A Rationale for Old Water Discharge Through Macropores in a Steep, Humid Catchment

Simultaneous observations of rapid preferential flow through macropores and isotopically “Old” water displacement remain unresolved in the Maimai (M8) catchment. Continuous, three‐dimensional soil moisture energy conditions were monitored in two discrete catchment positions for a series of storm events in 1987. Tensiometric response was related to the soil water characteristic curve, hillslope throughflow, and total catchment runoff. For events yielding ≪2 mm hr −1 peak runoff, near‐stream valley bottom groundwater systems discharged water volumes sufficient to account for storm period streamflow. This process was assisted by regular low (<−40 cm H 2 O) matric potential conditions and rapid filling of available soil water storage. For events yielding >2 mm hr −1 peak storm flow, hillslope hollow drainage into steeply sloping first‐order channels dominated old water production and most of the catchment storm flow. Highly transient macropore‐driven processes of crack infiltration (bypass flow), slope water table development, and lateral pipe flow enabled large volumes of stored water to be delivered to the first‐order channel bank at the appropriate time to satisfy catchment storm flow volumes and water isotopic and chemical composition.