EVALUATION OF PREFERENTIAL FLOW COMPONENT OF RZWQM IN SIMULATING WATER AND ATRAZINE TRANSPORT TO SUBSURFACE DRAINS

The ARS Root Zone Water Quality Model (RZWQM rer: 3.25) wets used to s imulate the effect of field measured macroporosity on atrazine transpo rt to subsurface drain lines. Field data on atrazine concentrations in subsurface drain flow from corn fields, for modified no-till (nNT) an d moldboard plow (MP) systems, were used to evaluate the performance o f the RZWQM for the growing seasons of 1990, 1991 and 1992, The model was calibrated using field data from 1990 and data from 1991 and 1992 were used to validate the model. Simulated subsurface drain flows and atrazine losses with and without macropore flow were compared with mea sured values. Although the preferential flow component slightly improv ed the predictions of peak subsurface drain flows for individual min s torms, it did not affect significantly the total annual flows. Simulat ed annual subsurface flows were within 11.6% of the observed values. S imulated atrazine concentrations in subsurface drain flows using mean values of macroporosity were ii? close agreement with the observed con centrations for 1990 (calibration year). Predicted total annual atrazi ne losses were also close to the observed values for 1990 (percentage difference 17.6% for mNT and 34.8% for MP system). For 1991 and 1992 ( evaluation years), with macropores, the simulated atrazine losses for mNT plots were within -9.9% of observed values and for MP plots were w ithin +12.0% of observed values (combined for two years), Th RZWQM pre dicted only trace amounts of atrazine in subsurface drain flows if mac ropores were not considered. The model showed sensitivity to lateral f low from macropores, K-sat of surface layer and macroporosity in decre asing order in simulating atrazine losses to subsurface drain flows. O verall, the RZWQM showed good potential for simulating atrazine losses with subsurface drain water as affected by tillage practices.