Uchimizu is an old Japanese custom of watering gardens and/or streets. It also means water to be used for watering.
Uchimizu is effective for preventing dust from rising and it is also done for the purpose of cooling down in summer. This is because water takes heat when vaporizing and it is effective, albeit only slightly, for lowering air temperature (the heat of vaporization: vaporization of one gram of water takes about 0.58 kilocalories of heat).
As a countermeasure for the urban heat island phenomenon, Tokyo Metropolitan Government and others are implementing the program in which hundreds of thousands of citizens conduct Uchimizu simultaneously. The Japanese Government also recommends Uchimizu as a part of its campaign against global warming and municipal governments as well as NPOs around the nation are also planning or implementing Uchimizu.
In these recent campaigns, a note of caution stating 'with environment in mind, please reuse remaining water of baths instead of fresh tap water' is often seen. By recommending citizens to recycle used water instead of using fresh water coming out of a tap, such campaigns aim to achieve a dual purpose of reducing waste and getting coolness. Needless to say, it is not recommended to sprinkle polluted water such as urine. It is common to reuse remaining bath water from the day before.
In recent years, some water works bureaus have been implementing such Uchimizu movements. One example is the donation of recycled waste water. As recycled waste water is not suitable for drinking, its use is necessarily restricted. The purpose of donation is to make use of recycled waste water in Uchimizu movements and the number of water works bureaus that donate such water has been increasing year by year.
Other than Uchimizu conducted by people, some municipalities additionally pave roads in order to improve water retention capacity or sprinkle water by sprinkler trucks.
As uchimizu has the implication of purifying a place, which derives from Shinto religion, it is done at the front entrance etc. for the purpose of 'consideration for guests.'
Although Uchimizu for the above purpose was once done as a Japanese custom, such custom has been declining recently.
Knack of doing Uchimizu
Although Uchimizu has the effects as mentioned above, such effects can not be achieved if it is done in the wrong way.
Firstly, it is generally believed that it is desirable to do Uchimizu in the morning or evening when the sun is low.
This is because Uchimizu is not effective if water is sprinkled on an asphalt-paved road in summer when air temperature is high since water vaporizes soon after sprinkled and the effect of inhibiting temperature rise by the heat of vaporization is minimal.
We can obtain lasting effects of Uchimizu by doing it in the morning and evening when air temperature is relatively low.
It is also not desirable to sprinkle a big quantity of water by the use of hose.
Taking advantage of its humid climate, another option available in Japan is to store water emitted from a dehumidifier and sprinkle it.
The knack of doing Uchimizu is to sprinkle, in the morning or evening when air temperature is low, water stored in a bucket little by little on the ground or the wall of a house.
Uchimizu of Sado (tea ceremony)
In Sado, Uchimizu is done, regardless of seasons, in the garden adjacent to a ceremonial tea house as sanro (three watering), namely hatsumizu, nakamizu and tachimizu.
On the occasion of chakai or chaji (tea ceremony), a host sprinkles water at the front entrance only when he has completed all preparations for receiving guests. Guests enter a house where chakai or chaji is held after confirming water has been sprinkled at the front entrance.
Uchimizu in Kyoto
Until mid-1970's, Uchimizu, along with kadohaki (sweeping the road in front of a house), was an important task done every morning in Kyoto's machinaka (in this context, it refers to a district densely-packed with residential houses and stores where a traditional community remains). In Kyoto, it seems that the term mizumaki has been used commonly instead of Uchimizu. Early every morning, each household's master, wife, retired people, children and employees (persons in charge are different depending on household) used to sweep the road in front of the house or store and sprinkle water after. As a result, the roads in the district were purified and became moist with water. However, mizumaki was not done in winter since roads could freeze.
Also, there was a tacit understanding that 'kadohaki' and 'mizumaki' should be done only in front of one's own house and doing it in front of neighboring house was considered to be impolite. As the time of morning cleaning differs depending on households, it may become a psychological burden for the neighbors if we sweep in front of neighboring houses after having finished our own cleaning. The above tacit understanding was based on such consideration.
In summer, Uchimizu was done not only in the morning but also in the evening. The purpose of the above was to bring coolness by cooling the heat of daytime. However, if water is sprinkled at wrong time, it may become more muggy due to water vapor. Therefore, it is not so easy to do Uchimizu in the evening since we have to subtly change the time depending on the weather of the day.
It was common to use a bucket and a dipper in sprinkling water up to mid-1960's, but households that sprinkle water using a hose connected to a tap increased in the 1970's. However, as traditional communities collapsed, a scene where all roads in the district are moist with water is seldom seen. At present (2005), only a small number of households and long-established stores are maintaining the old tradition of doing Uchimizu in the morning and evening.