3.2.1 Correlation of the raw series
Double cumulative curves were plotted to analyze the long-term variation and years with structural breaks for the relationship between the raw annual runoff series (X) and the raw annual sediment discharge series (Y) observed at Tangnaihai station, as illustrated in Figure 3.
—————————————place Figure 4 here——————————————-
Figure 4 shows that the runoff and sediment discharge measured at Tangnaihai station were significantly positively correlated. All the data points were located close to the trendline. The annual cumulative runoff and the annual cumulative sediment discharge formed a straight line in the Cartesian coordinate system. The coefficient of determination R2 equaled 0.9958, indicating that the runoff and sediment discharge in the source region were highly correlated. The overall fitted curve had a slope of 6.53, suggesting that 6.53 × 103 t of sediments would be generated for every 1 × 108 m3 of runoff. The curve during 1989–2004 shifted slightly upward, indicating more sediment was created per unit runoff. This was because the Longyangxia Reservoir started to operate in 1987, leading to river closure for some channels. This has gradually affected the water and sediment transport processes upstream, i.e., at Tangnaihai station. The slope variation suggested that the influence on the sediment discharge was greater than that of runoff. The curve during 2005–2013 shifted slightly downward, indicating less sediment was discharged per unit runoff. During this period, there was less runoff and upstream vegetation was affected by water and sediment retaining projects in the region. Hence, consistent variation in the runoff and sediment discharge was changed, and thus, their relationship was also altered.