Gravitational Effects on Groundwater Levels: Recent Research
Since writing my last two blogs (Groundwater Levels and Earth Tides - Part 1 and Part 2), I’ve received links to more recent research relating to semi-diurnal and diurnal groundwater level changes, including those caused by Earth Tides. I felt I should share these to end this series of articles. My thanks to Prof. Richard Carter (www.richard-carter.org) for bringing them to my attention.
All three of the following papers deal with semi-diurnal and diurnal fluctuations in groundwater levels from different perspectives. A common thread running through these is that high-frequency, small-amplitude variations in water levels are now being more commonly seen where pressure loggers are being deployed. Detection of these requires measurements to be taken at intervals of one hour or less.
Details for three technical papers are provided below, with some brief comments added with respect to Earth Tides. Web links are included for those of you who want to dig deeper.
Terrestrial water load and groundwater fluctuation in the Bengal Basin.
W. G. Burgess, M. Shamsudduha, R. G. Taylor, A. Zahid, K. M. Ahmed, A. Mukherjee, D. J. Lapworth & V. F. Bense, 2017. Scientific Reports Volume 7, Article number: 3872 (2017).
This is a fascinating paper which, whilst focussing on the more dominant impact of terrestrial water loading on groundwater level fluctuation, also assesses the impact of earth tides. Two locations were monitored: one in which groundwater levels were clearly influenced by ocean tides, and a second further inland where ocean tidal influences were absent. The second location is useful for comparison to the data in my earlier blogs.
Figure 2 and Table 1 of the paper summarise a mathematical correlation between different components of lunar and solar earth tides to the amplitude and frequency of groundwater level variations collected over a period of one year at hourly intervals.
The data from the inland site demonstrated a strong correlation with diurnal and semi-diurnal gravitational earth tides, with peak to peak pressure fluctuations of 0.4 to 0.6 mbar (equivalent to approx. 0.4 to 0.6 cm change in water level). In comparison the diurnal variations observed in my earlier blogs (see Earth Tides, Part 1) recorded peak-to-peak water level variations an order of magnitude larger than this (7 to 10 cm). It would be interesting to apply the same mathematical analysis to the data set in parts one and two of this blog.
Computer Note: Removal of Barometric Pressure Effects and Earth Tides from Observed Water Levels
Nathanial J. Toll and Todd C. Rasmussen 2007. Groundwater Vol 45, 101–105
This is a practical paper that describes the use of specialist software to remove the impacts of barometric pressure, time lag and earth tides from water level records derived from pressure loggers. The software is available for free download.
The discussion in the paper makes it clear that, “The software clearly removes noise caused by the earth tides; however, the theoretical nature of the earth tide response is not understood as clearly as the barometric response.”
I can’t help thinking that the removal of earth tide responses from raw data is debateable. In this respect it may be worthwhile (perhaps in a future research programme) to compare manual water levels taken at high frequency against the water levels interpolated from pressure loggers.
Diurnal fluctuations in shallow groundwater levels and streamflow rates and their interpretation – A review.
Gribovszki, Z, Szilágyi J, Kalicz, P., 2010. Journal of Hydrology, 385 (2010) 371–383. https://doi.org/10.1016/j.jhydrol.2010.02.001
This is a good “go to” paper for understanding climatic and hydrological reasons that cause diurnal fluctuations in groundwater levels. The magnitude of fluctuations shown in examples range from a few centimetres per day up to between 10 and 15 cm per day. Earth Tides are not considered, so the main value of this paper is in demonstrating alternative causes of diurnal fluctuations.
Links to previous Blogs
- Gravitational Effects on Groundwater Levels (part 1)
- Gravitational Effects on Groundwater Levels (part 2)
- Is Barometric Compensation Always Necessary?
- Monitoring Groundwater Levels in Sierra Leone
- Water Level Measurements – The Pocket Dipper
- Groundwater Level Measurements – being professional
Links to In-Situ level measurement equipment
Follow links here for water level measurement equipment.
©Peter Dumble 2019