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

Surface runoff sensitivity to spatial and temporal variability of rain fall is examined using physically based numerical runoff models. Rainf all duration t(r) and temporal sampling interval deltat are varied sys tematically, and normalized by the time to equilibrium t(e). The relat ive sensitivity R(s) is defined as the total volume of outflow variabi lity 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 spati ally variable rainfall, without temporal variability, on two watershed s 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-di mensional overland flow plane. These findings indicate that spatial va riability is dominant when t(r) < t(e), while temporal variability dom inates when t(r) > t(e), particularly for larger values of deltat/t(e) .