Quantifying the relative contributions of riparian and hillslope zones to catchment runoff

The spatial and temporal sources of headwater catchment runoff are poorly understood. We quantified the contributions of hillslopes and riparian zones to streamflow for two storm events in a highly responsive, steep, wet watershed located on the west coast of the South Island of New Zealand. We examined the spatial and temporal components of catchment storm flow using a simple continuity‐based approach. We tested this with independent isotopic/solute mass balance hydrograph separation techniques. We monitored catchment runoff, internal hydrological response, isotopic, and solute dynamics at a trenched hillslope, and at hillslope and riparian positions in a 2.6‐ha catchment. The gauged hillslope was used to isolate and quantify (by difference) riparian and hillslope zone contributions to the 2.6‐ha headwater catchment. Utilizing flow‐based approaches and a tracer‐based mass balance mixing model, we found that hillslope runoff comprised 2–16% of total catchment storm runoff during a small 27‐mm event and 47–55% during a larger 70‐mm event. However, less than 4% of the new water collected at the catchment outlet originated from the hillslopes during each event. We found that in the 27‐mm rain event, 84–97% of total storm runoff was generated in the riparian zone. In a larger 70‐mm event, riparian water dominated total flow early in the event, although the hillslope became the main contributor once hillslope runoff was initiated. Despite the large amount of subsurface hillslope runoff in total storm runoff during the second larger event, riparian and channel zones accounted for 96% of the new water measured at the catchment outlet. Riparian water dominated between events, throughout small runoff events, and during early portions of large events. While this sequencing of catchment position contributions to flow has been conceptualized for some time, this is the first study to quantify this timing, constrained by hydrometric, isotopic, and solute approaches.