Dealing with salinity in Wheatbelt Valleys - Department of Water

THE FUTURE
In looking at the future of the wheatbelt catchments,
it is not possible to divorce the health of the
ecohydrological system from that of the communities
that derive their living and wellbeing from these
catchments. The Avon Basin, for instance, produces
agriculture worth some $1.5 billion, and is home to
54,000 people. These rural stakeholders are affected
by declining terms of trade for agricultural products,
and the loss of people and services from rural
communities. At the same time, growing
environmental awareness regarding catchment
health, and a growing intention to assert their values,
translate to residents of Perth (1.3 million people)
having a larger influence on natural resource
management, including areas remote from Perth.
In any debate about wheatbelt catchment
management, it is crucial that an objective picture
regarding the future of the rivers be painted and
understood by everyone. The rivers do not look like
they once did, nor will they look like they do now in the
future. The decisions we take must be considered in
the context of the future state of these systems if we
do nothing, not against the background of how they
used to look or how they look now. Our technical
ability to paint this picture is limited but must be
developed.
As for what options are open to significantly improve
the state of the wheatbelt catchments, it is useful to
distinguish among the goals of reducing nutrient
loads, salinity, flooding and inundation, and
waterlogging. Each of these has a set of options that
may be more or less feasible and attractive.
Viney & Sivapalan (2001) concluded on the basis of
simulation modelling that strategic revegetation (of
particular sub-catchments) had the potential to
disproportionately reduce sediment and nutrient
delivery to the Swan estuary, although these results
would clearly not hold in the case of large flood
events arising from the upper catchment. It is
difficult to imagine revegetation at the scale of the
Avon that could intercept nutrients and sediment on
those occasions when summer rainfall inland leads to
the kind of flows and loads observed in January 2000.
It may be possible for new farming systems to reduce
the inputs of agricultural fertilisers in large scale
agriculture, but if the origin of most sediments
delivered to the Swan estuary are remobilised from
the river bed and banks from upstream, then the
source of sediment and particulate phosphorous may
in effect be inexhaustible.
– 13 –
Hatton and Ruprecht
The options for flood mitigation include revegetation
of the variable source areas (i.e. saline discharge
areas), increasing the storages along the chains of
lakes, or widespread revegetation with trees. These
options are not mutually exclusive, but the
impediments to adoption of the latter are widely
recognised. Lacking the development of
commercially attractive tree crops with a product
market that can sustain millions of hectares of
production, it is unlikely to ever happen and will
certainly not happen in the immediate future.
Revegetation of discharge areas (e.g. with salttolerant
vegetation) is more technically and
economically feasible, but the effectiveness of this
approach in mitigating flood risk is untested and is
unlikely to dramatically mitigate risk, particularly in
winter. The potential to create detention basins by
raising the outlets of natural lakes and thus mitigate
risk is higher, but also untested in practice. These
basins may create a temporary impoundment, which
releases water in times of “diluting high flows”, but
not floodflows.
The increasing likelihood of flooding affects not only
agricultural land, but has a major impact on
infrastructure such as roads, railways and townsites.
The potential economic value of protecting these
assets is probably much higher than for agricultural
land and as such there may be greater opportunity to
look at multiple objectives of mitigating the impacts
of floods on infrastructure and agricultural land.
The options for reducing inundation and
waterlogging are similar to flood mitigation.
However, the emphasis is more likely to be on
surface water management, water harvesting, and
engineering and vegetation options to increase
discharge.
Hatton & Salama (1999) concluded that neither
revegetation nor engineering was likely to recover
the wheatbelt rivers from salinity. However, it is
now widely recognised that engineering can be
effective in reducing the impacts and extent of land
salinisation on infrastructure and natural assets, as
well as in keeping land under crops. It is also
generally acknowledged that even if the long-term
strategy is to revegetate, the immediate protection
of land and assets can require engineering. Such
practices are already being employed in places such
as Lake Toolibin, where surface water diversions and
groundwater pumping are beginning to protect the
reserve ahead of intended revegetation in the
surrounding catchment.