City is one of the important anthropogenic modifications of land; therefore its hydrological aspects must be studied from the view point of environmental change. Extension of impermeable land surface modifies the runoff processes and water circulation of the area. City needs much clear water for the citizen and many kinds of activities, although the polluted water must be eliminated possibly soon. Therefore, urban hydrology should be studied from two stand points, namely, modification of runoff by built-up area and transport of municipal waters. In this article, these factors are explained based on examples in Tokyo area. It is also emphasized that cultural and historical aspects are also necessary to understand the water environment of a city.
The necessity of rainwater storage and infiltration facilities has increased in the measures and policies regarding the sustainable water cycle in urban areas as understood by the administrative history in the last 30 years regarding rainwater storage and infiltration technology. The deterioration of the water cycle due to urbanization is expressed as reduction of base runoff, increase in total flood discharge and peak flow discharge, lack of water resources, changes in ecosystem, deterioration of water quality and change in heat environment. The promotion of rainwater storage and infiltration facilities in the residential areas is supposed to be an influential measure to remediate those problems. Therefore, it is important to make clear the effectiveness and explain to inhabitants so that they could introduce those facilities in their own houses. The physically based distributed grid model is dealt with to estimate the effectiveness of infiltration facilities for individual houses on flood discharge, usual flow discharge, groundwater level and evapotranspiration. The distributed grid model was applied for the Azuma river basin which is located in Tokorozawa-city, Saitama prefecture. Applying the model in case that infiltration facilities are promoted in the residential area of Azuma river basin, it was confirmed that the promotion of infiltration facilities in the residential areas gives a certain influence on stormwater control, usual river flow, groundwater level and evapotranspiration.
As well as reducing groundwater recharge by precipitation, urbanization creates a new important source of water for groundwater recharge, that is mains leakage. Although it has been realized by a water balance method that urban groundwater recharge is often as high, or higher, than pre-urbanization rates owing to the downward percolation of mains leakage, the increases have rarely been quantified in a direct manner. In this study, contribution of mains leakage to the discharge of springs in the Kurome River basin and Shakujii River basin, both in Tokyo, were evaluated through a comparison among the δD and δ18O of spring water and mains leakage (tap water). In the moderately-urbanized Kurome River basin, all the springs represented almost 100% precipitation contribution. This is also the case in the more urbanized upper reaches of the Shakujii River basin, where precipitation alone could account for 85-100% of the discharge of springs. Precipitation was found to play a less important role in spring discharge in the highly-urbanized lower reaches of the Shakujii River basin (impermeable surface ratio: some 0.8 and more), were some springs contained substantial proportions of mains leakage of as much as 30-70%. On the other hand, it is worthy of notice here that, even in central Tokyo, the isotopic composition of most spring waters was indicative of predominant contribution of precipitation, which is contrary to the results of the previous studies on the basis of a water balance method.
Groundwater in urban area has a great potential in the near future for certain purposes including water for emergency uses, non-potable domestic consumption, landscape, and spraying. In order to evaluate the potential based on the groundwater quality, the concentration of dissolved iron and redox potential were measured with their spacial distribution in the aquifer using 103 groundwater samples in central Tokyo. Groundwater samples obtained from unconfined Musashino gravel layers and confined Kazusa layers had comparatively high redox potential and low concentration of dissolved iron, which met the drinking water quality standard of 0.3 mg⁄L as total iron. On the contrary, groundwater abstracted from unconfined Yurakucho layers and Tokyo layers, had low redox potential and high dissolved iron concentration most of which was in the form of ferrous iron. These results suggested that iron removal may be required for the future uses of groundwaters that contain high iron concentration in the central Tokyo district.
The nitrogen concentration in groundwater and stream water exceeds environmental standards at some location in the southern region of the Miura Peninsula. In this region, the influence of fertilizer and domestic wastewater is indicated as a cause of the groundwater and stream water contamination. In this study, to clarify the influence of domestic wastewater, the contribution rate of tap water to river water was calculated using stable oxygen isotopes. In stream water and tap water in Miura City, a difference was seen in the stable isotope ratio of oxygen because of a difference in mean elevation of the watersheds. The domestic wastewater contribution rate calculated from stable oxygen isotopes in deep groundwater and tap water ranged from 6 to 30 %.
Data of shallow unconfined groundwater levels were collected from several previous studies and were compiled to understand the quantitative change of shallow groundwater environment with urbanization in the Tokyo lowland that isthe urban center of the Tokyo Metropolitan Area, Japan. Also, risk of soil liquefaction in this area that was evaluated by the Institute of Civil Engineering, Tokyo Metropolitan Government (ICE, TMG) at 1987 was mentioned from the viewpoint of change of shallow unconfined groundwater level. In the central part of the lowland, shallow unconfined groundwater level was considered to be slightly changed from the 1920s to the 1980s. However, sources of shallow unconfined groundwater were drastically changed. Although main source of shallow unconfined groundwater was leakage from water mains from the 1950s to the 1990s, the main source was infiltrated precipitation in present. This change was caused by the decreasing of rate of the leakage (only 3.6% in 2006). On the other hand, groundwater table in the area that land subsidence was induced by extensive confined groundwater exploitation and the elevation became lower than sea level was distributed beneath the ground surface thorough this period. That is, shallow unconfined groundwater discharged to the sewerage system in this area. From the results of comparison of groundwater levels between the previous studies, groundwater table map that was used for evaluation of the risk of soil liquefaction was considered to not have enough accuracy in local scale. Therefore, it is essential to understand the distribution of present groundwater table for improvement of the evaluation of soil liquefaction. Also, monitoring of groundwater levels is necessary to evaluate future change of shallow groundwater environment caused not only by human activities but also sea level rise induced by global warming.
The author considered the new viewpoints of urban hydrology based on the comment in the symposium of Japanese Association of Hydrological Sciences in 2007. There are many subjects which urban hydrology holds. Those subjects are as follows; (1)Calculation of accurate water balance (2)Establishment of the technique and technology about urban hydrology (3)Monitoring of the observed data (4)Water amenity (5)Spatial change of hydrological environment (6)Serial change of hydrological environment, etc. If the above subjects are solved, the new viewpoints will be in sight.