In Japan we get precipitation as much as a man's average height per year (the average annual precipitation is 1700 mm). Of that, as much water as man's waist-deep runs off the land surface (the average annual runoff is 1070 mm). As much as knee-deep water is taken in for human use (the average annual usage is 300 mm).

The precipitation in the tropics can be as much as 2000 mm, however, the sun is much stronger in the tropics, resulting in more evaporation and less runoff. Rivers provide water, but it goes right back into the ocean if you don't have the means to use it. The world averages are; precipitation 850 mm; runoff 250 mm; usage 50 mm. Japan gets twice as much precipitation (the third in the world after Indonesia and Philippine), four times as much runoff (the second after Brazil), and six times as much usage (top in the world). The irrigated rice field is the biggest contributor to the high water usage. However, if you look at per capita statistics, a Japanese gets only 1/6 of the world average in precipitation.

In Europe, the most precipitation falls in winter. The annual average is 400 to 500 mm, and people rely on the meltwater from the Alps as their main source of water. During July and August, the plant growing season, precipitation approaches near zero.

The Annual precipitation in Los Angeles, America is also 400 to 500 mm. They also rely on the meltwater from the Rockies. Dams were built to irrigate water. The plants do not grow without sprinklers watering year-round.

Japan is indeed blessed with water. However, geographically, Japan is mountainous, and flat land is in short supply. The abundant rain fall often dumps too much water at once and cause disasters. Once, an engineer from Holland named Dreeke was visiting Japan to teach hydraulic engineering. The legend has it that he took one look at the Joganji river that originates from Tateyama and exclaimed "This is a water fall!" Too much untamed water is no blessing. Forests and irrigated rice fields have been playing important roles in taming water.

The fact that rivers in Japan are stable at their lower reaches today owes in large part to the water management works done all over Japan during the Edo era. They provided foundations for today's dams and embankments. The water management works done in the Edo era were excellent even in the area of civil engineering such as canal construction. The most superior aspect of their works was the idea that "water management is forest management".

They planted millions of trees upriver, where villages did not get enough water, to secure the river flow and to pull water into irrigation canals. Where the river flow was not stable, they took care of the forests upriver and built irrigated rice fields on the slopes(1)(2) to control water flow. Such water management practices reflect deep knowledge of the forest's ability to hold water, as well as the characteristics of the mountainous landscapes and the climate unique to Japan.

When you travel through southern Europe, you will often see bald hills and desert like landscapes next to lush tree foliage on some slopes and river banks. It is worst in Spain. What it indicates is that there used to be rich vegetation, but there were no people like Naotsugi and his son or Kumazawa Bansan who dedicated their lives to forest and water management for the sake of future generations. We are very fortunate to have had those noble pioneers. We owe them a lot. Currently in Spain, they are about to undertake a large scale tree planting project to reforest one million hectares of land as part of CAP (Common Agricultural Policy). They are trying to reforest the wasteland.

Now, looking at today's Japan, I cannot help but feel gloomy. It seems that we are behaving like the southern Europeans of the past.

Japan has been blessed geographically to get abundant water, however, it was man's wisdom that made water useful to us. Today, we face a new challenge. With the advent of water-supply systems, concrete streets and concrete landscaping, our relationship with water has changed drastically.

In cities, the ground surfaces and canals are lined with concrete. As a result, the rain water runs straight into the ocean without seeping down into the soil. The water that city people and factories use is the water that is gathered in a dam upstream after going through mountains and rice paddies. The cities downstream thus waste both the rain water and the water from upstream in their pursuit of convenience.

Finally, many people are starting to notice how the precipitation on cities is wasted. Not to waste rain water, Tokyo Dome is equipped to gather and store rain water for flushing toilets. It is high time that cities put more resources into research and development of such equipment for city-wide use.

The water shortage of western Japan in recent years has been attribute to abnormal weather. However, the more direct cause, in my opinion, is the overgrowth of downstream cities that consumes more water than the water supply they draw from upstream.

In pursuit of convenience, people move from upstream to downstream. The upstream villages become depopulated and no one is left to take care of the forests. Land slide is on the rise and the city's groundwater is depleting. Unless we put a stop to the overgrowth of cities and promote more even distribution of the population, the water supply will fail despite the abundant precipitation. We need to end the pattern of building new dams, vanishing villages under water, to compensate for the waste in cities.

We need to rethink the current practice of lining the river banks with concrete in the name of flood control. When you eliminate the direct contact between water and soil, the quality of water suffers. The micro organisms in the soil can no longer act on the water to purify it. In its natural state, water is alive, and dead water cannot support life.

There is a flood control method that is developed to suit the fast-flowing rivers commonly seen in Japan. It is the Shingen embankment built by Takeda Shingen. Guided by the idea of "control nature by using nature", Shingen devised a clever system of embankment that included side channels along side the main channel. What we need now is an approach that is rooted in the unique characteristics of Japanese landscapes. After the war, many traditional ideas have been thrown away as obsolete. Those ideas are the very resource we need to recycle and reuse.

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Water management of Edo era