Modelling groundwater flow in a variably-connected aquifer-stream system
- Publication Type:
- Thesis
- Issue Date:
- 2012
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Catchments with variably-connected surface and subsurface flow systems are not
uncommon in Australia or through various parts around the globe. Management of
the available groundwater and surface water resources in such generic types of
catchments is critical to ensure the sustainability of these valuable assets. This
requires a decision making to be based on quantitative estimates of available volumes
in the various interconnected water bodies, usually derived via suitable modelling.
Fully coupled modelling of such systems still faces several complications such as:
proper choice of spatial scale that better represents the interconnected system;
availability and ease of access to the data required; availability of capable software to
perform the simulation; and, occasionally certain jurisdictional conflicts- where these
water bodies cross the trans-boundaries between neighbouring regions. Under these
challenges, pseudo coupling- a less data-intensive but still rigorous modelling of
aquifer-stream system- can provide estimates of acceptable accuracy for management
decisions about the resource.
Therefore, the primary objectives of the present research were to investigate the effect
of variable model cell dimensions on the resulting simulated aquifer-stream water
balance estimates; and to develop a pseudo-coupled groundwater-surface water model
of a representative catchment with an unregulated intermittent stream.
The first objective is to prove the effect of grid resolution on the calculated water
balance components in modelling an interconnected aquifer-stream regime. Such
analysis is of high importance for resolving double accounting and related issues in
management of water allocations. This objective has been accomplished by pseudo
coupling of MODFLOW with RIV and STR algorithms and MODFLOW-SURFACT
with RIV applied to a synthetic aquifer-stream. Six cell resolutions (lOxlO, 25x25,
50x50, lOOxlOO, 150x150, and 250x250 m) were developed for pumping and nonpumping
scenarios. It was found that as the cell dimension increases, the difference
in the exchanged fluxes between the river and the aquifer also increases and could be
more than 100% of the base case (i.e. the 1 Ox 10 m mesh, which has cell dimensions
equal to the river width).
The outcomes of the grid variation experiment were applied to a three-dimensional
flow model with grid resolution of 250 x 255 m for the entire aquifer-stream system
in Zone 2 of Cox's Creek Catchment. The MODFLOW-SURFACT and RIV
algorithm has been utilised in the pseudo-coupled simulation of the groundwater and
surface water regimes over 24 years. The qualitative assessments and the quantitative
calibration measures illustrated that the model could reproduce the observed
groundwater level variations. The hydrographs support the observational inference
that the lower aquifers are probably used for irrigation more than the upper one.
The contribution of Cox's Creek to the total inflow recharge is about 2852 ML/yr,
which is nearly 13.4% of the total feed to the aquifers, and is around three times that
from rainfall. The aquifers recharge the Cox's Creek by approximately 111 ML/yr
(0.5% of the total groundwater outflow). The Creek receives the least amount of its
flow from the underlying aquifers, a finding which supports the work of other
researchers. The simulated monthly average leakage and baseflow of the Creek were
7.76 and 0.26 ML respectively throughout the simulation. These values provide
further evidence that the Creek is generally a losing stream.
While the simulation model has been designed for Zone 2 in the Cox's Creek region,
it has potential for application to other catchments with unregulated intermittent
streams. Such merits should prove helpful to decision makers in managing water
resources in regions of similar character.
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