Historical Development of Rice Paddy Irrigation System and Problems on Water Management in Recent Years : Yamada Diversion Dam Command Area in Japan

概要

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Productivity of rice in Japan has been increased rapidly by the land consolidation of rice paddy area. Modernized irrigation systems have been introduced to improve the irrigation and drainage efficiency. On the other hand, restriction of excessive rice production policy started in 1970 has brought the changes of exquisite water management once accomplished. Additionally, the increasing number of part time farmers has affected the characteristics of water management. To clarify the recent changes of water management, a typical rural area was selected. First, historical development of the rice paddy area supported by the Yamada diversion dam was studied. Second, characteristics of water management in 1981 and 1998 were discussed. The components of lot water requirement and delivery water requirement have shown drastic change, though water consumption rate has shown no change. Third, water intake rate changes at the Yamada diversion dam were studied. It is intersecting to note that the reduced area of rice paddy did not affect the water intake rate to this command area. This result would be caused by the need for keeping the water level in canals to deliver water adequately to each rice paddy fields and by the need for supplying water for increased lot water requirement.