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

Abstract

Identification of the major factors affecting herbicide transport through macropores improves our understandingof preferential flow and aids in the development and use of macropore-flow models, such as the Root Zone Water QualityModel (RZWQM). Recent research suggests that macropore-flow breakthrough time (bt) and the number of percolate-producingmacropores (nmacro) affect herbicide concentration in percolate (hc). Therefore, we investigated the effect of bt and nmacroon hc during the first storm after herbicide application using multiple regression and partial correlation. Observed datawere from five different leaching experiments that included five soil series 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 duringthe 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). Partialcorrelation 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) resultsin an eight-fold change in atrazine concentration (4.0 to 0.48 mg L-1). Clearly, breakthrough time during the first storm afterapplication affects herbicide transport.