PESTICIDES REPORT .35. OFFSITE TRANSPORT OF PESTICIDES IN WATER - MATHEMATICAL-MODELS OF PESTICIDE LEACHING AND RUNOFF

The process of modeling the leaching and runoff of pesticides is simpl e in concept but complex in execution. Models are physical, conceptual , or mathematical representations of reality. Screening-level models a re all appropriate first step for examining pesticide leachate and run off potential, as long as conservative input assumptions are used. The y may consist of comparisons of certain mobility and persistence prope rties with numerical criteria, or they may require pencil, paper, and a hand calculator. At a higher level of sophistication, a wide variety of computer models are available that can quantitatively simulate pes ticide leaching and runoff in the aqueous phase. It is important to pi ck ii model that has been validated in more than one study, has good u ser support, requires an amount of data input appropriate for the appl ication, and has a history of producing results acceptable to scientis ts slid regulatory authorities. Considering these various criteria for acceptability, EPA's PRZM2 model and tile German modification, PELMO, would be appropriate for evaluating leaching potential. The GLEAMS, L EACHM, and CALF models are also scientifically acceptable, but have no t been as widely used. Tile GLEAMS model is appropriate for quantifyin g runoff potential in simple, field-scale drainage patterns. The more complex SWRRBWQ model is more appropriate for watershed-scale assessme nts. The most appropriate use of these computer simulation models is t o rank the contamination potential of a particular pesticide at severa l sites or rank several pesticides at one site. Another excellent appl ication of these models is to calibrate them to fit the results of an intensive field study at one site, and extrapolate to other points ill time and space for the same pesticide. One should always recognize th e variability ill natural processes and field conditions, and use prob abilistic (stochastic) analysis whenever possible. More model validati on and calibration is needed in tropical climates and in special situa tions such as turf, Forests and orchards.