4.1 Trend
At all gauges combined by altitude groups, median values of SSLtrends increase with the height (Figure 6 ). This is a piece of
evidence that higher gauging stations are less exposed to a considerable
reduction in suspended sediment load. On the other hand, there is a lack
of suspended sediment measurements in the 2000s and 2010s in this
database. Our last findings (Tsyplenkov et al. , 2021) from the
Djankuat river station near Elbrus mountain (ca. 2700 m a.m.s.l.)
suggested a decline in 20-37% in suspended sediment load during the
2015-2019 period. Therefore, a tendency of declining absolute trend
values with the altitude should be discussed with caution.
We found that the mean annual SSD change pattern has considerably
altered after 1988-1994 depending on the gauging station from the
performed CUSUM analysis. Therefore, although it is hard to determine an
exact transition year for the whole Caucasus region, we can assume with
the high confidence level (cf. Table 3 ) that it was in the
1988-1994 period. This finding is in line with other studies on the
water runoff for the Caucasus region (Rets et al. , 2018, 2020),
whereas it was suggested 1986 as a transition year.
To a significant extent, calculated SSL trends correspond to
water discharge trends (Rets et al. , 2020) and suspended sediment
load changes (Gusarov et al. , 2021) calculated previously for the
Greater Caucasus (Table 6 ). Therefore, we performed a
correlation analysis using a nonparametric Spearman correlation
coefficient to compare our findings with previous studies. As a result,
we observed an insignificant negative correlation between the trendSSL values from our research and a change in suspended sediment
load from Gusarov et al. (2021) (Spearman r = −0.8, p = 0.1).
Also, a significant negative correlation between change in mean July’s
water discharge and SSL trends was observed (Spearman r = −0.7, p
= 0.06). The correlation with June’s (Spearman r = −0.45, p = 0.26) and
August’s (Spearman r = −0.42, p = 0.3) mean monthly discharges were
lower. Since the significance level is weak, this result should be
interpreted with caution. Nevertheless, it is likely that monthly runoff
in July to some extent more reflects the climatic sensitivity of
catchments in terms of sediment export than monthly runoff in June and
August. However, a comparison of peak water discharge trends with SSL
suggests that not always a drop in SSL trend results from water
discharge reduction. For example, the observed positive trend in peak
annual water discharge at Kam-Ol (+0.27% per year) corresponds to a
decline in annual SSL at −1% per year.
### FIGURE 6 ###
### TABLE 6 ###
### FIGURE 7 ###
### FIGURE 8 ###
We did observe a negative correlation between the SSL trend
values and the catchment area (Spearman r = −0.25, p = 0.17). However,
further exploration of this scale dependency indicated significant
differences, depending on the altitude zone. Hence, when only the low to
a middle mountain (500-1000 m) catchments were considered, a highly
significant positive correlation between SSL trend and catchment
area was observed (Spearman r = 0.86, p < 0.0001). The
catchment area explains 71% of the variation in SSL trend values
of low to middle mountain (500-1000 m) catchments (Fig. 7a ).
Since the observed total correlation is relatively weak and cannot be
straightforwardly explained by a transparent mechanism, this result
should be interpreted cautiously.
Nevertheless, likely, suspended sediment load trends are indeed
scale-dependent in Northern Caucasus. The gauging station altitude most
likely explains this scale-dependency since smaller catchments tend to
locate higher in the mountains and be more glacierized and less
cultivated (Fig. 7b-c ).
Similar results were obtained for 1000 generated sets of SSLtrend values (Section 2.6), confirming that these correlations are not
attributable to calculating or measuring errors (Fig. 8 ). We
found that SSL trend values of a low mountain (< 500 m)
gauging stations are significantly positively correlated to altitude and
glacier fraction and significantly negatively to cropland area. However,
in another altitude zone (500-1000 m), the catchment area became the
most valuable positive factor, while the correlation with altitude and
glacier should be considered insignificant. A correlation with
controlling factors in high mountain catchments was insignificant.