Using a Hydrological Model to Determine Environmentally Safer Windows for Herbicide Application
A modification of the GLEAMS model was used to determine application windows which would optimise efficacy and environmental safety for herbicide application to a forest site. Herbicide/soil partition coefficients were determined using soil samples collected from the study site for two herbicides (imazapyr, Koc=46, triclopyr ester, Koc=l038) and published values for two other herbicides (henarinone, Koc=54; triclopyr amine, Koc=20) were used in the model. Other site-specific characteristics were taken from catchment topographic maps and soil data. Long-term climatic records for the region were then used to provide meteorological data for use in the hydrology component of the model. The model was run with herbicide application for each day of the manufacturers' recommended growing-season application windows. Average surface run-off losses, expressed as a percentage of applied, were low for all herbicides modelled (hexazinone, 0.37%; imazapyr, 0.34%; triclopyr amine, 0.21%; hiclopyrester, I .85%). Model predictions of herbicide loss for each application day were then summarised and the application days with the lowest predicted loss within the manufacturers' application windows were identified as the environmentally safest days (environmental window)for application. This application of the GLEAMS model predicts an environmental window, for each herbicide for the site under consideration, during which the probability of adverse environmental impacts is at the lowest level achievable based upon long-term climaticrecords.The environmental window for pesticides other than herbicides can also be determined through modelling in a similar fashion. GLEAMS is the model chosen for this example, but other pesticide fate models may be equally applicable.