An Experimental Investigation of Runoff Production in Permeable Soils

In an area of low intensity rainfall and permeable soils, three hillside plots were instrumented for a study of runoff‐producing mechanisms. Runoff from the plots was measured at the ground surface, the base of the root zone, and in the zone of perennial groundwater seepage. Data on soil moisture, water‐table elevation, and piezometric head were also collected during natural and artificial storms. The data showed that, as the infiltration capacity of the soil exceeded the rainfall intensities that occurred and that were applied, overland flow generated by the mechanism described by Horton did not occur. Although soils and topography were those generally thought to be conducive to subsurface stormflow, the runoff produced by this mechanism was too small, too late, and too insensitive to fluctuations of rainfall intensity to add significantly to stormflow in the channel at the base of the hillside. When the water table rose to the surface of the ground, however, overland flow was generated on small areas of the hillside. Only when this overland flow occurred were significant amounts of stormflow contributed to the channel by the hillside. The return periods of storms that would produce such overland flow were found to be very large.