A mathematical model to estimate nitrogen (N) effluent from an upland field cultivated with barley, where under-drain pipes were installed 60 cm below the field surface, was developed and N effluents for several rainfall patterns during the cultivation period were analysed. The model is composed of the water drainage model and N cycle model. The water drainage model is made up of Sugawara's tank model in which the field is divided into two soil types, permeable soil and impermeable soil, and macro pore. The N cycle model can calculate the N reactions including nitrification, denitrification, mineralization, immobilization, urea hydrolysis and N transportation in the field. By using this model, N effluents caused by rainfall were analysed and characteristics of N effluent were clarified. The under drainages caused by heavy rains which occur around 60 days after fertilization contribute greatly to the amount of N effluent load during a cultivation period. Also, split application of fertilizer is not effective for the reduction of N effluents under any cultivation conditions, especially in winter crops. A large quantity of N effuses out of the field because the major portion of rainfall in a cultivation period occurs in the latter half of the period.

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