Change in Floodwater Retention Function of a Paddy Field Due to Cultivation Abandonment in a Depopulating Rural Region in Japan

Abstract

AbstractFlood control concept at watershed scale is used as an adaptation strategy for climate change; rainwater and floodwater should be stored in forests and agricultural lands in the watershed, rather than relying on only river systems; and paddy fields are important ecosystems for floodwater retention. The abandonment of paddy cultivation, however, is increasing year after year in Japan due to depopulation in rural regions. Through two-dimensional numerical inundation analysis, we evaluated the negative effects of topographical changes after abandonment on the floodwater retention function. The bipolarization of the inundation depth is apparent in the current topography. The areas with deepwater inundation are wider, particularly in the most downstream areas, whereas in the past, areas with shallow water were widely distributed. The peak water level in the current land use is 0.4 m higher than in the past. After the peak level is achieved, the water level decreases in a shorter time in the present land use than in the past. Owing to hydraulic changes after the abandonment of rice cultivation, the risk of flooding disasters for people living downstream has increased. To maintain floodwater retention function, one of the regulating services, by artificial wetlands, such as paddy fields, continuous management is essential to prevent vegetation succession. Thus, it is important to provide incentives for local people to perform the daily maintenance activities.