This study presents two new modeling strategies. First, a methodology for representing the physical process of subsurface storm flow within a TOPMODEL framework is developed. In using this approach, discharge at quick flow timescales is simulated, and a fuller depiction of hydrologic activity is brought about. Discharge of water from the vadose zone is permitted in a physically realistic manner without a priori assumption of the level within the soil column at which subsurface storm flow saturation can take place. Determination of the subsurface storm flow contribution to discharge is made using the equation for groundwater flow. No new parameters are needed. Instead, regions in excess of field capacity that develop during storm events, producing vertical recharge, are also allowed to contribute to soil zone discharge. These subsurface storm flow contributions to river runoff, as for groundwater flow contributions, are a function of catchment topography and hydraulic conductivity at the depth at which such regions in excess of field capacity occur. The second approach improves groundwater flow response through a reduction of porosity and field capacity with depth in the soil column. Large storm events are better captured and a more dynamic water table develops with application of this modified soil column profile (MSCP). The MSCP predominantly reflects soil depth differences in upland and lowland regions of a watershed. Combined, these two approaches, subsurface storm flow and the MSCP, provide a more accurate representation of the timescales at which discharge responds and a more complete depiction of hydrologic activity. Storm events large and small are better simulated, and some of the biases previously evident in TOPMODEL simulations are reduced.
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