Analysis of factors of fluctuations of water resource in a freshwater lens by frequency analysis techniques optimized for tides

2019 Fiscal Year Final Research Report

• Project/Area Number: 17K08011
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Rural environmental engineering/Planning
• Research Institution: National Agriculture and Food Research Organization
• Principal Investigator: Shirahata Katsushi 国立研究開発法人農業・食品産業技術総合研究機構, 農村工学研究部門, 上級研究員 (10648281)
• Project Period (FY): 2017-04-01 – 2020-03-31
• Keywords: 淡水レンズ / 時系列データ / 周波数分離 / 潮汐 / 降水

Outline of Final Research Achievements

Two-year-long continuous observations of multi-depth groundwater ECs and groundwater level (GWL) were made at an observation hole in a freshwater lens with auto-logging devices. The fluctuaions of GWL, ECs, and freshwater thickness derived therefrom were analyzed with comparison to precipitation data and others. The fluctuations of the freshwater thickness contained (1) increase shortly after a heavy rain event, (2) constant oscillations with two peaks and two troughs per about 25 hours affected by tidal oscillations, and (3) a few day-long temporary decrease accompanied with a temporary rise in sea level typically due to approach of a typhoon. Long-period fluctuation component extracted from the freshwater-thickness time series data showed positive relationship with one-year cumulative precipitation.

Free Research Field

地下水資源学

Academic Significance and Societal Importance of the Research Achievements

本研究で明らかに示したように、淡水レンズ地下水資源に潮位変動が大きく影響することには、淡水レンズ地下水を井戸等で取水して利用する場合に特に留意する必要がある。具体的には例えば、台風接近時等の潮位が高いときの淡水レンズからの取水では、井戸内に下層の塩水を引き込んで貴重な淡水資源を汚染する可能性が高くなる。また淡水レンズの厚さが長期間の累積降水量と正の相関をもつことは、市町村規模の地域全体の淡水レンズ地下水開発構想の検討のために必要となる、降水の淡水レンズ涵養率の推定に活用できると見込まれる。