The water level of ®ve river stages and seven groundwater wells in the Taipei Basin were analysed by spectral analysis in the frequency domain. The diurnal, semi-diurnal and quarter-diurnal tidal components of the Tanshui River appear to relate closely to astronomical tides as K 1 , M 2 and M 4 , respectively. It is also found that the diurnal component reveals a reversed phase angle in the middle section of the Tanshui River; the phase of the quarter-diurnal component is also found to be reversed at stations upstream in the Tanshui River and Hsintien Stream. It is believed that these phenomena could be caused by local variation in the river channel topography. The autospectrum and cross-spectrum between groundwater elevation and nearby river stage were observed to correlate highly with the frequency of the astronomical tides K 1 , M 2 and M 4 . From the study of the phase shift and time lag of water level¯uctuations at river stages and groundwater wells, it was found that the tidal eects of diurnal, semi-diurnal, and quarter-diurnal components were signi®cantly dierent. The relationships between phase and the¯uctuated range of atmospheric pressure and water level imply that change in atmospheric pressure does not aect water level¯uctuation in the river stage and groundwater well.
Water level data at 16 ground water wells and two sea water gauging stations, coupled with barometric measurements in an alluvial plain in the central‐west region of Taiwan, are analyzed using spectral analysis in the time and frequency domains. The semi‐diurnal component from water level station is observed to be the most noticeable signal while the diurnal component is the less distinct signal recorded at the water level stations. Both semidiurnal and diurnal components are coupled with atmospheric pressure measurements. From the atmospheric pressure data, spectral analysis indicates that both the raw and the pressure adjusted water levels are almost in phase and retain the same amplitude in this area. It implies that the effect of pressure variations is not significant for the sea water and ground water level nearby; the astronomical tidal components, as expected, are the main factor causing fluctuation of ocean water and ground water levels in the Choshuihsi alluvial plain.
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