IMPACT OF RETURN FLOW FROM PADDY FIELDS UNDER SPILL-OVER IRRIGATION ON WATER TEMPERATURE IN IRRIGATION-DRAINAGE CANAL

摘要

A recent concern in Japan is the high temperature damage to the ripening rice grains, and it may be magnifi ed in future due to climate change. It reduces rice yield and degrades rice quality in terms of lack of grain fi lling, cracked rice, and milky white kernel. Extremely hot summer in 2010 decreased the fi rst-class rice ratio down to 62% compared to about 80% of normal years (Nakagawa, 2013). The damage is mainly caused by high temperature after rice heading (Morita, 2008). In case of the milky white kernel, it looks chalky because of voids produced inside the amyloplast of rice endosperm (Tashiro and Ebata, 1975). Nowadays, some measures are proposed to prevent the damage such as cultivar improvement, fertilizer management, delay of rice planting, and water management. Among them, one of the simplest ways for famers is spill-over irrigation, in which paddy fi elds are simultaneously irrigated and drained over a certain period after rice heading. Nagahata et al. (2005) showed the spill-over irrigation management for 30 nights (from 6 pm to 6 am) after rice heading considerably reduced the occurrence of cracked rice and milky white kernel. However this method needs a large amount of cool irrigation water. Considering the capacity of irrigation facilities and water rights, it is obvious that every farmer cannot adopt this method. Additionally, since the temperature of irrigation water is a signifi cant factor for the effectiveness of spill-over irrigation, it is important to realize water temperature variation along the irrigation canal analysing possible infl uences. It is presumed that water temperature in open channel becomes higher in summer when water fl ows downstream. In addition, the return fl ow, that is, drained water from upstream paddy fi elds fl owing into downstream irrigation canal, probably has some infl uences on the downstream water temperature. Hence, in case of an irrigation-cum-drainage canal system, it is considered that in summer higher temperature water is supplied to downstream paddy fi elds