RUNOFF SENSITIVITY TO TEMPORAL AND SPATIAL RAINFALL VARIABILITY AT RUNOFF PLANE AND SMALL BASIN SCALES

Surface runoff sensitivity to spatial and temporal variability of rainfall is examined using physically based numerical runoff models. Rainfall duration t(r) and temporal sampling interval deltat are varied systematically, and normalized by the time to equilibrium t(e). The relative sensitivity R(s) 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 t(r) and deltat. An asymptotic R(s) value proportional to (deltat/t(e))1/2 is approached as t(r) >> t(e). Two-dimensional surface runoff simulations with spatially variable rainfall, without temporal variability, on two watersheds indicate that R(s) decreases as t(r)/t(e) increases. Normalized R(s) versus t(r)/t(e) curves are identical for two watersheds and a one-dimensional overland flow plane. These findings indicate that spatial variability is dominant when t(r) < t(e), while temporal variability dominates when t(r) > t(e), particularly for larger values of deltat/t(e).