Development characteristic of permanent gully morphology at the small watershed scale and its relations with driving factors along the Mollisols Belt of Northeast China

Gully erosion poses a significant threat to land, ecology environment and food security. However, current studies have predominantly focused on identifying the gully development and driving factors by selecting typical watersheds from broad regions, overlooking the spatial heterogeneity of gully erosion and the roles of driving factors. Therefore, to address this issue, this study aimed to investigate gully erosion and its driving factors by selecting representative small watersheds (0.48–2.93 km 2 ) along the Mollisols Belt of Northeast China. Gully morphology, topography, soil, climate and human activity (population and land use) data were obtained by unmanned aerial vehicle, field survey and spatial analysis. The results showed that gully linear density (GLD), gully areal density (GAD) and gully number density (ND) all exhibited a trend of initial sharp decrease followed by stabilization from south to north along the Mollisols Belt. The average gully length (L), width (W), depth (D), area (A) and volume (V) in the Mollisols Belt are 130.76 m, 6.42 m, 2.27 m, 1247.19 m 2 , 7158.12 m 3 , respectively. The gully L, W, D, A, and V are centrally distributed in the range of 0–300 m, 0.5–2.5 m, 1–5 m, 0–500 m 2 , 0–5000 m 3 , accounting for 68.5 %, 75.8 %, 68.3 %, 78.5 %, 83.3 %, respectively. The frequency distribution of five parameters showed great changes along Mollisols Belt. The gully volume can be estimated by V-L power function (V=a·L b , a=14.63–98.81, b=0.86–1.34). Gully erosion intensity reflected by GLD and GAD across all watersheds initially increased and then decreased with slope gradient and topographic wetness index (TWI), demonstrating distinct threshold behaviors, while gully erosion was more intense on sunny and semi-sunny slopes. GLD and GAD at small watershed scale were significantly and positively correlated with rainfall erosivity, mean annual precipitation, and mean annual temperature, population density, proportion of farm track area and watershed slope, while they exhibited the significantly negative correlations with MWD, soil bulk density, and soil shear strength and TWI. The topography, soil, climate and human activity factors collectively explain 83.51 % of the total variance in gully erosion, with the relative contribution of 17.8 %, 26.4 %, 27.4 %, 28.4 % respectively. These results can deepen the understanding of the characteristics and laws of gully erosion along the Mollisols Belt and provide scientific basis for the precise prevention and control of gully erosion. • Gully development characteristic is revealed along Mollisols belt of Northeast China. • V=a·L b is the optimal model for gully volume estimation of Mollisols belt. • Gully erosion in all watersheds exhibits threshold in response to in topography. • Climate, soil, topography, and human activity explain 83.51 % of gully erosion change.