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Herbicide leaching as affected by macropore flow and within‐storm rainfall intensity variation: a RZWQM simulation

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Within‐event variability in rainfall intensity may affect pesticide leaching rates in soil, but most laboratory studies of pesticide leaching use a rainfall simulator operating at constant rainfall intensity, or cover the soil with ponded water. This is especially true in experiments where macropores are present—macroporous soils present experimental complexities enough without the added complexity of variable rainfall intensity. One way to get around this difficulty is to use a suitable pesticide transport model, calibrate it to describe accurately a fixed‐intensity experiment, and then explore the affects of within‐event rainfall intensity variation on pesticide leaching through macropores. We used the Root Zone Water Quality Model (RZWQM) to investigate the effect of variable rainfall intensity on alachlor and atrazine transport through macropores. Data were used from an experiment in which atrazine and alachlor were surface‐applied to 30 × 30 × 30 cm undisturbed blocks of two macroporous silt loam soils from glacial till regions. One hour later the blocks were subjected to 30‐mm simulated rain with constant intensity for 0.5 h. Percolate was collected and analyzed from 64 square cells at the base of the blocks. RZWQM was calibrated to describe accurately the atrazine and alachlor leaching data, and then a median Mid‐west variable‐intensity storm, in which the initial intensity was high, was simulated. The variable‐intensity storm more than quadrupled alachlor losses and almost doubled atrazine losses in one soil over the constant‐intensity storm of the same total depth. Also rainfall intensity may affect percolate‐producing macroporosity and consequently pesticide transport through macropores. For example, under variable rainfall intensity RZWQM predicted the alachlor concentration to be 2.7 µg ml−1 with an effective macroporosity of 2.2 E−4 cm3 cm−3 and 1.4 µg ml−1 with an effective macroporosity of 4.6 E−4 cm3 cm−3. Percolate‐producing macroporosity and herbicide leaching under different rainfall intensity patterns, however, are not well understood. Clearly, further investigation of rainfall intensity variation on pesticide leaching through macropores is needed. Published in 2004 for SCI by John Wiley & Sons, Ltd.

Keywords: agricultural hydrology; infiltration and seepage; modeling; preferential flow; water quality

Document Type: Research Article


Affiliations: 1: USDA-Agricultural Research Service, National Soil Tilth Laboratory (NSTL), 2150 Pammel Dr, Ames, IA 50011, USA 2: Bennett & Williams Environmental Consultants Inc, Columbus, OH, USA 3: USDA-Agricultural Research Service, North Appalachian Experimental Watershed, Coshocton, OH, USA 4: USDA-Agricultural Research Service, Soil Drainage Research Unit, Columbus, OH, USA 5: USDA-Agricultural Research Service, Great Plains Systems Research, Fort Collins, CO, USA 6: USDA-Agricultural Research Service, Southeast Watershed Research Laboratory, Tifton, GA, USA 7: Environmental and Turf Services, Inc, Wheaton, MD, USA

Publication date: 2004-03-01

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