AGDISP Sensitivity to Crop Canopy Characterization

As computer-based decision and modeling systems become increasingly integrated into agriculture, it is important that users understand the effects that various inputs have on these systems. The objective of the work presented here is to quantify the effects that different crop canopy characteristics, such as height and canopy closure, have on aerially applied spray deposition and downwind movement and compare these results to the predictions of spray movement and deposition by the AGDISP computer model. Six trials were conducted in cotton fields ranging from bare ground up to 1 m in height and canopy closure ranging from 0% to 100%. Horizontal deposition measurements were used to verify that each of the applications made during each of the trials produced similar levels of deposition. Monofilament strings were placed at four sampling heights (1, 2 ,4, and 6 m) above the canopy at a distance of 50 m from the downwind edge of the spray swath to measure the airborne droplets at this distance. The vertical deposition values at 50 m for crop heights between 0.3 and 0.8 m were comparable between the field-collected and AGDISP-predicted data. The AGDISP model (version 8.08) overpredicted by a factor of 2 the levels of deposition at 1 m for the trials conducted with crop canopy height of 0 or 1 m as compared to field measurements; however, at 4 and 6 m above the ground, AGDISP and the field data agreed well. Users of AGDISP should be encouraged by the accuracy of the model but are cautioned when using the model with canopies that are more than 80% closed.

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