Background When identifying erosive events, scholars primarily focused on erosive rainfall, but ignored the importance of runoff erosivity, which is one of the key factors influencing sediment flow behavior. Effectively identifying the relationship between runoff and sediment in erosive runoff is beneficial to a thorough understanding of the law of flow and sediment movement on slopes. Therefore, the standard for erosive runoff should be proposed to further reveal the mechanism of erosion from slope runoff.

Methods The hydrological data used in the study was downloaded from National Earth System Science Data Center, National Science & Technology Infrastructure of China (http://www.geodata.cn). Erosive runoff standard was determined by the frequency analysis method. The procedures are: 1) M_s (area-specific sediment yield) was arranged in descending order of peak discharge or runoff depth; 2) summing the cumulative M_s in descending order, calculating the cumulative percentage of M_s to overall M_s of all runoff events; 3) optimal regression equations between cumulative percentage of M_s and peak discharge or runoff depth were acquired by regression analysis; 4) given that the cumulative percentage of M_s was 95%, the corresponding peak discharge and runoff depth were calculated through the optimal regression equations, then the erosive runoff standard was determined. Runoff-sediment relationship of erosive and non-erosive runoff events were analyzed at inter-and intra-events time scales through regression analysis method.

Results 1) In this study, the general reference standards for erosive runoff are: ① The runoff duration is at least 8 min; ② the peak discharge is above 2.3 dm³/s; ③ the runoff depth is greater than 2.3 mm; ④ the area-specific sediment yield is more than 0.3 kg/m² (equivalent to 300 t/km²). 2) The area-specific sediment yield by erosive runoff is mainly controlled by runoff depth whereas the mean sediment concentration is primarily driven by the peak discharge. On the basis of the erosive runoff standard, the sediment reduction benefit caused by regulating runoff depth is 8% higher than controlling peak discharge. 3) There is a power function relationship between instantaneous sediment concentration and instantaneous discharge in erosive events, and a stable tendency (approximately 490 kg/m³) emerges in instantaneous sediment concentration when instantaneous discharge exceeds a critical value (8-14 dm³/s). 4) There is a positive linear correlation between instantaneous sediment transport rate and instantaneous discharge in erosive events, and the positive linear correlation also appears between increment in area-specific sediment yield and increment in runoff depth. The sediment delivery capacity and sediment yield capacity of erosive runoff are 2.9 times and 2.7 times that of non-erosive runoff, respectively.

Conculsions Compared with non-erosive runoff, the capacity of sediment transport and sediment yielding from erosive runoff is larger and the runoff-sediment relationship is more stable. Therefore, the proposal of erosive runoff standard is feasible in the present study, and then runoff regulation in slope should be orientated at the conversion from erosive runoff to non-erosive runoff.