Development of a Regulatory Model to Simulate Deposition and Drift from Application of Pesticides

The USDA Forest Service has been working on the problem of simulating pesticide application from aircraft for over three decades. The object of this modeling is to predict landing position of sprayed material and ultimately the efficacy of the pesticide application. Over the past two decades, environmental concerns surrounding pesticide application have required expansion of the domain of the modeling. More recently, ground spraying has been added as a model scenario within these model templates. In the 1990's, the Forest Service model (AGDISP) was adopted under a cooperative development program between agricultural chemical manufacturers and the USEPA as a regulatory model (AGDrift).

These models are Lagrangian transport models which utilize an idealized wake algorithm to transport droplets. Single point meteorology is used and droplets are allowed to evaporate. Extensive data sets indicate that the three primary variables involved in spray transport are droplet size, wind speed and release height (relative humidity can be critical depending upon the volatility of the sprayed material). These models have between three and four dozen input variables and adjustable parameters ranging from detailed description of the aircraft (usually downloaded from an extensive library), through the release geometry to a description of the material being sprayed. The domain of these models is 1000m downwind, though AGDISP contains a straightline Gaussian model to extend its domain to O10km. The model is remarkably accurate in calculating deposition of larger droplets near the aircraft. As would be expected, the accuracy falls off with distance and atmospheric transport time which can be very long for fine droplets.

This model has been widely published but is relatively unknown in the meteorological field because it has focused in the past on near field deposition and mechanical aspects of the application technique. As concern increases regarding smaller amounts of material, transported as very fine droplets for greater distances, the need for sophisticated atmospheric modeling has increased as has the interest in this model from air pollution meteorologists.

The basic model physics will be discussed along with ongoing recent work in canopy interception, droplet neighborhood effects, extending the model to incorporate ground spraying, modification of the evaporation algorithms (including multi-component evaporation) and interfacing the model with more sophisticated meteorological modeling.