Abstract
Water quality was studied on the rice grown paddy field areas in Ishikari River basin of Hokkaido, Japan, with special concern to irrigation and drainage management. Irrigation water of the study sites show relatively good condition due to less effect of human pollution. Concentration and load of nitrogen(N) and phosphorus(P) are generally higher in drainage water than in irrigation water. Water quality of drainage water tend to fluctuate in wide range, with N and P in various form. Apparent discharge of N and P just after fertilizing were captured. During later period of rice growth, the accumulated N and P in form of organism in soil and water of paddy field, changes its form into inorganic matter, and discharges to the river. As discharge of N and P from rice paddy is restricted in particular period, thus, not only about the control of loss of fertilizer component, but about timing of fertilizing and way of water management will be also necessary for less load discharge.
