Abstract: Gravity erosion is an important agent for soilerosion and accounts for heavy portion of the total soilerosion amount in the Loess Plateau, and is one of main sources of the formation and maintenance of hyperconcentrated flow in the middle reaches of the Yellow River, thus the study on gravity erosion has great theoretical and application values. The finite difference software, i.e. FLAC3D, is used to simulate the distribution law of plastic yield zone of a small watershed model in the paper, and the systematic method of combining strength reduction method based on FLAC3D with simplified firstorder secondmoment method is applied to study the stability reliability and failure probability of the small watershed model. The results in the paper are as follows: firstly, the volume of shear plastic yield zone in the watershed model accounts for about 99.5 percent of that of total plastic yield zone in the watershed, while that of tension plastic yield zone accounts for only 0.5 percent, which reveals that shear plastic yield mainly leads to the failure of the slopes of the small watershed; secondly, from a view of the distribution of plastic yield zone applied to the Mohr-Coulomb yield criterion, the shear plastic zone is mainly distributed in the most area of slope and gully slope, while the tension plastic zone is mainly distributed on the top and slopes of barren hills; thirdly, according to the calculation results on simplified firstorder secondmoment method, the total failure probability of the small watershed model is up to 28.6 percent, i.e. much higher than 1.0 percent, which reveals that the small watershed is in nonacceptable risk level, as a result, some proper engineering measures should be taken to improve its stability; finally, more reasonable and reliable analysis results on slope stability of watershed could be obtained, throughout the application of the binary index evaluation system of combining traditional safety factor method and reliability analysis method. The numerical simulation results and those of stability reliability in the paper could be applied to study gravitational erosion in watershed on the Loess Plateau, and could offer scientific basis for deeply promoting the further study on the temporal and spatial distribution law of soil erosion and sediment yield and for comprehensive harness of soil and water loss in small watershed in the Loess Plateau.

Key words: gravitational erosion, watershed, stability, simplified firstorder secondmoment method, reliability