Prediction of the fate of agrochemicals based on model parameter estimation and GIS

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A mathematical model was developed to describe the agrochemicals runoff used in paddy fields into rivers and applied to the Oirase River Basin (main river length 70km, total basin area 844kmィイD12ィエD1, Aomori Prefecture). The data of concentration measurement of agrochemicals was collected for the year 1995 and 1996.The whole area of the river basin was divided into square meshes (5km x 5km) and the land coverage of each mesh was categorized as mountain and forest, paddy field, dry farm field, and town. The spraying schedule and the dosage of agrochemicals was estimated for 1995 and 1996 based on the marketing information of the commercial agrochemicals and the calendar for farmers. Eleven chemical compounds were selected as the target of analysis. The adsorbability and biodegradability in the soil was estimated by the physical-chemical and biological characteristics of each compound.Hydrometeorological data was collected from eight observation points in the river basin and allocated to each mesh by Tiessen's method. It was shown that the amount of evaporation and transpiration is best estimated by the method of Brutsaert and Strickerand among five estimation methods. It is also shown that the amount of snowmelt is precisely estimated by the temperature index method. The compartment model to simulate the behavior of agrochemicals needed at least six compartments, namely; river water phase, river bottom solid phase, paddy field water phase, paddy field bottom solid phase, soil-A phase and soil-B phase.Simulation results showed that the runoff behavior of the agrochemical compounds was largely affected by the spraying period and the amount of each chemicals, the period of rainfall, and the control of water discharge from paddy fields.