## 5 Discussion
### Influence of Coal Mining Subsidence on Soil Erosion  
The surface movement and deformation caused by coal mining subsidence will significantly change the original topography of the region, where the change in slope shape (slope gradient and slope length) is the most prominent. Slope and slope length characteristics are the key topographic factors affecting slope soil erosion, which has been the general consensus in the field of soil and water conservation [60]. Slope gradient has been repeatedly studied and confirmed as a key topographic factor affecting slope runoff and soil erosion. [PERSON] [61] studied the correlation between soil erosion and the topographic unit and slope in a watershed in northern Jordan. It was found that the soil erosion intensity on the slope was highly correlated with the slope, especially in the slope of 15-25\({}^{\circ}\). The intensity of soil erosion on the slope was significantly enhanced; [PERSON] et al. [62] carried out indoor experiments on loessial soil in the Loess Plateau and adjusted the experimental soil bin to three slopes of 5\({}^{\circ}\), 10\({}^{\circ}\), and 20\({}^{\circ}\). The study found that the slope can affect the total runoff and soil erosion by affecting the runoff. Under the condition of rainfall intensity of 1.5 mm min\({}^{-1}\), the total erosion amount of the slope reached the maximum when the slope was 10\({}^{\circ}\); [PERSON] et al. [63] conducted rainfall experiments on the typical red soil hilly area in the upper reaches of the Hanjiang River in the Pearl River Basin and used the PI10 index algorithm to calculate the rainfall erosion of each rainfall event. The study found that the soil erosion modulus variable and the slope under the condition of rainfall increased first and then decreased, and the critical slope value was about 10.75\({}^{\circ}\). By selecting the typical small watershed of Qilong Bay in the middle and upper reaches of the Yellow River Basin as the research area, and based on the observation data of 10 runoff plots from 2008 to 2016, [PERSON] et al. [64] analyzed that the amount of soil erosion increased with the increase in slope, and the amount of soil erosion was the largest when the slope was 30\({}^{\circ}\). The above research results are highly consistent with the results of the study on the increase in slope gradient caused by coal mining subsidence under the scale of annual erosion rainfall and the scale of typical field erosion rainfall, which leads to the increase in the slope soil erosion modulus. It can be seen that no matter what the slope shape is, the greater the influence of the same coal mining subsidence on the loess slope with larger natural slope, the more obvious the change in slope after subsidence and the resulting change in soil erosion intensity on the slope. As another important topographic factor, the effect of slope length on soil erosion is controversial. [PERSON] et al. [65] took the loess slope of Ansai in the loess hilly and gully region as the research object and studied the law of runoff and sediment yield on the loess slope under the two slope lengths of 5 m and 10 m through the indoor artificial simulated rainfall test. The results show that the increase in slope length will increase the rain-bearing area of the slope, resulting in an increase in the runoff rate per unit width and a corresponding increase in the total sediment yield. Therefore, the slope length and the slope erosion amount show a significant positive correlation. Based on the runoff and sediment yield data of eight runoff plots in Longfengling Soil and Water Conservation Science and Technology Demonstration Park in Mentougou District of Beijing from 2005 to 2009, [PERSON] et al. [66] concluded that slope length had a significant effect on runoff and sediment yield per unit area in a small runoff area and that the amount of soil erosion would increase with the increase in slope length.