# Temporal Variations of Sediment Provenance in a Karst Watershed, China
## 4 Discussion  
Ref. [33] studied the spatial correlations between lithology and rocky desertification in the karst area of Guizhou, and found that the degree of rocky desertification was obviously correlated with lithology, but did not consider the influence of human factors. The incidence of rocky desertification is highest for continuous carbonate rock, and the degree of desertification is serious [34]. With an increase in argillaceous content of carbonate rock, the percentage of acid-insoluble content in rock increases, the rate of soil formation speeds up, the occurrence rate of rocky desertification declines, and the degree of rocky desertification is gradually lessened. Continuous limestone bodies are the most prone to rocky desertification, followed by dolomite, and the sand-shale and mudstone units [35]. Relevant studies have been carried out on the temporal and spatial variation characteristics of rocky desertification in Guangxi [36; 37]. The largest area of rocky desertification occurred in Hechi, Guangxi Province in 2002, the second in 1988, and the smallest in 2015. In the past 32 years, the area of rocky desertification in Guangxi exhibited an increasing trend, followed by a decreasing trend [38]. This study shows that the contribution of background lithologies (limestone, dolomite, sand-shale and mudstone) to the entire catchment area was the highest in CGB1 (2008-2012), followed by EGMON (1992-1996) and CGB2 (2015-2019), which may closely related to the degree of rocky desertification in the region during these periods. Therefore, the results of this study could also be interpreted as indicating that, after years of governance in China, the degree of rocky desertification in karst areas has been greatly reduced. In the last ten years, the contribution of background geology to the sediments in the catchment area has decreased. Generally, limestone is very easily weathered compared to other rock types [39; 40]. More importantly, as shown in Figure 1, most limestone in this region is located in mountainous areas with strong hydrodynamic conditions, where a large amount of sediment will  
Figure 5: Contributions of each source to the catchment sediment samples collected during the three baseline project periods. be carried downstream by water flow. Limestone therefore makes the largest contribution to the catchment sediments. In addition, 45% of the total study area is comprised of limestone (Figure 1), which certainly leads to a greater contribution. The second largest contributor is dolomite, whose weathering capacity is lower than that of limestone but higher than that of sand-shale and mudstone. Although dolomite accounts for only 12% of the total study area, it occupies the steepest areas with the strongest hydrodynamic conditions; thus, its contribution is relatively high. Although sand-shale and mudstone occupy about 45% of the study area, they are more resistant to weathering. Moreover, as can be seen from the topographic map (Figure 1), these lithologies are located in a region with relatively flat terrain and poor hydrodynamics, so they contribute less to the catchment sediments than other rock types in the catchment. In addition to the background geology and geomorphology already discussed, land-use/vegetation change also plays an important role in the source changes of catchment sediments in karst areas. [41] studied the spatial and temporal variation of vegetation in the Guangxi Xijiang River catchment from 2007 to 2016, using MODIS EVI data, and found that the elevated vegetation index (EVI) generally declined during 2007-2011. One reason for this trend may be that during this time period, many severe natural disasters caused serious destruction of vegetation and crops, resulting in lower vegetation coverage. The EVI increased during 2011-2016, mostly due to vegetation protection measures such as afforestation and rocky desertification control projects in Guangxi (as shown in Figure 6).