The hydrologic properties of the forest floor under beech/podocarp/hardwood forest, North Westland

Abstract

A survey of the depths and nature of the forest floor on six small steep experimental catchments showed that the mean total depth of forest floor over the entire study area was 17 cm. Litter and fermenting layers were typically only 2-3 cm thick. The forest humus was characterised by an open, fibrous structure or low bulk density (0.14 g/cm³) and high porosity (mean 86%), with a wide variability in its physical properties over very short distances. Water retention tests showed that the forest floor was capable of retaining large quantities of water, (mean saturation capacity was 481% by weight, or 50% by volume). Much of the water above its field capacity of 45% by volume was held by large macropores which drained readily at very low tensions (0-20 cm water). The most frequent pore size class of the 15 samples tested was 250-300 fM. The Qnitial) saturated hydraulic conductivity of the forest floor was very high (0.14 cm/s, at 15°C). This reduced to a steady rate of 0.02 cm/s after several hours. The saturated hydraulic conductivity of the mineral soil was 0.007 cm/s. Field infiltration tests yielded an initial rate of 0.27 cm/s t which fell to a steady rate of 0.17 cm/s after approximately 5-10 minutes. Estimated quickflow yields in the rising limb of 12 storm hydrographs from one catchment (catchment 5), based on the physical and hydrologic properties of the humus, explained between 4% and 92% of the observed rising limb quickflow yield. Estimates became closer to the observed quickflow as the storm duration, time to peak and antecedent soil moisture content increased.