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Guidance on the Use of the Linked AGDISP-Gaussian Extension Models for Estimating Far Field Drift of Pesticides in FIFRA Ecological Risk Assessments

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Guidance on the Use of the Linked AGDISP-Gaussian Extension Models for Estimating Far Field Drift of Pesticides in FIFRA Ecological Risk Assessments (PDF) (2 pp, 47K, About PDF)

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Memorandum

March 2, 2010

SUBJECT: Guidance on the Use of the Linked AGDISP-Gaussian Extension Models for Estimating Far Field Drift of Pesticides in FIFRA Ecological Risk Assessments

FROM: Donald J. Brady, Ph.D.,Director, Environmental Fate and Effects Division (7507P)

TO: EFED Managers and Staff

I am issuing this technical guidance on the use of the linked AGDISP-Gaussian extension models for estimating far field drift of pesticides in pesticide risk assessments, consistent with senior science staff recommendations. This guidance is effective immediately.

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BACKGROUND

The Lagrangian model of AGDISP predicts near-field spray drift up to 800 meters (0.5 miles) downwind from the application site. The Gaussian Extension of AGDISP model can be used to predict pesticide spray drift up to 20 kilometers (12.8 miles) downwind from the application site. In response to concerns by some Environmental Fate and Effects Division (EFED) scientists, the EFED senior scientists re-evaluated the use of the Gaussian Extension. The concerns expressed centered on the model's lack of consideration of physical barriers (e.g. trees and topographic features, etc.) and meteorological conditions (e.g., crosswinds, humidity, etc.) beyond the application site affecting far-field drift of pesticides. The Environmental Fate and Effects Division (EFED) senior science staff has re-evaluated the use of the Gaussian Extension Model in AGDISP (Version 8.13, 2004) for ecological risk assessment. The senior science staff review noted three issues:

  1. there is significant uncertainty in estimating pesticide deposition at far-field distances (> 0.5 miles),

  2. the estimation of far-field drift distance under the current modeling assumptions is conservative and does not take into account potential physical barriers, and

  3. there is a lack of validation beyond 2 miles of the linked AGDISP-Gaussian Extension models for estimating far-field pesticide drift.

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GUIDANCE

Use of the linked AGDISP-Gaussian Extension models for estimating far field drift are to be used only for aerially applied mosquito adulticides or those pesticides aerially applied using a ASAE droplet size spectrum of very fine to fine, with a minimum pesticide release height of 50 feet above the ground with 10 feet above the plant canopy. Aerial spray conditions covered by the guidance include forests and tree orchards (walnut, pecans, etc). These use scenarios are most closely related to the parameters that have been validated and for these use scenarios the uncertainty relative to potential physical barriers at lower application heights or larger droplet sizes, will be minimized.

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DISCUSSION OF THE ISSUES

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References

lTeske, M.E. and H. W. Thistle. 2004. Aerial Application Model Extension into the Far Field. Biosystems Engineering. 89(1): 29-36.

2Woods, N., I. Craig, G. Dorr , B. Young. 2001. Spray Drift of Pesticides Arising from Aerial Application in Cotton. J. Environ. Qual. 30:697-701.

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