Submitted to: American Society of Agricultural Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 11, 2000
Publication Date: N/A
Interpretive Summary: Macropores are relatively large pores created in the upper soil profile as a result of root decay, earthworm activities, etc. Macropores are prevalent in no tillage crop production practices. Field studies have demonstrated that the presence of macropores can potentially enhance leaching of surface-applied agricultural chemicals through the crop's root zone and into the shallow groundwater. The goal of this study was to examine the capability of a newly developed model (MACRO model) in evaluating the significance of macropore flow on pesticide transport in the Coastal Plain Soils. The uniqueness of MACRO model is that it can be run in both one domain (without macropore flow component) and two domains (with macropore component). The measured field pesticide leaching concentrations were compared to predicted leaching concentrations using MACRO model with and without the macropore component. Results showed that MACRO model was able to reproduce the observed herbicide leaching pattern at the shallower soil depths. However, there were no significant differences between the model predictions and the actual observations whether or not the macropore component of the model was used due to the lack of macropores at deeper soil depths. The results indicated that macropore flow plays an important role in water movement and solute transport in the Coastal Plain Soils. The MACRO model with the macropore component is a more appropriate model for use in pesticide screening evaluations.
Technical Abstract: Preferential flow of water through macropores in soil has been shown to be a prime factor in transporting agricultural chemicals into deeper soil horizons. The objective of this study was to quantify the significance of macropore flow in pesticide transport in the Coastal Plain Soils by simulating a newly developed model that can be run in both, one domain (i.e., without macropore flow) and two domains (i.e., with macropore flow) component and compare the model simulations with field observatons. Field data used for this comparison are from four large sized field plots, located at the Coastal Plain physiographic region near Marlboro, Maryland with Monmouth fine sandy loam as a dominant soil and 2-5% slope. There are four fields. Fields 1 and 2 have been in no-till and fields 3 and 4 have been in conventional-till since 1989. The plots also contain suction lysimeters that were installed at six different locations at 30, 60, 90, 120, 150, and 180 cm soil depth. Each year herbicide atrazine was applied at the rate of 1.7 kg/ha to all four fields prior to the corn planting. Samples were collected from lysimeters throughout the corn growing seasons. For this model comparison, a thorough sensitivity analysis was performed in order to identify the model's most sensitive input parameters. The model was then calibrated with 1992 field 2 data. Finally, the model simulations of atrazine concentrations in 1993, 1994, and 1995 were compared to the 95% confidence interval of the lysimeter concentration data. On average, about 80% of the model predictions for the three years studied fell within the 95% confidence intervals of the measured field data. The MACRO model with two domains was able to reproduce the observed pattern of atrazine leaching.