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 known to concentrate rainfall and route it deep into the soil, yet other mechanisms of flow concentration are poorly understood. This study characterizes pour points, formed by the detachment of water flowing under 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. Throughfall depths at pour points were up to 15 times greater than rainfall and generally comparable to or greater than stemflow. Soil water content beneath pour points was greater than in adjacent controls, with 20%-30% of the seasonal rainfall volume infiltrated into the top 1 m of soil beneath pour points, compared to 5% in controls. Rainfall simulations showed that pour points amplified the spatial heterogeneity of throughfall, violating assumptions used to close the water balance. 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. When rain hits a tree canopy, it either wets the canopy, falls off, or flows along the tree's surfaces (leaves, branches, and trunk). This interaction changes the amount and location of water arriving at the ground. The water flowing underneath branches is may eventually reach the ground by flowing along the tree trunk as stemflow. Using a combination of field observations in seasonally dry Banksia woodlands and rainfall simulation experiments on tree branches, we show that this water may, alternatively, peel off the branch and reach the ground at a "pour point." Rain gauges placed under pour points recorded 1.5-15 times the water recorded at rain gauges under the open sky. We showed that the quantity of water arriving at the pour points varies with the rain volume, and with branch properties including the upstream leaf area, angle, and shape of the branch. The changes in the distribution of water received beneath tree canopies and deeper infiltration into the soil due to pour points proved their hydrological significance. Understanding pour points represents one path toward an improved characterization of the complex processes occurring when rain hits a tree canopy. Pour points occur when intercepted rain flowing under tree branches detach and their depths were 1.5-15 times the rainfall Pour points increase spatial heterogeneity of throughfall and enhance infiltration into the soil Rainfall simulation showed branch structure, foliation, and angle impose unclear controls on the volume of water received at the pour point