Spatial variation in the frequency and magnitude of mass movement in a semiarid, complex-terrain agricultural watershed on the Loess Plateau of China

The previous inventory datasets of mass movement typically include large-sized mass movements (e.g., landslides), whereas small ones are often missing due to either being too small to be interpreted from remote sensing images or too time-consuming to measure them in the field. Thus, the relative significance of different-sized mass movement in sediment production remains unknown. In this study, first, a total of 547 recent mass movements, with a volume ranging from 0.3 to 19,000 m³, are surveyed in a small complex-terrain watershed on the Loess Plateau of China; then, sediment contributions by mass movement of different magnitude and frequency are evaluated; furthermore, the effects of causative factors on both mass movement frequency and magnitude are examined and compared; finally, Geographic Information Systems (GIS) and statistical methods are employed to assess mass movement susceptibility and erosion risk. The results showed that majority of the mass movements are very small (<10 m³). Even though a large mass movement can produce a disproportionally high amount of debris on a per event basis, small- and medium-sized mass movements (<1,000 m³) are overall more important in sediment production in the watershed. The GIS-based modeling results also demonstrate that there is significant difference in spatial variation between the susceptibility and erosion risk of mass movements. Therefore, although the mass movement susceptibility map is very useful in prioritizing hazard prevention and mitigation, it should be combined with the mass movement erosion risk map in erosion control planning and design, which has not been done in the past.