Water management of paddy rice cultivation in Iwate


 In the Iwate prefecture, “Hitomebore” is the primary cultivar, with some “Akitakomachi” being grown in north areas. There are abundant water resources in the Tohoku region, and therefore serious droughts are rare. Soil fertility is high, and water drainage is about 10 mm per day in lowland areas and 30 mm per day in mountainous areas.
 Two types of water management, conventional agriculture and deep-water cultivation, are implemented in rice cultivation in Iwate. Conventional agriculture is mainly carried out in fertile soil, and its characteristic is that intermittent irrigation is started prior to the midseason drainage period. Oxygen is supplied to roots, and nutrient availability is coordinated using this method. Intermittent irrigation is done as often as 2 (in poorly drained paddies) to 3 (in well drained paddies) times a week. The pattern of irrigation is determined by the conditions in each area, such as “4 days wet, 3 days dry (fill paddy with water in amount of 4 days to drain, and again after 3 days dry)”, and “2 days wet, 2 days dry.” Water depth varies with the choice of farmers, but seems to be generally in the range of 30–40 mm. Alternatively, deep water cultivation is a method in which the water level is kept rather high. This inhibits tillering and retains heat, but in fertile places, it leads to the over-availability of nitrogen.
 Midseason drainage is carried out in both water management methods. However, if a paddy is dried too much, denitrification takes place and yield decreases. After midseason drainage, the rice is cultivated with high levels of water until the heading stage, in cases where cold weather may damage the crops. In this case, water level is set between 10 and 15 mm. If the soil requires oxygenation, irrigation is decreased and only applied intermittently. This is managed by filling paddies with water each time the water in footprints in the paddy soil dries. A high level of water is maintained around the heading season, as the water requirements of rice plants are higher during this period.