THE INFLUENCE OF MACROPORES ON RUNOFF GENERATION FROM A HILLSLOPE SOIL WITH A CONTRASTING TEXTURAL CLASS

The hydrological response of a well-structured hillslope soil under pasture is described. In this soil, during the winter recharge period, macropores provide pathways for water movement which effectively by-pass the soil matrix. In consequence, a perched water table does not develop within the soil profile. New input water is able to penetrate rapidly through the macropore system within the clayey B horizon to the soil/bedrock interface. A highly transient 'water table' is then established within the macropore system and overland flow commences only when the capacity of this system is exceeded. During the summer, this flow pattern changes and, although the soil might be expected to absorb water more readily than during winter, overland flow is the dominant runoff mechanism. The macropore-matrix dichotomy of the soil hydraulic properties is characterized in situ using auger hole and disc permeaters. The measured soil hydraulic properties are used in a finite element model to predict independently the subsurface runoff response during the winter recharge period and to check the adequacy of a throughflow collection system. The model predictions are in reasonable agreement with measured water tables during drainage.