TRANSPORT OF NUTRIENTS AND POSTEMERGENCE-APPLIED HERBICIDES DURING CORRUGATION IRRIGATION OF WHEAT

The nutrient and pesticide content in runoff water from surface irrigations can adversely affect the quality of receiving waters, and can also determine the suitability of the water for reuse in the subsequent downstream irrigation of other crops. In order to assess the magnitude of loss under normal corrugation irrigation practices, the transport of plant nutrients and postemergence-applied herbicides was monitored in runoff water from two irrigations of a 12.6-ha wheat (Triticum aestivum L. cv. Owens) field. Cumulative P and N losses in runoff water from both irrigations were 1007 and 2487 g, respectively, which corresponded to 0.29 and 0.13% of the amounts applied through fertilization. Loss of either nutrient in the first irrigation was approximately twice that in the second irrigation. Total losses of dicamba (3,6-dichloro-2-methoxybenzoic acid), MCPA [(4-chloro-2-methylphenoxy)acetic acid] and diclofop{(+/-)-2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid} from the experimental site were 3.0, 9.3, and 22.8 g, respectively, which corresponded to approximately 0.2% of the amount of each herbicide applied to the wheat. Of these losses, the majority (almost-equal-to 97%) was transported during the first irrigation. Irrigation efficiency was 70% for the first irrigation and mean nutrient fluxes in the resulting runoff were 0.79 and 1.92 g ha-1 h-1 for P and N, respectively, with corresponding weighted concentration means of 253 and 617 mug L-1. Herbicide fluxes were 3.1, 9.8, and 23.3 mg ha-1 h-1 for dicamba, MCPA, and diclofop, respectively. Corresponding weighted concentration means were 1.0, 3.2, and 7.7 mug L-1. Nutrient and herbicide weighted concentration means were below Canadian Water Quality Guidelines for both drinking and irrigation water. Maximum P and diclofop concentrations exceeded drinking water guidelines, whereas those for dicamba and MCPA exceeded interim guidelines for irrigation water.