Abstract
Nitrate concentration in the ground water under the paddy field has a seasonal change and relatively high concentration was observed in the summer. There is a weak relationship between water level of the ground water and nitrate concentration. The concentration of dissolved oxygen in the ground water is nearly 0 mg·l-1 in summer but about 10 mg·l-1 in winter. This means that denitrification is easier in summer. The change of denitrification rate could not explain why relatively high concentration of nitrate observed in summer.
A numerical model is presented. The model incorporates nitrification of fertilizer in the paddy field, penetration of nitrate into the soil and change of ground water quantity. The model is useful to interpret the mechanism of seasonal nitrate concentration change in the ground water under the paddy field.
