## 3 Results  
### Spatio-Temporal Variation of Soil Erosion from 2000 to 2020  
The spatial distribution of annual rainfall erosivity was investigated, and the spatial distribution of annual rainfall erosivity factors in 2000, 2005, 2010, 2015, and 2020 was obtained by grid calculation (Figure 3). The results showed that the overall spatial distribution of rainfall erosivity in the study area increased from northwest to southeast. In 2000, the high rainfall erosivity was mainly distributed in the eastern and southern regions, but in 2020, the degree of erosivity decreased significantly, and most of the rainfall erosivity was distributed in the southeastern corner of the study area. The area of low rainfall erosivity expanded from the northwest corner of the study area to the central area. Globally, the mean rainfall erosivity was estimated to be 2190 MJ mm ha\({}^{-1}\) h\({}^{-1}\) yr\({}^{-1}\)[47], with the highest values in South America and the Caribbean countries, Central east Africa, and southeast Asia. China has a mean value of 1600 MJ mm ha\({}^{-1}\) h\({}^{-1}\) yr\({}^{-1}\) but exhibits high variability with zero erosivity in the arid northwest areas (Taklimakan desert) and extreme erosivity (>15,000) in the southeastern coastal zones. Regional studies conducted by [PERSON] and [PERSON] [48] and [PERSON] et al. [49] show very similar spatial patterns compared to our rainfall erosivity distribution in China. According to the geographical distribution of soil erosion (Figure 4), there are significant differences between the research area's north and south. The northern portion of IV11-1, IV11-3, and IV11-5 is where the majority of the regions with the higher soil erosion intensity level are located, where the landscape is more undulating, and the vegetation cover is sparse. In the southern regions IV12-1 and IV12-2, the terrain is flatter, the slope is even milder, the soil thickness is deeper, and the vegetation is mainly temperate broad-leaf forest, which is less prone to erosion. In general, in the past 20 years, the greening process of the country has been accelerated by key provincial and municipal afforestation projects, such as forest rehabilitation, natural forest protection, and the Three-North Shelter Forest Program (TNSFP), and the area and intensity of soil erosion have been doubly reduced. This shows that over this time, the ability to conserve soil and water has substantially increased, and forest rehabilitation has been effective, which is consistent with [PERSON] et al. [50]. The results of [PERSON] et al. [51] are compatible with the regional distribution pattern of the severity of soil erosion in this area, and the temporal variation pattern is basically the same as that of [PERSON] et al. [12]. Figure 3: Temporal and spatial distribution of rainfall erosivity factors in the research region from 2000 to 2020. The average soil erosion modulus (t/ha per year) in the field of study is 4866, 3435, 3423, 3474, and 3787 in 2000, 2005, 2010, 2015, and 2020, respectively. Soil erosion modulus classes vary significantly over time, and the percentage of area eroded by each class is shown in Table 2. The soil erosion intensity categorization in the research area during the five years had the largest percentage of the entire area (60.69%, 60.85%, 60.25%, 59.40%, and 57.45%, respectively). The percentages of soil erosion mild and above are in the order of 9.65%, 10.65%, 10.47%, 10.81%, and 10.01%. Erosion that is slight or mild affects more than 67.46% of the total area. The stability rate (a constant percentage of erosion severity) of slight erosion in the research region from 2000 to 2020 is 75.32%, 81.18%, 71.12%, and 81.45%, respectively (Figure 5). There tends to be an increase and, subsequently, a reduction in the percentage of land subject to mild to moderate erosion. The proportion of the area that has very strong and extreme erosion shows first a decreasing and then an increasing trend. The percentage of strongly eroded land that has been significantly degraded is steadily increasing.