Towards Improving Pesticide Dispersion Models: An Evaluation of Spray Cloud Thermodynamics

Spray drift models are commonly used to determine buffer distances between target areas (e.g., crops, forests) and non-target areas (e.g., streams, neighboring crops, urban areas). Most current modeling of spray drift uses a single droplet approach that treat the atmosphere around the droplet as if it were not influenced by other droplets. This lack of a ‘neighborhood' or ‘cloud' effect leads to the over estimation of evaporation as the elevated humidity caused by the other droplets in the droplet cloud is not considered. Further, the effects of expansional cooling and evaporative cooling are not included in the current applied models. These effects not only directly influence evaporation but also change the temperature of the aggregate cloud and may cause slumping.

Dense Gas models, such as SLAB and DEGADIS, have existed for many years and are used to model such phenomena as material releases due to the collapse of a tank storing a volatile liquid, the crash of a tanker truck, and jet release from valve ruptures. These models were originally developed to have low computational requirements. As such, most Dense Gas models are so-called “1-D models” that capture downstream dispersion of a denser than air cloud. Dense Gas models handle the cloud thermodynamics in some form. For example, SLAB captures changes in cloud temperature and turbulent mixing due to liquid droplet formation and evaporation. Although relatively simple, these models may represent significant improvements to current spray drift applied models through improved thermodynamics.

Here, we evaluate spray cloud thermodynamics in an effort to improve spray drift models. We compare pesticide drift models to dense gas models over a range of application rates, droplet size distributions, and ambient meteorological conditions for simple spray configurations (i.e., one nozzle with zero crosswind). Evaporative loss and fraction of applied to the ground surface as a function of downwind distance from the source are presented.