Technical Note: Model Physics and Collection Efficiency in Estimates of Pesticide Spray Drift Model Performance

Highlights Recent large field programs are re-examined in the context of model development. Details of plant canopy wind fields are discussed. Collection efficiency of rotorods is discussed in detail, and the theory is used to re-examine field data. The approach used in the AGDISP model to simulate canopy wind fields is discussed in detail. Abstract . Recent field studies provided data to evaluate the performance of the aerial spray deposition algorithm in AGDISP. Those studies provided data for forest canopy settings that are either outside the stated domain of AGDISP or where assumptions in the model greatly impact the model performance. The two data sets were collected with the intention of providing input to drive model upgrades, but data limitations restricted that objective. Rather, this technical note shows that collection efficiency (CE) must always be considered (the model currently adjusts for CE only if the modeled output is canopy capture). One of the previous studies showed that the model substantially overpredicted droplet flux 65 m downwind of the spray line. Consideration of CE resolves some of this overprediction, but the model physics employed in AGDISP remain a substantial simplification of the complex flows that transport droplets in the atmospheric boundary layer near and in deep, three-dimensionally varying forest canopies. Keywords: Aerial application, AGDISP, Model, Spray drift.