Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2016
Publication Date: 11/14/2016
Citation: Alfieri, J.G., Prueger, J.H., Gish, T.J., Kustas, W.P., McKee, L.G., Russ, A.L. 2016. A note on the effective evaluation height for flux-gradient relationships and its application to herbicide fluxes. Agricultural and Forest Meteorology. 232:682-688.
Interpretive Summary: A dominant pathway for pesticides to enter the environment, where they can adversely impact human and ecological health, is volatilization. In order to understand, predict, and mitigate the negative effects of pesticides usage, accurate estimates of the pesticide losses due to volatilization are needed. One common method for determining pesticide volatilization is the flux-gradient method. Historically, the effective measurement height used with this approach has been calculated based on a number simplifying assumptions. These assumptions can cause significant errors in the estimates of both the evaluation height and pesticide losses due to volatilization. As a part of this study, a method for calculating the exact evaluation height was identified. Using data collected over an eight-year period at a USDA-ARS experimental watershed, this study compared the pesticide volatilization losses computed using the exact evaluation height to those determined using the current approaches. The results of the study demonstrated that the current approach can underestimate volatilization losses by as much as 30%. Based on these results, it is recommended that the exact evaluation height is used with the flux-gradient technique. These results will benefit both scientists seeking to understand the impacts of pesticides on the environment and policy makers, resource managers, and agricultural producers who need accurate information to decide how best to minimize the negative effects of pesticides on the environment.
Technical Abstract: Volatilization represents a significant loss pathway for many pesticides, herbicides and other agrochemicals. One common method for measuring the volatilization of agrochemicals is the flux-gradient method. Using this method, the chemical flux is estimated as the product of the vertical concentration gradient and a turbulent-transfer coefficient (eddy diffusivity). For computational simplicity, the evaluation height needed to calculate the eddy diffusivity is typically approximated as either the geometric or logarithmic mean. Both of these estimation methods are based on simplifying assumptions and can be a significant source of error, particularly when the separation distance between the measurement heights is large. Using data collected over an eight-year period at the USDA-ARS OPE3 experimental watershed, this study compared fluxes of metolachlor, a commonly-used herbicide, computed using the approximated evaluation heights with those calculated using the exact evaluation height. While it was found that the primary factor influencing the accuracy of the flux estimates using the approximate evaluation heights was atmospheric stability, errors in the estimate of the evaluation height can result in significant (>10%) errors in the flux estimates. Based on these results, it is recommended that the exact evaluation height is used with the flux-gradient technique.