Variation trends of runoffs seasonal distribution of the six larger basins in China over the past 50 years

Based on the observed monthly streamflow of 19 key hydrological control stations in the six larger basins in China since 1950,the uneven coefficient of seasonal distribution,concentration rate,concentration period and variation range are analyzed,and the Mann-Kendall test method is employed to detect trends of inner-annual runoff.Results indicate that since 1980s,the observed monthly streamflow in the six larger basins has generally changed,except of Shixiali station,Xiangshuipu station in Hai river basin and Huayuankou station in Yellow River basin.For other stations,the uneven coefficient of inner-annual distribution and relative concentration rate are larger,particularly in Songliao river basin.Moreover,the monthly runoff in winter and spring (from December to March) in southern basin increases and decreases in May and increases in July while in winter and spring (from January to May) in northern basin it decreases evidently in Hai river basin,Tieling station in Liao river basin,Lijin station in Yellow River basin and Wujiadu station in Huai river basin.