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 ###