To predict nitrogen discharges from fields is important to make a plan of water quality preservation in an agricultural region. Then a prediction model that is simple and easy to treat is useful. The objective of this study is to develop a simplified estimation model for nitrogen discharge from a barley field. The simplifying processes are neglecting some nitrogen transformation processes included in nitrogen cycling model reported by authors by sensitivity analysis and lumping some nitrogen

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... ms. The unique feature of this model is to divide whole nitrogen cycling in soil into two subsystems, nitrogen cycling system of soil fertility (SFC system) and chemical fertilizer nitrogen cycling system (CFC system). The nitrogen losses with discharge and plant uptake are assumed to be from CFC system and inorganic nitrogen in this system organizes into SFC system with bacterial uptake. And it is assumed SFC system supplies mineralized nitrogen to CFC system as zero-order kinetics. This model is easy to apply to various cropping conditions because the government equation of the model is a simultaneous equation with urea N, ammonium N and nitrate N so that the initial condition for calculation can be given as fertilized nitrogen. The model yields reasonably good fits to the observed data.