Comparison of rill flow velocity regimes between developing and stationary rills

Abstract Flow velocity is fundamental in calculating related parameters of physical soil erosion models. It has been well known that rill flow velocity varies with developing rill form, which complicates and impedes the development of soil erosion models. In filed, rainfall erosion frequently occurs with pre-existing rills not damaged by tillage practices. However, the rill flow velocity in this case has received little attention. In this study, we investigated rill flow velocity regimes for both developing and stationary rills using 5 m-length plots in the laboratory. The experiments included two consecutive rainfalls, which had similar runoff rate. The first rainfall had an intensity of 90 mm h −1 , which generated well-developed rills; the second had an intensity of 60 mm h −1 and did not damage the rills formed by the first rainfall. The results show that, for a developing rill, rill flow velocity is higher in the lower slope than that in the upper slope, and rill flow velocity in the lower slope fluctuates much more over time as a result of the variation of rill morphology following rill development. With rill development, the rill flow velocity of the lower slope decreased responding to the stabilizing rill channel and exhibited a similar trend to that in upper slope. For a stationary rill, the rill flow velocities at both sections varied little over time, demonstrating that rill flow velocity is primarily influenced by rill morphology. Hence, the results suggested that the strength of fluctuation in rill flow velocity could be used to characterize the instantaneous intensity of soil erosion and active level of rill development. The channel of the stationary rill simply acted as transport pathways of sediment supplied by inter-rill erosion. The physical models of soil erosion should take into account the micro-topography on land surface related to the antecedently formed rills.