Abstract:The dynamic change of rainfall erosivity and river sediment discharge as well as the correlation between river sediment discharge and rainfall erosivity were analyzed based on daily rainfall data and yearly sediment discharge of 12 rainfall stations located in He-Long reach of the Yellow River from 1957 to 2011. Meanwhile, the impact and contribution of rainfall erosivity changes and human activities on the river sediment discharge changes were quantitatively evaluated. The main methods used were the moving average, linear trend estimation, Mann-Kendall nonparametric test, cumulative departure curve and double mass curve. The results showed that rainfall erosivity in He-Long reach from 1957 to 2011 shifted from 378.1 MJ ·mm/(hm 2 ·h ·a) to 2 324.6 MJ ·mm/(hm 2 ·h ·a) with a mean of 1 319.7 MJ ·mm/(hm 2 ·h ·a), and rainfall erosivity did not exhibit significant downtrend. The decrement of rainfall erosivity per year in 1957—2011 was 9.7 MJ ·mm/(hm 2 ·h ·a). The changing trend of rainfall erosivity was divided into three stages during past 55 years in He-Long reach, which consisted of a rapid declining trend from 1957 to 1974, a slow decreasing trend from 1975 to 1999, and a slow increasing trend from 2000 to 2011. The declining rates of the first and second stages were 83.7% and 66.6%, respectively, and the increasing rate of the third stage was 42.7%. With rainfall erosivity in 1957—1969 as a reference, rainfall erosivities in the 1970s, 1980s, 1990s and the first 12 years of the 21st century were decreased by 15.9%, 19.5%, 27.5% and 22.7%, respectively. The river sediment discharge in He-Long reach from 1957 to 2011 shifted from 9×10 6 t to 2.14×10 9 t with a mean of 5.60×10 8 t, and river sediment discharge showed highly significant downtrend. The decrement of river sediment discharge in 1957—2011 was 1.9×10 7 t/a. With river sediment discharge in 1957—1969 as a reference, river sediment discharges in the 1970s, 1980s, 1990s and the first 12 years of the 21st century were decreased by 27.3%, 64.1%, 54.8% and 88.7%, respectively. The Mann-Kendall nonparametric test and the cumulative departure curve showed that the abrupt change of river sediment discharge appeared in 1979. There was a better linear correlation between river sediment discharge and rainfall erosivity. According to the double mass curve equation, the contributions of rainfall erosivity to the river sediment discharge changes in the 1980s, 1990s and the first 12 years of the 21st century were 22.6%, 44.3% and 19.0%, respectively; while the contributions of human activities to the river sediment discharge changes were 77.4%, 55.7% and 81.0%, respectively. Therefore, the impact of human activities on the river sediment discharge changes was dominated. Although rainfall erosivity in two periods of 1980—1989 and 2000—2011 was similar, river sediment discharge in 2000—2011 was decreased by 67.6%, compared with that in 1980—1989. Thus, the river sediment discharge changes were mainly caused by human activities and the decrement of river sediment discharge per year caused by human activities was 2.5×10 8 t in 2000—2011.
