This study performed flood analysis, considering the water storage of irrigation reservoirs, to realize flood control by reservoirs, which are traditional irrigation facilities. The effect on flood control was determined through flood analysis performed by varying the reservoir storage rates. Based on the flood analysis results, Japan’s flood damage costs were estimated. Using three Shared Socioeconomic Pathways (SSPs) (SSP1-2.6, SSP2-4.5, and SSP5-8.5), we evaluated the flood control potential of reservoirs under future climate conditions. When the reservoirs were empty, the damage cost reduction rate resulting from the use of reservoirs across Japan was small, ranging from 1.1% to 2.3%, but as the damage cost reduction rate did not change under all future scenarios, reductions in flood damage were not affected by changes in rainfall under future climate conditions. Moreover, some prefectures showed high damage cost reduction effects. In northern part of Japan and numerous prefectures in western Japan, the potential to reduce damage through flood control by irrigation reservoirs was high. Some prefectures experienced similar reduction levels as those under alternative adaptation strategies. In the Kanto region, flood control by irrigation reservoirs had low potential for reducing damages. Even among prefectures with low water storage capacities of irrigation reservoirs, the potential to mitigate the effect of damage was high.

To implement strategic countermeasures against flood damage, assessing flood damage and its countermeasures is necessary. However, in Japan, there is a lack of scientific knowledge on how to implement strategic measures considering climate and demographic changes. In this study, we aimed to evaluate the impacts of climate and demographic changes on fluvial and pluvial flood damage and the effectiveness of adaptation measures across Japan. The main novelty of our methodology is the incorporation of Coupled Model Intercomparison Project Phase 6-based climate scenario data and the Japanese version of shared socioeconomic pathways into damage cost calculations based on inundation analysis to consider both climate and demographic changes. Our results showed that although fluvial and pluvial flood damage could increase in almost all prefectures due to climate change, damage could decrease in approximately 70 % or more of prefectures by the end of the 21st century after considering demographic changes. In contrast, flood countermeasures are necessary, as the per capita fluvial and pluvial flood damage costs are estimated to increase. Furthermore, our assessment of adaptation measures suggested that demographic change causes a reduction in the effectiveness of adaptation measures. Therefore, we emphasise the need to consider not only climate change but also demographic change in adaptation planning.

Dietary carbon sources in headwater stream food webs are divided into allochthonous and autochthonous organic matters. We hypothesized that: 1) the dietary allochthonous contribution for fish in headwater stream food webs positively relate with canopy cover; and 2) essential fatty acids originate from autochthonous organic matter regardless of canopy covers, because essential fatty acids, such as 20:5ω3 and 22:6ω3, are normally absent in allochthonous organic matters. We investigated predatory fish Salvelinus leucomaenis stomach contents in four headwater stream systems, which are located in subarctic region in northern Japan. In addition, stable carbon and nitrogen isotope ratios, fatty acid profile, and stable carbon isotope ratios of essential fatty acids were analyzed. Bulk stable carbon analysis showed the major contribution of autochthonous sources to assimilated carbon in S. leucomaenis. Surface baits in the stomach had intermediate stable carbon isotope ratios between autochthonous and allochthonous organic matter, indicating aquatic carbon was partly assimilated by surface baits. Stable carbon isotope ratios of essential fatty acids showed a positive relationship between autochthonous sources and S. leucomaenis across four study sites. This study demonstrated that the main supplier of dietary carbon and essential fatty acids was autochthonous organic matter even in headwater stream ecosystems under high canopy cover.

共有社会経済経路（SSP）別の気候シナリオデータを用いて，気候変動と土地利用変化が洪水被害に与える影響を全国評価した．二次元不定流モデルを用いて洪水氾濫による浸水深を算出した．また，浸水した土地の資産額に浸水深に応じた被害率を乗じることにより洪水被害額を算出した．将来気候における極値降雨の推定に5つの全球気候モデルの出力値を用いた．基準気候（1981年から2000年）から近未来気候（2031年から2050年）にかけて年期待被害額は，SSP1-2.6において2%増加，SSP5-8.5において7%増加すると推定された．基準気候から21世紀末気候（2081年から2100年）にかけて年期待被害額は，SSP1-2.6において33%減少，SSP5-8.5において11%減少すると推定された．