THE LAND DEGRADATION SITUATION
OF THE
M U L A LANDS OF SOUTlET WEST
QUEENSLAND
R L MILES
QUEENSLAND DEPARTMENT OF PRIMARY INDUSIlUES
ARID ZONE I N S m U T E
PO BOX 282
Arid Land Administrates Conference
Charlde, 3-5 April 1990
The Mulga Lands of south west Queensland cover an area of approximately 150 000km2.
The area produces 25% of the state's wool and 4% of the state's beef cattle. Concern is now
held for &e sustainability of this production as the area is degrading.
It is the purpose of this paper to provide a brief overview of recent research findings into
the degradation of the muiga lands of south west Queensland.
In the disastrous 1964-66 drought, graziers in western Queensland called for an investigation
into the economic plight of their industry. The producers of the south west were managing
a drought when faced with high accumulated debts, interest charges and low commodity
prices.
This investigation included a full physical survey of the region (Western Arid Land Use
Survey, 1974). The survey covered over 60 million hectares and took over 15 years to
complete. The results showed that while most of the area was still in a stable productive
state, degradation was evident. The area most severely affected was the mulga lands. The
mulga lands comprise of the shallow infertile red earths with the mulga tree (Acacia anuem)
the dominant tree species. Within this area extensive sheet erosion and woody weed invasion
was evident
The re%!.; s f this snrve:. ~ r o m ~ t efurther
d
research into !he mob!== ~d! ! i 19% 2 inint
investigation into the degradation by the Queensland ~edartmentof Lands a n d the
Queensland Deparunent of Primary Industries was conducted (Mills ct a1 1989).The results
of the survey indicated that in the mulga lands west of the Warrego River:
44% of the area had a substantial woody weed problem;
79% was sufferins from over-utilisation of mulea:
64% of the
in the area had 60% of ;heir land
grazed bare. thus predisposing this land to erosion and high rates of runoff; and
45% of the properties were suflerinc from sub\tantial shec! erosion with a further
16% di~~la$ng~minor
symptoms of Grosion.
In summary over two thirds of the area was degrading. Thirty percent of the area was
eroded and twenty percent of the area had greater than 5,000 turkey bushes per henare.
Having identified the extent of the problem research was undertaken to evaluate the effect
of the degradation on production. This research was funded by the National Soil
Conservation Programme. .
In each of thc five landscape units soil samplcs were taken in one cemimetrc increments10
a depth of ten centimetres and the samples analyscd for the ;!mount of caesium 137. The
values obtained were analysed slatinicnlly using a split plut design.
The research was conducted in an 80 000km2area west of the Warrego River. The resear*
quantified the amount of soil loss, the effect of soil loss on soil nutrient status, changs to
the surface hydrology and the relationship between soil and nutrient loss on
productivity on the five major landscape units that the mulga lands comprise. These u,,jg
can conveniently be described as:
The results have shown that areas invaded by woody weeds, such as turkey bush, have in the
last 30 yrs lost 2 cm more soil than grass or mulga tree areas; bare ground 2 to 3 cm and
eroded areas greater than 5 cm of soil (Figure 2). Work on entire catchments has shown
that the material is not deposited in the gullies but entirely removed from the area (Miles
and Campbell 1990) (Figure 3).
.* areas of mulga trees;
' grassed areas;
woody weed invaded areas:
* bare ground; and
* areas showing signs of erosion.
These signs include lips and ledges in the soil, exposed roots, sheet wash and soil deflation
These landscape units formed the basis of comparison for the study.
I
Soil Lms
The measurement of soil erosion in a historical sense was achieved by the use of the radio
active isotope Caesium 137. This isotope is a by-product of atmospheric nuclear weapoe
testingand nuclear accidents since the 1950's. The isotope remains in the atmosphere until
it is washed to the ground in known quantities when it rains. The isotope binds to the clay
fraction in the soil and can only be removed by physically removing the soil. The amount
of caesium found in the soil decreases ~?nnnefltia!lvwith deoth. ?T.Pctlx!!~!eriitic hofldi.~
arnngcment of caesium with tlie soil provides a means of estimating soil loss by measuring
the amount of caesium left in the soil (Campbell 1982)( Figure 1.)
~.~
~
~
~
Figure 2 Caesium levels in the soil prome for five landscape units in a catchment in the
mulga lands of Queensland.
,
.
. .
8
,.'
..
_ ,....
I
Middle
I)ppEr
Lower
o,:;;:inn
-
L l * <I:,2.UW
LS
Figure 1. Schematic presentation of how C,-137 b used to measure soil erosion
C2t>
.
>.8859
Figure 3. Caesium lwels *om W l o p e positions in the catchment
-
XI ~ o
'AL
CONDUCTIVIT'
l lI-.
; r ~ . s =
x2
" s,"r::
Soil Nutrient%
3:
.,*
z*,=
The results from over 2400 soil samples for nutrient determination supported the findin*
of the caesium assays. The results of the nutrient assays were similarly analysed using a sp[il
plot design and were sampled from the same sites as the caesium samples.
Major losses of both mobile and nonmobile nutrients have occurred in the sheet eroded
areas. The nutrient losses recorded are sufficient to have a slgnlficant effect on planl
establishment and productivity. The changes in soil pH with soil loss further cor?pound lh,
problem by limiting the availability of soil nutrients for plant uptake. Alurmmum and
manganese toxicities are also likely (Miles and Baker 1990).
The magnitude of the differences between the landscape units suggest that the vast areas
of turkey bush (Erentopkillagiiesii)may still be capable of supporung peremlal grasses. ?he
future of the eroded areas is not so clear however it is unlikely that they can be
economically reclaimed in the immediate future. The bare areas appear tra~sltlonaland
at risk. If these areas are not revegetated further degradation is probable (Figure 4).
Lx
I..
--
-
n_i
.,m>
-
EXTRACTABLE ACID P
-- .,- *m5.
mom ,z,=,.:2e
2.873 9=
., s8 ,ST
',,
CHLORIDE
ws-
,so
tx 1.w U I.>&,
3z aooz,
9x
z
~
Within the first ten centirnetres of the soil profile 75% of the nutrients were found in the
first 2 cm of soil, 90% in the first 3 cm of soil. The high level of accumulation of,the
nutrients in the surface soils indicates the potential impact on plant production from mnar
losses of soil by erosion. ?he soils were found to he extremely infertile by world standards.
-- -,
-
I"!
TOTAL PHOSPIIOILUS
u
M
.
mo7.
,za*,
<x*mrn
*, . o-,w
:I
REPL4CEABLE K
- --,_,
L C .
0
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111
TOTAL SLII.I'IIUII
-.-,
..-:-hs,%"m:
.>cs,
;
:
0
,.;
--
-:-,?: . .
l l Gill l Z
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TOTAL NITROGEX
elsr
.I:
ax :tat 3z s".
--
<-5
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TOTAL L~+ASSI;IM
--
%!a
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cmm. ,.sn,3
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LEGEND
-
Figure 4. Soil nutrient characterisations of five landscape units in the mdga lands of
Queensland
Figure 4.(mnt) Soil nutrient characterisations of five landscape units in the mnlga lands of
Queensland
82
SURFACE HYDROWGY
The Paroo rivcr has its catchment entirely contained within the mulga lands. A,
examination of the streamnow records since 1926 and rainfall records since 1880 show,
distinct change in the streamnow but no disccrnable change in rainfall pattern. While the
records are not complete enough to show volume discharged, the cumulative stage height!
recorded for each year would indicate that stream discharge has dramatically and continuall)
increased since the 1950's (Figure 5).
Rainfall simulators were used to determine if the major landscape unilr yielded differences
in surface hydroloa. The resullr from a uniform miniall intensiry of 7OmmJhr suggmted
that grass offered the greatest protection from runoff followed by turkey bush and pure
mulga leaf litter (Miles and Glanville 1990) (figure 6).
The relationship between percentage of vegetation cover, runoff and soil loss was
investigated on four one hectare catchments over a three vear ~ e r i o dand a ranee of
milnagiment and cover conditions. Summarily the study found'that ;oil loss OII areas 2 less
than 20% cover are ten times the rate experienced on areas with approximately 4010 cover.
lntiltratiun losses are on average 40% of the annual minfdl for areas of lcss than 20% cover
which equates to a potentiaiproduction loss in dry matter of in excess of 400kg/ha.
Infiltration losses of only 10% are experienced in areas of greater than 40% vegetation
canopy cover. These relationships clearly demonstrate the importance of conservative
stocking and the maintenance of minimum cover levels (Miles and Johnston 1990).
Figure 5(a) Cumulative stage heigbts for the Parw river a t Eulo.
::1,
.
!
,.
o mulga
0
10
z0.2,.
20
30
o.,,,,..
,=, .o.,,,,,
50
60
Surlace cover (%)
40
70
80
90
1
100
Figure 6. The relationship behveen
cover and runoff for the major landscape units
of the mulga lands of Queensland
THE EFFECX OF SOIL LOSS ON PLANT PRODUCITON
I
Based on this data, up to 30% of the area may be experiencing as much as 84% reductiol
in productivity. Preliminary analysis, while still being evaluated, suggest that a combinatiot
of soil loss, rainfall runoff and nutrient losses may be costing the x e a $32 million pe
annum in lost meat and fibre production. These estimates are based on the followin]
assumptions:
In order to evaluate the effect of soil loss and nutrients on plant produdon a,g]ass house
pot trial was undertaken using two native grass species grown in undisturbed s?d cores. The
replicated pot trial evaluated plant growth under soil loss in one c e n q e t r e Increments to
a depth of ten centimetres. All pots were 20 an in diameter and contmfd a 20 cm depth
of soil. AU plants were exposed to the same conditions and had a non lirmnng water supply.
The results of the pot trial indicated that a one centimetre loss of soil resulted i? a 64%
reduction in plant produnivity. Soil losses in excess of three centimetres resulted In more
than a 84% reduction in productivity (Figure 7).
COST OF DEGRADATION
I
the 30% of the area that is eroded is experiencing an 84% reduction in potential
production;
the 20% of the area that has greater than 10% canopy cwer of woody weeds ( > 5 000
turkey bushes per hectare) is experiencing a 64% reduction in potential productivity;
50% of the area is relatively stable and productive; and
* the value of current production is $88.5 million per annum in meat and fibre
* each mm of rainfall lost s runoff represents a 2.04kgIha of loss in biomass production
* a dry sheep equivalent (DSE) consumes 4SOkg of biomass per year
domestic livestock consume 30% of the pasture produced annually.
The figure derivsd is co~rideredconservative and is based rrr! !oss~rdue !n ho!h nlr!n'ent
and runoff
It is clearly trident that degradation is a major problem in the mulga lands of south west
Qucensland. Land administrators have a major role lo play in curbing this degradation. It
is imperative that leases have the corren economies of size in terms of what constitures a
conservative living are;i for 3 single family unit, A responsibiliry also exists for the land
administrators to ensure consenrative and sustainable management is practised on all leases.
Figure 7. The effect of soil loss on dy matter production