Current State of Water Resources in Japan

Water Balance in Japan

Available Amount of Water

Annual precipitation in Japan is approximately 650 billion m3 (average figure over the 30-year period from 1971 to 2000), of which approximately 230 billion m3 (35%) is lost through evaporation. The remaining 420 billion m3 is theoretically the maximum amount that can be used by humans and is referred to as the "inventory of water resources". The inventory of water resources decreases in years of low precipitation, reduced to 280 billion m3 in the year of water shortage occurring once in about 10 years.
The amount of water actually used (intake amount in 2004) is approximately 83.5 billion m3, which is equivalent to roughly 20% of the mean inventory of water resources. This ratio is referred to as the "water resources utilization rate". The water not used, amounting to 300 billion m3 or more, runs off into the sea as river water or flood is stored as groundwater.

History of Water Use

Ancient times to Edo Era: Development of water use in agriculture

Water use in Japan developed in close association with the production of rice paddies from ancient times. After rice paddy cultivation was introduced, the construction of small irrigation ponds was started, and then, with expansion of the manorial system and later backed by efforts of warring feudal lords to maintain and increase rice production, the use of water from small and medium rivers expanded. In the Edo Era flood control works on major rivers such as the Tone River were enhanced and plans to develop new paddy fields were implemented, resulting in the rapid development of paddy fields in alluvial plains. Meanwhile, the first construction of water supply systems such as the Kandagawa and Tamagawa Waterways were started in major cities including Edo (old Tokyo).

Meiji Era through to the Pre-World War II Period: Modernization of Japan and formation of the basis of socioeconomic development

The demand for industrial water rapidly increased with the drastic growth of heavy and chemical industries following the implementation of government policy of industrial promotion. Modern water supply systems were enhanced in urban areas including Yokohama in response to the increase in population and the need for prevention of infectious diseases. At the same time, in response to the increased demand for electricity with the progress of urbanization and industrialization, major advances were made in the hydroelectric power generation sector.

Post-World War II Period to the Present: A vital role in socioeconomic development

Since the demand for domestic, industrial and agricultural water use grew sharply due to rapid economic growth and population increase, comprehensive development of water resources such as multipurpose dams was promoted in order to secure stable water supply.
In the meantime, legislative systems were developed; laws concerning water resources development such as dam construction, laws for individual purpose of water use, and laws for prevention of ground subsidence were established by the end of 1960s, while laws concerning development of reservoir areas, and laws for water quality and environmental preservation were put into place after 1970.

State of Water Use

Water for Domestic Use

Water used in households is called "household water", while water used in offices, hotels, restaurants and so forth, is called "water for use in urban activities", and they are collectively called, "water for domestic use". The daily per capita amount of domestic water use doubled in the period between 1965 and 2004 due to changes in lifestyle (e.g. spread of flush toilets).
With population increase and expansion of economic activities over the same period, domestic water use increased by more than three times, though it showed a slightly increasing trend or remained static in recent years. A large proportion of household water is consumed for cleansing, as in bath (approx. 24%), toilets (approx. 28%), and laundry (approx. 17%).

Water for Industrial Use

"Industrial water" refers to water supply for industrial activities of manufacturing industry and so on, and is used as raw material or for product processing, cleansing, cooling, heating by boilers and others. The amount of industrial water use includes the amount of reuse of water. The percentage of the amount of water recycled in the total amount of use is called "the recycling ratio". The volume of Water for Industrial Use has increased almost three-fold from 1965. On the other hand, due to advances in water recycling, the amount of water taken in newly from rivers, etc. (referred to as "freshwater replenishment") has decreased or remained static since1973.

Water Resources Development

Needs of Water Resources Development

Japan has relatively high precipitation and plenty of water resources per square meter of its territory, compared to the global standard. However, river flows largely fluctuate throughout the year; high in the spring thaw, the rainy season from June to July, and the typhoon season, and low in other periods. On the other hand, the amount of water for domestic and industrial uses does not fluctuate as largely as the river flow, though it fluctuates seasonally and by days of the week.
In order to secure stable water supply, it is necessary to sustain stable intake of river water all the year round irrespective of fluctuations in the river flow. For this reason, water resources development facilities such as dams and weirs have been constructed, so that the required amount of water is made available in all seasons of the year.

Basic Idea for Water Resources Development

Let us assume the river flow at the point of water intake as in the conceptual diagram below. River flow in the natural state without dams or other water reservation facilities is shown by the undulating line. Generally, the amount of water flow is large in the rainy and the typhoon seasons, but otherwise, it is small. To take in a certain amount of water constantly throughout the year in the natural condition, the level indicated by line A in the diagram would be the maximum possible intake amount. When a dam is constructed, water can be stored at times of high flow such as in the rainy and the typhoon seasons (reservation: green areas in the diagram) and partly discharged from the dam at times of low flow (replenishment: blue areas in the diagram). The modified river flow is indicated by the red line. It is clear that with a dam the maximum possible level for constant water intake throughout the year becomes higher (A+B). The amount of water stored in a dam is referred to as "developed water through construction of a dam" (B in the diagram).

Water Resources Development Facilities

Water resources development facilities include the following. Apart from them, water channels are necessary for introducing river water to places of use (agricultural land or water purification plants).

• Dams and barrages - Sameura Dam, Chikugo Large Barrage, etc.
  Along with dams and barrages constructed to secure agricultural water, domestic water or industrial water, there are those constructed as multipurpose facilities with objectives of flood control, maintenance of normal functions of water flow, hydroelectric power generation and so on. The amount of downstream river flow is stabilized by adjusting the water flow through reservation and discharge of water in dams, enabling an additional amount of water to be obtained for use.
• Lake and marsh development facilities - Facilities for Lake Biwa, Lake Inbanuma, etc.
  These facilities, like dams, artificially control water levels of lakes and marshes for stabilizing downstream river flow as well as increasing the amount of water for use.
• Water transfer canals to regulate flow state - Kitachiba Water-Introductory Canal, Kasumigaura Water Introduction, etc.
  These are connecting facilities between two or more rivers with different annual fluctuations of water flow, to stabilize the river flow as well as increase the amount of water for use. These introduce additional flow from one river to another river when the river flow is insufficient in the latter.

Importance of Water Resources Development Facilities

So far in Japan, approximately 789 multipurpose dams and 1,878 single-purpose dams for agricultural, domestic or industrial water supply have been constructed. A steady supply (approx. 17.8 billion m3 per year) of "water for domestic and industrial use" has been secured, but unstable intake water has been partly taken.
Currently, approximately 28.3 billion m3 of water for domestic and industrial use is used in Japan, and 75% of it is taken from rivers, 83% of which (57% of overall amount of urban water) have been developed through construction of such water resources development facilities. In particular, in the Kanto coastal region where population and economic activity are highly concentrated, about 91% of water for domestic use from rivers is newly secured through construction of water resources development facilities.

Scale of Water Resources Development Facilities in Japan

The total storage capacity (active storage capacity) of dams constructed up to this date in Japan, including those used for purposes of power generation, flood control, water utilization and so on, amounts to approximately 20.4 billion m3.
As the territory of Japan is relatively small and rivers are generally short and steep, construction of giant reservoirs is difficult. Accordingly, even though numerous dams have been constructed in Japan, their combined storage capacity is less than that of the Hoover Dam in the United States alone and only 6% of the total storage capacity of the dams under the control of the four federal organs of the United States.