Shear stress partitioning of overland flow on disturbed and undisturbed rangelands

Authors: AL-HAMDAN, OSAMA - University Of Idaho; Pierson Jr, Frederick; Nearing, Mark; Stone, Jeffry; Williams, Christopher - Jason; KORMOS, PATRICK - Boise State University; BOLL, JAN - University Of Idaho; Weltz, Mark

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/7/2011
Publication Date: 9/18/2011
Citation: Al-Hamdan, O.Z., Pierson, F.B., Nearing, M.A., Stone, J.J., Williams, C.J., Kormos, P.R., Boll, J., and Weltz, M.A. (2011) “Shear Stress Partitioning of Overland Flow on Disturbed and Undisturbed Rangelands”, Abstract, in the International Symposium on Erosion and Landscape Evolution, Anchorage, Alaska, USA, American Society of Agricultural and Biological Engineers and Association of Environmental and Engineering Geologist, Edited by Ascough II C. and Flanagan, D. C. pp 110.

Interpretive Summary: In physically-based hillslope erosion models, only overland flow shear stress exerted on soil aggregates (grains) is used to estimate concentrated flow soil detachment rates and sediment transport capacity. However, on vegetated hillslopes, only overland flow total shear stress can be obtained using traditional methods. Accordingly, partitioning of total shear stress into components exerted on soil, vegetation, and rock cover is a key element for these erosion models. The objective of this study is to estimate the components of shear stress of overland flow on disturbed and undisturbed rangelands using field experimental data. In addition, this study investigates the vegetation cover limit at which the soil shear stress component is substantially reduced limiting the erosion rate. In this study, the soil shear stress component was estimated based on the assumption that the ratio of soil shear stress to the total shear stress is equal to the ratio of hydraulic friction factor of soil to the friction factor of the composite surface. The friction factor of the composite surface was estimated using an empirical equation (R2=0.56) that was developed based on field experimental data over diverse rangeland landscapes within the Great Basin Region, United States. This equation logarithmically correlates the composite surface friction to the vegetation cover (plant base and plant litter) and rock cover components. Moreover, the hydraulic friction factor of each cover element was estimated based on its parameter in that equation. The soil hydraulic friction portion was assumed to be the logarithmic difference between the total friction and the friction of the cover elements. Also, an empirical equation that predicts the ratio of soil shear stress to the total shear stress in terms of bare soil fraction of total area was developed (R2=0.96). This equation is used to partition the measured shear stress from field experiments. The equation is applicable across a wide span of ecological sites, soils, slopes, and vegetation and ground cover conditions and can be used by physically-based rangeland hydrology and erosion models. The results from the developed equation show that the shear stress exerted on soil grains is reduced significantly when the bare soil area is less than 40% of the total surface area. This percentage value could be an indicative measure of recovery for disturbed rangelands such as burned sites. In this study, the percentage of recovery for the studied burned sites was reached two seasons after fire.

Technical Abstract: In physically-based hillslope erosion models, only overland flow shear stress exerted on soil aggregates (grains) is used to estimate concentrated flow soil detachment rates and sediment transport capacity. However, on vegetated hillslopes, only overland flow total shear stress can be obtained using traditional methods. Accordingly, partitioning of total shear stress into components exerted on soil, vegetation, and rock cover is a key element for these erosion models. The objective of this study is to estimate the components of shear stress of overland flow on disturbed and undisturbed rangelands using field experimental data. In addition, this study investigates the vegetation cover limit at which the soil shear stress component is substantially reduced limiting the erosion rate. In this study, the soil shear stress component was estimated based on the assumption that the ratio of soil shear stress to the total shear stress is equal to the ratio of hydraulic friction factor of soil to the friction factor of the composite surface. The friction factor of the composite surface was estimated using an empirical equation (R2=0.56) that was developed based on field experimental data over diverse rangeland landscapes within the Great Basin Region, United States. This equation logarithmically correlates the composite surface friction to the vegetation cover (plant base and plant litter) and rock cover components. Moreover, the hydraulic friction factor of each cover element was estimated based on its parameter in that equation. The soil hydraulic friction portion was assumed to be the logarithmic difference between the total friction and the friction of the cover elements. Also, an empirical equation that predicts the ratio of soil shear stress to the total shear stress in terms of bare soil fraction of total area was developed (R2=0.96). This equation is used to partition the measured shear stress from field experiments. The equation is applicable across a wide span of ecological sites, soils, slopes, and vegetation and ground cover conditions and can be used by physically-based rangeland hydrology and erosion models. The results from the developed equation show that the shear stress exerted on soil grains is reduced significantly when the bare soil area is less than 40% of the total surface area. This percentage value could be an indicative measure of recovery for disturbed rangelands such as burned sites. In this study, the percentage of recovery for the studied burned sites was reached two seasons after fire.