Digital Filters to Eliminate or Separate Tidal Components in Groundwater Observation Time-Series Data

Abstract

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.

Skew Surge and Storm Tides of Tropical Cyclones in the Delaware and Chesapeake Bays for 1980–2019

“…Closer inspection of the NTR time series did reveal small oscillations at tidal frequencies. Low-pass filters designed to remove these components could be applied to the NTR time series (Shirahata et al, 2016), however, filters can easily decrease amplitude of the signal and care must be taken to not remove water level oscillations (e.g., surge) caused by TCs moving quickly through the region. Additionally, for TCs with durations of multiple tidal cycles, maximum NTR often occurs over low predicted tide, and not indicative of amount of flooding over the next (or previous) high tide.…”

Analyzing Recharge Dynamics and Storage in a Thick, Karstic Vadose Zone

“…The extent of groundwater fluctuations from tides are a result of the hydraulic connectivity and distance to the ocean (Ferris, 1951). Numerous methods exist to remove the periodic tidal signal, whereby a Godin filter is recommended by the National Oceanic and Atmospheric Association (Pagendam & Percival, 2015; Shirahata et al., 2016; Walters & Heston, 1982). We therefore applied a Godin filter to the groundwater level data to smooth the signals using N = 24‐point (for ∆ t = 30 min) window to remove the semi‐diurnal tidal signal.…”

Technical note: Removing dynamic sea-level influences from groundwater-level measurements

“…Groundwater and tidal data were inspected prior to analysis and no issues (e.g., gaps, spikes, steps) were found. Barometricpressure and precipitation data were examined using an automated evaluation and correction procedure by the data provider The low-pass finite-impulse-response filter, "LP241H079122kM3" from Shirahata et al (2016) was applied to groundwater and sea levels for comparison with regression deconvolution results. The filter uses a ten-day symmetric window designed to remove diurnal and semi-diurnal tidal constituents as well as their higher harmonics.…”