Rain and City
There is a popular Japanese song entitled "Tokyo Sabaku" or Tokyo Desert. One may be surprised to learn that the image of the title is not just a songwriter's whimsy, but also what actually exists. The average annual humidity in central Tokyo is only 64% though 78% was recorded a century ago. In suburban Tokyo, on the other hand, annual humidity reaches 78%, showing that the urban center is evidently becoming arid. The main cause of this is that the ground is paved with concrete - consequently decreasing its water permeability.
The "coefficient of discharge" is a technical term for the rainwater percentage that courses all the way to a river. Although the figure changes depending on topography and soil characteristics, the average is approximately 0.1 in sand, 0.2 in forest, 0.4 in a cultivated field, 0.5 in residential districts, 0.6 in commercial business districts, and 0.9 on a paved concrete surface.
Therefore, in central Tokyo, where most land is paved, 90% of rainwater goes directly to sewage. It may mean that only 150 millimeters of rain out of about 1,500 millimeters of annual rainfall every year actually soaks into the ground. However, as some of rainfall becomes groundwater, 80 millimeters of rain stays in the ground surface as a matter of fact. Then the surface water humidifies the atmosphere through evaporation. Compared with forests and farmlands, where 800 millimeters of water becomes atmospheric moisture, urban land area contributes only 10% to evaporation. The less evaporation there is, the hotter it becomes because the evaporation can't decrease temperature so much. This is one of the main causes of the so-called "Heat Island Phenomenon", a meteorological occurrence and environmental problem that increases urban center temperatures to a degree much higher than their surroundings. Also, the higher the temperature, the lower relative humidity drops.
Disappearance of rivers worsens matters. Many rivers in Tokyo have been destroyed and reclaimed over the years following the early days of landfill projects that were undertaken for construction of the JR Yamanote Line. Despite the fact that many people didn't appreciate these medium to small sized stinking rivers filled with human sewage, their loss has been significant in accelerating desertification. Words of the second verse to "Tokyo Sabaku" read "Rivers between buildings don't flow", but it's not an issue of water "flow"; rather, what matters is that there aren't many rivers out there.
Aridity in urban areas affects our health, too. One reason for increased hay fever incidence is that cedar pollen, the histamine of hay fever, is incessantly airborne because the ground surface is very dry.
Urban dryness is to blame for an increase of city flooding. There were many floods in Fukuoka and the Tokai region of Japan last year. A flood occurring in medium and small rivers is called an "Inland Water Flood". Most contemporary floods are inland water ones. Residential areas are developed in such suburban areas as farmlands and forests, while at the same time, inner city residential districts turn into apartments. This leads rainwater to flow into rivers and sewers. In spite of improvement work for major rivers which eventually prevented floods with high concrete embankments, many waters end up in such rivers and people came to need to drain some water off with pumps to medium and small rivers. This suggests that when pumping facility capacity is reached, these rivers become flooded. In worst cases, the pumping facilities themselves could be submerged.
Among other causes for growing likelihood of flooding is the fact that rainfall characteristics are becoming "tropical" due to global warming and the heat island phenomenon. In other words, the atmospheric shell holds more water at higher temperatures, which turns genial rains that we normally have into tropical squalls.
There are three ways to prevent urban flooding: first, by maintaining the coefficient of discharge; second, by creating retarding basins and storing water therein temporarily; and finally, by upgrading as many rivers and sewers as possible so that water can be streamed there. The first measure is the best of all in the sense of recharging ground water. The use of permeable paving and leader heads is growing, yet not widespread. Permeable paving with wider gaps is used for freeways, pedestrian paths, and parking lots. However, it isn't widespread enough because it requires frequent cleaning to get rid of dust and mud.
As far as river improvement work is concerned, it is hard to expand rivers inside the city because many houses occupy most areas. Also, there are many rivers flowing underground. Many of the underground flows and sewers are located even deeper than sea level. In such cases, the water eventually needs to be pumped up after all.
Retarding basins are required to be installed in major development sites. Some of them are built along middle and lower courses of a river and are usually used as sports fields. Yokohama International Stadium, for instance, is in the middle of the Tsurumi River retarding basin and its parking lots can store much water when it rains heavily.
In my house, I make use of rainwater for our domestic water supply. When we moved to a new house, I came up with this idea: if we store rainwater in a tank and use it for our living water, it would help to prevent urban flooding and desertification. My house is located along the Tsurumi River. According to the Land, Infrastructure and Transportation Ministry, the river "originates in Machida City, Tokyo and it runs through Tama Hills, the Tsurumi Industrial Belt, Yokohama, and ends up in Tokyo Bay. It is one of the most typical rivers in an urban city."
"Urbanization rates in the third decade of the Showa Era (1955-1964) were only about 10% and population along with a river accounted for about 200,000. But in the fourth decade (1965-1974), about 85% of the lands were rapidly urbanized and those who lived near the river increased up to some 1.84 million". Though we don't have many floods recently, the Tsurumi River used to be a frequently flooded waterway. In addition, it is thought to be the dirtiest of all class "A" rivers in Eastern Japan.
I thought of using the water for the washroom first. It is said that average households, with four people, consume 220 liters of tap water a day. We use 25 cubic meters of tap water monthly, so we represent average consumers of tap water. Out of that total, a third is used for the toilet. For a family of four, 240 liters are used every day. So, I calculated how big the water tank ought to be in order to store water for our washroom regularly. Of course it doesn't rain constantly every day. Annual rain precipitation in Tokyo amounts to approximately 1,400 millimeters. It rains a lot during Tsuyu, Japan's rainy season (June to July), and from September to October, though in winter it doesn't rain that much. In Owashi, Mie, on the other hand, precipitation comprises 4,000 millimeters, while there are only 800 millimeters in Abashiri, Hokkaido, one fourth that of Owashi.
Provided that we use 240 liters of water a day for the washroom, we need a tank that can store six cubic meters of water so that the rainwater can provide three-fourths of the water demand. Here is a problem that I encountered: The tank itself together with the water inside weighs some five tons and it is physically impossible for it to be situated on the roof. You could put it on the floor, but considering gravity, it is not wise to set it below the lavatory: you need to pump the water up again. It costs a lot, too: they say you need to pay as much as one million yen to get the concrete-made tank and the water-ducts equipped. Even if you compromised and decided to supply two-thirds of water demand with rainfall, three cubic-meter tanks would still be required. With that system, you would only save 14,000 yen a year!! There is no economy there. What is even worse is that if you carry out Life Cycle Analysis (LCA), it tells you my attempt would negatively affect our environment, taking into consideration the fact that the tank would also become "waste" in the future.
In this way, my first wonderful thought of utilizing rainwater for our living wound up being a "deep-thought". Then I downgraded the plan: I made up my mind to use the water for car washes and sprinkling the lawn, for which you need about 100 liters. So, if you have a 700 liter tank, you can provide 80% of the water needed for those purposes with rainwater. It's good that you may use a plastic tank that is a lot cheaper and lighter, although you can only save 2,000 yen a year in terms of home economics. Luckily, I was given a "waste" or used plastic tank, and I was able to cheaply build my own rainwater utilization system with less environmental impact. Some municipalities in Tokyo such as Sumida Ward are actively encouraging the use of similar systems and some even provide subsidies. I am sure you will find it fun to try to do it yourself.
Floods in urban cities are increasing.
The graph shows the size of the tank needed (horizontal scale) and the percentage of collectable rain (vertical scale) when collecting rainwater with a 60 square meter roof tank, in the case of using either 120 liters or 230 liters of water daily.
The size of a tank is so small that three of the tanks are stacked.
Plastic tanks that were industrial waste have been recycled. The front spigot is for rainwater drainage.