Spectral analysis of water level fluctuations in aquifers

Abstract

The water level of a seawater gauging station and 18 groundwater wells coupled with atmospheric pressure in southwestern Taiwan are analyzed by using spectral analysis in time and frequency domain. The semidiurnal component is found to be the most significant signal from the measurement of water level and atmospheric pressure, and the diurnal component is less distinctive in part of water level and atmospheric pressure record. Although auto-spectral and crossspectral density functions are significant in atmospheric pressure and water level, the pressure variations do not significantly affect the seawater and the majority of groundwater level in the study area with amplitude of time series observations. The astronomical tidal components are likely the main factor causing seawater and groundwater level to fluctuate in Pingtung, Taiwan. Time lags are estimated from 20 min to a few hours in aquifers. It concludes that the disturbance on groundwater levels from the effect of oceanic astronomical tide is different from the varying hydrogeological characteristics of aquifer. In this study, the spectral analysis of water level in time and frequency domains gives strong indications of sensitive variations to water level fluctuation.

Application of spectral analysis to determine hydraulic diffusivity of a sandy aquifer (Pingtung County, Taiwan)

“…The studies of spectral density functions of water level for demonstrating important groundwater aspects has been conducted in the northern and the central-west region of Taiwan (Shih et al, 1999Shih and Lin, 2002). The results show that the effect of pressure variations is not significant on seawater and groundwater level.…”

“…The coherence of the KOS well to other wells also reaches the non-zero coherence significant level. Shih and Lin (2002) concluded that the barometric variations do not affect seawater and groundwater levels. Barometric data (KOSp) at KOS is analysed to show the affect of the signal on water level.…”

“…The 95% confidence interval for the autospectral density function and the non-zero coherence level for cross-spectral density function are also evaluated to pick up significant components in the frequency domain. The basic development of spectral analysis and its expansion to the application in tidal hydrological aspects can be demonstrated in relevant literature (Bendat and Piersol, 1991;Shih et al, 1999Shih et al, , 2000Shih, 2002;Shih and Lin, 2002) …”

“…From the viewpoint of the application, Equations (9) and (10) are suitable for groundwater flow that encounters different time-dependent water level boundaries at both ends, and Equation (11) (1991). Shih has undertaken a wide range of applications of spectral analysis to identify fluctuation frequency and phase propagation in tidal water level of aquifers and river (Shih et al, 1999Shih, 2002;Shih and Lin, 2002) Autospectral density function is used to detect the stronger signal in time series, whereas cross-spectral density and coherence are measured to identify intensity of specific components between two time-series. The 95% confidence interval for the autospectral density function and the non-zero coherence level for cross-spectral density function are also evaluated to pick up significant components in the frequency domain.…”

“…Conventional tests and methods for finding aquifer parameters can be found in the literatures (Bear, 1979;Freeze, 1979;Driscoll, 1986;Batu, 1998). For aquifers adjacent to an ocean, river or lake the groundwater table fluctuates with the change of water level in the nearby surface water (Godin, 1972, p. 9;Shih et al, 1999Shih et al, , 2000Shih, 1999bShih, , 2002Shih and Lin, 2002). Such a disturbed groundwater table can be expressed mathematically providing relevant boundary conditions, which can be analysed by the spectral relationships through time-frequency domain of tidal effects.…”