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This paper discusses digital low-pass filters for application to tidally fluctuated groundwater observation data. Three types of filters that are commonly used, mainly for oceanography, and newly produced filters are comparatively evaluated with a focus on their ability to eliminate major diurnal and semidiurnal tidal components. All the digital filters presented are the nonrecursive type that can easily be used with spreadsheet software. Newly produced low-pass filters are excellent tide-killer filters with a length of 241 hours applicable to hourly sampled time-series data. The new filters suppress eight major diurnal and semidiurnal tides to practically negligible magnitudes (10 -8 order input), with longer-period components (longer than two days) being nearly completely preserved. High-pass filters transformed from these new tide-killer low-pass filters can separate the components of semidiurnal to diurnal tidal periods from other longer-period components, keeping approximately the same magnitude as in the input data for eight major tides. Therefore, the use of the new high-pass filters prior to quantitative analysis of major tidal components in groundwater observation data should effectively improve the accuracy of analysis.

Paddy rice fields on an alluvial fan not only use groundwater for irrigation but also play an important role as groundwater recharge sources. In this study, we investigated the spatial distribution of isotopic and hydrochemical compositions of groundwater in the Nasunogahara alluvial fan in Japan and applied a self-organizing map (SOM) to characterize the groundwater. The SOM assisted with the hydrochemical and isotopic interpretation of the groundwater in the fan, and clearly classified the groundwater into four groups reflecting the different origins. Two groundwater groups with lower isotopic ratios of water than the mean precipitation values in the fan were influenced by the infiltration of river water flowing from higher areas in the catchments and were differentiated from each other by their Na+ and Cl− concentrations. A groundwater group with higher isotopic ratios was influenced by the infiltration of paddy irrigation water that had experienced evaporative isotopic enrichment. Groundwater in the fourth group, which was distributed in the upstream area of the fan where dairy farms dominated, showed little influence of recharge waters from paddy rice fields. The findings of this study will contribute to proper management of the groundwater resources in the fan.

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