3.3 Cumulative sum charts
Further analysis of suspended sediment discharge was carried out for
eight gauging stations (Ard-Ta, Bel-Ko, Che-Ba, Cheg-Nc, Kam-Ol, Mal-Ka,
Sun-Br, Uru-Kh, see Fig.1 for location) with the most extended series of
observation and not altered by HPP. Table 3 summarizes the
results for each analyzed gauging station. The CUSUM charts are reported
in Fig. 4 ; the change points are indicated with grey (Taylor,
2000) and red (Pettitt, 1979) lines.
The CUSUM chart analysis demonstrates similarities between the seven
river basins except for Uru-Kh. Indeed, all series showed a change point
approximately within the same range of years, increasing trends from the
1950s and decreasing trends from 1988-1994.
It can be noticed that mean annual SSD values tend to decline at
the beginning of the observation period at Ard-Ta, Cheg-Nc, Sun-Br, and
Bel-Ko (from 1945 to 1950s). For example, the analysis highlighted that
the SSD at Sun-Br underwent a significant change in 1958
(confidence interval 1957–1977) at a confidence level of 82%. After
that, an increasing trend occurred at Ard-Ta, Cheg-Nc, Sun-Br, and
Bel-Ko stations. A similar trend fluctuation was observed at the Che-Ba
and Mal-Ka stations, where the average annual SSD has been subject to a
moderate increase up to the ca. 1990.
According to Taylor’s (2000) change point method, a decreasing trend in
annual average SSD occurred during time window 1988-1994 at
almost every gauging station with a 99.9-100% confidence level (see CI
intervals in Table 3 ). In addition, at several stations (i.e.,
Ard-Ta, Cheg-Nc, Sun-Br, Mal-Ka), the change point location in the 1990s
was proven with both methods.
A slightly different pattern is observed on the Kam-Ol and Uru-Kh
stations. From the 1960s to the 1980s, the average annual SSD has
been subject to a sharp decrease followed by an increase up to 1994
(Kam-Ol) and 2005 (Uru-Kh). Thus, these trends resulted in a change
point at a 99.9% confidence level for the Kam-Ol series. However,
Uru-Kh’s change point in 2005 was proven neither by Taylor (2000) nor
Pettitt (1979) techniques. After 1990 (or 2005), a moderate decrease
occurred up to 2018.
### FIIGURE 4 ###
### TABLE 3 ###