1983 Volume 38 Issue 1 Pages 20-35
At present the total number of the ponds above I ha of benefited area in Japan is about 97,500 and the benefited area about 1,140,000 ha. The authors have analyzed their development, irrigation, conservation, maintenance and policies. (1) A large number of them are situated in some prefectures along the Inland Sea of Seto from the Kinki district. The reason is, it seems, not only there is frequent rainfall, but also they have no large rivers for water sourses, though they have the land fit for paddy field, and people there have relied upon ponds. (2) To consider periods in which those ponds were built. Before the Edo period, were built in the Seto-Inland-Sea district (e.g. prominently in Kagawa Prefecture) and the Kinki district; and they were distributed sporadically in Aichi and Fukushima Prefectures. After the Edo period, were built in the northeastern part of Japan and fewer in the southwestern part. After the World War II, large reservoirs were made in the valleys by the modern-type dams and the benefited area per one reservoir was strikingly extended. (3) At present, 87% of the total number of the ponds are small-scale ones with the capacity of less than 30,000 m^3. Building and maintenance of them are made dominantly by buraku (or village communities) or water-use associations in accordance with the buraku administration. But the maintenance has been made difficult by decreasing the function of the rural communities, due to either depopulation of the farmers, in the case of the ponds in the remoter areas, or the increase in the number of worker farmers and transferred uses for houses and factories, in the case of the ponds in the urbanized areas. (4) Present study, it was emphasized to increase of their uses as living environments: multipurposes, such as their uses as landscapes, drainage, open space or fire water, should be considered. It is expected that we should establish systematic scheme for their appropriate management by local governments. (5) To consider the improvement situations of the ponds in relation to natural hazards. Old ponds built before the Edo period are as many as 34% of the total ponds, but only 20% have been improved. Many of them are situated in the southwestern part of Japan where earthquakes, typhoons and land slides occur more frequently. So the authors have proposed that local government should not only proclaim the maintenance regulations of the ponds, but also check their uses and conservation periodically based on the standard of the classified measures for conserving them against natural hazard.