AlHamdan, Osama   
Pierson, Frederick  
Nearing, Mark  
Williams, Christopher  
Stone, Jeffry  
Kormos, Patrick   
Boll, Jan   
Weltz, Mark 
Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal Publication Acceptance Date: January 20, 2013 Publication Date: April 26, 2013 Citation: AlHamdan, O.Z., Pierson Jr, F.B., Nearing, M.A., Williams, C.J., Stone, J.J., Kormos, P.R., Boll, J., Weltz, M.A. 2013. Risk assessment of erosion from concentrated flow on rangelands using overland flow distribution and shear stress partitioning. Transactions of the ASABE. 56(2):539548. Interpretive Summary: In this study, we developed an approach for risk assessment of erosion due to concentrated flow on rangelands. First, we investigated ground cover and flow conditions under which flow transitions from sheet to concentrated flow. Then, a logistic equation was developed to estimate the probability of overland flow to become concentrated. Also, we developed equations for partitioning shear stress of concentrated flow on rangelands using readily available vegetation and rock cover data. These equations were used to estimate the effective shear stress portion which is exerted on soil and causes erosion. For the purpose of risk assessment modeling, the developed logistic equation can be used for estimating the risk of concentrated flow formation in a specific rainfall scenario for a selected site. And then an assessment of how well that site is protected against such concentrated flow can be investigated using the shear stress partitioning equations. Technical Abstract: Erosion rates of overland flow on rangelands tend to be relatively low, but under certain conditions where flow is concentrated, soil loss can be significant. Therefore, a rangeland site can be highly vulnerable to soil erosion where overland flow is likely to concentrate and exert high shear stress on soil grains. This concept is commonly applied in cropland and wildland soil erosion modeling using predictions of flow effective shear stress (shear stress applied on soil grains). However, historical approaches to partition shear stress in erosion models are computationally complex and require extensive parameterization. Furthermore, most models are not capable of predicting the conditions in which concentrated flow occurs on rangelands. In this study, we investigated the rangelands conditions at which overland flow is more likely to become concentrated, and developed equations for partitioning shear stress of concentrated flow on rangelands. A logistic equation was developed to estimate the probability of overland flow to become concentrated. Total shear stress of rangeland overland flow was partitioned into components exerted on soil, vegetation, and rock cover using field experimental data. In addition, we investigated the vegetation cover limit at which the soil shear stress component is substantially reduced, limiting the erosion rate. The results from the partitioning equations show that shear stress exerted on soil grains was relatively small in sheet flow. Shear stress exerted on soil grains in concentrated flow was significantly higher when bare soil exceeded 60% of the total surface area, while reduced significantly when bare soil area was less than 25% or when the plant base cover exceeded 20%. These percentages could be used as relative measures of hydrologic recovery for disturbed rangelands or triggers that indicate a site is crossing a threshold where soil erosion might accelerate due to the high soil shear stress.
