Saltwater intrusion in Southern Eyre Peninsula, December 2009

Recharge estimates were adopted from numerical modelling where it exists (Uley
South) and from the Water Allocation Plan in other areas (Coffin Bay and Lincoln B).
In Uley South, numerous and varying recharge predictions exist. Zulfic et al. (2006)
used a uniform recharge value of 100 mm/yr, while the value of ~140 mm/yr has
been adopted by DWLBC for the purpose of setting water allocations. Recent work
under the Research Fellowship investigating the spatial variability of recharge
indicates that recharge along the coastal fringe of Uley South may be lower than
other areas of the basin, therefore we will adopt the value of 100 mm/yr for the
purpose of this modelling exercise.
Lateral inflows from inland (q i )
Once the other parameters have been measured or estimated, the lateral inflows can
be calculated from average hydraulic head h i observed at the inland boundary of the
model according to the following formula (from Ward et al., In Prep.):
q
i
K
2x
i
h
i
2
1
B
2
Rxi
K
2
(8)
Please note that q i is not necessarily the total inflow to the “basin”, but is the inflow at
the inland boundary of the cross-section defined for the analytical model (see Figure
11).
4.3.2 Regional Water Balance
From (4) and (5) we can see that the ratio of inflows entering the aquifer from inland
(q i ) to inflows entering the coastal zone as recharge (Rx i ) is critical to the resultant
seawater intrusion extent. This means that the breakdown of the water balance in the
coastal zone is significant. Along the whole coast in Uley South, the heavily
vegetated hilltop is understood to lead to lower recharge (Ward et al., 2009). Given
the large area of comparatively high recharge inland in the less vegetated central
portion of the lens, this would suggest q i > Rx i and hence F > 1.
4.3.3 Inland Boundary Condition
When considering the impact of climate change, one has a choice of inland boundary
conditions (specified head versus specified inflow) as investigated by Werner and
Simmons (2009) and Ward et al. (In Prep.). A specified head condition could occur
due to a high inland water table that allows evapotranspiration to maintain the
groundwater at an approximately constant level near the ground surface.
Alternatively, excessive pumping could create drawdown of groundwater to an
approximately constant level. However, in the present study, the most appropriate
inland boundary is thought to be one of specified inflow (q i ), dominated by recharge
occurring inland. For this reason, the inflow q i is specified to reduce by the same
proportion as any reduction in coastal recharge (R) under climate change. In other
words q = R . Climate change scenarios are described in section 5.3.1.
i
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