INCORPORATING PREFERENTIAL FLOW AND HERBICIDE FATE AND TRANSPORT INTOTHE DRAINAGE MODEL

The DRAINAGE model was modified by incorporating a pesticide component and a preferential flow component in order to simulate pesticide conc entrations in subsurface drain flows. Field data on subsurface drain f lows and their atrazine concentrations were used to calibrate and vali date the enhanced DRAINAGE model for growing seasons of 1990, 1991, an d 1992. Simulated subsurface drain flows and their atrazine concentrat ions were compared with the measured values. Predicted daily flows by the modified DRAINAGE model were close to the observed values (differe nce over all years + 6.3%). Overall mean difference (Md) and correlati on coefficient (R-2) were + 0.1 mm and 0.70, respectively. The predict ed atrazine concentrations in subsurface drainage water followed the o bserved trends well except in 1992. The overall timings of pesticide a ppearance in the drain water were predicted well by the model. The ann ual atrazine losses with subsurface drain flows predicted by the model were also in close agreement with the observed losses for 1990 and 19 91 (with 1.1% difference). The results of this study indicated that th e modified DRAINAGE model has good potential for simulating atrazine c oncentrations for normal rainfall years when a substantial amount of p esticides may be lost in the subsurface drainage water (overall M-d = 2.0-3 mu g/kg and R-2 = 0.58).