Characteristics and influencing factors of proximity distance index on the northern Shaanxi Loess Plateau in China

Topographic feature points and lines are the framework of topography, and their spatial distance relationship is an breakthrough in the study of topographical geometry, internal structure and development level. Proximity distance (PD) is an indicator to describe the distance between the gully source point (GSP) and the watershed boundary. In the upstream catchment area, PDs can be expressed by the streamline proximity distance (SPD), as well as by the horizontal proximity distance (HPD) and the vertical proximity distance (VPD) in the horizontal and vertical dimensions, respectively. The series of indicators (e.g., SPD, HPD and VPD) are important for quantifying the geomorphological development process of a loess basin because of the headward erosion of loess gullies. In this study, the digital elevation model data with 5 m resolution and a digital topographic analysis method are used for the statistical analyses of the SPD, VPD and HPD in 50 sample areas of 6 geomorphic types in the Loess Plateau of northern Shaanxi. The spatial characteristics and the influencing factors are also analysed. Results show that: 1) Central tendencies for the HPDs and the VPDs for the whole study area and the six typical loess landforms are evident. 2) Spatial patterns of the HPDs and the VPDs exhibit evident trends and zonal distributions over the whole study area. 3) The HPDs have a strong positive correlation with gully density (GD) and hypsometric integral. The VPDs also correlates with GD to an extent. Vegetation cover, mean annual precipitation and loess thickness have stronger effects on the VPD than on the HPD.