Simazine (6-chloro-N,N'-diethyl-1,3,5-triazine-2,4-diamine) and diuron (N'-(3,4-dichlorophenyl)-N,N-dimethylurea) are applied as pre-emergence herbicides to control weeds in over 45,000 hectare of citrus in the San Joaquin Valley of California. Growers usually apply 2.0 kg active ingredient (ai)ha(-1) simazine and 2.0 kg ai ha(-1) diuron together in the fall and winter seasons. Surface water and groundwater contamination, ecological, and health damage have led to increased regulatory attention and a search for changes in current weed management practices. Reduced application rate of simazine on runoff losses generated by simulated rainfall was studied in experiment 1. Treatments included a control without herbicide application and application of herbicides at 0.5, 1.0, 1.25 and 2.0 kg ai ha(-1). Three generated runoff events were produced using simulated rainfall with 540 1 water. For each simulated rainfall event, the simazine concentration in runoff water was greatest at the first volumetric sampling interval, decreasing gradually in subsequent samples. Simazine concentration in runoff water for the three runoff events was undetectable for the control treatment. For the first simulated rainfall event, simazine concentrations in runoff water averaged 0.55, 1.07, 1.27, and 2.12 mg l(-1) for treatments receiving 0.5, 1.0, 1.25 and 2.0 kg ai ha(-1) herbicide application, respectively; simazine concentration in runoff water from the second event one week later averaged 0.16, 0.27, 0.36, and 0.58 kg ai ha(-1) and two weeks later the concentration was reduced to 0.05, 0.10, 0.14 and 0.22 mg l(-1), respectively. Total simazine mass recovered in runoff water from the three simulated rainfall events was estimated approximately 13.8, 26.3, 32.3 and 53.8 g ha-1 for the treatments receiving herbicide application at 0.5, 1.0, 1.25 and 2.0 kg ai ha(-1), respectively. Field evaluation of weed density showed that pre-emergence herbicides applied at reduced rates (1.0 and 1.25 kg ai ha(-1)) were as effective as the standard rate (2.0 kg ai ha(-1)) for weed control in experiment 2.