Location: Northwest Watershed Management Research
Title: Rangeland Hydrology and Erosion Model (RHEM) enhancements for applications on disturbed rangelands Authors
|Al-Hamdan, Osama -|
|Hernandez, Mariano -|
|Boll, Jan -|
Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 29, 2014
Publication Date: March 1, 2014
Citation: Al-Hamdan, O.Z., Hernandez, M., Pierson Jr, F.B., Nearing, M.A., Williams, C.J., Stone, J.J., Boll, J., Weltz, M.A. 2014. Rangeland Hydrology and Erosion Model (RHEM) enhancements for applications on disturbed rangelands. Hydrological Processes. doi: 10.1002/hyp.10167. Interpretive Summary: In this study we enhanced the applications of the RHEM model on disturbed rangelands where concentrated flow plays a major role in the soil erosion process by incorporating a new concentrated flow erosion modeling approach. The new approach addresses the increase in erodibility due to the elevation of exposed bare soils caused by disturbance such as fire or tree encroachment. The approach also addresses the instantaneously elevated sediment pulse of limited supply caused by fire by using a dynamic erodibility concept where erosion starts at high rates and then decreases due to the decline of sediment supply. The enhancements of RHEM create a practical management tool for quantifying erosion and assessing erosion risk following rangeland disturbance. The enhancements expand the model applicability as a practical land management tool for conservation planning and quantifying environmental benefits of alternative conservation practices. The tool can use vegetation and ground cover data to determine the degree of disturbance impact on erosion and track the rate of site recovery. The enhanced RHEM model is easily parameterized using readily available vegetation, soils, and ground cover data.
Technical Abstract: The Rangeland Hydrology and Erosion Model (RHEM) is a new process-based model developed by the USDA-ARS. Disturbance such as fire or woody plant encroachment can amplify overland flow erosion by increasing the likelihood of concentrated flow formation. In this study, we enhanced RHEM applications on disturbed rangelands by using a new approach for the prediction and parameterization of concentrated flow erosion. The new approach was conceptualized based on observations and results of experimental studies on rangelands disturbed by fire and/or by tree encroachment. The sediment detachment rate for concentrated flow was calculated using soil erodibility and hydraulic (flow width and stream power) parameters. Concentrated flow width was calculated based on flow discharge and slope using an equation developed specifically for disturbed rangelands. Soil detachment was assumed to begin with concentrated flow initiation (i.e. no threshold concept for initiating detachment was used). A dynamic erodibility concept was applied where concentrated flow erodibility was set to decrease exponentially during a runoff event due to declining sediment availability. Erodibility was estimated using an empirical parameterization equation as a function of vegetation cover and surface soil texture. 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 enhanced version of the model was evaluated against rainfall simulation data for three different sites that exhibit some degree of disturbance by fire and/or by tree encroachment. The coefficient of determination (R2) and Nash-Sutcliffe efficiency (NSE) were 0.78 and 0.71, respectively, which indicates the capability of the model using the new approach for predicting soil loss on disturbed rangeland. The enhanced model can be a practical tool for quantifying erosion and assessing erosion risk following rangeland disturbance (e.g. wild fire, prescribed fie, and/or by tree encroachment), using readily available vegetation and soils data.