阿蘇火山白川地域と大野川上流区域の灌漑について
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概要
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The average volume of ruuning water in the upper reaches of Shirakawa River watering Nango valley in volcanic Mt. Aso is not so small but the area irrigated by the river is not extensive. Besides, in this area water is being supplied additionally with aid of pumps from another river system, with the result that in the event of drought troubles are bound to arise in order to get enough supply of water. This is because the paddy-fields in tablelands and valleys along the middle reaches of the river use up a great deal of water for their areas while no part of supplied water goes back to the river. A peculiarity of the river should be taken into consideration that its long channel stretches over the lava fields where paddy fields are to be supplied with water by using ditches. Already nnder the clan government people living along the upper and the lower reaches of the Shirakawa River used to meet in conference in time of diminution of water to find a solution to difficult water distribution. For a long time it was an agreement between that upon request of the people of the lower stream the people of the upper stream would stop using water of the river for a certain time, thereby allowing sufficient volume of water to flow downstream.On the other hand, a remakable development of stairslike paddy fields can be seen in the areas along the upper reaches of the Ono River that drains the east-side foot of Mt. Aso. What is characteristic of the irrigation of these rice fields is that water is supplied through tunnels made in the lava bed. This method of irrigation, rare in Japan, was devised to surmount topographical difficulties in those areas. The tunnels are well taken care of to prevent leakage of passing water. There are few rivers having the catchment area of great extent flowing on the slope of a volcano, and in addition, the flat land, if any, on it where it is possible to grow rice is in need of quantities of water. Such a land will be best utilized by growing two crops of rice cultivated on dry land and wheat in succession and theredy stretching out the irrigated area.