Gravitational effects on Groundwater Levels
In my last blog, Groundwater Levels and Earth Tides - Part 1, I presented a six-month record of hourly groundwater level measurements from the Indian sub-continent showing daily cyclical variations of approximately 10 cm per day (Figure 1).
Figure 1: Daily Groundwater Level and Barometric Pressure Variations (One day in June 2018)
I ended the blog by stating that there are online references that might explain how these water level fluctuations are produced and why they differ from the more familiar diurnal atmospheric pressure variations observed in barometric logger records for the site (approximately four cm per day at this site - Figure 1).
Earth Tides in Literature
Enter “Groundwater Levels and Earth Tides” into Google and this produces a few notable geophysical references to research going back to the 1960s. Some of these seem to relate solely to confined aquifers (which is not the case here). Most are highly technical but some papers on unconfined aquifer responses offer what appear to be plausible explanations for daily cyclical changes.
Perhaps the most useful reference I found is a paper by Maréchal et al., 2002. This paper is based on observations at a well located at the National Geophysical Research Institute in Hyderabad, India. The location (several hundred kilometres inland from the coast) and geology (granitic basement) are very different, but the aquifer is unconfined. The data analysed was measured every 42 minutes over a period of 12 days in June 2000.
Water levels (Figure 2) demonstrate semi-diurnal variations in the order of 10 cm (similar in amplitude to the daily variations recorded in Figure 1). The paper does not provide any barometric data for comparison, nor does it state whether the loggers are compensated against atmospheric pressure (which is presumed to be the case).
Maréchal et al. explain Earth Tides as follows.
“Earth tides are the result of a visco-elastic deformation of the Earth under the action of gravitational pull of Moon and Sun. Among the 386 existing tide waves, only high amplitude waves have an effect on the aquifers. Melchior (1978) indicates that five large main waves are responsible for almost 95% of water level fluctuations observed in wells. These waves can be divided into two groups: tesseral waves of daily period and sectorial waves of semi-daily period.”
The paper concludes from spectral analysis of the water level data that there is a high correlation with earth tides and states
“… tesseral waves of daily period (O1, K1) and sectorial waves of semi-daily period
(N2, M2) are identified as the origin of the water level fluctuations.”
Daily tesseral O1, and K1 waves have periods of 1.07 and 0.99 days respectively, whilst semi-daily sectorial waves N2 and M2 have periods of 0.53 and 0.52 days respectively.
For more on spherical harmonics and a useful image explaining the difference between “sectoral” and “tesseral” go to: https://mysite.du.edu/~jcalvert/math/harmonic/harmonic.htm
Interpreting Data – Earth Tides or Ocean Tides?
The centimetre-scale changes in groundwater levels induced by earth tides have little significance in most groundwater resource assessments and will only ever be identified when carrying out high-frequency groundwater level measurements with data loggers. (In this case an In-Situ Rugged Troll logger was used to record at hourly intervals.)
Knowing earth tides exist is important, particularly when abstractions are close to coastal areas and where any cyclical variations in groundwater levels could be misinterpreted as being caused by ocean tides. However, the absence of cyclical variations does not rule out
the possibility of seawater saline intrusion, particularly where over-abstraction of groundwater is occurring.
When it comes to assessing the sustainability of an aquifer, the development of a good conceptual model that describes the hydrogeology of the aquifer is vital. Monitoring data is one line of evidence informing such models, and it is important these data are interpreted within a broader understanding of all influences on groundwater levels. Be careful!
- J.C. Maréchal, M.P. Sarma, S. Ahmed and P. Lachassagne, 2002: Establishment of earth tide effect on water level fluctuations in an unconfined hard rock aquifer using spectral analysis. Current Science, Research Communications Vol 83, 61-64. https://pdfs.semanticscholar.org/8959/b540aa24d24674317c76751fbfb23ad2e4ca.pdf
Melchior, P. 1978. The Tides of the Planet Earth. Pergamon, Paris, 609 p
For more on groundwater level measurements see earlier blogs:
- Gravitational Effects on Groundwater Levels (part 1)
- 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 2018