Dealing with salinity in Wheatbelt Valleys - Department of Water

eliminates drainage through the next crop and fallow
from the shallow duplex soil, provided it can extract
water to a depth of ~1 m. A third strategy is the
encouragement of weedy fallows, which may have a
similar effect to pastures in increasing drying of the
soil profile prior to the break of season. This could
have an effect of 5-10 mm/yr, depending on the
weed cover. However, the implications for disease
carry over to the subsequent crop and deleterious
effects on its nutrition need to be examined before
this practice is considered seriously. Summer
cropping may have a similar effect, but has a
restricted application. The key feature of this
discussion is that changes in crop and pasture
management are unlikely to achieve the kind of
recharge reduction required to significantly reduce
the amount of land at risk of salinisation in the longer
term. It must be emphasised, however, that
recharge reductions can extend the time it will take
for salinisation to progress to its full extent. This has
value.
Extensive afforestation will reduce recharge to preclearing
levels in all probability. The feasibility of
such an endeavour is questionable. We note that
even with the majority of the landscape under dense
plantation, water levels in the lower parts of the
landscape will continue to rise for a long time as
groundwater already in the uplands continues to
discharge down slope.
Drainage/Groundwater Pumping
Any response to coping with high saline water tables
must consider the value and effectiveness of
engineering options. It may be possible to maintain
cropping through the judicious use of drainage and
surface water control. More intensive engineering
interventions (e.g. groundwater pumping) may be
justified where built or natural assets at risk have high
value. The effectiveness of these more intensive
interventions will depend crucially on their design
and location with respect to key subsurface
hydrogeological features. At present, there is
insufficient information about the region to
confidently design a drainage or pumping scheme.
The low aquifer transmissivities mean that
groundwater pumping is unlikely to be an attractive
option except in areas with a more transmissive
palaeochannel system underlying the present land
surface. These systems have been discovered over
most parts of the Wheatbelt and are currently under
investigation to assess their capability for increasing
the discharge capacity of the system by drains and/or
pumps. The difficult problem of disposal of the saline
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Morrell, Hatton and Curry
water remains and this must be included as a key
consideration in any analysis of the attractiveness of
engineering options. It is, however, feasible to save
local remnants on a small scale with pumping, at least
at a technical level.
Managing saline land
It is clear that the catchment will have more salinised
land and water than it does at present, regardless of
treatments aimed at recharge control. As the area
developing high saline water tables spreads (as it will
to some large degree under any reasonable land
management scenario), it must be emphasised that
not all options for productive use will be closed off.
There is ample evidence in the region that saline
agriculture (principally, saltland grazing systems) is a
feasible and attractive option for many salinity
affected soil types (particularly the lighter textured
soils). Recognising the likely extent of high saline
water tables in the future, plans should be put into
place that will facilitate the adoption of new farming
systems for saline lands.
Serious consideration and development of industries
based on salinised resources is recommended. At
present, the National Dryland Salinity Program is
awaiting the outcomes of a scoping study aimed at
assessing the options and feasibility of such industries.
It is recommended that the local community obtain
the results when available and assess the local
feasibility and attractiveness of saline industry options
in the context of the above salinity forecasts.
Status quo
For the status quo option there will be large
increases in the areas of shallow saline water tables.
Most of the catchment will be at risk of shallow
water tables in the next 20–40 years, although only
an unknown (but probably large) fraction will actually
become saline enough to impact crops.
FUTURE MONITORING
It is essential that a continuous monitoring system be
established in selected sub-catchments to monitor
the water level trends with respect to recharge and
the different management options applied in the
catchment. The monitoring system should include all
the modelled catchments to monitor the rise in
water level and compare it with the modelling
results. This will especially include the Magenta
Reserve and the sub-catchment downstream of the
reserve that showed the highest rise in water levels.
In addition, at least one of the wells for the Rural