Surface runoff sensitivity to spatial and temporal variability of rainfall is examined using physically based numerical runoff models. Rainfall duration tr and temporal sampling interval Δt are varied systematically, and normalized by the time to equilibrium te. The relative sensitivity Rs is defined as the total volume of outflow variability over 50 Monte Carlo simulations normalized by the rainfall volume and the coefficient of variation of rainfall. Relative sensitivity to temporal rainfall variability increases with both tr and Δt. An asymptotic Rs value proportional to (Δt/te)1/2 is approached as tr≫te. Two-dimensional surface runoff simulations with spatially variable rainfall, without temporal variability, on two watersheds indicate that Rs decreases as tr/te increases. Normalized Rs versus tr/te curves are identical for two watersheds and a one-dimensional overland flow plane. These findings indicate that spatial variability is dominant when trte, particularly for larger values of Δt/te. ¿ American Geophysical Union 1993<