Water resources in the Japanese Islands

Due to its limited land area and limited range of natural resources (particularly fuel), Japan has developed a highly efficient economy in terms of resource utilization. This also applies to water resources. For sustainable use of water resources in the Japanese Islands, integrated and unified analyses of the data of groundwater by the nation and local governments have been needed. Land area of the Japanese Islands is 377,000 square kilometers, which is equivalent to the area of the state of Montana, but extends for 3,600 kilometers along the margin of the Eurasian continent. Mountainous areas separated by isolated, narrow plains make up 80 % of the land area. Due to the topography of Japan, rivers are generally short with steep grades, the longest being only 367 kilometers in length. Average annual precipitation is 1,600 millimeters but is highly seasonal. The annual water demand was approximately 87 billion cubic meters during the past 25 years, which represents 21 % of the total usable water. The water demand for agriculture makes up 66 % of the total water demand, and 96 % of the water for agricultural uses is used for the irrigation of rice paddies. Municipal and industrial uses make up 15.4 and 18.9 % of the demand, respectively (as of 2000). Nearly 80 % of the water used by industry in recycled. Approximately 87 % of the water demand is supplied from surface water with the rest from ground water. Because of its mountainous topography, the extent of individual aquifers is far smaller than in United States. Groundwater basins in the Japanese Islands are classified into the following six types: plain type (thick Quaternary strata); basin type (intermontane terraces and fans; hill type (highly eroded old volcanoes); volcano type (permeable lava and pyroclasitc flows comprising Quaternary strato volcanoes); pyroclastic type (thick tuff associated with large caldera formations); and limestone type (limestone blocks with karsts). Of the above types, the only major groundwater aquifers are the plain type (e.g., Kanto plain) and the volcano type (e.g., Mount Fuji). Ground water usage in 1998 was divided between 28 % domestic, 31 % industrial, and 23 % agricultural, and 18 % other. In the Japanese Islands, due to the small drainage areas of rivers, limited storage volume of aquifers, and variable seasonal and annual precipitation, droughts are common in some regions. Excessive pumping of groundwater in the 1950s to 1970s caused local land subsidence and salt water intrusion.