The role of upslope soil pore water pressure on lateral subsurface storm flow dynamics is poorly understood. Further development of hillslope hydrologic models requires new understanding from field understanding. In particular, we need new, quantifiable measures that link upslope soil pore pressure and water table dynamics to the timing and volume of subsurface storm flow. Here we examine the relationship between hillslope-scale pore pressure and lateral outflow from slope base using the fine-temporal-resolution hydrometric data (10 min interval) from two steep unchanneled concave hillslopes, one hillslope (Fudoji) covered by relatively high hydraulic conductivity sandy soil and the other (Toinotani) covered by relatively low hydraulic conductivity clay soil. In both hillslopes, pore pressures in the area close to the slope base were only weakly related to subsurface storm flow dynamics. During periods of storm flow production, hillslope discharge was strongly related to the cross-sectional area of the upslope saturated layer. During slope seepage periods between events, hillslope discharge from the highly permeable hillslope was still related to the upslope cross-sectional subsurface saturated area. However, during this same period at the low-permeability site, hillslope discharge was not related to the upslope subsurface saturated area. Through intersite comparison we show that the soil matrix permeability has a large impact on the hydrological extension of preferential flow and hence the linkage between upslope pore pressure and subsurface storm flow dynamics.