Storm Runoff Generation in Humid Headwater Catchments: 2. A Case Study of Hillslope and Low‐Order Stream Response

Previous hydrometric and dye tracer studies in Maimai 8, a highly responsive catchment in the Tawhai State Forest, Westland, New Zealand, suggest that storm runoff generation is by rapid transmission of “new” (current storm rain) water to the stream via macropores. We used hydrometric and natural tracer (oxygen 18, deuterium, electrical conductivity, chloride) observations in two first‐ and one second‐order stream and in six throughflow pits, to evaluate the roles of “old” (stored) and new water during three storm events (return periods from 4 weeks to 3 months) in September 1983. New water contributions were small (<25% of the hydrograph volume) and could be accounted for by saturation overland flow. Hillslope response varied areally but all sites issued old water‐dominated throughflow. Ridge top sites had larger new water contributions (∼ 30–40%) than valley sites (< 10%). Macropore flow of new water therefore cannot explain streamflow or throughflow response in the Maimai catchments.