DISTURBANCE ASSESSMENT AND MITIGATION OF GREAT BASIN RANGELAND
Location: Watershed Management Research
Title: Modeling overland erosion on disturbed rangeland
Submitted to: Trans American Geophysical Union
Publication Type: Abstract Only
Publication Acceptance Date: September 28, 2012
Publication Date: December 3, 2012
Citation: Al-Hamdan, O.Z., Hernandez, M., Pierson, F.B., Nearing, M., Stone, J.J., Williams, C.J., Boll, J., and Weltz, M. 2012. Modeling overland erosion on disturbed rangeland. Presented at the American Geophysical Union Annual Fall Meeting, December 3-7, 2012, San Francisco, CA.
The Rangeland Hydrology and Erosion Model (RHEM) is a new process-based model developed by the USDA-ARS primarily for undisturbed rangeland. Greater sediment detachment rates are usually generated by concentrated flow rather than by sheet flow. Disturbance on rangeland such as fire and tree encroachment can increase overland flow erosion rate by increasing the likelihood of concentrated flow formation on a more erodible surface. In this study, we made advancement to RHEM by developing a new version of the model to predict concentrated flow erosion rate from disturbed rangelands. The model was conceptualized based on observations and results of experimental studies on rangelands disturbed by fire and/or by tree encroachment. A logistic equation was used to partition overland flow into concentrated flow and sheet flow. The equation predicts the probability of overland flow to become concentrated based on slope angle, percentage bare soil, and flow discharge per unit width. Sediment detachment rate from concentrated flow was calculated using soil erodibility of the site and hydraulic parameters of the flow such as flow width and stream power. Soil detachment was assumed to start when concentrated flow starts (i.e. no threshold concept for initiating detachment was used). Width of concentrated flow was determined by flow discharge and slope using an equation which was developed specifically for rangeland. A dynamic erodibility concept was used where concentrated flow erodibility was set to be high at the beginning of the event and then decrease exponentially due to the reduction of availability of disturbance-source-sediment. Initial erodibility was estimated using an empirical parameterization equation as a function of readily available vegetation cover and surface soil texture data. Detachment rate from rain splash and sheet flow was determined by rainfall intensity and sheet flow discharge. A dynamic partial differential sediment continuity equation was used to model the total detachment rate of concentrated flow and rain splash and sheet flow. The model was evaluated on a plot scale (32.5 m2) using data obtained from rainfall simulation experiments on two sites, where one was disturbed by fire and the other was disturbed by tree encroachment.