Evaluation of preferential flow component of RZWQM in simulating water and atrazine transport to subsurface drains

The ARS Root Zone Water Quality Model (RZWQM ver. 3.25) was used to simulate the effect of field measured macroporosity on atrazine transport to subsurface drain lines. Field data on atrazine concentrations in subsurface drain flow from corn fields, for modified no-till (mNT) and moldboard plow (MP) systems, were used to evaluate the performance of the RZWQM for the growing seasons of 1990, 1991 and 1992. The model was calibrated using field data from 1990 and data from 1991and 1992 were used to validate the model. Simulated subsurface drain flows and atrazine losses with and without macropore flow were compared with measured values. Although the preferential flow component slightly improved the predictions of peak subsurface drain flows for individual rain storms, it did not affect significantly the total annual flows. Simulated annual subsurface flows were within 11.6% of the observed values. Simulated atrazine concentrations in subsurface drain flows using mean values of macroporosity were in close agreement with the observed concentrations for 1990 (calibration year). Predicted total annual atrazine 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 within +12.0% of observed values (combined for two years). The RZWQM predicted only trace amounts of atrazine in subsurface drain flows if macropores were not considered. The model showed sensitivity to lateral flow from macropores, Ksat of surface layer, and macroporosity in decreasing order in simulating atrazine losses to subsurface drain flows. Overall, the RZWQM showed good potential for simulating atrazine losses with subsurface drain water as affected by tillage practices.