3.3.2. Single-Factor Analysis

Elevation, rainfall, slope, fractional vegetation cover (FVC), karst rocky desertification (KRD), and land use cover/change (LUCC) were analyzed by the geographic detector. As shown in Figure 8, the overall contribution of the six individual factors to soil erosion results during the study period was slope > LUCC > KRD > FVC > rainfall > elevation, but there were minor differences between years. The *q* value of KRD gradually decreased but also contributed to the spatial diversity of soil erosion only less than slope and LUCC. In 2000, the *q* value for KRD was 0.15, which was only lower than the slope. From 2005 onwards, the contribution of KRD to soil erosion results was lower than that of LUCC. LUCC was the environmental factor that had the greatest influence on soil erosion results apart from the slope, with a relatively stable *q* value. From 2000 to 2005, the *q* value for FVC decreased from 0.1 to 0.03. Moreover, from 2005 to 2020, the *q* value for FVC increased from 0.03 to 0.05. The contribution of rainfall to soil erosion results in the study area was low, but in individual years, it had an important influence. For example, the *q* value for rainfall in 2020 was as high as 0.08, contributing more to the soil erosion results than LUCC, KRD, and FVC. The effect of elevation alone on soil erosion results was not significant.

**Figure 8.** Single-factor trends.

**Figure 7.** Factor classification and mean soil erosion values.

#### 3.3.3. Factor Interaction Analysis

The interaction results showed a decreasing trend in the *q* values of the interaction of KRD with the other four factors, except the slope factor. Their contribution to the spatial divergence of soil erosion gradually decreased. The interaction of rainfall with slope, elevation, and LUCC, which had a very low *q* value, increased the *q* value significantly and had a weaker and stronger effect on the spatial variation of soil erosion. The interactions of both LUCC-slope and vegetation-slope also gradually increased. In 2000 and 2005, the interactions of KRD-slope, elevation-slope, and rainfall-slope were the main influencing factors on the spatial variation of soil erosion. However, from 2010 to 2020, the interaction of slope-KRD was not significant and was replaced by slope-LUCC. The factor combinations with the highest *q* values in each period and the most significant increase in *q* values compared to the sum of the *q* values of the single factors were selected and are shown in Figure 9. X1 was the sum of the *q* values of the two factors and X2 was the *q* value after the factor interaction. The dominant factor influencing soil erosion varied between years. The years 2000 and 2005 were KRD-slope, explaining 60% and 49% of the spatial distribution of soil erosion, respectively. The year 2010 was for LUCC-slope, with a *q* value of 0.64. Meanwhile, the years 2015 and 2020 were for elevation-slope, and the *q* values after the interaction were higher than the sum of the single factors. In comparison with the sum of the *q* values of the individual factors, the *q* value of the interaction between elevation and rainfall increased most significantly throughout the whole period.

**Figure 9.** Factor interaction variation. X1 is the sum of the two-factor *q* values. X2 is the *q* value of the interaction of the two-factor.

#### **4. Discussion**

Soil erosion processes and driving mechanisms in karst areas are not yet understood due to the special karst structure and complex erosion patterns. Most of the existing modeling studies do not take into account the grade of rocky desertification, leading to large errors in results. In addition, the current soil erosion results based on administrative divisions and raster networks can hardly meet the requirements of refined soil erosion control. In this study, soil erosion in karst areas was estimated using an optimized RUSLE model with the karst rocky desertification factor. On this basis, the spatial and temporal dynamics of soil erosion in the study area in the last two decades were studied on the basis of slope units, while the soil erosion driving factors were quantitatively identified with the geographic detector.

#### *4.1. Spatial and Temporal Dynamics of Karst Soil Erosion*

Understanding the dynamic evolution of soil erosion is not only the basis and prerequisite for the prevention and control of soil erosion but also has great significance in the conservation of soil resources and ecological restoration. In this study, the erosion

area of the study area was mainly no erosion or light erosion, while medium erosion and strong erosion were less. It differs from Guizhou as a whole, which is predominantly light to medium. This difference may be influenced by the level of rocky desertification development at different study scales. The limited erodible soil sources in the mediumintensity rocky desertification area constrain the development of soil erosion. There is a need for small-scale studies in karst areas with highly heterogeneous geography, and the results of these studies are important in tailoring soil and water conservation efforts to local conditions. The no erosion area decreased during the study period, with the highest proportion of light to medium erosion. The reduction of no erosion area was the ecological restoration effect of the national implementation of the comprehensive rocky desertification management project [49]. The highest percentage of light to medium erosion shows that controlling light to medium erosion is the key to effectively managing soil erosion in the region. The high proportion of light erosion is mainly due to the large proportion of light erosion areas, and it is recommended to reduce unreasonable human actions to allow the natural ecosystem to repair itself. The small area of moderate erosion or high erosion required soil and water conservation measures to maintain the stability of the soil in the area.

Soil erosion fluctuated and increased in the last two decades, unlike the findings of some karst areas where soil erosion had been decreasing. This difference might reflect the unreasonable human activities in the early part of the research area and the good management effect of the later management project. Soil erosion tended to increase, but the natural ecosystem developed benignly. The area was ecologically fragile and had low agricultural productivity. In the early years, people plundered land resources, causing massive soil loss, and eventually there was not even soil left to erode. In the later period, with the rocky desertification control project, soil resources gradually recovered. The highest soil erosion in 2005 was related to the high number of heavy rainfall events in that year. Extreme precipitation is often considered to be an important factor influencing erosion processes. Soil erosion under extreme precipitation conditions may account for the majority of annual soil erosion [50]. This is also confirmed on the basis of the proportion of sediment produced during the 3 and 10 largest erosion events [51].

The conversion in soil erosion classes over the whole period occurred mainly between no erosion, slight erosion, and light erosion. The conversion from no erosion to slight erosion and light erosion was related to the decrease in the area of rock desertification and the increase in eroded soil sources as described above. The reciprocal transfer between slight and light erosion may have been influenced by long-term rocky desertification management projects. The high slope unit is a susceptible area for soil erosion, and the local government should coordinate with multiple departments to implement the following to restore agricultural land to forest, alleviate the ecological carrying capacity, and reduce soil erosion. The erosion cold spot areas occurred in the low slope-low elevation unit, mainly because the soil resources were lost due to unreasonable human activities in the early years, and it was difficult to restore the soil resources in the short term. The progression of the erosion cold spot confidence level from 95% to 90% is evidence of the effectiveness of ecological restoration over time. In the case of meeting human needs, it is appropriate to convert the low-slope units into terraces; strengthen agroforestry cultivation [52], which can increase the harvested area of arable land [53]; and meet living needs while enhancing water and soil conservation measures.
