| The Toyohira River runs through Sapporo、 Hokkaido. The upper reaches of the basin are densely forested、 and the river merges at its lower reaches with the main stream. Sapporo (pop. 1.82 million) lies on an alluvial fan formed by the Toyohira River. The city has relied on the Toyohira for most of its water resources for drinking and hydroelectric power. However、 population increases have necessitated the development of artificial elements in the hydrologic cycle、 such as the public water supply system and the sewerage network. In recent years、 rivers in northern Sapporo have experienced drought and have experienced water quality deterioration.We researched the hydrologic cycle in Sapporo toward finding solutions to these water environment problems、 focusing on the artificial elements of the hydrologic cycle、 such as the public water supply system and the sewerage network、 for quantitative and qualitative assessment. The assessment revealed the following characteristics of the hydrologic cycle in Sapporo:1) There are two dams at the upper reaches of the Toyohira River. Their presence makes for a ratio of annual mean discharge to 355 day discharge、 which indicates dry stage、 of 3.9、 versus an estimated value of 6.7 if these dams were absent. This suggests that the dams contribute to stable supply of water from the Toyohira River.2) The water quality (BOD) of the Toyohira River was assessed based on the per capita volume of organic matter in the municipal water system. Although the two dams mitigate drought、 the BOD required for native fish species including salmon to live in the river and migrate up it is 3.0 mg/L or less. To meet this requirement、 the river discharge must be approximately three times the wastewater discharge from the municipal water system、 to allow the river water to dilute the municipal wastewater. For the Toyohira River to satisfy the above environmental requirement and to receive all the municipal water emitted from Sapporo、 the catchment basin at the upper reaches would need to be 2.3 times its current size.3) 2/3 the volume of wastewater discharge from the municipal water system is channeled into other rivers、 and the remaining 1/3 into the Toyohira River. The load on the Toyohira River is reduced in this way to meet the above environmental requirement.4) The Barato River、 which runs through low flatlands in northern Sapporo、 receives 1/3 the volume of wastewater discharge from the municipal water system. Construction of sewerage facilities has enabled the reduction of organic pollutant emission、 but eutrophication remains a problem.Based on the population and water hydrologic of Sapporo、 we have clarified how the water environment in the river basin is maintained and the problems in maintaining that environment. |
