Assessing the Impact of Climate Change and Demographic Shifts on Japan's Small Agricultural Reservoirs

Despite the fact that Japan generally receives a considerable amount of rainfall annually, the steep terrain and limited size of rivers results in rapid outflow to the sea. Consequently, in numerous regions of Japan, small-scale agricultural reservoirs have emerged as the predominant water storage infrastructure and the primary source of water for irrigating rice paddies. It is estimated that there are approximately 150,000 such reservoirs in Japan, with the majority having been constructed over a century ago. This study investigates how changes in rainfall patterns due to climate change and the aging of the population, as well as the accompanying changes in agricultural practices, particularly in rural areas, are affecting the fate of Japan's agricultural reservoirs.

In the present study, the primary objective was to establish a comprehensive database of reservoirs. Despite the availability of official data on reservoirs, it has been observed that this data does not accurately reflect the current situation, particularly in the case of small reservoirs. This is primarily due to the large number of reservoirs and the limited capacity of the government to effectively monitor these bodies of water. Additionally, the database has not been updated to reflect the changes in the reservoir environment that have occurred in tandem with the rapid advancements in agricultural practices and the shifting social landscape, characterized by declining birth rates and an aging population. To address these challenges, we have developed a technology capable of detecting these reservoirs through the analysis of aerial images and satellite observations.

The results of case studies of several regions based on the database suggest that the background to the abandonment and abandonment of small reservoirs is the decline in demand and the existence of modern irrigation facilities that make it possible to use the water of large-scale, highly efficient reservoirs rather than small reservoirs nearby. A significant proportion of these reservoirs, constituting the bulk of the total number of reservoirs, have been either abandoned or are on the verge of being abandoned, particularly those with a capacity of less than 10,000 m3. This is primarily attributable to their suboptimal utilization and management efficiency. Moreover, it has been observed that the recent increase in precipitation in the form of heavy rainfall has accentuated the risks associated with reservoirs. Consequently, stakeholders are being prompted to make decisions that lead to the abandonment or closure of these reservoirs. This presentation will draw from the trends in the fate of reservoirs for agricultural use in Japan, with a focus on abandonment and closure, to discuss sustainable and effective water management and the various environmental and socio-economic factors that affect reservoirs.