RUNOFF IN SHALLOW SOILS UNDER LABORATORY CONDITIONS

Laboratory experiments of runoff production in shallow soils were conducted on plots consisting of a bedrock layer, an intermediate soil layer, and a crushed stone cover layer. Artificial rainfall was supplied in varying amounts and time sequences; and surface runoff, soil layer interflow, and throughflow were measured. The purpose of the experiments was to investigate infiltration mechanisms in shallow soils and to characterize the effect of stone cover on the partitioning of rainfall at the soil surface. Two soil types were used: (1) A sandy loam; and (2) a silty clay loam. In the case of the sandy loam, the results showed that the majority of rainfall infiltrated readily into the soil, where it subsequently moved downward to the bedrock as throughflow. Strong throughflow response was attributed to preferential flow through large soil pores. Because these macropores formed naturally during the experiments, this work serves as a plot-scale demonstration of the role of rainfall as a factor of change of soil hydrology. For the silty clay loam, the majority of rainfall exited the plot as surface runoff from the upper surface layer, while the relative magnitude of interflow and throughflow components of storm response increased with rainfall duration. Increasing the slope of the soil layers resulted in increased surface runoff and throughflow and reduced interflow. The influence of surface storage as represented by the thickness of the stone cover was found to be minimal for both soil types. This suggests that runoff production in both cases is controlled by the infiltration capacity of the soil layer alone.