Abstract
The Katsuragi Fan, situated in the south-western of the Yamato Basin is an alluvial fan well cultivated into rich rice-field. To irrigate these fields is used the irrigation tunnel named “Yokoido”, the characteristics of which are as follows;
(1) Most of the“Yokoidos” may be found in the central part of the fan. They have been constructed by digging the underground straight from the scarp of the terraced field or along the river bank, and are generally about one meter in width and depth and from 60 to 80 meters in length.
(2) The temperature of these water is rather low in summer and its quantity, being much varying, is controlled by the natural circumstances. It is to help the shortage of water supply from the pond and the rivers.
(3) The “Yokoidos” seem to have been developed because of the abundance of underground water resourses as well as the gradual increase of rice planting in this area. And it must be noted that they were mostly constructed in Meiji Era.
(4) Since the soil of the fan is much permeable it must be so often consolidated, or mended with the earth brought from other area.
