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.
River water quality changes markedly due to water-rock interaction and groundwater recharge through the river course from headwaters to river mouth. In particular, the composition of dissolved inorganic ions of a river running through hills and uplands in urbanized areas is changed by the confluence of tributaries with different water qualities due to various land uses, and also by groundwater recharge from the riverside uplands and hills. In the case of such rivers, highly accurate measurement of dissolved inorganic ion load is an essential problem for discussing the water quality formation processes. The present study proposes an improved method of measuring dissolved inorganic ion load in stream with heterogeneous distribution of dissolved ions along the cross section of the channel. The accuracy of measurement was examined in terms of comparing the material balance between upstream and downstream reaches at the confluence of rivers. For rivers without the transformation of dissolved materials by precipitation or chemical reaction, the measurement errors by the improved method are less than 20%, while the measurement errors by the ordinary method show around 40% with the maximum error of 80%. Comparison of discharges between the upstream reaches and the downstream ones at confluences shows that water discharge measurement errors amount to about 15%, due to the heterogeneous distribution of current velocity along the cross section of the channel, indicating that the measurement errors of ion concentration are 10% or less. The improved method presented in this paper is practically useful for evaluating the processes forming the water quality of inorganic ion composition through a river course.