## 3 Results and Discussion
### Influence of Environmental Factors on Subsurface Erosion
In addition, precipitation in summer is also a feature of this climate type. Annual precipitation is 400-500 mm [59]. In June, July and August, the precipitation reaches two-thirds of the annual precipitation. The average annual precipitation in summer is 359.9 mm [60]. Concentrated rainfall in a short period causes the exposed loess-like soil to be eroded. With the occurrence of rainfall infiltration and subsurface erosion, the geomorphology of mud forest changes and promotes the evolution and degradation of the mud forest landscape. The formation and development of the mud forest geomorphic landscape is also closely related to the occurrence and movement of groundwater around it. Dabusu Lake and its surrounding areas are not only the center of precipitation and surface water collection in the Songnen alluvial plain, but also the center of groundwater collection. In addition, it is a confined water basin with multiple aquifers, which is rich in groundwater. Due to the uplift of the crust during the uplift period and the new faulting activities, the shallow groundwater was exposed. The Quaternary confined water may also  
Figure 11: Profile of Dabusu Lake and its banks. be exposed. So, the gully where the mud forest geological and geomorphic landscape is located is almost always filled with water. This provides a long-term underground water-rich environment for the formation of subsurface erosion geomorphology. The buried groundwater level in the study area is shallow (Figure 11). The average groundwater level is 15.2 m. So the hydraulic gradient near the lakeshore is high. This leads to serious seepage damage of the soil. The subsoil of loess-like soil is a fine sand layer with good permeability. The water in the river is complementary through this highly permeable layer. Moreover, due to the high permeability of soil layer, groundwater has a higher velocity, and the decomposed soil particles can be carried away in time. As shown in Figure 12, infiltration of the precipitation converges with groundwater flow, continuously eroding loess-like soil, and over time, the mud forest landform landscape is formed. The content of sodium ion in water is higher, and it has the ability to disperse colloidal substance. At the same time, as this type of water circulates through the soil, it can increase the soluble salt content in the soil. While this is also an important reason for the degradation and destruction of the mud forest landscape. The results of the soil particle analysis tests showed that the soil sample contained the largest proportion of fine particles. While fine and very fine sand fractions were most important in controlling soil erosion [61], the high content of fine grained soil and weak hydrophilicity of the silt promoted the development of subsurface erosion in mud forest areas. Soil has no plasticity, more prone to subsurface erosion damage. Larger porosity of loess-like soil in mud forest area can promote the occurrence and development of subsurface erosion. Because the pores provide channels for the erosion products, accelerating the subsurface erosion process. The soil in mud forest area is dispersive (Figure 10). Thus, loess-like soil has a great propensity to produce a colloidal dispersion when groundwater flows through. After colloidal dispersed, small particles are carried along the pores by the water flow, forming cracks. Dispersive clays develop large pipes and erosion tunnels through rapid enlargement of small cracks and fissures as a consequence of the spontaneous dispersion of clays lining the fissure walls when these come in contact with water. This promotes subsurface erosion, thereby exacerbating the degradation of the mud forest landscape. Figure 12: Groundwater flow and precipitation infiltration in mud forest area.