Water management of paddy rice cultivation in Kagawa


 In the Kagawa prefecture, which is located southeast of the Japanese islands, “Hinohikari” and “Koshihikari” are the primary cultivars. Although the permeability of the soil is normal, the coastal regions of the Seto Inland Sea make up one of the areas receiving the least precipitation in Japan. Thus, droughts frequently damage crops. Water resource management is remarkable in this area.
 Reservoirs and Kagawa Yosui are essential to mention water during discussions of resources in Kagawa. Although some of the reservoirs are principally made in the Edo period, the idea of building ponds to collect and hold rainwater is an ancient one. For example, Mannou Ike, which is said to be the largest reservoir in Japan, was made in 8th century, went out of use in 12th century after which a colony was built inside it; however, it was repaired and ultimately came into use again in 17th century.
 Droughts are frequent in Kagawa, in spite of these efforts, but the completion of Kagawa Yosui in 1974 improved the status of the water resource supply drastically. The structure is a canal used to divert the Yoshino River into the Kagawa prefecture via a tunnel. The Yoshino River moves water from the Sameura dam in the Kochi prefecture to the Tokushima prefecture; therefore, Kagawa Yosui is an epochal project which conveys water to different river systems. Negotiation over water rights is difficult settle, but during the Yoshino River Comprehensive Development Project, the parent of the Kagawa Yosui project, it was decided that the negotiations among all 4 prefectures in Shikoku was settled, since Kagawa, which suffers from droughts, and Tokushima, which suffers from the flooding of the Yoshino River shared a mutual interest, leading to the completion of the project involving the study of water management practices. However, droughts still occur, with serious droughts taking place in 1994 and 2005. In 2005 in particular, the exhausted water supply of the Sameura dam was replenished in only 1 day by a typhoon, illustrating the instability of water resources.
 For these reasons, people in Kagawa have a high awareness of droughts, and they come up with elaborate methods of using scant water resources effectively. For example, midseason drainage is generally not performed. Paddies are kept wet to maintain their absorption capacity. During droughts, water management is switched to a water-saving method. In terms of agricultural water supply, detailed management methods such as rotation irrigation (set each user’s time within 24-hours period and use water by rotation) are used.