[M-IS22] 歴史学×地球惑星科学

[MIS22-06] Flood protection in a changing society: an agent-based model focusing on historical agriculture transformation during 1931 and 1954 floods in Yangtze River Basin, China

*劉 暢¹、川崎 昭如²、城山 智子³ (1.東京大学社会基盤学科、2.東京大学未来ビジョン研究センター、3.東京大学経済学研究科)

キーワード：中国近代史、長江流域、洪水被害、社会変動、社会水文学、水災リスク管理

As the longest river in Asia, the Yangtze River has shown its impact on human societies with floods recorded since 12th century. In 1931, the Yangtze River has manifested its force again with one of the deadliest floods ever recorded in Chinese history, causing 422,499 deaths, damages to more than 25.2 million people and 58.7 billion m² farmland. Similar flood occurred again in 1954, resulting in 31,762 deaths, damages to 18.9 million people and 31.7 billion m² farmland. Researches have shown that 1954 flood being larger and higher compared to 1931 flood. However, it is still unclear for what reason that a more severe flood leading to less damage. Was it because of the change of policy and what can we learn from it? To evaluate the effect of society transformation we developed an index called the potential crop production (PCP). Furthermore, an agent-based model (Farmer Landlord Inundation Production, FLIP) was constructed to simulate the agricultural transformation and its impact on residents’ response. The results have shown that traditional countermeasures were of certain effect. For example, the reinforcement of levees in 1950s was more effective in reducing inundation area of 8% compared to 1931, while the construction of detention basins accounted for 2%. However, with only traditional countermeasures failed to explain the relative success of agricultural product in 1954. Which, according to PCP result, the observed rice production was 12% higher than potential in 1954, while it was 29% lower in 1931. Here we assumed such difference could be ascribed to drastic society transformation in 1930s and 1950s (e.g., increase of absentee landlords in 1930s, land reform movement in 1950s). The effect of which was partially demonstrated with FLIP model, indicating an increase of crop production after eliminating landlords during floods. Our results demonstrate how society transformation are likely to affect the damage of and response to floods in a different (sometimes more important) way from traditional countermeasures in modern Chinese history. We anticipate our research to be a starting point towards deeper understanding of human and hazard, and the knowledge of which is likely to be applicable to many other regions and times.