SPATIAL, STRUCTURE OF THE IRRIGATION SYSTEMS IN THE HOKURIKU DISTRICT

Abstract

The most important farm activity in Japan has been paddy rice cultivation based on efficient irrigation systems. The irrigation systems often play a major role in defining not only orga nizations of rice production but also socio-economic structures of paddy regions. Many geographers in Japan have discussed on regional differences or characteristics of paddy regions inn terms of irrigation systems. It is difficult, however, to find the regional differences in irrigation systems because of the lack of appropriate approaches.
The objective of this paper is to analyse regional differences in irrigation systems in the Hokuriku District through employing a newly proposed approach named “the spatial structure of irrigation systems” which integrates hierarchical organization of spatial irrigation units. In order to set up the spatial structure of irrigation systems in a region, first of all, spatial units are defined based on functional territories of irrigation facilities, management areas of irriga Lion cooperatives, and areas of traditional rights of water usage or a traditional way of water use. These spatial units are picked out from individual farmers' level to villages, and municipalities' levels. For a example a first order spatial unit is formed by a small sum of paddy fields where irrigation water is supplied by the smallest ditch. The ditch is mainly controlled by a group of farmers concerned. A second order spatial unit is formed by a territory of a channel which supply water to the lower order spatial units. The second order spatial units is often equivalent to an area of a rural settlement. Then a third order spatial unit conMains some second order spatial units and it is formed by a territory where a main canal distributes water to the channels. In this way, finally, we arrive at the highest order spatial unit, which is an area irrigated by a dam diverting water from the river to main or trunk canals. The spatial arrangement of hierarchical organizations composed of various order spatial units in a river basin can be defined here as the spatial strucuture of irrigation systems. The Kurobe alluvial fan, the Tedori alluvial fan, and the Takada plain were selected as study areas out of many alluvial plains in the Hokuriku District, because the three plains are nearly same in size and have elaborate irrigation systems.
On the Tedori alluvial fan the first order spatial units are formed by small sums of paddy fields with one to three hectares of areas irrigated by small ditches. Groups of farmers concerned maintain and control the irrigation facilities. The second order spatial units are territories to which secondary channels distribute irrigation water. Their areas vary from 10 to 50 hectares and the irrigation facilities are controlled by rural communities having farmhouseholds ranging from 30 to 100. When Bansui (irrigation by rotation) is carried out during drought season, a leader of each community makes a decision in order to distribute evenly the limited water into all paddy fields. The third order spatial units are areas served by the eight main canals on the Tedori alluvial fan. The maintenance of the canals was done by lower branches of the Tedori River Seven Canals Land Improvement District in the northern part of the fan and the Miyatake Irrigation Canal Land Improvement District in the southern part of the fan. When Bansui is necessary because of the water shortage, the terTitory of each lower branch and the Miyatake Irrigation Canal Land Improvement District are divided into three parts and from one part to another water is served by rotation. These parts are named sub-third order units because their positions are between the second and the third ones. The fourth order spatial unit is an area irrigated by the trunk canal which integrates seven main canals from among eight canals on the Tedor 1 alluvial fan.