Technical Abstract: During rainfall events, surface roughness affects runoff generation by providing surface storage in the depressions and altering the flow direction on the surface. The process of runoff initiation involves the gradual filling of individual depressions and the connection of those overflowing depressions toward the outflow boundary. The study examines the role surface roughness on runoff initiation. Rainfall simulation experiments were conducted on a 2.4-m x 2.4-m soil box exposed to a sequence of four rainfall events with two levels of initial roughness and under two slope gradients. Surface microtopography was digitized by a laser scanner and variogram was used to express the scale-dependent roughness function. Variogram showed a gradual lowering indicating all scales of roughness were affected by rainfall similarly. Analyses of runoff triggering, using the conditional walker model, showed a major effect on runoff initiation from a small modification of micro- topographic structure. Even if the storage capacity is an important parameter of the runoff characteristics, large differences are observed between initial stages of each experiment and final stages. We attributed these differences to the creation of preferential connections between topography depressions due to the material redistribution. Since the variogram analysis may not be sensitive toward the development of preferential flow path in a local scale, additional methodologies, such as the conditioned-walker analysis, need to be incorporated in quantifying the role of surface microtopography on the dynamics of runoff generation.