The relationship between hydrologic processes and scale is one of the more complex issues in surface water hydrology. Plot scale measurements of infiltration and runoff were made on 2 m by 6 m rainfall simulator plots on an instrumented sub-watershed within the USDA-ARS Walnut Gulch Experimental Watershed (WGEW). Rainfall and runoff data have been collected for over 30 years on the 1.9 ha sub-watershed using a recording rain gage and instrumented weir. Detailed soils, vegetation, and topography data have been collected at both the plot and watershed scales and input into a GIS. Plot scale infiltration and runoff measurements were made using a variable intensity rainfall simulator at a range of intensities and two different soil moisture conditions within the watershed. A distributed, process-based hydrologic model coupled with a GIS is used along with measured plot characteristics to determine distributed Green and Ampt infiltration parameters using the plots as micro-watersheds. The same hydrologic model is used to determine distributed infiltration parameters using a range of spatial configurations at watershed scale. The level of discretization necessary to model the observed infiltration and runoff processes at both the plot and small watershed scales are compared.