Quantitative evaluation of the role of rice paddy agriculture in the hydrological cycle at the basin scale using stable isotope ratios of water

2023 Fiscal Year Final Research Report

• Project/Area Number: 21H02308
• Research Category: Grant-in-Aid for Scientific Research (B)
• Allocation Type: Single-year Grants
• Section: 一般
• Review Section: Basic Section 41030:Rural environmental engineering and planning-related
• Research Institution: Osaka Metropolitan University (2022-2023); Osaka Prefecture University (2021)
• Principal Investigator: Nakagiri Takao 大阪公立大学, 大学院農学研究科, 准教授 (80301430)
• Co-Investigator(Kenkyū-buntansha): 堀野 治彦 大阪公立大学, 大学院農学研究科, 教授 (30212202) ; 濱 武英 京都大学, 農学研究科, 准教授 (30512008) ; 櫻井 伸治 大阪公立大学, 大学院農学研究科, 講師 (30531032) ; 吉岡 有美 島根大学, 学術研究院環境システム科学系, 助教 (40753885)
• Project Period (FY): 2021-04-01 – 2024-03-31
• Keywords: 水の酸素・水素安定同位体 / 水田農業 / 河川流況 / 水質分析 / 水田還元水 / 流域水循環モデル

Outline of Final Research Achievements

Based on field surveys in the Chikusa, Hii, and Kuma Rivers, it was shown that the proportion of paddy field-reduced water in river water at stable flow rates can be quantitatively evaluated with reasonable accuracy by using the stable isotope ratio of oxygen and hydrogen in water as an indicator.
It was shown that this method can be applied not only to the Chikusa River basin, which has no large dams or other water utilization facilities, but also to the Hii River basin, which has a larger area. However, when there is a relatively large dam, it is necessary to take appropriate measures.
By applying the Gonfiantini equation to an existing rice paddy runoff model, we were able to accurately estimate the variation of stable isotope ratios of oxygen and hydrogen in rice paddy surface water, and we were able to show the direction of development for quantitative evaluation using the model in the future.

Free Research Field

農業水文学

Academic Significance and Societal Importance of the Research Achievements

水田農業が河川流況を初めとする流域水循環に及ぼす影響の大きさについては認識されつつも，その定量評価は困難とされてきた中で，水の酸素・水素安定同位体比を指標とすることでその可能性を実証的に示せたことの社会的意義は大きい．また，比較的面積規模の大きな流域や，ダムなどの水利施設が存在する場合の適用可能性について言及できたことで実用性という観点での検討もできた．さらに，既存のモデルに比較的簡単なモデル式を組み込むだけで，地表水の同位体比の動態を比較的精度良く再現できることが示されたことは，今後，モデル開発の新たな展開の方向性を示すことができたといえ，学術的意義も有するといえる．