INFLUENCE OF TRACTOR WHEEL TRACKS AND CRUSTS SEALS ON RUNOFF - OBSERVATIONS AND SIMULATIONS WITH THE RZWQM/

In simulations on the fate of agricultural chemicals applied to crops, accurate partitioning of rainfall between infiltration and runoff is fundamental to chemical runoff predictions. We evaluated the Root Zone Water Quality Model (RZWQM version 3.1) against measured runoff from two field plots (15x45 m with 3% slope) on a Tifton loamy sand (fine-l oamy, siliceous, thermic Plinthic Kandiudult). Six simulated rainfall events, each 25 mm h(-1) for 2 h, were applied to maize (Zea mays, L.) each year. In the uncalibrated mode, RZWQM under-predicted runoff by 40% on average, with the closest fit for events that occurred after fu ll canopy. Saturated hydraulic conductivity (Ks) accounted for the maj ority of the uncertainty in predicted runoff. When Ks of the surface c rust was back calibrated from the measured runoff, RZWQM predicted run off closely for the remaining plots and events. Alternatively, using d ifferent Ks values for wheel track and crop beds, running the model fo r each and then, proportionally assigning runoff also led to predictio ns that agreed with measured runoff. When spatial and temporal changes in Ks were calibrated to specific conditions at the site, RZWQM effec tively predicted runoff. (C) 1998 Published by Elsevier Science Ltd. A ll rights reserved.