The age, origin and pathway of subsurface stormflow in a steep humid headwater catchment.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A combined recording tensiometric, hydrometric, chemical and stable isotope tracing approach was used to identify the age, origin and pathways of subsurface stormflow in a small (3.8 ha) catchment in the Tawhai State Forest, Westland, New Zealand. Eleven storm runoff events were intensively monitored during the August to December 1987 study period. Of the 7 events suitable for isotopic mass balance hydrograph separation, the volume fraction of groundwater, or pre-event water, averaged 10% of stormflow volumes. New water contribution could be accounted for by saturation overland flow and headwater subsurface flow. Hillslope flow was dominated by old water contributions of between 70 and 100%, which increased downslope. Within storm deuterium variations in rainfall were high (4.4 to 93.5%/oo), reflecting changes in air mass trajectory and rainfall intensity. Soil water and groundwater deuterium concentrations showed a dampened response to rainfall input, and mean residence time varied from approximately 12 to 100 days, depending on soil depth, slope position and distance downslope. Soil physics analyses were conducted in valley-bottom nearstream, midslope hollow and upslope hollow locations. Tensiometric results showed that the unsaturated zone overlying near-stream groundwater was rapidly changed to positive matric potential due to the limited storage characteristics of the local soils. This response produced an early and steady increase in old water exfiltration into the stream channel as a groundwater ridge developed along the channel margin. As rainfall depth increased above approximately 10 mm, contributions from mid-slope and upslope hollows dominated channel stormflow, and most of the subsurface water was delivered to first order channels via continuous pipes occurring at the mineral soil-bedrock interface. Limited storage effects augmented the development of a perched water-table in the mid-slope hollow, which was quickly dissipated by lateral pipeflow. Bypass flow down cracks was observed during some events with high (8 to 14 mm hr⁻¹) short-term rainfall intensity bursts. The relatively low frequency of high hourly rainfall intensities, however, ensures that bypassing is not a regular occurrence in the M8 catchment. Results from ten hillslope water injection experiments in various slope positions showed that input water isotopic signatures attained an old water status very quickly, due to mixing with a near-saturated soil matrix along crack and pipe walls. The major implications of this work are twofold: (i) it has demonstrated that stream-based isotopic and chemical tracing can be reconciled with hillslope-oriented hydrometric work, and (ii) it has revealed some of the important runoff processes that link the idea of macropore flow with the notion of old water displacement, and which satisfies information on soil water physics and water isotopic concentrations.