Quantifying and modeling sediment yields from interrill erosion under armouring

Authors: COCHRANE, THOMAS - University Of Cambridge; YODER, DANIEL - University Of Kentucky; Flanagan, Dennis; Dabney, Seth

Submitted to: Soil & Tillage Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/9/2019
Publication Date: 8/28/2019
Citation: Cochrane, T.A., Yoder, D.C., Flanagan, D.C., Dabney, S.M. 2019. Quantifying and modeling sediment yields from interrill erosion under armouring. Soil & Tillage Research. 195:104375. https://doi.org/10.1016/j.still.2019.104375.
DOI: https://doi.org/10.1016/j.still.2019.104375

Interpretive Summary: Soil erosion is a serious problem throughout the world, with erosion by water being a major process. In particular, detachment of soil particles by raindrops and by concentrated water flows can degrade soil quality where the erosion is occurring, as well as negatively impact water quality when the sediment moves off-site. Rocks in a soil can act to protect, or “armour” the surface from the raindrops, and typically the more rock cover present on a soil surface, the lower the soil erosion rates. In this study, we examined the effects of different rock content amounts and different rainfall intensity rates on the amount of runoff, soil loss, and level of soil surface armouring. On fresh soil from a mine site containing rocks, the greatest initial soil loss occurred from the greatest intensity rainfall. However, this high rainfall also exposed more rock cover, so that in subsequent rainfalls these soil boxes had the lowest rates of sediment loss because they were armoured more. A second set of experiments with mixtures of glass beads and rock contents of 0%, 20% and 40% clearly showed how the greater rock contents resulted in reduced runoff and sediment loss. It was also possible to use two different erosion prediction models to fairly well approximate the observed sediment loss as affected by the armouring. This research impacts scientists and others involved in modeling or assessing the impacts of rocks on runoff and soil loss caused by raindrop impact and shallow overland flow.

Technical Abstract: Surface soil properties can change as a result of soil disturbances, erosion, or deposition. When soils contain rock, surface soil properties can also change over time as a result of the process of soil armouring, which is the selective removal of finer particles by erosion, leaving an armoured layer of coarser particles that may reduce further soil loss. Rapid armouring is typically reported in steep and bares slopes on mine sites, construction sites, road embankments, and also rangelands. Changes in surface soil properties over time induced by armouring are not accounted for in current erosion models such as WEPP or RUSLE2 because little is known about rates of armouring over time as a function of rainfall intensity, rock content, slopes, and other factors. In this paper we simulate soil armouring induced by interrill erosion in two sets of experiments and propose ways to account for the process in WEPP and RUSLE2. The first set of experiments was conducted to demonstrate and quantify the effect of armouring on sediment yields under varying rainfall intensities. Rainfall with intensities ranging from 22 to 80 mm h-1 were simulated on 0.56 m2 plots at slopes of 18 degrees (32.5%) using topsoil with high rock content from a mine restoration site. Results showed a clear relationship between rainfall intensity and armouring. There was an over 75% reduction in total soil loss under 22 mm h-1 rainfall between freshly applied soils and highly armoured soils at the same slope. A second set of experiments was conducted to understand the relationships between soil rock content, rate of surface rock cover change, slope change and sediment yields. Sediment yields and surface rock cover were quantified for non-cohesive soils consisting of glass beads with a diameter range of 45 to 90 µm and 0, 20 and 40 % rock content in 0.22 m2 plots at a 15 degree (26.8%) slope under 80 mm h-1 simulated rainfall. Linear relationships were observed between cumulative sediment yields and % rock cover, and exponential relationships between total runoff and % rock cover. The armouring process was modeled with WEPP and RUSLE2 by iteratively altering the % rock cover and the slope over time. WEPP event based simulations resulted in reasonable predictions of sediment yields throughout the armouring process, and RUSLE2 required modification of soil erodibility values to account for high rock content and changes in runoff over time. Automation of this process, however, would require modifications of the models to expose rock cover and change slope over time. A method for doing this is discussed for interrill processes. Interactions between rill erosion and armouring need further study.