Dealing with salinity in Wheatbelt Valleys - Department of Water

unworkable lands, he would conclude that the drain
is working effectively. However, another measure of
success could be the effective lowering of the water
table irrespective of whether or not a crop is able to
survive.
According to Speed & Simons (1992), a drain is
termed ineffective if it does not lower the water
table significantly and at a notable distance from that
drain. Anecdotal evidence from various sites in the
wheatbelt suggest that many drains in the wheatbelt
valleys did not impact on the water table (per se) but
had significant impacts on the period for which the
areas were waterlogged and rate at which surface
runoff was removed (Coles et al. 1999). A drainage
option may be termed as effective if it enables the
land to economically grow crops on a sustainable
basis (irrespective of its impacts on the water table).
Deep drainage is often not seen as a preferred tool
to manage salinity, as it does not address the root
cause, i.e. recharge. It is expensive, not always
effective and has high risk factors associated with onsite
and downstream impacts (Drainage Taskforce
2000). However, as recent studies have suggested,
re-vegetation strategies are likely to have only
localised impacts in the near future (George et
al.1999) with long term benefits for areas affected by
regional groundwater systems to accrue over 100
years or more (Hatton & Nulsen 1999;
Pannell 2001), drainage is more often than not seen
as the ‘quick fix’ to the immediate salinity problem.
Past Research on Drainage
The research-based assessment of deep drainage in
WA was instigated in the late seventies. Deep
drainage, as a method for treating salt land, was
assessed by Bettenay (1978) at farm scale level at the
Yalanbee research station and by George (1985;
1991) in the Moora district. These investigations
concluded that severely degraded land will take many
years to reclaim - if at all - and that marginal or
recently degraded land should be targeted for
drainage. Three issues were identified as being
critical to the success of drainage: (1) an assessment
of the most likely impacts of a drain on the shallow
and deeper groundwater table; (2) the disposal of
drainage water, which may be highly saline; and (3)
cost/benefit including construction, maintenance and
return on investment. The first two are site specific
whilst the third is likely to depend on personal choice
and perceived net benefit. Despite problems such as
relatively flat landscapes in the wheatbelt, deep
drains are increasingly seen as a viable option in this
region (Luke 2000).
– 3 –
Ali and Coles
Cox & McFarlane (1995) evaluated inverse seepage
interceptor and other shallow drains in a number of
sub-catchments at Mt Barker and Narrogin. The
spacing of these drains varied between 70 and
140 m. They concluded that the drains helped in
reducing waterlogging immediately downslope in
18 of the 21 transects in which the problem of
waterlogging was present. There were similar
reductions in waterlogging at both Narrogin and Mt
Barker during wet years. Another drainage study at
Bulyee (Berhane 1999) assessed that the shallow
drains (less than 1.8 m), constructed on the lower
slope and in valley near the break of slope in heavy
clay soils, had no measured effects on salinity and a
limited impact on waterlogging. Some leaching
occurred in the upper perched system
(20-40 cm bgl) but it was difficult to determine if salts
were being removed from the deeper profiles. It
was difficult to attribute the apparent changes in soil
salinity and salt distribution to the construction of
drain. These drains did not have any appreciable
impacts on shallow water table and it was concluded
that water in the drain was derived from surface
runoff and subsurface flow from the perched aquifer
system or via preferential pathways (Berhane 1999).
Coles et al. (1999) reviewed the efficacy of deep
open drains in the wheatbelt. Their appraisal was
based mainly on existing hydrological investigations
and interpretations and anecdotal evidence from
landholders. This review suggested that deep drains
might reduce waterlogging and improve leaching and
plant survival at distances greater than those
predicted by drainage theory. They were of the
view that improvements in the yields (recounted by
farmers at some sites) were due to amelioration of
waterlogging, the management of surface runoff and
changes in the behaviour of water storage in the
unsaturated zones. The location of the drains was
considered an important factor, with drains
constructed at the break of slope appearing to have a
greater impact because of their location at the point
of stronger upward flow. They concluded that a
careful evaluation of the feasibility of drainage is
required for each location and placement in the
landscape was as critical a factor as design in
determining the effectiveness of a particular drain.
Only limited in-depth scientific studies have been
completed on drainage of dryland areas of WA.
Large-scale drainage designs need to be evaluated at
the catchment scale to ensure that the effectiveness
and impacts of these systems are fully understood.
Current studies on drainage address some of these
issues. Guidelines, drafted after proper design
evaluations, will help minimise sedimentation,