Water Management of Small Reservoirs distributed over the Middle and Mountainous Areas in Noto Region, Ishikawa Prefecture

Abstract

The number of small reservoirs, which are distributed over the middle and mountainous areas in Ishikawa Prefecture, is around 3, 000. Of all the prefecture, 80% of that type of small reservoir is located in Noto. The high number of small reservoirs in those areas provides us with a good reason to choose this region as a research model. We chose 17 areas in four cities and towns in Noto, and carried out investigation on the water management of those small reservoirs through questionnaire. The results are as follows:
1) Every small reservoir in each of those area has its own function and is operated by small reservoir operators. Several small reservoirs are managed as a group in order to gain complementary merits. This average volume of small reservoir is 231 mm in depth. In general, the inlet water, which is 2 to 3 times as much as individual small reservoir volumes, flows into small reservoirs from catchment areas.
There are 4 kinds of water resources: river water, underground water, water coming from mountain streams and the water from small reservoirs. In combination with these differences of water resources, each area utilizes small reservoirs as integral water resources.
2) In order to puddle and make ridges between rice fields, farmers primarily use rain water stored in the paddy fields and water from mountain streams. After puddling, small reservoir water is begun to use. The standard releasing time of individual small reservoirs is limited to within daytime hours. And, in general, each beneficiary takes irrigation water into paddy fields. Some communities, however, have 60-70-year-old specialists as watermen who are in charge of supplying water into paddy fields.
3) In an average year, precipitation is 496 mm in depth if it is calculated on the condition of 80% as an effective ratio. The amount of irrigation water released from a small reservoir is 613 mm. Therefore, the total amount of irrigation water corresponding to demand is 1, 109 mm. In 1994, the drought year, the amount of both precipitation and small reservoir water was 60% less than those of the average year. That resulted in 665 mm of in-depth water shortage for paddy fields. That was the reason why great efforts were demanded on the part of community farmers to get irrigation water from mid-June to the end of the irrigation period.