The Impact of Stability Corrections for Flux-Gradient Relationships on Measurements of the Volatile Loss of Pesticides from Agricultural Ecosystems

Volatilization represents the primary pathway loss for many pesticides. One common method for measuring pesticide volatilization is the aerodynamic flux-gradient method. Using this method, the pesticide flux is estimated as the product of the vertical pesticide concentration gradient and a turbulent-transfer coefficient (eddy diffusivity). Depending on the underlying conceptual framework used to develop the flux gradient relationship, the exact evaluation height needed to calculate the eddy diffusivity is typically approximated as either the geometric or logarithmic mean. Using data collected over an eight-year period at the USDA-ARS OPE3 experimental watershed, this study compared pesticide volatilization fluxes computed using the approximated evaluation heights with those calculated using the exact evaluation height. While the primary factor influencing the accuracy of the flux estimates was correctly accounting for atmospheric stability, the method used estimate the evaluation height could introduce significant (>10%) errors in the flux estimates. When the evaluation height was approximated by the geometric mean, errors were introduced under all stability conditions; when the logarithmic mean was used, the magnitude of the error tended to be smaller and was evident only under unstable atmospheric conditions. Based on these results, it is recommended that ether the exact evaluation height or approximation using the logarithmic mean be used with the flux-gradient technique.