Numerical experiments of streamflow generation in steep catchments

SummaryThe role played by subsurface flow in the rainfall–runoff transformation is still poorly understood. In particular, two aspects are highly debated. The first one is the relatively fast response of the catchments to rainfall input which is often observed. The rapid response contradicts the general notion of slow movement of water in the subsurface. Another important issue is the control of pre-event water to streamflow. In the present work, we perform a series of 2D numerical simulations in order to gain understanding on the above problems. The platform is the experimental site described in Montgomery and Dietrich [Montgomery, D.R., Dietrich, W.E., 2002. Runoff generation in a steep, soil-mantled landscape. Water Resour. Res. 38(9), 1168, doi:10.1029/2001WR000822]. The aim is not to replicate exactly the experimental conditions but rather to use the model as a numerical laboratory for investigating streamflow generation processes. The simulations show that a Darcian formulation of subsurface flow in a combined soil–bedrock system can generally reproduce fast catchment streamflow responses comparable to observations. The principal mechanisms for the streamflow generation are subsurface flow along the soil–bedrock interface combined with groundwater ridging in the vicinity of the hillslope base. Numerical analysis of solute transport indicates high proportions of pre-event water in the resulting hydrograph.