Introducing Pour Points: Characteristics and hydrological significance
of a rainfall-concentrating mechanism in a water-limited woodland
ecosystem
Abstract
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.