Dealing with salinity in Wheatbelt Valleys - Department of Water

Morrell, Hatton and Curry
GROUNDWATER MODELLING DISCUSSION
Based on the results of the FLOWTUBE and HARSD
modelling, the following preliminary conclusions
were drawn:
• Recharge takes place over most of the
catchment (although higher in some places) and
most of the groundwater discharge ends up in
the lake areas.
• Most of the catchment (50%) will be at risk of
shallow water tables in the next 20–40 years.
The water levels in the unconfined and perched
aquifers can only be controlled by reducing
recharge through better agronomic
management. If the recharge rate can be
reduced to 10 mm yr –1 , the area affected by
salinity will be reduced to less than 20% of the
total catchment area after 40 years.
• The predictive scenario modelling showed that
reducing recharge by 90% will be effective in
reducing the water levels to several metres
below the surface in most parts of the
catchment, especially in the high recharge areas.
However, due to the gradients that have been
created since clearing, groundwater discharge
will continue in the lower areas of the
catchments for a very long time.
• Although surface and groundwater discharge can
be greatly reduced by planting of trees, water
level rises in cleared areas of the catchment will
not be greatly affected and the process of
salinisation in the other parts of the catchment
will continue largely unabated for a long period
of time.
Table 2: Land Monitor Statistics
Non-Affected Land 72.6%
Perennial Veg 12.1%
Salt Affected 1987–91 9.8%
Salt Affected 1994–97 1.6%
Combined Salt affected area 11.4%
Lakes 3.9%
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ASSESSMENT OF SALINITY MANAGEMENT
OPTIONS
Salinity management options generally consist of:
• Recharge reduction;
• Groundwater pumping and drainage to lower
water tables ;
• Management of the saline land; and/or
• Status quo.
Recharge reduction
The discharge capacity for the deep groundwater
systems is estimated to be equivalent to 1–5 mm/yr
recharge (i.e. the recharge under native vegetation).
The deep drainage under most cropping/pasture
agronomic systems generally exceeds that for native
vegetation and hence will inevitably lead to rising
water tables and increased land salinisation. Hence,
as long as agriculture occurs within the catchment,
we would expect salinity to also occur.
Recharge control is unlikely to reverse groundwater
trends in the foreseeable future. It has been shown
that a large reduction in recharge is required to
reduce the subsurface lateral flow to the numerous
lakes. Even if this is achieved, due to the sluggish
gradients and the non-flushing capacity of the system,
the salinity already developed in some of parts of the
catchment will be difficult to reduce. In nearly all
wheatbelt catchments for most years rainfall exceeds
the amount of water that can be used by the winter
cropping systems and annual pastures. As well as
inducing deep drainage and recharge, this can result
in waterlogging which affects crop performance.
There are three management strategies that can
reduce excess water under winter crops. The first is
to ensure the crops have adequate nutrition. The
second is to plant the winter crops into drier soil.
Annual pastures can create such a buffer on the deep
duplex soil but not on the shallow duplex soil.
Lucerne can create a dry soil buffer that essentially