1996 Volume 105 Issue 4 Pages 459-474
Two aspects of the hydrological change in Tokyo are analyzed in this paper. The first is the changes in river courses, canals and open aqueducts from ca. 1890 to ca. 1985. The position of surface waters were traced from the topographical maps. Average stream density in the 23 special wards area except for three wards in the eastern lowland was 1.4 km-1 during a period from 1890 to 1910, while it reduced to 0.49 km -1 in 1985. In several wards, the density reduced to less than 10 % during this 100 years. The diminution of small rivers and canals began as early as 1920's, and many of them disappeared by 1985.
The reduction of surface waters is closely related to the diffusion of municipal water supply and sewerage system, therefore, the water balance caused by the man-induced water transport was estimated for the recent about 40 years. The water balance was calculated for each ward and city for each 10 years interval. Data were taken from several kinds of statistics on waterworks and “Statistic Yearbook of Tokyo”. The total water supply includes the municipal water and the industrial water. The annual amount of total supply expressed by water depth (height) was about 6 m during 1960's and 1970's in the central district of Tokyo, that is, about four times of the annual rainfall in Tokyo. The depth of total water supply exceeded the annual rainfall in all the 23 special wards in 1970's. The consumption and supply of water have been reduced since 1980's according to the establishment of recycling system. Because of the high supply rate, the depth of seepage from underground pipes of the waterworks amounted to 1 my-1 in 1975 in the central wards.
The first sewerage system was established in 1920's, but the coverage for all the areas in Tokyo was not completed until the recent. Therefore, more than 1 my -1 of the untreated water had been discharged from several wards before 1980 causing the high pollution of rivers. Historical changes in surface waters and their pollution can be explained in connection with the changes in man-induced water balance. It must be emphasized, that the artificial water transport is a crucial element in understanding the urban hydrology.
