SURFACE-ROUGHNESS CHANGES AS AFFECTED BY RAINFALL EROSIVITY, TILLAGE,AND CANOPY COVER

Surface roughness and canopy cover are important factors in preventing soil erosion. There is limited information on how soil surface roughn ess changes as a function of natural rainfall erosivity and canopy cov er by plants. We hypothesized that canopy cover, tillage systems, and cumulative rainfall erosivity (CRE) would have unique effects on rough ness. We tested this hypothesis on a Miami silt loam soil (fine-silty, mixed, mesic Typic Hapludalf) using a portable laser microtopographer . Tillage treatments of conventional (moldboard plowing + disking), ch isel plowing, and chisel plowing + dragging a chain produced three ini tial roughness levels. Surface cover was none (fallow) or soybean [Gly cine max (L.) Merr.]. Random roughness (RR), standard deviation (SD), tortuosity (T), and fractal roughness functions, expressed by the frac tal index (D) and the crossover length (l), were calculated from micro topography data. Chisel tillage had the greatest initial values of sur face roughness, followed by chisel + chain and conventional tillage, a s measured by the l index. All indices but D generally decreased with CRE. The RR and SD indices decreased quadratically with CRE, with decr eases of 38 and 36%, respectively, from initial values after 200 units of CRE, while the T and l indices decreased exponentially, with decre ases of 40 and 60%, respectively, from initial values after 200 units of CRE. Soybean cover lowered soil surface roughness 7% less than fall ow, as measured by the l index. The l index was 50, 71, and 205% more sensitive to changes in CRE than RR, SD, and T indices, respectively. The fractal roughness functions, with D and l indices calculated, were the best approaches to characterize surface roughness at small scales , such as existing plant rows, mainly due to l index sensitivity to ch anges in CRE.