A PRAGMATIC FIELD-SCALE APPROACH FOR MODELING PESTICIDES

Environmental assessments of the fate of pesticides and other agricultural 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 predictive tools that include regression equations, empirical relationships, and simulation models. Comprehensive simulation models are attractive from a scientific standpoint, since they consider leaching of the chemical by water as well as the various chemical, biological, and physical processes that affect the chemical's fate during its transport. A number of simulation modeling approaches have been developed to describe soil-water-chemical systems, but a fundamental question remains regarding the degree of model complexity required to simulate agrochemical movement to groundwater. Most model complexity in soil-water-chemical simulation models arises from the manner in which water flow and chemical transport are considered. It is therefore logical, especially for management purposes, to consider alternative, pragmatic, and vet sufficiently accurate approaches to the description of water flow and chemical transport. Such approaches would require less characterization of soil hydraulic and physical/chemical properties and would reduce computation time. A method for such simplification and use is described, starting with LEACHP, a model based on the Richards and convection-dispersion equations. The resulting model is shown to be sufficiently accurate for many management decisions.