## 1 Introduction  
Soil erosion is a global problem and poses major issues in many countries [1, 2]. Soil erosion can increase run-off by changing soil properties and particularly by destroying topsoil structure and reducing water holding capacity [3, 4]. Soil erosion is not only a land degradation process, but also a geomorphological appearance [5]. When the rate of soil erosion exceeds the rate of soil formation, the process of land degradation helps shaping the natural geological landscape [6]. In other words, while soil erosion causes land degradation, it also forms some typical landscapes (Table 1). [PERSON], et al. [7] studied the effect of rainfall on the Zin Valley badlands in Israel. \"Calanchi\" and \"biancane\" are two typical badland landscapes in Italy. [PERSON], et al. [8] analyzed the evolution of \"biancane\" landforms, whereas [PERSON], et al. [9] investigated how topography and rainfall may affect runoff and slope processes in \"calanchi\" landforms. Most of the soil erosion landscapes studied above are gully shaped or round enveloped landscapes formed by surface erosion. The research object of this paper is a columnar geological landscape formed by both surface and subsurface erosion. As seen in Table 1, the studied landscapes are mainly composed of earth pillars, arranged together looking like a forest, hence it was named \"mud forest\" [10, 11]. The process of erosion is caused by both surface and subsurface processes. Over the last decades, most studies on soil erosion have focused on surface processes, such as sheet, rill and gully erosion [5; 14]. Although subsurface erosion has been reported to be a significant and widespread process, the disproportion in the number of studies on surface erosion compared to those on subsurface erosion is striking [5; 15]. Subsurface erosion generally refers to various forms of erosion caused by groundwater below the surface [16]. This process and its effects have been described under a variety of names: subterranean erosion [17], subcutaneous erosion [18], sinking of the ground [19; 20; 21], sink-hole erosion [22; 23], tunnel erosion [24], pothole gullying [25], tunnel-gully erosion [26; 27], tunneling erosion [28], piping [29], soil piping [30], pothole erosion [31], and suffusion [32; 33; 34]. While degradation occurs in all kinds of landscapes over the world, the drivers of degradation vary from region to region [35]. Due to the complexity of subsurface erosion, no single factor can be held responsible for subsurface erosion development [5]. [PERSON], et al. [36] pointed out that the \begin{table}
\begin{tabular}{c c c} \hline \hline
**Name** & **State** & **Picture** \\ \hline Zin Valley badlands & Israel & \\ Badland landscape & Spain & \\ “Calanchi” badland landform & Italy & \\ “Biancane” badland landform & Italy & \\ “Biancane” badland landform & Italy & \\ “Biancane” badland landform & Italy & \\ “Biancane” badland landform & Italy & \\ “Yuanmou Dry-hot Valley & China & \\ “Mud forest landscape & China & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Several typical geological landscapes formed by soil erosion. [7; 8; 9; 12; 13]. occurrence of subsurface erosion is controlled by soil and climate features [37; 38; 39; 40; 41]. The occurrence of subsurface erosion is also closely related to topography [42; 43; 44; 45]. In 2009, the Qian-an mud forest landscape was officially approved as a national geological park in China. Due to its unique landform caused by erosion, this landscape has become the only protected \"mud forest\" site in China with these geological features, giving the area a high aesthetic and scientific investigation value [46]. However, mud forest is a very fragile geological landscape and suffers from serious soil erosion. In order to maintain mud forest stable for a long time, it is necessary to understand the causes of such landscape degradation. Previous researches on mud forest landscape have been carried out, including the formation mechanism, development and utilization, slope failure.