AGDRIFT/AGDISP Model Capabilities

    This page provides an overview of the aerial application models AGDRIFT and AGDISP for the accurate prediction of deposition of spray material released from aircraft. These models address the behavior of aerially released material from a wide range of aircraft (Figure 1) beginning after nozzle breakup into droplets, through interaction with the aircraft wake and the atmosphere, to deposition through a canopy and onto the ground. This physics-based model follows the droplets with a Lagrangian particle solution technique validated against a series of 27 field studies conducted by the USDA Forest Service (FS) and its cooperators (1972-1995), and extensive industry Spray Drift Task Force (SDTF) field studies (1992-1993).

Figure 1: Aerial applicators modeled by AGDRIFT and AGDISP.

    The key motivation for use of these models is that the uniform aerial spray application of pesticides within a clearly defined spray area must be at levels sufficient to achieve efficacy. At the same time, drift of pesticides from the target site during these applications is a source of environmental concern due to its potential human health impacts, downwind damage to crops and livestock, and endangerment of ecological resources, especially streams and water bodies. AGDRIFT/AGDISP comprise a method for evaluating spray area deposition patterns of aerial pesticide applications, and deposition off-site and downwind, bypassing the need for exhaustive flight testing. (Note: The AGDISP code is the development platform for FS, while AGDRIFT is the SDTF regulatory version of the model).

    The AGDSIP model had its roots in U.S. Army modeling activity in the 1960s, in which Gaussian modeling techniques were used to account for the loss of material by gravitational setting of droplets from elevated spray clouds, and to predict the resulting surface deposition patterns. This work was largely a collaboration by the FS and the US Army with H.E. Cramer and his associates. The computer code that resulted was called FSCBG (for Forest Service Cramer-Barry-Grim after its developers). In 1979 C.D.I. began developing a Lagrangian model for the dispersal of spray material, culminating in a model for NASA known as AGDISP (for AGricultural DISPersal). This approach included models for aircraft wake effects (vortices, propellers, jet engines) and evaporation, and subsequently became the "near-wake model" for FSCBG. Further development and refinement led to the SDTF regulatory model AGDRIFT.

    AGDISP predicts the motion of spray material released from aircraft, including the mean position of the material and the position variance about the mean as a result of turbulent fluctuations. Released spray material is modeled as a discrete set of droplets, collected into categories, and called a drop size distribution. Each drop size category is defined by its average diameter and volume fraction, and is examined sequentially by the model. A Lagrangian approach is used to develop the equations of motion for discrete droplets released from the aircraft, following the droplets as they move in the wake of the aircraft and into the atmosphere, accounting for local winds (Figure 2). The novel feature of the AGDISP methodology is that the dispersion of a group of similarly sized droplets resulting from turbulent fluid fluctuations in the atmosphere is quantitatively computed within the aircraft wake as the group of droplets descends toward the ground, with evaporation accounted for during this transit (Figure 3). AGDRIFT/AGDISP are PC-based codes, operating in a Windows environment with a highly capable GUI to guide user operation (Figure 4). Over 1000 copies of these models are in use worldwide.

Figure 2: Droplet trajectories predicted by AGDISP's Lagrangian model in a crosswind.

Figure 3: Typical particle evaporation history during transit to the ground

Figure 4: Main screen for Windows-based AGDISP GUI.