The basin factor is a hydrologic parameter that represents the overall impedance to flow of a watershed in the PC-Hydro peak discharge model for small ( < 10 mi2) semi-arid watersheds in Pima County, Arizona. To improve peak discharge estimates and provide validation of the basin factor as a physically-based parameter, basin factor values were "back-calculated" for return period and observed events on six undeveloped rangeland watersheds and correlated with Manning's n roughness coefficients, rainfall intensities, and hydraulic radii. Observed basin factor values displayed a positive trend with observed peak discharge. The results suggest that the correlated variables can predict the basin factor on small undeveloped sub-basins in Pima County and the basin factor is measurable as a physical parameter. Empirical models for basin factor prediction are proposed. The study may also be useful for estimating hydraulic roughness in hydrologic methods such as the kinematic wave time of concentration equation.