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.
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Figure 4
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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.