Background Soil erosion has been recognized as the most important environmental problems worldwide. Rainfall is acknowledged as the direct dynamic factor of soil erosion on the Loess Plateau. The obtainment of erosive rainfall standard is essential in calculating rainfall erosivity and predicting soil erosion. But not all of the sediment caused by rainfall will arrive the outlet section of small watershed (< 100 km²), thus the rainfall event under which sediment can reach to the outlet section of small watershed is called sediment generating rainfall of small watershed. There have been extensive studies on erosive rainfall standard on the Loess Plateau, but the research on the standard of sediment generating rainfall has been rarely reported.

Methods On the basis of the observation of 20 small watersheds on the Loess Plateau, we determined the standards of sediment generating rainfall in small watersheds by fitting the regression relationship between rainfall amount calculated with area weighting factor method and the proportion of cumulative sediment yield to the total sediment yield at the inter-event timescale based on observations of not less than five years. The amount standards were obtained by the regression relationship when the proportion was given to 95%. Besides, the response of the standards of sediment generating rainfall to underlying surface conditions of small watersheds were also analyzed.

Results 1) The amount standards of sediment generating rainfall in 20 small watersheds ranged from 7.48 to 33.51 mm. 2) In the Loess Hilly-gully Region, the amount standards of sediment generating rainfall in ungoverned watersheds and governed watersheds were 7.5-14.0 mm and 15.6-16.4 mm respectively. In the Loess Plateau-gully Region, the amount standards of sediment generating rainfall in governed and forested watersheds were 14.1-16.8 mm and 33.5 mm respectively. 3) The amount standards of sediment generating rainfall in governed watersheds were all higher than those of their paired ungoverned watersheds. In small watersheds with larger area, the implementation of soil and water conservation such as check dams and reservoirs significantly increased the amount standards of sediment generating rainfall. While, in smaller watersheds, soil and water measures in slope (such as revegetation and terraces) also increased the amount standards of sediment generating rainfall. 4) The amount standard of sediment generating rainfall in one watershed was not a fixed value, varying dynamically with the modification of underlying surface conditions.

Conclusions There is a large difference in the amount standards of sediment generating rainfall in different watersheds. In general, the amount standards of sediment generating rainfall will increase with the soil and water conservation in small watersheds. This study may provide valuable information for calculating rainfall erosivity and improve the prediction accuracy of sediment yield in small watershed scale. The results of this study can also provide a reference for the evaluation of soil and water conservation benefits on the Loess Plateau.