The e}ects of slope\ cover and surface roughness on rainfall runo}\ in_ltration and erosion were determined at two sites on a hillside vineyard in Napa County\ California\ using a portable rainfall simulator[ Rainfall simulation experiments were carried out at two sites\ with _ve replications of three slope treatments "4)\ 09) and 04)# in a randomized block design at each site "9=53 m 1 plots#[ Prior to initiation of the rainfall simulations\ detailed assessments\ not considered in previous vineyard studies\ of soil slope\ cover and surface roughness were conducted[ Signi_cant correlations "at the 84) con_dence level# between the physical characteristics of slope\ cover and surface roughness\ with total in_ltration\ runo}\ sediment discharge and average sediment concentration were obtained[ The extent of soil cracking\ a physical characteristic not directly measured\ also a}ected analysis of the rainfallÐruno}Ðerosion process[ Average cumulative runo} and cumulative sediment discharge from site A was 76) and 131) greater\ respectively\ than at site B[ This di}erence was linked to the greater cover\ extent of soil cracking and bulk density at site B than at site A[ The extent of soil cover was the dominant factor limiting soil loss when soil cracking was not present[ Field slopes within the range of 3Ð05)\ although a statistically signi_cant factor a}ecting soil losses\ had only a minor impact on the amount of soil loss[ The Horton in_ltration equation _t _eld data better than the modi_ed Philip|s equation[ Owing to the variability in the {treatment| parameters a}ecting the rainfallÐruno}Ðerosion process\ use of ANOVA methods were found to be inappropriate^multiple!factor regression analysis was more useful for identifying signi_cant parameters[ Overall\ we obtained similar values for soil erosion parameters as those obtained from vineyard erosion studies in Europe[ In addition\ it appears that results from the small plot studies may be adequately scaled up one to two orders of magnitude in terms of land areas considered[