Subsurface flow velocities through selected forest soils, South Island, New Zealand

Abstract

Subsurface flow through soils in Tawhai, Big Bush and Craigieburn State Forests has been studied by applying water at a line source 1 m upslope from a pit, in the base of which an intercepting trough is located. By measuring lag times between the centres of mass of input and outflow and between the start of input and start of outflow, minimum estimates of mean and maximum flow velocity, V ̄ and V max, at 51 locations were obtained. Mean values for the Tawhai sites were for V ̄ 0.3 cm/s and for V max 0.42 cm/s, but a considerable degree of variability was present, with coefficients of variation up to 90%. A number of different pathways through the soil are followed by flowing water; macropore networks (root channels, etc.) are effective transmitters of water and at some sites conveyed up to 40% of the input rapidly to the interception trough. Variability in flow velocity and the proportion of the input appearing as rapid outflow is a function of antecedent moisture conditions and of the relative importance of the various pathways at a given site, which is in turn a function of the characteristics of the soil, the macropore network and the parent material at the base of the soil. At sites where the soil had an open structure and the parent material was shattered or permeable, the macropore network was a less important control upon soil hydrological behavior than where the subsoil had a less open structure and was underlain by impermeable bedrock. Measurements of flow velocities on undisturbed, logged, and logged/burned/planted sites were made at Tawhai SF, but the spatial and temporal variability was such that no statistically significant differences could be discerned. The time lapse since logging may be insufficient for changes in the root systems to be having a hydrological impact, but the high variability would require a sample size of over 1000 to show a significant difference in velocity of even 10%. For rapid flow through macropores to have a significant influence on streamflow, saturation of the soil is necessary before water can enter the macropores. Soil, topographic, geologic and climatic conditions in Tawhai SF are such that saturation of the lower layers of the soil regularly occurs, so that rapid flow through macropores can contribute to storm-period streamflow. Water may reach the channel system from a proportion of the catchment which varies with antecedent moisture conditions and storm duration and intensity, but which may attain 100% in moderate-to-large rain events. Because of the drier climate and deeper soils, subsurface flow contributions to stormperiod streamflow should be less important in the Big Bush area and, particularly, in the Craigieburn area where the parent materials — shattered sandstone or ancient scree deposits — do not provide an impermeable layer which diverts flow laterally.