Typical arguments at international water fora advocate a competitive relation between agricultural water use and other water use, including for ecosystems. This premise is generally applicable to the discussion on irrigation in arid and semi-arid regions where water is constantly scarce. However, it is unsuitable for humid regions such as the Asian monsoon region where paddy rice culture has been developed for thousands of years, using ample natural water from rainfall, including flooded water and artificially irrigated water from various water sources such as streams, ponds and rivers. The inundated water in paddy fields and flowing water in irrigation and drainage canals serves as a network of wetlands and waterways to create another excellent secondary natural environment outside the river. Furthermore, paddy fields stretching along a river serve as a retardant reservoir that at once receives outflow from the mountainous hinterlands and irrigated water drawn from the river, and that gradually supplies the water to groundwater aquifers and the downstream river. This paper, in the context of the impact of irrigation on the environment, reviewing studies and reports of recent years on quantifying hydrological characteristics on a basin scale and identifying services for secondary natural environment in Japan, shows the unique natural features and cultural climate in paddy field irrigation in humid regions contrasting with those in irrigation in arid and semi-arid regions. It also describes international activities among rice growing countries, regions, international organisations and research institutions, namely the INWEPF, and the Japanese policy direction “Shifting to agriculture, thinking much of preservation of the environment”, and gives recommendations for future challenges.