MODELING HYDROLOGY, METRIBUZIN DEGRADATION, AND METRIBUZIN TRANSPORT IN MACROPOROUS TILLED AND NO-TILL SILT LOAM SOIL USING RZWQM

Authors: Malone, Robert - Rob; Ma, Liwang; Wauchope, Robert - Don; Ahuja, Lajpat; Rojas, Kenneth; MA, QINGLI - ENVIRONMENTAL AND TURF; WARNER, RICHARD - UNIVERSITY OF KENTUCKY; BYERS, MATT - ZOELLER COMPANY

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/20/2003
Publication Date: 5/7/2004
Citation: Malone, R.W., Ma, L., Wauchope, R.D., Ahuja, L.R., Rojas, K.W., Ma, Q., Warner, R., Byers, M. 2004. Modeling hydrology, metribuzin degradation, and metribuzin transport in macroporous tilled and no-till silt loam soil using RZWQM. Pest Management Science. 60:253-266.

Interpretive Summary: Models are important tools to investigate agricultural water quality issues because they can efficiently investigate pesticide transport under multiple scenarios (e.g., soils, climates, tillage/management). To correctly predict pesticide movement, however, it is important to use the correct model. Several commonly used models don't simulate macropore flow, but research suggests that most pesticide movement to tile drains and shallow groundwater is through macropores (root channels, worm burrows, cracks in soil, etc). It is also important to correctly simulate pesticide sorption, desorption, and degradation. Therefore, we used the Root Zone Water Quality Model (RZWQM) to investigate pesticide movement through macropores, pesticide sorption and desorption, and pesticide degradation. Our results show that when macropore flow and hydrology are accurately simulated, pesticide transport in the field can be accurately simulated using a relatively simple pesticide model. This work clearly shows the importance of accurately simulating macropore flow when modeling pesticide transport and suggests that complex sorption, desorption, and degradation models may be less important. These results may help pesticide transport modelers focus resources on macropore flow rather than complex pesticide processes. Moreover, these results are key to understanding pesticide movement into the environment and will help in designing farming practices that reduce pesticide concentrations in streams and groundwater. This work will help scientists as well as decision-makers and farmers reduce pesticide contamination in the environment.

Technical Abstract: Due to the complex nature of pesticide transport, process-based models can be difficult to use. For example, pesticide transport can be effected by macropore flow. Moreover, sorption, desorption and degradation can occur at different rates in different soil compartments. We used the Root Zone Water Quality Model (RZWQM) to investigate these phenomena with field data that included two management conditions (till and no-till) and metribuzin concentrations in percolate, runoff, and soil. Metribuzin degradation and transport were simulated using three pesticide fate models: a) instantaneous equilibrium-only (EO); b) equilibrium-kinetic (EK, includes sites with slow desorption and no degradation); c) equilibrium-bound (EB, includes irreversibly bound sites with relatively slow degradation). The results indicate that 1) simulated metribuzin persistence was more accurate using the EK (RMSE=0.03 kg/ha) and EB (RMSE=0.03 kg/ha) models compared to the EO (RMSE=0.08 kg/ha) model and 2) simulating macropore flow resulted in predicted metribuzin transport in percolate over the simulation period to be within a factor of two of observed using all three pesticide models. Also, little difference in simulated daily transport was observed between the three models except the EB model substantially under-predicted metribuzin transport in runoff and percolate >30 days after application when transported concentrations were relatively low. This suggests that when macropore flow and hydrology are accurately simulated, metribuzin transport in the field may be adequately simulated using a relatively simple pesticide model (EO).