Hydraulic Roughness Coefficients for Native Rangelands

A subfactor-based regression technique for estimating hydraulic roughness coefficients for shallow overland flow was developed from simulated rainfall runoff plots originally collected for erosion studies. The data were collected from 14 different native rangeland areas in the western United States. Rainfall was applied at a constant intensity of 65 mm hr from a rotating-boom rainfall simulator. Surfaces evaluated ranged from smooth bare soil to gravelly bare soil and sparsely to densely vegetated rangeland areas. A reference table of "effective roughness" coefficients for shallow overland flow is presented with a description of site characteristics. The derived roughness regression equations predict an "effective Darcy-Wiesbach roughness coefficient" for native rangeland (r2=0.70) that incorporates the effect of raindrop impact, soil texture, random roughness, rocks, litter, and canopy and basal plant cover. The sites evaluated in the paper covered a wide range of vegetation types and included short-, mid-, and tallgrass prairies; desert shrubs and sagebrush; and oak and pinyon-juniper woodlands. No trend in effective roughness coefficient associated with type of vegetation (grass or shrub) or soil texture was apparent.