A mathematical model is developed to predict the runoff of pesticides from paddy field to a river. The model describes the long term runoff as a function of physical-chemical properties of pesticides, cultural practices and climatic conditions. It is assumed that the adsorption and desorption of pesticides between soil and water phases are in equilibrium state, while the degradation in water and soil and the evaporation from water to atmosphere are of first-order reaction. The model is used to examine the relationships between the seasonal amounts of pesticide runoff and their environmental dynamic factors. A four-stage evaluation system is proposed to assess the impact of the runoff on drinking water by taking into account the amount of application, runoff ratio, toxicity and the possibility of being removed from raw water by water purification processes. The model simulates the daily variation of contaminant level in river water, thereby providing a basis for making recommendations for pesticide usage and water management for rice cultivation.

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