C. J. Williams

RISK ASSESSMENT OF EROSION FROM CONCENTRATED FLOW ON RANGELANDS USING OVERLAND FLOW DISTRIBUTION AND SHEAR STRESS PARTITIONING

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 the 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 effective 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 but decreased significantly when the 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 as triggers that indicate a site is crossing a threshold beyond which soil erosion might accelerate due to the high effective shear stress.

Shear Stress Partitioning of Overland Flow on Disturbed and Undisturbed Rangelands

Physically-based hillslope erosion models commonly estimate soil detachment and transport capacity based on overland flow shear stress applied to soil aggregates. However, vegetation and rock cover counteract the shear stress of overland flow where they occur. Accordingly, partitioning of total shear stress into components exerted on soil, vegetation, and rock cover is a key element for the 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. 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 total friction factor of the composite surface was estimated using empirical equations 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. The result of this assumption was used to develop empirical equations that predict the ratio of soil shear stress to the total shear stress of concentrated flow and sheet flow in terms of bare soil fraction of total area. The predicting equation of total friction factor was improved by adding the slope and the flow discharge variables. The predicting equations of soil shear stress as a function of bare soil fraction did not change significantly when changing the assumption of a rectangular shape of cross section to a parabolic shape. The developed shear stress partitioning equations in this study are 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 equations show that shear stress exerted on soil grains is significantly higher when bare soil exceeds 60% of the total surface area, while reduced significantly when bare soil area is less than 25% or when the plant base cover exceeds 20%. These percentages could be used as relative measures of hydrologic recovery for disturbed rangelands or triggers that indicate that a site is crossing a threshold where soil erosion might accelerate due to the high soil shear stress.