3.4 Double Mass Curves
According to CUSUM analysis, there is evidence that mean annualSSD values significantly decline after 1988-1994 depending on the
gauging station. Transition years for every gauging station identified
by Taylor’s method are summarized in Table 4 . To further
quantify the sediment discharge changes before and after the transition
years, double mass curves, along with the linear regression lines, were
plotted in Figure 5 . There are precise breakpoints between the
two regression lines, suggesting that the selected transition years are
correct and meaningful. The regression lines’ slopes were 1.5-4.5 times
lower after the breakpoints or transition years (i.e., at higher
cumulative precipitation values) than before (see equations inFigure 5 ).
To estimate the relative reduction of total sediment discharge for the
period after the transition years, linear models describing the
cumulative SSD before the transition years were used to further
extrapolate the cumulative sediment up until 2018 (dashed line inFigure 5 ). Compared with the extrapolated cumulative sediment
discharge (SSDC ), observed cumulative sediment
discharge (SSDO ) reduced by 11-43% in various
basins (Table 4 ).
### TABLE 4 ###
### FIGURE 5 ###
We further calculated suspended sediment discharge for the period after
the transition years using the regression equations established from the
double mass curve of precipitation-sediment before the transition years
(cf. Figure 5 ). The difference between observed values\(\overset{\overline{}}{\text{SS}D_{O}}\) before the transition year and
estimated \(\overset{\overline{}}{\text{SS}D_{C}}\) after the transition
year is due to precipitation change. However, the difference between the
estimated mean values \(\overset{\overline{}}{\text{SS}D_{C}}\) and
observed values \(\overset{\overline{}}{\text{SS}D_{O}}\) in the same
period is the result of other factors (e.g., human activities, glacier
shrinkage, sediment source depletion, etc.). The results are shown inTable 5 .
The impact of additional factors was dominant in all cases for the
sediment discharge reduction. Their average contribution rate is 87.8%
which is more robust than the average precipitation rate (12.2 %).
Indeed, the impact of precipitation varies from basin to basin and can
explain up to 21% of sediment discharge reduction (e.g., Fia-Ta).
Contrariwise, the Kam-Ol basin precipitation events almost did not alter
the sediment discharge (5 %). Therefore, there is evidence that
precipitation played a minor role in the suspended sediment discharge
reduction in the upper Terek basin during the last decades.
### TABLE 5 ###