Estimating Small Grain Equivalents of Shrub-Dominated Rangelands for Wind Erosion Control

Awind erosion equation, which estimates average annual erosion, requires that all vegetative cover be expressed as dry biomass per unit area of flat small grain equivalent (SG)e. For a standing vegetative canopy, the (SG)e depends on the magnitude of the friction velocity reaching an underlying erodible surface. The soil surface friction velocity, and thus (SG)^, was shown to be a function of aerodynamic roughness length of the canopy and the product of a drag coefficient and plant area index. Aerodynamic roughness, as well as canopy silhouette area and mass distribution, were measured in sand sagebrush {Artemisia filifolia Nutt.) and yucca {Yucca elata Englem.) canopies. Estimating equations were developed to predict (SG)^ of the sagebrush and yucca canopies using either above-ground dry biomass or plant area index as inputs. Additional estimating equations were developed to predict plant area or plant mass from simple geometric measurements of yucca and sagebrush. Finally, for shrub or stubble canopies in which (SG)e prediction parameters have not been measured, a way to approximate the prediction parameters using an estimate of canopy aerodynamic roughness length was developed.