In the San Joaquin Valley of California the presence of preemergence herbicides in runoff water from citrus orchards during winter rainfall can cause surface water and groundwater contamination. In 1999 and 2000 this study was conducted in a citrus orchard to evaluate the effect of irrigation incorporation with rotor sprinklers on simazine losses in runoff water. Four treatments included the application of simazine at 2.0 kg ai ha⁻¹ without irrigation incorporation and the application of simazine at 2.0 kg ai ha⁻¹ with three levels of irrigation incorporation (0.5, 1.25, and 1.75 cm of water). Simulated rainfall with 3.5 cm of water was then applied on the plots to generate surface runoff. Simazine concentrations in runoff water from the first simulated rainfall event averaged 0.85, 0.77, 0.45, and 0.30 mg L⁻¹ for treatments without irrigation incorporation and for those receiving 0.5, 1.25, and 1.75 cm of water incorporation, respectively; simazine concentration in runoff water from the second event applied 1 wk later averaged 0.42, 0.38, 0.21, and 0.16 mg L⁻¹, respectively. Total simazine mass recovered in runoff water from both simulated rainfall events was estimated to be 6.5, 6.0, 3.6, and 2.5% of the application for treatments without irrigation incorporation and for those receiving 0.5, 1.25, and 1.75 cm of water incorporation, respectively. Applying sufficient water for herbicide incorporation into the soil matrix is a practical approach to mitigate off-site movement from the soil.

Nomenclature: Simazine.