In this study, water balance including municipal and irrigation water have been analyzed quantitatively in the Koise River Basin, Ibaraki Prefecture. The amounts of artificially supplied and drained water in each grid was estimated on the basis of the water balance method. The amount of municipal water supply increased from 1.8×106m3/y in 1965, to 15.6×106m3/y in 1995. However, sewerage systems in the area were not constructed prior to the 1990s. Therefore, the greater portion of discharge water was left untreated. The amount of sewerage discharge water into the sewerage was 10.3×106m3/y in 1995. Since the sewerage treatment was developed every year thereafter, the untreated discharge water has reduced accordingly, and the amount became 5.4×106m3/y in 1995. The amount of irrigation water supply tends to be influenced by precipitation. Water consumed and drained in the paddies is proportional to irrigation water supply. When comparing the artificial water supply to the natural runoff amount, it appears that irrigation water does not exceed the effective precipitation in most of the river basin. The rate of untreated discharge water to natural runoff was examined for each grid. On the whole, the influence of artificial discharge is small at present, but it was observed that discharge water influences water balance in some parts of the river basin. Artificial water supply, including municipal and irrigation water, has been increasing every year in each grid of the Koise River Basin. In conclusion, it can be said that artificial water is today an important factor for domestic life and agriculture in the Koise River Basin.
