The age, origin and pathway of subsurface stormflow in a steep humid headwater catchment.

摘要

A combined recording tensiometric, hydrometric, chemicaland 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 theAugust to December 1987 study period. Of the 7 events suitable for isotopic mass balance hydrograph separation, the volumefraction of groundwater, or pre-event water, averaged 10% of stormflow volumes. New water contribution could be accounted forby saturation overland flow and headwater subsurface flow.Hillslope flow was dominated by old water contributions ofbetween 70 and 100%, which increased downslope. Within stormdeuterium 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 adampened response to rainfall input, and mean residence timevaried from approximately 12 to 100 days, depending on soildepth, 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 overlyingnear-stream groundwater was rapidly changed to positive matricpotential due to the limited storage characteristics of the localsoils. This response produced an early and steady increase in oldwater exfiltration into the stream channel as a groundwater ridgedeveloped along the channel margin. As rainfall depth increasedabove approximately 10 mm, contributions from mid-slope andupslope hollows dominated channel stormflow, and most of thesubsurface water was delivered to first order channels viacontinuous pipes occurring at the mineral soil-bedrock interface.Limited storage effects augmented the development of a perchedwater-table in the mid-slope hollow, which was quickly dissipatedby lateral pipeflow. Bypass flow down cracks was observed duringsome events with high (8 to 14 mm hr⁻¹) short-term rainfallintensity bursts. The relatively low frequency of high hourlyrainfall intensities, however, ensures that bypassing is not aregular occurrence in the M8 catchment. Results from tenhillslope water injection experiments in various slope positionsshowed that input water isotopic signatures attained an old waterstatus very quickly, due to mixing with a near-saturated soilmatrix along crack and pipe walls.The major implications of this work are twofold: (i) it hasdemonstrated that stream-based isotopic and chemical tracing canbe reconciled with hillslope-oriented hydrometric work, and (ii)it has revealed some of the important runoff processes that linkthe idea of macropore flow with the notion of old waterdisplacement, and which satisfies information on soil waterphysics and water isotopic concentrations.