Effects of Fibrous Roots of Four Herbaceous Species on Water Flow Velocity and Rill Detachment Capacity

Vegetal species are potentially effective at stabilising soil against water erosion due to the beneficial effects of their root system. In this regard, fibrous roots increase soil aggregation and improve its physical properties. However, the literature about the variability in particle detachment capacity in rills (the prevalent erosion form on steep and long hillslopes) with species with fibrous roots is not abundant. To fill this gap, this study has analysed the effects of fibrous roots of four herbaceous species (Hypericum androsaemum, Urtica dioica L., Epipactis latifolia All. and Danae racemosa (L.) Moench.) that are typical of Northern Iran on water flow velocity (V) and rill detachment capacity (Dc) by flume experiments. The results showed that plants with fibrous roots significantly reduced the mean V, but not Dc (except for Danae racemosa (L.) Moench.) between two positions located upstream and downstream of a rill. Only Dc of Epipactis latifolia All. was significantly different (-30% to -40%) compared with the other species. High correlations were found among the plant root characteristics. The associations between those vegetal parameters and Dc were significant but weaker. Linear regressions between Dc and shear stress allowed the estimation of rill erodibility and the critical value of the shear stress for all species, while equations set up to predict Dc from the stream power of the water flow showed a very high accuracy in all cases. Despite some limitations (linked to the small scale of the experiments), the study has demonstrated that the root system of herbaceous species may play an important role in rill erosion, but this erosion form may be driven by factors other than root system morphology.