Two methods were used to obtain the sensitivity of chemical leaching depth to variations in the input parameters of the Pesticide Root Zone Model (PRZM). First a Plackett-Burman (PB) screening design was used to vary 35 PRZM inputs over seven ranges around a nominal value. Six of the seven ranges were approximately 0.1, 0.25, 0.5, 1.0, 5.0, and 15%, the seventh range was chosen to cover a range appropriate for a soybean herbicide applied preemergence in the Midwestern region defined by the USDA–SCS land resource region M. Next, Fourier amplitude sensitivity testing (FAST) was then used to vary from 19 to 25 parameters over four of the ranges previously tested. For the smaller parameter ranges the two methods typically gave equivalent results but the PB method required far fewer simulations. For the simulation of the Midwestern region where some parameter varied by larger amounts the relative magnitudes of the sensitivity coefficients obtained by the two methods were similar but the magnitude of the coefficients obtained using FAST were smaller than those obtained using PB.
This paper traces the development of modelling to assess the environmental fate of agricultural chemicals within DowElanco's Environmental Fate Group. Field monitoring of late was the initial tool for assessment, but inefficiency of the process and poor data interpretability turned the group's attention to benchmarking. This, too, proved to be an inadequate and frustrating process as it evaluated fate only relative to other chemicals and did not allow in an absolute sense the assessment of risk of contamination away from treated locations. It also did not allow the evaluation of management programmes for minimization of environmental impact. Currently, probability modelling is being evaluated to assess the environmental fate of chemicals and the likelihood of attaining a given concentration at a specified site. It allows the handling of variability and provides estimates of likelihood for environmental events to take place in designated areas of the United States. Through the use of Fourier Amplitude Sensitivity Test and Monte Carlo sampling techniques, ranges of inputs are used to drive environmental models to provide frequency distribution data for output. The process appears useful for assessing environmental impact of chemicals because it allows whole range evaluation with data that are readily available, and provides information appropriate for best management practices.
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