Watershed from runoff from thunderstorms is heavily dependent upon the rainfall intensity and the infiltration or loss rate of the watershed. Although techniques are available for determining infiltration rates of soils, these techniques are inadequate when dealing with vegetation-soil complexes found in arid and semi-arid environments. A more representative technique is on-site measurements of runoff from natural and/or simulated rainfall. Analysis from natural and simulated rainfall-runoff plots located on a bajada (alluvial fan) northeast of Las Cruces , New Mexico, indicate that the sparse desert vegetation does not have a significant effect on runoff depth (natural plots), vegetation has a slight but significant effect on infiltration rate (simulator plots), and a seasonal effect on infiltration losses.