Comparison of Herbicide Runoff and Volatilization Fluxes Over Multiple Years.

Surface runoff and volatilization are two processes critical to herbicide off-site transport.  To determine critical field scale processes influencing off-site herbicide transport, runoff and turbulent vapor fluxes were simultaneously monitored on the same site for eight years.  Site location, herbicide formulations, and agricultural management remained unchanged throughout the duration of the study.  Metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] and atrazine [6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine] were co-applied as a surface broadcast spray.  Herbicide runoff was monitored from a month before application through harvest.  A flux gradient technique was used to compute volatilization fluxes for the first 5 days after application using herbicide concentration profiles and turbulent fluxes of heat and water vapor as determined from eddy covariance measurements.  Results demonstrate that volatilization losses for these two herbicides were significantly greater than runoff losses (P< 0.007) even though both have relatively low vapor pressures and are considered "non-volatile".  The largest annual runoff loss for metolachlor never exceeded 2.5% while atrazine runoff never exceeded 3% of that applied.  On the other hand, herbicide cumulative volatilization losses after 5 days ranged from about 5 to 63% of that applied for metolachlor and about 2 to 12% of that applied for atrazine.  Additionally, daytime herbicide volatilization losses were significantly greater than nighttime vapor losses (P<0.05).  This research confirms that vapor losses for some commonly used herbicides frequently exceeds runoff losses and that herbicide vapor losses on the same site and with the same management practice can vary significantly year to year depending upon local environmental conditions.