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Title: RELATIONSHIP BETWEEN HERBICIDE CONCENTRATION IN PERCOLATE, PERCOLATE BREAKTHROUGH TIME, AND NUMBER OF ACTIVE MACROPORES

Author
item Malone, Robert - Rob
item Shipitalo, Martin
item Meek, David

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/14/2004
Publication Date: 6/14/2004
Citation: Malone, R.W., Shipitalo, M.J., Meek, D.W. 2004. Relationship between herbicide concentration in percolate, percolate breakthrough time, and number of active macropores. Transactions of the ASAE. 47(5):1453-1456.

Interpretive Summary: Most herbicide movement to tile drains and shallow groundwater is through macropores (e.g., worm burrows, cracks, and root channels). Water transport through macropores is complex and identification of the major factors affecting macropore flow improves our understanding of preferential flow and helps in the development and use of macropore flow models such as the Root Zone Water Quality Model (RZWQM). Few studies have addressed the combined effect of percolate breakthrough time (bt) and number of percolate producing macropores (nmacro) on herbicide transport through macropores. Therefore, we developed a regression model to predict herbicide concentration in percolate based on variables that included nmacro and bt. The equation is especially sensitive to bt because varying bt from 1.4 to 25 min. results in an eight-fold decrease in predicted atrazine herbicide concentration (4.0 to 0.48 mg L-1). This clearly shows the importance of breakthrough time on herbicide concentration in percolate and will help scientists, model users, and model developers better understand and predict herbicide transport. A more complete understanding of macropore flow will enable us to design agricultural systems that reduce pesticide transport off-site, which will result in reduced risk to humans and the environment.

Technical Abstract: Identification of the major factors affecting herbicide transport through macropores improves our understanding of preferential flow and helps in the development and use of macropore flow models such as the Root Zone Water Quality Model (RZWQM). Recent research suggests that macropore flow breakthrough time (bt) and the number of percolate producing macropores (nmacro) affect herbicide concentration in percolate (hc). Therefore, we investigated the effect of bt and nmacro on hc during the first storm after application using multiple regression and partial correlation. Observed data were from five different leaching experiments that included five soil types and two tillage types (no-till and moldboard plow). Multiple regression was used to develop a model to predict herbicide concentration (hc) in preferential flow at 30 cm during the first rainfall after application using data from the five studies. The ln(hc) in macropore flow at 30 cm was related to bt (min.) at 30 cm, ln(nmacro) at 30 cm, herbicide type (alachlor or atrazine), and herbicide application rate (R2 = 0.96). Partial correlation confirmed the relationship between ln(hc) and bt (P<0.0001; r=-0.77) and ln(nmacro) (P<0.0001; r=-0.46). A single variable sensitivity analysis on the regression equation suggested that the observed range of bt (1.4 to 25 min.) results in an eight-fold change in atrazine herbicide concentration (4.0 to 0.48 mg L-1). Clearly breakthrough time during the first storm after application affects herbicide transport.