A Simple Model of Alachlor Dissipation
- D. A. J. Weed,
- R. S. Kanwar * and
- R. J. Salvador
Alachlor [2-chloro-N-(2,6-diethylphenyl)-N-(methoxymethyl) acetamide] dissipation in the field shows two characteristics: (i) rapid, initial loss followed by slower degradation and (ii) sensitivity to environmental conditions including precipitation patterns, soil temperature, and soil-water content. Empirical models, such as a first-order equation, are simple and easy to use, but they do not accurately predict alachlor dissipation in the field. Complex mechanistic models provide theoretical mechanisms for dissipation, but their usefulness can be limited because they typically require data that is difficult to acquire under field conditions. We developed a hybrid model (M2CM) for alachlor dissipation based on a two-compartment model (2CM). Our model requires only easily-collected weather, soil, and pesticide information. It predicts the daily amount of alachlor in the soil and calculates times for 50 and 90% dissipation. We calibrated it with data from 30-cm-deep field-soil samples and weather records from the 1993 growing season. Using 1992 and 1994 environmental data, the model calculated dissipation values that fit field-sample data accurately (0.95 ≤ r2 ≤ 0.99). Results from the 2CM were not as consistently satisfactory (0.64 ≤ r2 ≤ 0.97). According to the M2CM, the alachlor half-life (time to 50% dissipation) was 1 d for all 3 yr. Time to 90% dissipation was 18 d for 1992, 23 d for 1993, and 15 d for 1994.
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