A partially coupled hydro-mechanical analysis of the Bengal Aquifer System under hydrological loading

Woodman, Nicholas D.; Burgess, William G.; Ahmed, Kazi Matin; Zahid, Anwar

doi:https://doi.org/10.5194/hess-23-2461-2019

Articles | Volume 23, issue 5

https://doi.org/10.5194/hess-23-2461-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/hess-23-2461-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 23, issue 5

Research article

|

22 May 2019

Research article |

| 22 May 2019

A partially coupled hydro-mechanical analysis of the Bengal Aquifer System under hydrological loading

Nicholas D. Woodman, William G. Burgess, Kazi Matin Ahmed, and Anwar Zahid

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Final revised paper (published on 22 May 2019)
Supplement to the final revised paper
Preprint (discussion started on 25 Jul 2018)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of hess-2018-304', Anonymous Referee #1, 24 Aug 2018
- AC1: 'Reply to referee 1', Nick Woodman, 04 Sep 2018
- AC3: 'Summary of responses', Nick Woodman, 18 Oct 2018
RC2: 'A partially-coupled hydro-mechanical analysis of the Bengal Aquifer System under hydrological loading', Garth van der Kamp, 14 Sep 2018
- AC2: 'Reply to referee 2', Nick Woodman, 18 Sep 2018
- AC3: 'Summary of responses', Nick Woodman, 18 Oct 2018
- AC1: 'Reply to referee 1', Nick Woodman, 04 Sep 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by editor and referees) (15 Nov 2018) by Monica Riva

AR by Nick Woodman on behalf of the Authors (11 Dec 2018) Manuscript

ED: Referee Nomination & Report Request started (21 Jan 2019) by Monica Riva

RR by Garth van der Kamp (15 Feb 2019)

Suggestions for revision or reasons for rejection

This paper provides a case study of hydraulic head changes in a deep confined aquifer, including the poroelastic stress effects of changes of total water storage above the aquifer. It demonstrates that the measured changes of hydraulic head in the deeper portions of the BAS largely or completely reflect mechanical loading by the changes of total water storage (TWS) above the formation, rather than reflecting changes of the water table elevation that diffused downwards by transient flow (although the two are usually related). Data on changes of the water table at the field sites are not presented and, regrettably, are presumably are not available. Such information on the water table changes, taken together with the measured hydraulic head changes at greater depths, would greatly help to distinguish TWS effects from water table effects. The conclusions of the paper should include a recommendation to the effect that both water table data and deeper head data should be obtained to support analysis of the groundwater dynamics of large layered aquifers, such as the BAS.
Line 133-137. Equations for the one-dimensional specific storage coefficient and loading efficiency are given in the discussion following equation 1and are formulated in terms of 3D poroelastic equations. This formulation is not incorrect, but it is needlessly complex and is likely to confuse many readers, who may be left wondering whether these two parameters, as defined in this paper, are the same as the familiar parameters which go by that name in established groundwater texts and literature. It is preferable to present the equations for these two parameters in the conventional form of standard groundwater theory, well-known and accepted since Jacob (1940). How these are derived from the general 3-dimensional poro-elastic equations can be left to the appendix and has in any case been shown in previous literature.
Line 165. The hydraulic diffusivity is in fact just қ /Ss and қ (hydraulic conductivity) has already been introduced in equation 1. It is a needless complication to now introduce the factor 𝑘/𝜌𝑔 𝜇 without even mentioning that 𝑘𝜌𝑔 𝜇 = қ. Therefore 𝑘/𝜌𝑔 𝜇 should be replaced by қ.
L 208-213 (Fig. 3). This figure presents the 3D loading efficiency, but the analysis and discussion of the field data are in terms of the 1D loading efficiency. Therefore Fig 3 would be more relevant and useful to the analysis and discussion if it were to present the 1D loading efficiency instead.
L 224. The reference to “field methods” here appears to refer to the high-pressure dilatometer measurements at Padma bridge (Da Silva et al), measurements that presumably were short-term and carried out with stress changes that are far larger than the ambient stress changes due to barometric pressure changes and TWS changes. It is not clear why the dilatometer results, which suggest a much more compressible material, would be more appropriate to use for analysis of the TWS effects. At the very least it would be instructive to carry out the simulations with a range of values for E (and Ss and 1D loading efficiency) that reflects both the barometric and dilatometer field results.
L 413-416 It is questionable to extrapolate the 0.06 ratio for daily (tidal) variations to the annual variation. The response of the piezometer to the longer-term annual changes of the Rupsa River is likely to be much greater and may well be close to the annual change of the river stage, depending on the hydraulic connections between the aquifer and the river. Therefore the assumption that piezometer KhPZ60 reflects a pure TWS signal is questionable. This assumption also implies that the surface moisture load changes by about 2 m annually, which seems an unrealistically large change unless much of the area is flooded by the end of the monsoon season. A much more sophisticated 2D or 3D simulation is likely required, as pointed out in other parts of the paper and as should be clearly stated in the discussion of the Khulna piezometer data.
L 477 The finding that “For LkPZ244 the simulated heads are an excellent match with measurements over the entire period” is partly due to the fact that the measurements for LkPZ244 were used as TWS input for the simulation. The excellent match reflects the choices of Kv and Ss values such that changes of head at the upper boundary do not penetrate to the depth of LkPZ244, as also suggested by the simulation results presented in Fig 4c. But it is not entirely obvious that this perfect match would obtain for all values of Kv and Ss within reasonable ranges for the BAS.

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ED: Publish subject to minor revisions (review by editor) (15 Mar 2019) by Monica Riva

AR by Nick Woodman on behalf of the Authors (22 Mar 2019) Author's response Manuscript

ED: Publish as is (08 Apr 2019) by Monica Riva

AR by Nick Woodman on behalf of the Authors (16 Apr 2019)

Short summary

We show that a conventional hydraulic understanding of groundwater level fluctuation is too simplistic for the extensive floodplains of Bangladesh and West Bengal. This is crucial because 150 million people of the region rely on groundwater for drinking and irrigation. We describe a more complex situation: the coupled hydro-mechanical action of surface water coming and going as the seasons change. Our model results will assist sustainable management of groundwater resources across the region.