Concentrated flow erosion rates as affected by rock fragment cover and initial soil moisture content

Concentrated flow experiments using a small hydraulic flume and a constant flow discharge and bed slope have been conducted in order to investigate th e effects of rock fragment cover (Rc) on sediment yield for an initially we t and an initially air-dry loamy topsoil. The experimental results indicate that Re reduces concentrated flow erosion rates (E) in an exponential way (i.e., E = e(-bRC)), which is similar to previously reported relations for other water erosion processes such as interrill erosion and sheet-rill eros ion measured on runoff plots. The decay rate (b) of this exponential relati onship increased throughout the experiments because of scour-hole developme nt and bed armouring. The concentrated flow erosion rates and b-values also depend on the initial moisture content of the topsoil. Depending on Re, me an concentrated flow erosion rates were 20% to 65% less on initially wet co mpared to initially air-dry topsoils. The mean value for b was 0.032 for th e initially wet, but only 0.017 for the initially air-dry topsoil, indicati ng that a rock fragment cover is less efficient in reducing concentrated fl ow erosion rates when the topsoil is initially air-dry than when it is init ially wet. The results help explain the data scatter in reported relationsh ips between Re and inter-rill-rill erosion rates. They also indicate that a given surface rock fragment cover will offer more protection to wet topsoi ls than to dry topsoils, which are very common in Mediterranean environment s. Event-based water erosion models should incorporate effects of anteceden t soil moisture content as well as those of Re on concentrated flow erosion rates. (C) 1999 Elsevier Science B.V. All rights reserved.