Seiya KINOSHITA* Yoshio TOKUNAGA**
* Director, Water Resources Planning Division, Water Resources Department, Land and Water Bureau, Ministry of Land Infrastructure and Transport (MLIT)
** Deputy Director, Water Resources Planning Division, Water Resources Department, Land and Water Bureau, MLIT
Water is circulated on the earth by solar energy and gravity. Its total quantity is assumed to be about 1.4 billion km3, about 97.5% of which is sea water, and about 2.5% fresh water. Most fresh water exists as ice in the Arctic and Antarctic regions, with the river water and groundwater that can be used easily by human beings comprising about 0.8% of global water. The river water and groundwater that can
be utilized as water resources are delivered mainly as rainfall and snowfall, and due to varied topological conditions and precipitation water resources are not evenly distributed around the globe.
The greater part of Japan is located in the Asian monsoon zone, supplied with sufficient precipitation. Its average annual precipitation is about 1,700 mm/year, almost double the world’s average annual precipitation of about 970 mm/year. However, Japan’s land area is small compared to its population, which makes the total annual average precipitation per person (the value obtained by multiplying precipitation by national land area and dividing by population) about 5,200 m3/year/person, about 1/5 of the world average of about 27,000 m3/year/person.
Japan is a long, narrow nation, extending from north to south, in which spine-like mountain ranges produce a steep topography. Compared to other nations, its major rivers are short with sharp gradients (Fig. 1-1). In addition, rainfall concentrated in the rainy season and typhoon season makes a seasonally uneven distribution of precipitation, and river flow rates fluctuate greatly. Due to these factors, various efforts, including the construction of ponds and dams, have been made to overcome these conditions disadvantageous to the utilization of water resources.