Abstract: The Chinese Loess Plateau has been widely acknowledged as one of the world's mostly eroded areas and thus characterized by a fragmented and complex terrain. During the past decades, various monitoring methods, such as field investigations, erosion pins, manipulation experiments and tracer studies, have been employed to monitor soil erosion on the Loess Plateau. However, due to the limitation in the monitoring range of the above methods, previous soil erosion studies have been primarily undertaken at an erosion plot scale, while the catchment scale erosion monitoring was seriously lacking. In recent years, emerging remote sensing technologies, such as airborne light detection and ranging (LiDAR), have provided a promising means for an effective monitoring of soil erosion process over a large area (i.e. the catchment scale). However, little was known about the uncertainty of topographic changes detected by the airborne LiDAR for topographically complex areas, and thus the ability of airborne LiDAR to monitor soil erosion remained unclear. In the study, four airborne LiDAR flights were undertaken during a period without topographic change using an unmanned aerial vehicle (UAV) platform to acquire point clouds for a typical slope-gully system (consisting of hillslopes and gully slopes) in a small catchment (i.e. Dongzhuanggou) of the gullied Loess Plateau. Digital elevation models (DEMs) were produced based on the acquired point clouds using the triangle irregular network (TIN) algorithm. The uncertainty of topographic change detections were then derived as the DEM of difference (DoD) through subtracting the DEMs derived using the point clouds acquired by different flights from one another. The spatial pattern of the DoD uncertainty for the slope-gully system was investigated, while the ability of airborne LiDAR to detect soil erosion was assessed through comparing the magnitude of DoD (DoD_ua) with soil erosion rates collected from literature. Results showed that ① The DoD_ua for different flight combination was generally insignificant, while the spatial pattern of DoD uncertainty derived based on different flight combinations was similar, with apparent difference only emerging in certain places. ② The DoD_ua on the hillslope ranged between 0.023 m and 0.034 m, which was much lower than that of gully areas (0.057 m~0.077 m). The peak values of DoD_ua were normally found on steep-sloping gully walls. The area percentage decreased with the increase of DoD_ua, with a<0.05 m DoD_ua occupying over 40% while a>0.3 m DoD_ua accounting for less than 7% of the study area. ③ In terms of a comparison of DoD_ua and measured soil erosion rates, the UAV LiDAR was found to be able to detect soil erosion of permanent gullies at an event scale and deep-seated/shallow landslides. The results also showed that the UAV LiDAR may be able to monitor erosion of shallow (ephemeral) gullies, and was not able to monitor rill erosion. Our results provided a useful reference for the catchment-scale soil erosion monitoring and erosion process studies over topographically complex areas.

Key words: soil erosion monitoring, catchment scale, Loess Plateau, airborne LiDAR, DoD uncertainty