Impact droplets and the protection of soils by plant covers

Abstract

Impact droplets, the fine drops splashed from the impact sites of larger drops on plant elements, were investigated experimentally to assess their role in water erosion. It was found that impact droplets rarely strike the soil surface before decelerating markedly from their sometimes high initial velocities. This, and their small sizes, render them almost non-erosive. Their production therefore represents a major soil-conserving mechanism in that large, potentially erosive drops are converted to almost harmless drizzle. Conversion rates of incident-drop water to impact droplets were found to be about 10-25% by weight for large, effectively semi-infinite targets, but rose rapidly to about 90% as impact points approached within half a drop diameter of target edges. Most impacts on plant elements are edge-affected and elements of about the same diameter as a raindrop (e.g. those of fine grasses) produce double-edge effects, almost entirely converting incident drops into impact droplets. These results, combined with previous findings, allow consideration of the abilities of different arrays of plant elements (i.e. 'covers') to combat water erosion generally. Closely spaced, near-ground, plant elements, about 1-3 mm in diameter, display a peculiar overall effectiveness in this respect.