Introduction Herbaceous plants regulate the generation of runoff and sediment and the anti-erosion ability of soil by altering vegetation coverage and root systems, thereby influencing soil erosion. However, the variation in soil erosion reduction benefits and the contributions of stem-leaf and roots under diverse vegetation spatial distribution patterns on the Loess Plateau have not yet been understood.Methods In the current study, alfalfa spatial distribution patterns with vegetation coverage of 50% (T1 treatment; LS, downstream of hillslopes; MS, middle of hillslopes; US, upper of hillslopes; and SS, equidistant planting), four alfalfa spatial distribution patterns without above-ground biomass (T2 treatment), and bare soil (CK) were designed. Rainfall simulations with slope gradients of 8.75%, 17.63%, 26.80%, and 36.40% and a rainfall intensity of 90 mm h-1 were carried out on runoff plots (2 m & times; 1.0m) filled with clay loam. The initial runoff generation time (RT) was recorded, and runoff sediment samples were collected.Results The results showed that the RT for the T1 and T2 treatments was delayed by 1.47 to 2.51 and 0.75 to 1.65 min, respectively, compared with CK. The mean runoff rate for the T1 and T2 treatments ranged from 0.18 to 1.15 and from 0.61 to 1.16 L m-2 min-1, respectively. The mean sediment rate for the T1 and T2 treatments ranged from 1.50 to 18.63 and 3.01 to 20.01 g m-2 min-1, respectively. The runoff reduction benefits (RRB) for the T1 and T2 treatments ranged from 44.20% to 84.67% and from 34.40% to 43.66%, respectively. The sediment reduction benefits (SRB) for the T1 and T2 treatments ranged from 25.01% to 86.48% and from 9.30% to 71.30%, respectively. The contribution rates of stem-leaf and root to RRB ranged from 3.87% to 30.14% and from 10.10% to 30.80%, respectively, and those for SRB ranged from 9.75% to 24.65% and from 7.28% to 37.75%, respectively. Principal component analysis indicated that the slope gradient explained the greatest variation in runoff, followed by the spatial distribution pattern and vegetation traits.Conclusion The research results provide references for vegetation protection and management in areas with severe soil erosion and enrich the understanding of soil erosion mechanisms.