Aerial Spray Patterns in A Forest Canopy

Abstract

ABSTRACT SPRAY distribution experiments were carried out in a level stand of Douglas-fir (Pseudotsuga taxifolia) and western larch (Larix occidentalis). Ground deposition was measured on a rectangular grid of 81 points for each of ten trials. Dosage was estimated from the relative catch of droplets on cylinder pairs hung in the canopy. Wind speeds, direction, and temperatures above the canopy were measured on a tower above the site. The results indicate a division of the spray into a part moved directly into the forest canopy by aircraft wake flows and another which is advected with the ambient wind and diffuses above the canopy in approximate ac-cord with published models for elevated sources above rough surfaces. The relative proportion of the material in the latter component increases with wind speed. The dosage pattern within the canopy is analyzed in terms of a two layer entrainment model for droplet sizes between 8 and 52 yacn and between 52 and 140 \Am. The entrainment coefficient appears to vary quadratically with speed for the smaller droplets but linearly for the larger. An estimated bulk interception coefficient varies ap-proximately as the 3/2ths power of the speed for the larger droplets and as the second power for the smaller droplets.