Abstract
The annual precipitation(P), evapotranspiration(Ep) and runoff(R=P-Ep) averaged for 5 years from 1990 to 1994 in the Iwaki River basin were estimated as 3704, 1403 and 2301×106m3, respectively. The annual utilizations of water for rice cultivation, human life and industries were 736, 46, and 14 x 106m3, respectively. Because of the heavy snowfall and the limited rainfall in spring and its large increase from summer to autumn, the runoff is large and constant in winter and then significantly decreases from March and reaches nearly zero in May and June, then increases again from July. The actual flow of the river, which is affected by the rapid thaw from the mountainous region in early spring and by the discharge from the dams according to the rice cultivation, greatly increases from February and reaches its maximum in April and on the contrary decreases significantly and reaches its minimum in May and June, while it increases again from July. The utilization of water for the rice cultivation begins from April, reaches its maximum in May and slowly decreases till August. The actual flow of the river coincides with the utilization of agricultural water in May, while in June it is only half and again becomes similar to the utilization of agricultural water at during July and August. These results indicate that the utilization of the runoff water for the rice cultivation is highly efficient. The environmental pollutants in the drained water which are born from fertilizers and agricultural chemicals utilized for the rice fields are concentrated and scarcely diluted when the water is returned again to the river. In conclusion, the intensive utilization of the runoff water for the rice cultivation greatly lowers the level of water flowing in the middle of the river and results in degradation of the natural environment in this river basin.