Hydrogeological controls on the representativeness and commensurability error of aggregated groundwater level changes

Oshinlaja, Nurudeen A. 2023. Hydrogeological controls on the representativeness and commensurability error of aggregated groundwater level changes. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Groundwater is the largest store of global freshwater, and fundamental to its sustainable management is understanding and evaluating spatiotemporal groundwater storage (GWS) changes. However, this endeavour is subject to two major unresolved uncertainties stemming from (1) the selection of observation wells which accurately represent water table fluctuations, and (2) commensurable aggregation of such ‘point’ data for use in larger scale resource studies or for comparison to larger scale data and modelling outputs. Here, a range of new analytical and numerical groundwater models were derived and developed to address these two research gaps. For a given observation well, characteristic relationships were found to exist between the monitoring depth, the hydraulic diffusivity, and the temporally variable hydrologic inputs enabling the analytical models to constrain the hydrogeological conditions under which water table fluctuations or water loading effects dominate measured groundwater levels (GWL), in one or two-layered systems. This yields a robust and easily applied method for assessing the ‘representativeness’ of a given GWL hydrograph, allowing more confident interpretation of the causes of observed groundwater dynamics. A dimensionless groundwater response index (GRI), here defined as the ratio of groundwater response time to the period of the hydrologic forcing, is shown to be a primary control on the probability of obtaining insignificant commensurability error in aggregated GWS change estimates from point GWL observations. Heterogeneities in recharge and hydraulic properties, and the conductance of any separating layer between connected surface water bodies and the aquifer are of secondary importance. It is also demonstrated that the use of interpolators is superior to the most commonly applied arithmetic averaging method for aggregation of GWL or GWS time series. The results of the thesis improve our ability to interpret observations of groundwater dynamics and associated estimation of groundwater storage changes more robustly at a range of scales.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Earth and Environmental Sciences
Date of First Compliant Deposit:	31 May 2023
Last Modified:	31 May 2023 11:48
URI:	https://orca.cardiff.ac.uk/id/eprint/160070

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