Changes in leaf inclination angle as an indicator of progression toward leaf surface storage during the rainfall interception process

Abstract

During rainfall events, rain and throughfall drops impact on individual leaves exerting a force that divides the drops into multiple droplets and produces momentary increases in the leaf inclination angle. A portion of the splash residue from the raindrop impact accumulates on the leaf surface and adds mass to the lamina of leaf. This lab-based study examined changes in leaf inclination angle after the impact of a sequence of raindrops of two different volumes (10 µl and 30 µl) on leaf surfaces of three different tree species (Acer saccharinum L., Ulmus pumila L., and Quercus gambelii Nutt.). Leaf inclination is measured as the angle between the lamina surface normal and the horizontal. The differences between the initial leaf inclination angle before raindrop impacts and the steady-state leaf inclination angles after each raindrop impact were examined to explore the hypothesis that rainsplash residue accumulated on leaves after raindrop impact will incrementally increase the steady-state leaf inclination angle as the leaves approach leaf surface storage. The difference between the initial leaf inclination angle and the steady-state leaf inclination angle after two 10 µl raindrops increased by 0.23° for Q. gambelii, 0.84° for A. saccharinum, and 1.29° for U. pumila. The difference between the initial leaf inclination angle and the steady-state leaf inclination angle after two 30 µl raindrops increased by 0.56° for Q. gambelii, 1.45° for U. pumila, and 2.05° for A. saccharinum. For each species, the mass of the accumulated water incrementally increased the steady-state leaf inclination angle after each raindrop impact. As expected, larger raindrops produced more rainsplash residue on the leaf surface based on the incremental increase in steady-state leaf inclination angle after sequential raindrop impacts. With repeated raindrop impacts, leaves in the canopy accumulate more water (mass) as the maximum leaf surface storage is achieved. Observing changes in steady-state leaf inclination angles after raindrop impact may serve as an indicator for rainfall interception totals during rainfall events.