The change in water quality for irrigation in Tochigi prefecture [Japan]

We investigated the quality of irrigation water for 14 elements including nutrient salts in Tochigi Prefecture from 1996 to 1998. The irrigation water was sampled at 85 points 5 times during the period from transplanting to ripening of rice plants. The drainage water was sampled at 6 points 7 times during the period from transplanting to November. The percentage of the water with the value within the standard value for irrigation was 70, 88, 84, 30 and 100 in pH, EC, COD, T-N and SS, respectively. The water quality was, the best in the Naka basin, followed by was the Kinu basin and was the worst in the Watarase basin. The range of seasonal changes in water quality was narrow. The point with the most polluted water was in the area downstream from cities in the southern and western part of Tochigi Prefecture. The nutrient salt concentration of the drainage was higher than that of irrigation, except for SiO2. The nutrient salt concentration in 1996-1998 tended to be lower than that in 1986-1988, especially in T-N.