Introducing Pour Points: Characteristics and Hydrological Significance of a Rainfall-Concentrating Mechanism in a Water-Limited Woodland Ecosystem

The interception of rainfall by plant canopies alters the depth and spatial distribution of water arriving at the soil surface, and thus the location, volume, and depth of infiltration. Mechanisms like stemflow are well known to concentrate rainfall and route it deep into the soil, yet other mechanisms of flow concentration are poorly understood. This study characterises pour points, formed by the detachment of water flowing on the lower surface of a branch, using a combination of field observations in Western Australian banksia woodlands and rainfall simulation experiments on Banksia menziesii branches. We aim to establish the hydrological significance of pour points in a water-limited woodland ecosystem, along with the features of the canopy structure and rainfall that influence pour point formation and fluxes.Pour points were common in the woodland and could be identified by visually inspecting trees. Water fluxes at pour points were upto 15 times rainfall and were usually comparable to or greater than stemflow. Soil water content beneath pour points was greater than in adjacent control profiles, with 20-30% of seasonal rainfall volume infiltrated into the top 1m of soil beneath pour points, compared to 5% in controls. Rainfall simulations showed that pour points amplified the spatial heterogeneity of throughfall, violating water balance closure assumptions. The simulation experiments demonstrated that pour point fluxes depend on the interaction of branch angle and foliation for a given branch architecture. Pour points can play a significant part in the water balance, depending on their density and rainfall concentration ability.