Location: Watershed Management Research2013 Annual Report
1a. Objectives (from AD-416):
The Rangeland Hydrology and Erosion Model (RHEM) is a new process-based model developed by the USDA-ARS. The model has been developed for assessing the runoff, soil erosion rate, and sediment delivery rate and volume on rangelands at the hillslope scale for a rainfall event. Evaluation of RHEM performance is a key element for its acceptance as a useful tool. Therefore, the objective of the research work is to evaluate the performance of RHEM and its new components especially when it is applied to disturbed rangelands.
1b. Approach (from AD-416):
In order to accomplish the objective of the research, the following tasks will be performed: 1. Evaluate the new width (w) and hydraulic friction (ft) equations for concentrated flow in RHEM using independent plot-scale data. 2. Evaluate the current parameterization equation for splash and sheet erodibility (Kss), and hydraulic conductivity (Ke) in RHEM on disturbed rangeland, and if necessary develop new parameterization equations specifically for disturbed rangeland. 3. Evaluate the RHEM concentrated flow erodibility approach (Kw(max)) and its parameterization equations using independent data. 4. Evaluate the performance of RHEM for disturbed rangelands. The performance of the model will be evaluated without calibration as well as by optimizing some of the sensitive parameters such as Ke .The new probabilistic approach for partitioning overland flow into concentrated and sheet flow as well as parameterization of hydraulic friction (ft) for routing the overland flow will be also tested.
3. Progress Report:
The Rangeland Hydrology and Erosion Model (RHEM) is a new process-based model developed by the USDA-ARS. Past enhancements to RHEM for application to disturbed rangelands were conceptualized based on observations and results of extensive experimental studies on rangelands disturbed by fire and/or by tree encroachment. Concentrated flow width (w) was calculated based on flow discharge and slope using an equation developed specifically for disturbed rangelands. Hydraulic friction for total overland flow and for concentrated flow (ft) was estimated by equations developed specifically for rangelands. For erosion prediction at burned sites, a dynamic erodibility concept was applied where concentrated flow erodibility was set to be maximum (Kw(max)) at the beginning of runoff event and then decreases exponentially during the runoff event due to declining of the instantaneously elevated sediment pulse of supply caused by fire. Average erodibility (kw) approach was used for the unburned sites. 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. This year the enhanced version of RHEM was evaluated against rainfall simulation data for three different sites that exhibit some degree of disturbance by fire and/or by tree encroachment. The model performance for predicting erosion was evaluated with optimizing the hydraulic conductivity (Ke) for total runoff. The performance evaluation was conducted with and without calibrating the erodibility parameters in order to evaluate the parameter estimation equations. Evaluation of the enhanced version of RHEM including associated parameter estimation equations using plot scale experimental data at the three different sites indicated the ability of the model to predict erosion at the plot scale with a satisfactory range of error. The new version of the model was able to match the predicted effect of disturbances and treatments across a wide range of ecological sites and vegetation and ground cover conditions. The enhancements of RHEM create a practical management tool for quantifying erosion and assessing erosion risk following rangeland disturbance, as well as, 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. The above-mentioned research work for enhancing the RHEM has been submitted for the journal of Hydrological Processes for review. Additional research work was conducted to develop a risk index of soil erosion due to concentrated flow formation and high effective flow shear stress on disturbed rangeland. 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. This research work has been published in the Journal of Transactions of the American Society of Agricultural and Biological Engineers. This agreement was established in support of objective 1 (Sub-Objective 1.B)of the in-house project, the goal being to develop management tools for hydrologic and erosion assessment of the impacts of fire, weed-invasions, and conservation practices on Great Basin sagebrush steppe rangelands to aid public and private land managers in formulating conservation strategies and selecting effective conservation practices by enhancing RHEM for assessing hydrology and erosion responses associated with management of disturbed states and transitions occurring on sagebrush steppe ecological sites.