Coupled surface-subsurface response to rainfall: Runoff nonlinearity and scale-dependence in a topographically complex catchment

Storm hydrograph peak and timing are controlled by the interaction of rainfall characteristics, such as storm intensity and duration, with the antecedent wetness in a topographically complex catchment. This study investigates the transient runoff response to storm events of varying properties using a physically-based, distributed model of the coupled surface-subsurface system which incorporates the observed spatial variability in topography, soils and vegetation. We demonstrate the significant control that differential runoff partitioning (infiltration-excess runoff, saturation-excess runoff, perched return flow and subsurface stormflow) has on the nonlinearity in the rainfall-runoff transformation and its scale-dependence over multiple internal basins. Antecedent wetness conditions imposed through a distributed water table position are also varied to illustrate its important effect on runoff generation. Differences in rainfall partitioning into surface and subsurface flows explain the nonlinearities in the hydrograph characteristics over a range of basin scales. In particular, storm events excite specific components of the surface-subsurface model leading to variable flood responses over the complex terrain. An analysis of the spatial organization of runoff production also illustrates that multiple runoff mechanisms have specific catchment niches and can occur simultaneously within the basin. In addition, catchment scale plays an important role in the spatial-temporal distribution of individual runoff mechanism as topographic complexity is varied.