18.4 Discharge to surface water
a. Groundwater model
The Department of Land Resource Management use the Finite Element Subsurface Flow system (FEFLOW) to estimate the natural water movement to and from the groundwater store within the Daly region.
FEFLOW estimates groundwater movement for the entire extent of the Oolloo aquifer and the Tindall aquifer, which extends beyond the Daly region boundary as shown in the figure below. The volumes reported in this account refer to the natural water movement that occurs within these aquifers within the Daly region boundary.
Map of the groundwater model area relative to the Daly region boundary
The Oolloo and Tindall aquifers are karstic and were modelled as an equivalent porous media with relatively limited storage. The groundwater model was calibrated using regional aquifer parameters to reproduce the observed groundwater levels and discharge to the rivers, as outlined in Knapton et al. (2010).
b. Supporting information
This line item volume (1,219,000 ML) represents the total volume of groundwater discharge to rivers within the Daly region during the 2011–12 year.
Several of the rivers and streams within the Daly region have persistent streamflow or baseflow during the dry season (May–October) due to the groundwater discharge from regional aquifers. Knapton et al. (2010) estimated that approximately 5–10% of the total annual runoff in the Daly region is baseflow.
The flow between groundwater aquifers and rivers within the Daly region is estimated using FEFLOW in combination with the one dimensional river hydraulic model MIKE 11. The flow interactions between surface water and groundwater are estimated where the surface water channels in the MIKE 11 model are coupled to the aquifer boundaries in the FEFLOW groundwater model.
Detailed information on the model calibrations are provided in Knapton et al. (2010).
Assumptions, limitations, caveats and approximations
There is limited understanding of actual river/aquifer interactions, especially with respect to flows from the river to the groundwater system.
It is likely that, when FEFLOW is coupled with MIKE 11, groundwater recharge from rivers is overestimated during large flood events.
Derived from modelled data. The uncertainty estimate was not quantified.