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

The nutrient and pesticide content in runoff water from surface irriga tions can adversely affect the quality of receiving waters, and can al so 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 aest ivum L. cv. Owens) field. Cumulative P and N losses in runoff water fr om both irrigations were 1007 and 2487 g, respectively, which correspo nded to 0.29 and 0.13% of the amounts applied through fertilization. L oss 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] a nd {(+/-)-2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid} from the e xperimental site were 3.0, 9.3, and 22.8 g, respectively, which corres ponded to approximately 0.2% of the amount of each herbicide applied t o the wheat. Of these losses, the majority (almost-equal-to 97%) was t ransported during the first irrigation. Irrigation efficiency was 70% for the first irrigation and mean nutrient fluxes in the resulting run off were 0.79 and 1.92 g ha-1 h-1 for P and N, respectively, with corr esponding weighted concentration means of 253 and 617 mug L-1. Herbici de fluxes were 3.1, 9.8, and 23.3 mg ha-1 h-1 for dicamba, MCPA, and d iclofop, respectively. Corresponding weighted concentration means were 1.0, 3.2, and 7.7 mug L-1. Nutrient and herbicide weighted concentrat ion means were below Canadian Water Quality Guidelines for both drinki ng and irrigation water. Maximum P and diclofop concentrations exceede d drinking water guidelines, whereas those for dicamba and MCPA exceed ed interim guidelines for irrigation water.