Many transport processes on or across the soil surface boundary are controlled by surface microtopography, or roughness. How roughness affects the transport process depends on the length scale of the process. The most commonly used method of expressing soil surface roughness, the roughness length or random roughness, is contrained by the measurement technique and does not embody the concept of scale. The structural function, or variogram, plotted on a log‐log scale was used in this study to express the surface roughness at different scales. With the aid of a laser scanner, surface topography was measured down to 0.5‐mm grid spacing. Data collected from a variety of surface conditions showed that soil roughness can be quantified by a combination of fractal and Markov‐Gaussian processes at different scales. Potential applications of the roughness quantification were also discussed.

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duction into sediment production, that is, the reduced erosion is caused by a reduced water runoff. Hence it Most of prior research showed increasing soil roughness delayed does not differentiate between the roughness effect on runoff and reduced total runoff and sediment yields but failed to water runoff and the roughness effect on sediment prodifferentiate roughness effects on water runoff and on sediment production. duction. This study was conducted to assess separately the effects of soil surface depressions on runoff initiation and water and particle Even in a process-based model, such as the Water fluxes. A 5-m long soil box, filled with a silt loam, was split into 0.6-m Erosion Prediction Project (WEPP), where the process wide paired smooth vs. rough plots with manually formed depressions, of runoff production supposedly has been isolated from and subjected to a sequence of 24 mm h Ϫ1 simulated rainstorms at 5% the sediment production, an increased surface roughslope. Eight experiments were conducted under different upstream ness also results in an overall reduction in sediment inflows and subsurface regimes (drainage or seepage). Collected data delivery. In WEPP, an increased surface roughness include time to runoff initiation and fluxes of water and particles after causes a decrease in interrill sediment delivery and an an apparent steady state was reached. Depressions delayed the runoff increase in critical shear resistance in the rills (Flanagan initiation by storing water into puddles and enhancing infiltration. and Nearing, 1995). Once runoff reached an apparent steady state, surfaces with initial Despite the dominance of research results and predepressions produced 10% greater water flux than the initially smooth surfaces, regardless, the subsurface moisture regime. Roughness had dictive models showing that an increased roughness deno significant effect on steady-state particle flux and concentration. creases erosion, there is evidence pointing the other Our results indicate that the only assured soil and water conservation direction. Burwell et al. (1968) and Burwell and Larson benefit from surface depressions is due to the delay in runoff initiation (1969) showed that after runoff had initiated, a rougher at the beginning of the rain event before the entire surface is contributsurface might not have the distinctly higher infiltration ing to runoff. as a smooth surface as shown before runoff. The laboratory study of Helming et al. (1998) showed that while runoff was marginally affected, rough surfaces did show

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