Hydraulic Roughness Coefficients for Native Rangelands

Abstract

A subfactor-based regression technique for estimating hydraulic roughness coefficients for shallow overland flow was developed from simulated rainfall/runoff plots originallycollected for erosion studies. The data were collected from 14 different native rangeland areas in the western United States. Rainfall was appliedat a constantintensity of 65mm/hrfrom 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 char acteristics. The derived roughness regression equations predict an "effective Darcy-Wiesbach roughness coefficient" for nativerangeland (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 shrubsand 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.

Organic litter: dominance over stones as a source of interrill flow roughness on low‐gradient desert slopes at Fowlers Gap, arid western NSW, Australia

“…These plots carried some surface stones and a 10-40 per cent vegetation cover. Weltz et al (1992) derived a relation for the component of f attributable to organic litter, treated separately from soil grain or stone cover roughness:…”

Spatial structures of stream and hillslope drainage networks following gully erosion after wildfire

“…Thus, Abrahams and Parsons (1990) indicate that, when the zero depths are not included, the corrected value of h hill ranges from 0·00424 to 0·00709 m, and therefore h hill = 0·005 m was used for these calculations. Hillslope friction is typically 0·50 m −1/3 s for woody brush, litter and duff, or shrubland and grassland (Weltz et al 1992;NRCS, 1986). Critical shear stresses measured for agricultural soils range from about 0·8 to 6·5 N m −2 (Elliot et al, 1989), and Dietrich et al (1992) reported values that range from 20 to 100 N m −2 for saturated overland flow on grasslands.…”

“…Fires decrease the hillslope friction by removing obstructions, which reduces the friction to values typical of bare soil (n hill = 0·01 m −1/3 s; Weltz et al, 1992;NRCS, 1986). A consequence of removing these obstructions is that the time for runoff to reach the channel decreases.…”

Hydraulics of interrill overland flow on rough, bare soil surfaces

“…Various attempts have been made to predict f, from data on soil cover, stoniness and flow Reynolds number using regression analysis (e.g. Gilley et al, , 1992Weltz et al, 1992;Abrahams et al, 1994). Extrapolation of these equations to surfaces other than those they were developed for is questionable, considering their empirical nature and the fact that different factors may control hydraulic roughness on different surfaces.…”