Discretization for a spray deposition model: Criteria for temporal and spatial differencing

• Studied simulated midair spray mass sensitivity of model to time and space difference.
• Sensitivity observed under varying time and space step sizes combinations.
• Sprayer air velocity found very important in model.
• Criterion for selecting optimum time step was established.

A spray deposition model was developed to simulate spray dispersion in open space and deposition within a citrus tree canopy. It was validated by field tests reported in earlier publications. The model is based on a compartment modeling approach where the spray cloud is spatially partitioned into several compartments, with equal thickness but increasing cross section in the direction of application. The stability of the simulation curves depends concurrently on the compartment thickness (Δx) and time step (Δt) used. Simultaneously, the needed computer memory and computational time greatly depend on Δt. In this application note, the influence of these two parameters on simulated airborne spray mass has been explored and a criterion for selecting a pair of them to optimize simulation output has been established. The criterion, Δt = Δx/ua, establishes a strong relationship of these two parameters with sprayer air velocity, ua, stressing the importance of ua in the model. The results provided critical understanding about the sensitivity of the model to the studied parameters and tend to be crucial for successful implementation of the model.