Dealing with salinity in Wheatbelt Valleys - Department of Water

preventative impacts on salinity could be achieved
by clever selection and placement of relatively
small-scale treatments, or by changes to the
management of traditional annual crops and
pastures (in all but the most localised and smallscaled
groundwater flow systems). The new
consensus is that large proportions of land in
wheatbelt catchments would need to be
revegetated with deep-rooted perennial plants
(shrubs, perennial pastures or trees) for at least
part of the time. The perennials would need to be
integrated with engineering works, particularly
shallow drainage for surface water management.
(Deep drains are discussed below under the
heading of “Repairing Salinity”)
Even with major revegetation and surface drainage
Salinity risk in 2100 (% of catchment)
35
30
25
20
15
10
5
0
Limit with alley-farming/annuals Limit of tree belts &
perennials in alleys
Pannell
efforts, the degree of salinity prevention in the
long run in wheatbelt valleys will probably be less
than we would like. Figure 1 shows the results of
hydrological modelling for several catchments in
Western Australia (George et al. 1999). These
results indicate that if recharge across a catchment
were reduced by 50%, implying perennials on
more than 50% of the land, the eventual area of
salinity in the catchment would be reduced by 3 to
12% of the catchment.
(Strictly, the indicator of salinity risk shown in
Figure 1 is not “area” but “flowtube length”. The
results are reasonably indicative of those which
would be obtained from an analysis based on area.
Responsiveness to recharge reductions would be
slightly greater on an area basis.)
All trees or
engineering
0 10 20 30 40 50 60 70 80 90 100
Reduction in recharge (% of current level)
Figure 1: Responsiveness of dryland salinity to reduced recharge (e.g. from perennials or drainage) in a
range of catchment types in Western Australia, assuming that “business as usual” would result in
salinisation of 30% of the catchment. (Source: based on George et al. 1999)
These are modelling results, rather than field
measurements, so perhaps the reality will not be
so severe. Nevertheless, even if the true
responsiveness of salinity to preventative
treatments is twice as great as shown in Figure 1,
the economics of perennials look adverse, unless
the perennials themselves can generate income
directly.
– 3 –
How much income would perennials need to
generate to be worth growing? Figure 2 shows
results of calculations done by Bathgate & Pannell
(2001) to answer this question from the
perspective of farmers. The answer depends on
issues like: