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Title: HERBICIDE LEACHING AS AFFECTED BY MACROPORE FLOW AND WITHIN-STORM RAINFALL INTENSITY VARIATION: A RZWQM SIMULATION

Author
item Malone, Robert - Rob
item WEATHERINGTON-RIC, J - BENNETT & WILLIAMS
item Shipitalo, Martin
item Fausey, Norman
item Ma, Liwang
item Ahuja, Lajpat
item Wauchope, Robert - Don
item MA, Q - ENVIRONMENTAL AND TURF

Submitted to: Pest Management Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/31/2003
Publication Date: 5/7/2004
Citation: Malone, R.W., Weatherington-Ric, J., Shipitalo, M.J., Fausey, N.R., Ma, L., Ahuja, L.R., Wauchope, R.D., Ma, Q. 2004. Herbicide leaching as affected by macropore flow and within-storm rainfall intensity variation: a RZWQM simulation. Pest Management Science. 60:277-285.

Interpretive Summary: Pesticide moves to tile drains or to shallow groundwater mainly through macropores (root channels, worm burrows, cracks in soil, etc). The effect of within storm rainfall intensity variation (natural storms) on pesticide transport through macropores, however, is uncertain. Midwestern storms generally have higher rainfall intensity at the beginning of the storm than at the end of the storm, whereas much macropore flow research is based on constant intensity storms. We investigated the effect of variable rainfall intensity within a storm on alachlor and atrazine transport through macropores. It was observed that Midwestern storms increase pesticide transport through macropores up to two-fold compared to constant intensity storms. These results indicate that experiments and models based on constant intensity storms can under predict pesticide transport under more natural conditions of variable rainfall intensity. This research will mostly benefit scientists studying herbicide transport but will eventually help decision makers recommend farming practices that will reduce pesticide movement into the environment.

Technical Abstract: Pesticide transport to ground water and tile drains may increase when macropores and fractures are present in glacial till derived soil, and when rainfall intensity varies within a storm. Most studies of pesticide transport through macropores use constant rainfall intensity. Because field and lab data are scarce, models are an efficient way to investigate these processes. A model that handles pesticide transport through macropores under variable rainfall intensity is the Root Zone Water Quality Model (RZWQM). Our objective was to use the RZWQM to investigate the effect of variable rainfall intensity within a storm on alachlor and atrazine transport through macropores. Data was used from an experiment in which atrazine and alachlor were surface applied to 30 x 30 x 30 cm undisturbed blocks of two silt loam soils from glacial till regions. One hour later the blocks were subjected to a 0.5 h, 30 mm simulated rain with constant intensity. Percolate was collected and analyzed from 64 square cells at the base of the blocks. The median Midwest storm, which has higher rainfall intensity at the beginning of the storm than at the end, was predicted (using RZWQM) to increase pesticide transport through macropores up to two-fold compared to constant intensity storms. These results indicate that experiments and models based on constant rainfall intensity may under predict pesticide transport. Because of variable rainfall intensity, storms are more typical of natural conditions, more field and lab studies are needed to adequately simulate macropore transport of herbicides.