The effective evaluation height for flux-gradient relationships and its application to herbicide fluxes

- Effect of the calculation method for evaluation height using the flux-gradient approach was tested.
- The analysis focused on metolachlor data collected over an eight year period.
- Under unstable conditions, historical estimation methods caused a 15% error in the flux, on average.
- The results indicate that the exact evaluation height should be used whenever possible.

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 be used with the flux-gradient technique.