A PRAGMATIC FIELD-SCALE APPROACH FOR MODELING PESTICIDES

Environmental assessments of the fate of pesticides and other agricult ural chemicals usually require estimation of chemical concentrations, with some indication of their accuracy. These estimations are usually based on a few measurements combined with existing data bases and pred ictive tools that include regression equations, empirical relationship s, and simulation models. Comprehensive simulation models are attracti ve from a scientific standpoint, since they consider leaching of the c hemical by water as well as the various chemical, biological, and phys ical processes that affect the chemical's fate during its transport. A number of simulation modeling approaches have been developed to descr ibe soil-water-chemical systems, but a fundamental question remains re garding the degree of model complexity required to simulate agrochemic al movement to groundwater. Most model complexity in soil-water-chemic al simulation models arises from the manner in which water flow and ch emical transport are considered. It is therefore logical, especially f or management purposes, to consider alternative, pragmatic, and vet su fficiently accurate approaches to the description of water flow and ch emical transport. Such approaches would require less characterization of soil hydraulic and physical/chemical properties and would reduce co mputation time. A method for such simplification and use is described, starting with LEACHP, a model based on the Richards and convection-di spersion equations. The resulting model is shown to be sufficiently ac curate for many management decisions.