Impact of Grass Root Mass Density on Soil Detachment Capacity by Concentrated Flow on Steep Slopes

Abstract

Abstract. The effects of root systems on soil detachment, which are closely related to plant species and flow properties, are not well documented under a wide range of flow hydraulics. The objective of this study was to quantify the effects of root mass density of a versatile switchgrass species (Panicum virgatum) on soil detachment capacity, rill erodibility, critical shear stress, and relative soil detachment in a laboratory flume under a wide range of hydraulics using undisturbed soil samples collected from one bare plot (control) and five grass plots with different plant densities. The flow rates varied from 1.32 A— 10 -3 to 6.58 A— 10 -3 m 2 s -1 , slope gradients from 17.4% to 42.3%, and root mass densities from 0.25 to 17.98 kg m -3 . The results showed that soil detachment capacity decreased exponentially with increasing root mass density. The average detachment capacity of soil samples with roots was only one-fourth that of the control, and the rapid decline mostly occurred in the root density range of 0 to 4 kg m -3 . Rill erodibility declined exponentially as root mass density increased (R 2 = 0.595, n = 409), whereas no consistent trend for critical shear stress was found. Live roots reduced soil erodibility up to 78% compared with the control. Relative soil detachment decreased as an exponential function with root mass density (R 2 = 0.395, n = 409). However, the exponential decline was less pronounced compared to other studies, probably caused by differences in plant species, amounts of dead root or residue, and experimental conditions.