Abstract
This study was conducted in order to understand the conditions and amounts of irrigation water used in large, sub-irrigated rice paddy fields. On-site monitoring of subsurface water levels was conducted, as well as the delineation of: irrigation water supply routes; the quantity of irrigation water at each crop growth stage; and the time/rate of each water intake. It was found that when sub-irrigation is carried out during initial irrigation, the irrigation water quickly raises the level of subsurface water in the underdraining filter material, rising to the plow layer level before spreading horizontally. Results indicate that even in large rice paddy fields, a mostly temporally uniform water supply to the entire field is possible. In direct-seeding cultivation, the amount of water per intake was large at the beginning of irrigation and in subsequent intake for shallow-water management. Between mid- and late May, a significant spike in regional water demand may be seen because water is required both for land preparation in fields used for transplant cultivation and for the beginning of irrigation and shallow-water management in fields used for direct-seeding cultivation.
