The Impact of Various Environmental Changes Surrounding Paddy Field on Its Water Demand in Japan

This review article aims to summarize and analyze comprehensively the impacts of various environmental changes surrounding the paddy fields after the World War II on each component of the irrigation water requirement in the paddy field. And it will serve as an introduction to typical water management and irrigation planning for paddy field in Japan. As a result, it could be explained that many factors such as “separated irrigation and drainage canals”, “pipeline irrigation”, “well-drained field”, “a large-size lot”, “rotational use of paddy field”, “direct seeding”, and “urbanization” enhanced to increase the water requirement and decrease the reuse of irrigation water, which resulted in promoting to increase the total water demand for the paddy field. For this reason, it is difficult to reduce the irrigation water demand with the decrease of paddy field areas.