Effects of spatial discretization on the simulation of surface water features in groundwater flow models

Author: Saeed Ghaderi

Ghaderi, Saeed, 2017 Effects of spatial discretization on the simulation of surface water features in groundwater flow models, Flinders University, School of the Environment

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Interaction between groundwater and surface water is an integral part of regional groundwater models. Effective representation of surface water features can improve the accuracy of the groundwater model’s ability to produce exchange values and correspondingly inform water resources managers and policy makers to make more effective decisions.

In regional models, it is common to deal with cases where there is a dimensional difference between the river and the underlying cell. Moreover, the stream is assumed to be in the centre of the finite difference cell, which might not be representative of the physical reality. The primary focus of this study is to investigate the impacts of horizontal discretization and river position on groundwater-surface water exchange quantification. This is achieved by comparing an exact analytical solution to a numerical version of an identical setting.

The exchange between the river and aquifer is classically estimated using riverbed conductance, but due to difficulties in measuring the parameters it is treated as an equivalent parameter and achieved through model calibration. This approach makes the riverbed conductance dependant on flow conditions and limits the predictive capabilities of the calibrated model. The secondary objective of this study is to investigate the suitability of the equivalent parameter approach and to observe how it functions in response to variations of other parameters.

From the first analysis, it was found that the errors induced to the model from horizontal discretization is minor and dominated by the errors from vertical coarsening for narrow rivers, but for wider rivers the errors from horizontal discretization were found to be significant. Secondly, it was shown that the maximum error occurs when the stream is attached to the border of the river cell, which is possibly due to the inability of the numerical model to capture the vertical flows in these arrangements. Also, it is shown that the equivalent conductance can vary as a function of several parameters, such as regional flows, river geometry, horizontal discretization and the location of river, the influence of which needs to be taken into consideration when defining riverbed conductance. The findings of this analysis show that the accuracy of the model output could be improved through the implementation of a riverbed conductance definition that adopts the dependence on regional flows and is also dependent on the scale of the cell.

Keywords: Groundwater, surface water, discretization, modelling

Subject: Environmental Science thesis

Thesis type: Masters
Completed: 2017
School: School of the Environment
Supervisor: Okke Batelaan