 years

Based on runoff and meteorology observation data of Xilinhe River,the characteristics and variations of runoff and climate in the basin were examined by M-K trend analysis and double mass curve method.The results showed that the runoff in Xilinhe River Basin in recent 50 years slightly reduced( P 0.05),while runoff significantly decreased( P 0.05) from 1958 to 1990.After 1990 s,annual runoff was significantly higher than that in the early period.At the beginning of 21 th century,runoff significantly declined( P 0.05).Seasonal variations of runoff presented the bimodal runoff process which was specific in the mid-latitude semi-arid areas and formed by snowmelt and precipitation,respectively.There was an evidence suggested that Xilinhe River Basin was turning to dry and warm during these 50 years with reducing precipitation and increasing temperature.In most seasons,evaporation increased and precipitation decreased especially from July to September during recent five decades.These observations suggested that dry and warm climate was one of the important factors that changed the timing and quantity of the runoff.The change of precipitation in flood season had a leading role and the ice and snow melting in warming winter had a supporting role to the change of river basin runoff.Runoff coefficient declined since the 1960 s and reached to the lowest during the 1980 s,which suggested that the less runoff was produced by the same rainfall.The runoff coefficient increased in the 1990 s and decreased again at the beginning of 21 th century.The double cumulative curve showed that compared with the 1960 s,the accumulated value of runoff had significant deviation since the 1970 s,especially after the 1980 s which indicated that the runoff in the basin was not only impacted by precipitation,but also affected by human activities.The dry and warm climate and increasing human activity were the main reasons of the water cycle change in the Xilinhe River Basin.