Abstract
Rice cultivation in alluvial fan areas along large rivers in Japan has rapidly de-veloped in the past one hundred years. This development chiefly depends upon the improvement of irrigation systems with large-scale land improvement works. In this paper, the author attempts to analyze the process of the irrigation systems in the Kurobe Alluvial Fan after the 1910's from the spatial viewpoint. The study area is located in Central Japan facing the Japan Sea and more than 95% of its arable land is devoted to paddy fields. Irrigation water taken from the Kurobe River is distributed through several diversion dams to many tributary channels and paddy fields. Establishment of spatial orders should, the author thinks, be dependent upon the follow-ing principle of hierarchical organization irrigated areas by canals directly conducted from the Kurobe River form the highest spatial order of irrigation system. Water management of this order of irrigation system is carried out by irrigation cooperatives or administrative agencies. The second spatial order is formed by areas of the tributary channels of the above-mentioned major canals. Areas served by the sub-divided tribu-taries are classified to be the third spatial order ones. Finally, a small sum of paddy fields of each farmer forms the lowest spatial order area. Water supply of this order is controlled by the farmers owning those fields. This hierarchical structure can be defined to be the Spatial Organization of Irrigation System. Considering changes in the network of irrigation channels and their supply areas, three different stages of spatial organization in the irrigation system can be found in this region. The First Stage (ca. 1890-1930): there were fifteen areas of the highest spatial order, each of which had its own intake and they were rather independent on each other in terms of the supply and demand system (Fig. 2). Their major concern was directed to the maintenance of intakes and diversion dams within their areas, otherwise they would have had to face water shortage. The Second Stage (ca. 1930-1969): after the construction of small hydro-electric power plants on this fan, the irrigation water system changed. Thirteen intakes out of the fifteen were abolished, and two large intakes were newly constructed at the dam-site of power plants. Thus, the thirteen areas of the highest spatial order were re-organized into two by the newly established two irrigation cooperatives along the new canals originated from the diversion dam. The remaining two areas still kept their original intakes and did not join the new cooperatives. The number of the highest order became four in this stage (Fig. 8), which fact means that even larger areal units of the spatial organization appeared and the spatial independence of the thirteen areas became less strong keeping their initial function to irrigate paddy fields to some extent. The Third Stage (After 1970): with the development of land consolidation, the network of irrigation channels has improved considerably by reforming the meticulous flow of canals and ditches. In this process, areas of the lowest spatial order have been newly delineated in accordance with the improved channels (Fig. 14). Nevertheless the areas of the highest spatial order have not changed, the Third Stage could be established, because of the reformation of the lowest order groups. Fig. 15 shows the process of the spatial organization of irrigation system in the Kurobe Alluvial Fan.
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