SIMULATING ATRAZINE TRANSPORT USING ROOT-ZONE WATER-QUALITY MODEL FORIOWA SOIL PROFILES

The pesticide component of the Root Zone Water Quality Model (RZWQM) w as calibrated and evaluated for two tillage systems: no-till (NT) and moldboard plow (MB), The RZWQM is a process-based model that simulates the water and chemical transport processes in the soil-crop-atmospher e system. Observed data on atrazine concentrations in the soil profile , for model calibration and testing, were obtained from a field study in Iowa. Two statistical parameters, maximum error (ME) and coefficien t of determination (CD), were used to evaluate the ability of the RZWQ M to predict atrazine concentrations in the soil profile, The ME, CD, and other statistical tests indicated that there was a significant dif ference between predicted and observed atrazine concentrations. Compar ison of simulated vs. observed atrazine concentrations with 1:1 line s howed that atrazine concentrations were overpredicted, especially in t he later part of the growing season. However, the model correctly pred icted depth of atrazine penetration in the soil profile. Also, the ran ge of predicted atrazine concentrations was within the same order of m agnitude as observed concentrations. Although observed atrazine concen trations were usually higher in surface layers under MB than in NT tre atment, the model did not show any consistent tillage effects on atraz ine distribution in the soil profile, The results from this simulation study indicated that the following factors may be critical and should be considered when simulating pesticide transport in the subsurface e nvironment: (i) macropore flow, (ii) variation in K-oc and pesticide h alf-life with depth, and (iii) interception of pesticide by surface re sidue during application.