Mechanisms of Spring Discharge at the Headwater of the Shiokawa River: Quantitative Verification by Short-Term Intensive Observation and a Numerical Model

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  • 天然記念物塩川の湧出機構——短期集中観測と数理モデルによる定量的検証——
  • 天然記念物塩川の湧出機構 : 短期集中観測と数理モデルによる定量的検証
  • テンネン キネンブツエンガワ ノ ユウシュツ キコウ : タンキ シュウチュウ カンソク ト スウリ モデル ニ ヨル テイリョウテキ ケンショウ

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Abstract

<p>The Shiokawa River in Okinawa prefecture, a natural monument designated by Japanese government in 1972, is unique from a hydrological viewpoint since the spring at its headwater contains a large amount of salt. To understand quantitatively the mechanisms of the spring discharge of salt water, intensive observations of the discharge rate and water quality were performed, focusing on their short-term variations. Based on the results, a numerical model of spring discharge was constructed; it is the so-called tank model composed of four tanks (i.e., precipitation, groundwater, seawater, and cave tanks) and capable of reflecting density-driven flow. Simulation results of the discharge rate and concentration of major ions dissolved in the spring water were consistent with observed ones. Diurnal variation in the discharge rate was also well reproduced by involving tidal effects. These results confirmed quantitatively the mechanism by which precipitation (on direct infiltration areas), groundwater (within matrix/fractures), and seawater mix within an underground cave(s) to form spring water that flows to the ground surface from the cave(s) due to the driving forces of 1) the density difference between seawater and saline cave water and 2) the difference in elevation between the discharge point and water table of the surrounding area. However, further improvements of the model with long-term observational data are needed, since there are still some uncertainties concerning the size and architecture of the cave(s) and interannual change in spring water quality.</p>

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