*3.1. Spatial and Temporal Dynamics of Soil Erosion*

The research region contains both karstic and non-karstic areas. Therefore, the rationalized RUSLE model was used to estimate soil erosion in karst areas, and the conventional RUSLE model was used for soil erosion in non-karst areas. Soil erosions in the study area were 6.11, 9.35, 7.49, and 8.88 t·ha−1·a−<sup>1</sup> in 2000, 2005, 2010, 2015, and 2020, respectively. This result was close to the average soil erosion in the Beipanjiang basin for the last 20 years published in the Guizhou Water Resources Bulletin (http://mwr.guizhou.gov.cn/ (accessed on 28 October 2020)) of 5.29 t·ha−1·a<sup>−</sup>1.

On the basis of the Standards for Classification and Gradation of Soil Erosion (SL190- 2007) issued by the Ministry of Water Resources of the People's Republic of China, soil erosion in the study area was classified into seven classes: no erosion (construction land, water, and exposed bedrock), slight, light, medium, strong, very strong, and severe (Figure 3). The no erosion zone was the most widespread, accounting for approximately 50% of the total area, and was mainly located in the central, western, and north-western parts of the study area. Starting in 2010, there was a significant decrease in no erosion in the west and north-west, with no erosion mainly in the central region. This was followed by slight erosion and light erosion, accounting for about 40% of the total area, which was more evenly distributed in the study area. Medium erosion and strong erosion were less frequent and were mainly distributed along the southeastern edge, northwestern edge, and northern edge of the study area. Very strong and severe erosion accounted for less than 1% of the study area, with almost no extreme or severe erosion occurring in the research region.

**Figure 3.** Spatial and temporal characteristics of soil erosion in 2000–2020.

As shown in Table 3, regional soil erosion fluctuated and increased during the study period, with the highest percentage of light and medium soil erosion overall, and the most extensive area with no erosion and light erosion. The ratio of different intensities of erosion showed an increase in medium, strong, and very strong erosion, with the greatest increase in intensity and extreme intensity in 2005, with 23.39% and 8.77% increases, respectively, compared to 2000. The amount of light erosion decreased, especially by 31.2% in 2005, compared to 2000. The area of no erosion showed the greatest variation in the different intensities of erosion, with a general trend of increasing and then decreasing; the area of no erosion increased by 7.26% from 2000 to 2005, while it decreased by 21.28% from 2005 to 2020. Unlike the erosion-free area, the erosion area ratio generally tended to increase at all other intensities over the study period. In addition, we found that in 2005, the area without erosion was the most extensive, while the total soil erosion was the highest, with medium and strong erosion accounting for more than 60% of the total regional erosion, and the area with strong erosion increased by 3.72% compared to 2000.



#### *3.2. Regional Differentiation Based on Different Slope Units*

The study area was divided into 2491 slope units on the basis of hydrological processes (Figure 4). The minimum cell size was 1.17 × <sup>10</sup><sup>2</sup> <sup>m</sup>2, while the maximum cell size was 9.72 × <sup>10</sup><sup>4</sup> <sup>m</sup><sup>2</sup> with an average cell size of 2.07 × 104 <sup>m</sup>2.

**Figure 4.** Division of slope units.

3.2.1. Soil Erosion Class Transfer Based on Slope Units

The mean soil erosion values in terms of slope units are available in six classes: no erosion, slight, mild, medium, strong, and very strong. To further understand the quantitative changes in soil erosion in the region, we produced soil erosion grade transfer maps for four time periods: 2000–2005, 2005–2010, 2010–2015, and 2015–2020 (Figure 5). The result shows that the transfer in soil erosion levels during the study period occurred mainly between no erosion, slight erosion, and light erosion, with medium, strong, and very strong erosion remaining relatively stable. We found that 17.5% and 9.8% of no erosion changed to slight and light erosion, respectively, during 2000–2005. Slight erosion converted mainly to no erosion and light erosion by 18.66% and 31.9%, respectively. From 2005 to 2010, 37.87% and 24.56% of no erosion transformed to minor and minor erosion, respectively. Slight and medium erosion moved mainly to light erosion, with 27.81% and 71.65% transfers, respectively. Soil erosion transferred in a similar direction for both the 2010–2015 and 2015–2020 periods. The soil erosion classes were relatively stable, except for some of the slight erosion transferring to light erosion, with soil erosion transition occurring mainly between the same soil erosion grade.

**Figure 5.** Soil erosion class transfer.

3.2.2. Hotspot Analysis Based on Different Types of Slope Units

We used a slope of 25◦ and an elevation of 896 m as the distinction. If the slope was less than or equal to 25◦, then it was defined as low slope, and otherwise as high slope. An elevation less than or equal to 896 m was defined as low elevation, otherwise as high elevation. The slope units were divided into four unit types: low slope–low elevation, low slope–high elevation, high slope–low elevation, and high slope–high elevation. The

four unit types were overlaid with the results of the soil erosion hotspot analysis as shown in Figure 6. The result shows that the hotspots of erosion remained relatively stable and the erosion coldspots decreased significantly during the study period. In general, erosion hotspots were mainly located in the high slope–low elevation and high slope–high elevation units along the northern, north-western, and south-eastern edges of the study area, with a few erosion hotspots also located in the high slope–high elevation units in the south. Coldspot areas of erosion were mainly found in the central and western low slope–low elevation units. From 2000 to 2020, the erosion coldspots decreased from the central and western low slope–low elevation units to the central low slope–low elevation units, and the erosion cold point confidence level decreased from 95% to 90%.

**Figure 6.** Hotspot analysis under different slope units.

#### *3.3. Quantitative Attribution of Soil Erosion Variability*

The mean values of soil erosion and the dominant values of the environmental factors within each cell were assigned to the corresponding cell. The contribution of each environmental factor to soil erosion (*q*) was quantified with the help of the geographical detector, and the results showed significant (*p* < 0.05) confidence in the *q* values for all factors.

#### 3.3.1. Soil Erosion Risk Analysis

The risk detector of the geographical detector can detect potential relationships between factor variation and soil erosion risk by analyzing the mean soil erosion values for each interval within the factor. As shown in Figure 7, the differences in soil erosion risk under each factor sub-interval were significant. There was no significant pattern in the mean soil erosion values within the rainfall intervals, and the differences in soil erosion risk were not significant. Soil erosion risk increased with increasing vegetation cover, but the trend of increasing soil erosion risk decreased after the vegetation cover exceeded 80%, and the maximum soil erosion value did not exceed 15 t·ha−1·a<sup>−</sup>1. The risk of soil erosion increased and then decreased with increasing altitude. When the elevation was below 1170 m a.s.l., the risk of soil erosion increased as elevation increased. When the elevation was higher than 1170 m a.s.l., the risk of soil erosion decreased as the elevation increased. Soil erosion risk was graded between different land use zones, with soil erosion risk ranked as grassland > wood land > utilized land > water > arable land > construction land. The risk of soil erosion gradually changed from high to low from no rocky desertification to heavy rocky desertification. The average soil erosion was 11.76 t·ha−1·a−<sup>1</sup> in areas without rocky desertification and 6.46 t·ha−1·a−<sup>1</sup> in areas with heavy rocky desertification. The soil

erosion risk increased with slope, and the growth of increase in soil erosion risk increased when the slope increased, with a maximum value of 38.57 t·ha−1·a<sup>−</sup>1.
