Journal of Arid Environments(1994) 28:173-187
Long-term contemporary erosion rates in an arid rangelands
environment in western New South Wales, Australia
Patricia Fanning
School of Earth Sciences, Macquarie University, New South Wales 2109, Australia
(Received I1 February 1993, accepted 21 June 1993)
Rates of soil loss were determined using erosion pins on a severely eroded surface
in a small (19km 2) arid rangelands catchment in western New South Wales,
Australia, over a 10-year period. Rates of up to 209tha-lyear -1 on rilled
surfaces, 59"5 t ha- I year- ~ on fiat surfaces, and 30"6 t ha- ~year- 1 on vegetated
hummocky surfaces were calculated. The initiation of this erosion is attributed to
overgrazing by sheep and rabbits in the late nineteenth century, and its
amelioration is precluded by hydraulic factors which prevent the use of
reclamation techniques like waterponding.
Keywords: erosion rates; soil loss; arid rangelands; overgrazing
Introduction
Information on soil erosion rates in different environments and under different land
management practices is becoming increasingly important in the light of the current
concern with land degradation worldwide. In Australia in the late nineteenth century,
overgrazing by sheep, and rabbits in plague proportions, supposedly led to dramatic
increases in soil erosion in the arid and semi-arid rangelands of the interior of the continent
(Noble & Tongway, 1986). Various indicators, such as the erosion of creeklines, burial of
fences, infilling of dams and exposure of tree roots, have been used to show that
considerable erosion has occurred in the past 100 years or more (e.g. Pickard, 1990, 1992).
There are still relatively few studies, however, which investigate current erosion rates in
arid rangelands, especially over the long term (i.e. more than a few years). This paper
presents the results of monitoring of surface lowering by sheetwash, rilling and deflation
over a 10-year period in a small (19 k m 2) catchment in the Barrier Ranges, north of Broken
Hill, N S W , Australia.
The Fowlers Gap study site
The study site (Fig. 1) lies on the eastern margin of Australia's arid interior, with a hot dry
climate characterized by a mean annual rainfall of 195 m m , with a variability coefficient of
44% (Bell, 1973). Rainfall over the 10-year study period (1981-91) was above average
(mean annual rainfall = 234 mm), but variability was just as high, with the yearly total
ranging from 9 4 r a m in 1982 to 405 m m in 1987 (University of NSW, 1991). The mean
0140--1963/94/030173+ 15 $08'00/0
~ 1994 AcademicPress Limited
174
P. FANNING
./
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Figure 1. Location and geology of the Fowlers Gap study area. The position of the erosion pin plot
in the Homestead Creek catchment is indicated.
annual pan evaporation exceeds 2m (Cordery et al., 1983; Dunkerley, 1992), and the
estimated potential evapotranspiration at Broken Hill is 1515 mm (Bell, 1972).
The study site is located on the valley floor adjacent to Homestead Creek, an upland
tributary of Fowlers Creek, which flows in a north-easterly direction through the northern
Barrier Ranges towards Lake Bancannia, about l l0km north of Broken Hill, NSW
(Fig. 1). The broad strike valley of Homestead Creek is underlain by low grade
metasediments of Precambrian (Adelaidean) age (Beavis & Beavis, 1984), comprising
mostly Sturts Meadows Siltstone, with Faraway Hills Quartzite and Frieslich Quartzite
forming the steep bounding ridges to the east and west respectively. The main valley floor
EROSION RATES IN ARID RANGELANDS
175
consists of a narrow strip of alluvium up to 270 m wide and 5 km long into which
Homestead Creek is entrenched, and which is currently undergoing severe erosion.
The valley fill consists of a sequence of up to three clearly recognisable layers. The
Lower Unit (as first described by Fanning, 1984) is between 30 cm and 140 cm thick and
contains up to 70% silt and clay. A free subangular blocky structure has developed in the
top of the unit, which gradually becomes more massive with depth. The carbonate content
appears to increase correspondingly, coating ped faces and as occasional nodules towards
the top of the unit, and as a fine powder throughout the base of the unit. The Lower Unit
unconformably overlies weathered siltstone, and often contains imbricated gravels
cemented with carbonate at its base, and therefore is interpreted to be of alluvial origin.
The upper boundary of the unit is clear or abrupt, depending on whether it is overlain by
the Middle Unit in the former instance or the Upper Unit in the latter.
Outcrop of the Middle Unit is discontinuous and limited (Fanning, 1984). It is mainly
found as an isolated remnant on the severely stripped surface of the valley floor. Its field
texture is a loamy sand to a fine sandy loam (after Northcote, 1979), but samples subjected
to chemical and mechanical dispersion and particle size analysis exhibited silt loam
textures, possibly indicating the presence of aggregates in the field samples. The Middle
Unit comprises two parts: a darker, looser upper part and a lighter, massive hardsetting
lower part containing abundant voids. Together, they represent the A1 and bleached A2
horizons of a soil formed in the surface of the valley fill (P.B. Mitchell, pers. comm.).
Testing of samples of the lower part of the Middle Unit with hydrochloric acid failed to
elicit any reaction which would indicate the presence of calcium carbonate, and the nature
of the cementing agent therefore remains uncertain.
The Upper Unit is between 10 cm and 95 cm thick and bulk samples exhibit a loamy
sand texture (Fanning, 1984). However, the unit contains abundant sedimentary
structures in which the individual laminae and strata are all well sorted. Particle sizes range
from rare gravel through granules, abundant coarse and fine sand, to silt, the latter
occurring most frequently as thin laminae between slightly thicker fine sand laminae.
They resemble successive surface crusts. Lenticular stratification, inclined stratification,
low-angled cross-lamination and micro cross-lamination are the most common
sedimentary structures. Fine gravel is found in lenses throughout the unit, but more
commonly towards its base. Cut-and-fill sequences are also common, the larger ones
corresponding with traces of narrow, sinuous palaeochannels on the valley floor surface,
truncated by the present channel system. The presence of gravel and of cut-and-fill
structures in the unit, and the interbedding of sand and silt, suggests that the bulk of the
unit is waterlaid. However, there is also likely to be a considerable aeolian component,
especially as the fine sand laminae resemble those accumulating around the base of
saltbush plants as a result of wind transport under present conditions.
The surface of this alluvial sequence has been severely degraded by sheetwash, rilling,
gullying and aeolian deflation, leaving extensive bare areas, commonly referred to in
Australia as 'scalds '1. The Upper Unit remains on only 8% of the valley floor surface, the
remainder having been stripped, exposing the Lower Unit beneath. Exposure of the
Middle Unit is limited to a pale coloured apron no more than 20 cm wide between the
Upper and Lower Unit surfaces. Dispersal of the sediment on the Lower Unit surface,
through raindrop impact and sheet flow, has formed a smooth surface crust exhibiting a
fine cracking pattern (Fig. 2), reflecting the pedal structure of the material beneath. Lag
deposits of very fine to medium gravel are scattered across the surface. Rilling and gullying
of this surface is very intense close to the main channel, and resembles typical 'badlands'
topography. As illustrated later in this paper, the rill network is rapidly expanding and will
eventually result in the complete removal of the valley fill sediments.
1A scald is definedby Houghton & Charman (1986)as a bare, unproductivearea producedby the removalof
the surfacesoil by wind and/or water erosion. The result is the exposureof the more clayeysubsoil which is, or
becomes, relativelyimpermeableto water. They are a typical erosionform on texture contrast soils in arid and
semi-arid regions of Australia (Abrahams, 1988).
176
P. F A N N I N G
Figure 2. View of part of the erosion pin plot, showing the condition of the pins in 1986, half way
through the study period. The vegetated hummocky surface remnant, composed of Upper Unit
material, at the eastern end of the plot can be seen in the middle distance. Lower Unit materials
comprise the flat surface in the foreground. The fine cracking pattern reflects the pedality of the
material beneath the surface crust.
The vegetation cover in the Homestead Creek catchment bears only slight resemblance
to that existing prior to European settlement in the mid-nineteenth century. The bladder
saltbush/copperburr (Atriplex vessicaria/Bassia sp.) community dominates, with limited
amounts of tussock grasses, such as Mitchell grass, in the less grazed areas remote from
watering points. There are scattered Mulga (Acacia aneura) and Belah (Casuarina cristata)
trees on the steeper slopes, remnants of a much more widespread community which was
largely cleared to provide timber for fence posts. The main creeks are lined with River Red
Gum (Eucalyptus camaldulensis), and Prickly Wattle (Acacia victoriae) is found along the
smaller tributaries. The study site itself is largely devoid of vegetation, with the growth of
saltbush restricted to the h u m m o c k y surface of the Upper Unit.
Methods
In order to determine the rate of surface lowering and volume of soil loss from the
'badlands' area adjacent to the main channel of Homestead Creek, a grid system of erosion
pins was established on the eastern bank of the creek, approximately 1 "5 k m downstream
of the divide (Fig. 1). It comprised a rectangular plot, 35 m × 15 m, in which 198 thin steel
rods, 2 0 0 m m in length, were installed. The rods were driven into the ground leaving
10 m m protruding above the surface. A galvanized washer was placed over the rod head,
which was split to prevent the washer from slipping off.
Figure 3 illustrates the original layout of the plot. Two long east-west oriented transects
each containing 36 pins were installed 15 m apart, extending from the base of the closegullied channel bank, across the valley floor surface, to the base of the adjoining hillslope.
A further nine north-south-oriented transects containing 14 pins each were established
between these at approximately 5 m intervals. Pins were emplaced at 1 m intervals on all
transects. The grid was established in such a way that the rates of erosion and surface
EROSION RATES IN ARID RANGELANDS
E8°S
0 1
2
3
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177
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F i g u r e 3. Layout of the erosion pin plot, showing pin locations in relation to the distribution of
surface types in 1981, at the b e g i n n i n g of the study.
178
P. FANNING
lowering on a variety of surface types could be monitored at the same time. As indicated on
Fig. 3, Transects C, D and E and the first 11 pins of Transects A and B were located within
the close-gullied surface adjacent to the main channel. Transects F, G, I, and J, and most
of Transects A, B and K were located on the flat, smooth, surface of the Lower Unit, while
Transect H and parts of Transects A and K were located on the hummocky, vegetated
surface of the Upper Unit, described above.
Transects A, C, D, E and F were installed in July 1978, and the remainder in January
1980. Measurements of erosion and deposition at each pin were made at 6-monthly
intervals until February 1981, and again in May 1991 (a 10-year interval). The amount of
erosion at each pin was determined by measuring the distance between the base of the split
in each pin head (i.e. the 10mm mark) to the base of the washer using a metal measuring
tape. Because the ground directly under the washer is protected from rainsplash impact,
a pedestal of material under the washers sometimes prevented them from dropping down
the pins. In this case, measurement was made to the base of the pedestal. Measurements
were made to the nearest millimetre. Deposition at each pin could also be determined by
measuring the thickness of any material deposited on top of the washer.
Sources of error are considerable with this type of experimental procedure, and a variety
of measures were employed to try to keep these to a minimum. The recommendations
made by Haigh (1977), in relation to pin type, installation, and measurement, were closely
adhered to in this study. All measurements were made by just one operator (the author)
using the same measuring tape. All readings were made on the upslope side of the pin, as it
was considered to be the least influenced by the presence of the pin itself. Deliberate
disturbance of the plot surface by humans was prevented, but trampling by stock and
native and feral animals was not, because they constitute part of the contemporary erosion
system.
The clay-sized fraction of the Lower Unit materials into which the bulk of the pins were
installed, consist of 2 : 1 lattice clay minerals, which swell and shrink on wetting and
drying. Thus there is a risk that the position of the pins could change with movements of
the substrate. A survey of the plot was therefore made at the time of installation, and
repeated in November 1979 and again in 1991. At each re-survey, the base of the staffwas
gently placed on the top of each pin so that any changes in the vertical position of the pin
could be detected. Apart from gross changes, such as that caused by erosion or stock
trampling, no alterations in pin positions could be detected at the level of accuracy used (to
the nearest 5 mm). Figure 2 shows the condition of some of the pins in 1986, halfway
through the period of the study,
Results
Rates of surface lowering
Average yearly rates of surface lowering and of soil loss have been determined at each pin,
on each surface type and over the whole plot, and are presented in Table 1. An average,
over the whole plot, of 50 mm of surface lowering occurred over the 10-year period, giving
an annual rate of 5 mm year-1. There was, however, a considerable variation in the amount
of surface lowering between the different surface types. The gullied surface exhibited the
greatest amount of surface lowering, averaging 123 mm over the 10 years, or 12"3 mm
year -1. The flat surface averaged 35 mm or 3"5 mm year -1, while the hummocky surface
averaged only 18 mm or 1 "8 mm year -1 (standard errors are given in Table 1).
The figure for the hummocky surface needs some qualification, however. Up to 53 mm
of material accumulated over some individual pins on this surface, through aeolian
deposition, thus reference here to 'mean surface lowering' is not strictly correct, as erosion
occurred at some sites and deposition at others. This is reflected in the variability
coefficient of 305%. In addition, measurements from only 10 pins were included, the
remaining nine having been lost due to erosion or stock trampling.
EROSION RATES IN ARID RANGELANDS
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179
180
P. FANNING
Loss rates
Soil loss rates for the whole plot have been calculated using the above figures (Table 1).
The area of the plot which is gullied is 10m x 15 m or 0"00015 km 2. Thus, 123 m m of
lowering represents a total soil loss of 18"45 m 3 over this area, or 12,300 m 3 km -2 year -1
For the flat surface (area 0"00033 km2), the loss rate is 3521 m 3 k m -2 year -1, and for the
h u m m o c k y surface (area 0"00004km2), it is 1810m 3 km -2 year -1. Assuming a bulk
density of l ' 7 g c m -3 (after Wasson and Galloway, 1986), these figures convert to
209 t ha -1 year -1, 59"5 t ha -1 year -1 and 30"6 t ha -1 year -~ respectively (Table 1).
The three surface types have been mapped over the whole valley floor of Homestead
Creek and their percentage areas calculated. As mentioned previously, the h u m m o c k y
surface remains on only 8% of the valley floor, or 4" 86 ha. There are 39 ha of flat surface and
17"7 ha of gullied surface. If it is assumed that the soil loss rates calculated from this one
plot are representative of loss rates from the whole of the valley floor, then the average
annual soil loss from the valley floor, comprising 61" 56 ha or just 3% of the total catchment
area, is estimated to be 6168"5 tonnes year -~. There are no figures available at the present
time for soil loss rates elsewhere in the catchment, but they are expected to be somewhat
lower than for the valley floor. This is because the vegetation and stone cover is much more
extensive on the hillslopes, providing greater protection from the erosive power of rainfall
and runoff. In a trial of simple runoff traps in the same catchment, Fanning (1984) found
that those installed on the bare surface consistently produced more runoff and more
sediment than traps on vegetated, h u m m o c k y surface and stony surface, as indicated in
Table 2. Moreover, runoff and sediment was collected from the bare surface even during
small events (5 m m or less of rainfall) while the other traps collected nothing. This
indicates that the bare surface adjacent to the main channel, on which the erosion pin plot
is located, is very responsive to rainfall, and is likely to produce runoff and sediment under
a wider range of rainfall conditions than the more stable surfaces further away from the
main channel.
Patterns of erosion
Comparison of Figures 3 and 4 illustrates the changes in surface condition which occurred
on the erosion pin plot between 1981 and 1991. T h e gullied surface extended headwards by
up to 3 m, while the h u m m o c k y surface was reduced in area from 45 m 2 to 21"5 m 2. In
addition, some shallow 'channels' containing deposits of material derived from erosion of
the h u m m o c k y surface began to form at the eastern end of the plot on the flat surface, as
shown by the dashed lines in Figure 4.
However, erosion was not the only geomorphic process occurring. Figure 4 and the
measurement data for the erosion pin plot indicate that deposition of material occurred at
a n u m b e r of pins. Some of this was by sheetwash from surfaces upslope (e.g. at pins B 26 &
I 1), but a considerable amount of aeolian deposition must have occurred, because a
n u m b e r of pins on the h u m m o c k y surface were buried even though they were located
above any source areas for sheetwash (pins A 32 to A 35). Presumably, the wind-blown
sediment accumulated around the base of the ephemeral plants growing on this surface.
Discussion
Comparison with ratesfor similar environments elsewhere
There are very few studies reported in the literature which use erosion pins to determine
erosion rates and soil loss. In a recent review of the measurement of soil erosion, Loughran
EROSION RATES IN ARID RANGELANDS
181
(1989) listed eight studies in which erosion pins were used, alongside other techniques.
However, few are directly comparable with the Fowlers Gap study (Table 3).
O f the eight studies listed by Loughran (1989), Crouch (1987) used pins to measure
gully sidewall retreat rather than surface lowering, and Gupta et al. (1981) used them to
measure wind erosion on a desert sand plain. Others tried the technique on land subject to
cultivation but found that it was unsatisfactory for long-term monitoring (McFarlane,
1984; Freebairn & Wockner, 1986). Mackay et al. (1985) considered that the technique
was limited by its lack of sensitivity to small changes: measurement to the nearest
millimetre is equivalent to a soil loss of 1 kg over 1 m 2, assuming a bulk density of 1 g cm -3
(Loughran, 1989). In a study of soil erosion and sediment yield from cultivated land in
Table 2. Runoff volume and sediment concentration values from runoff traps on
various surface types for a range of rainfaU events in Homestead Creek catchment
(from Fanning, 1984)
Event date
Total
rainfall
(ram)
22/4/80
30"9
26/5/80
5"3
24/7/80
35"2
19/8/80
19"8
11/1/81
3"8
21/1/81
12"5
Plot number
& type
Total runoff
(litres)
Sediment
concentration
(g 1-1)
1 (stony)
2 (bare)
3 (bare)
4 (vegetated)
5 (vegetated)
6 (stony)
1 (stony)
2 (bare)
3 (bare)
4 (vegetated)
5 (vegetated)
6 (stony)
1 (stony)
2 (bare)
3 (bare)
4 (vegetated)
5 (vegetated)
6 (stony)
1 (stony)
2 (bare)
3 (bare)
4 (vegetated)
5 (vegetated)
6 (stony)
1 (stony)
2 (bare)
3 (bare)
4 (vegetated)
5 (vegetated)
6 (stony)
1 (stony)
2 (bare)
3 (bare)
4 (vegetated)
5 (vegetated)
6 (stony)
19" 1
19-6
20"5
16"0
20"6
20"0
0
5"0
4"3
0
0
0
18"7
19"7
19"5
19"0
19"2
18"9
19"4
20"2
20" 1
15"4
16"0
19"6
0
6"5
5"7
0
0
0
7"4
15"4
16"9
14"7
11"2
8"6
1"04
28"02
26"35
1"73
2"23
0"73
0
4"5
3"46
0
0
0
3" 19
10"33
5"70
0" 10
4"33
3"97
0"43
8"76
6"82
0"64
0"99
0"49
0
8"43
8"21
0
0
0
0"56
18"86
36"63
11" 15
9"69
1"49
182
P. FANNING
E8os
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8 9 10 11 12 13 14 15
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Flat surface
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1
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This row removed by channel erosion
0 S8°W
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p i n p l o t i n 1 9 9 1 . T h e extension of the r i l l n e t w o r k , and the degree of removal
of the H u m m o c k y Surface over the 10-year period can be clearly seen.
Figure 4. Theerosion
EROSION RATES IN ARID RANGELANDS
183
Table 3. Comparison of the remits of this study with soil loss rates determined elsewhere
by various methods. The time period involved in each study is indicated
Site of study
Erosion pin studies
Gupta et al. (1981)
Rajasthan, India
Millington (1981)
Sierra Leone
Mackay et al. (1985)
SE Australia
Freebairn & Wockner (1986)
Darling Downs, Qld,
Australia
Fowlers Gap, NSW,
Australia
Fanning (this paper)
Runoffplots
Alchin ( 1983)
Reservoir surveys
Wasson & Galloway (1986)
Surface reconstruction
Neil & Fogarty (1991)
Soil loss rate
325-615 t ha- ~for wind erosion on
sand plains; (75 days)
1"5-55"1 t kin-2 year-~ for
cultivated land;
1"0-1"2 tkm -~ year -1 for
uncultivated land; (< 3 years)
Unsatisfactory due to bulk
density changes
Unsatisfactory due to cultivation
of plots
30"6-209 t ha -~ year -~ for scalded
surface; (10 years)
Hay, NSW, Australia
0-46 t ha-i unscalded;
8"65 t ha -~ scalded; (1 event)
Umberumberka, NSW,
Australia
5-2 t ha -1 year -1 (1915--41);
2"0 t ha -~ year -~ (1941-82)
Upper Yass River, NSW,
Australia
102 t ha- 1year- 1for seepage scald;
(30 years)
Sierra Leone, Millington (1981) was able to directly compare erosion rates obtained from
pin measurements with those from runoff plots by measuring surface lowering along
transects of erosion pins laid parallel to the slope within the erosion plots. He found that
soil losses obtained from pin measurements tended to be much higher in most cases,
although he presented no statistical analyses of the results. However, Mtakwa et al. (1987)
found that there was no significant difference (p < 0"05) between soil erosion determined
by pins and by runoff plots in cropped and bare-faUow sites in Western Nigeria.
Direct comparison of the results obtained from this study with those reported in the
literature is also difficult because of differences in the method used, as well as differences
in environmental conditions. Haigh (1977) and Toy (1983) both comment that while
simple in concept, there has been considerable variation in pin use in practice, ranging
from variations in the type and dimensions of the pins through methods of installation to
techniques of actual measurement. As mentioned above, all of Haigh's recommendations
were followed for this study.
Soil loss estimates from three other Australian studies which are of some relevance to
this study are also listed in Table 3, namely those of Wasson & Galloway (1986), Neil &
Fogarty (1991) and Alchin (1983). Wasson & Galloway (1986) compared erosion rates
before and after European settlement in the catchment of U m b e r u m b e r k a Reservoir near
Broken Hill, approximately 100 k m south of Fowlers Gap. Estimates of the former were
made by calculating the volume of sediment of known age deposited on an alluvial fan.
Repeated surveys of the reservoir were used to estimate the more recent sediment yield.
T h e authors found that the average post-settlement yield was about 50 times greater than
the average yield for the 3000 years preceding settlement, and attributed the difference to
overgrazing by sheep and rabbits.
184
P. F A N N I N G
The 15- to 100-fold difference in the soil loss estimated by this study and that of Wasson
& Galloway (1986) is a consequence of the differences in the methods used to determine
sediment yield. Wasson and Galloway's study is a whole catchment study, whereas this one
focuses on just a small part of the Homestead Creek catchment, and the most severely
degraded part at that. Lower values for sediment yield are usually obtained for reservoir
sedimentation studies because the spatial and temporal variations in erosional and
depositional processes within the catchment are smoothed out.
Erosion rates on severely stripped surfaces in Australia have been reported by Neil &
Fogarty (1991) and by Alchin (1983). As part of their study of land use and sediment yield
on the southern tablelands of NSW, Neil & Fogarty (1991) calculated soil loss on a seepage
scald (a bare area caused by seepage of saline groundwater) by using relict soil pedestals to
estimate the elevation of the original soil surface. The rate they obtained
( 1 0 9 t h a - l y e a r -1) is comparable with the Fowlers Gap study reported herein (302 0 9 t h a -~ year-l). Alchin (1983) used runoff plots to estimate the sediment yield on
uneroded, scalded (i.e. bare) and reclaimed sites near Hay, NSW. However, the rates
quoted (8"6tha -1) are for individual rainfall events and are therefore not directly
comparable.
Implications for land management
In the 120 years since this area of the Australian arid rangelands was first opened up to
livestock grazing, dramatic changes have been wrought to the creeklines along this, and
other, headwater catchments (see Pickard, 1992). The consequences of these impacts are
still being felt. The figures quoted above for this study show that around 6000 tonnes
year -1 of soil is lost from just 3% of the total area of this small catchment. It is estimated
that, at these rates, the whole of the narrow valley fill section of the Homestead Creek
catchment will be removed by the processes of sheetwash, gullying and aeolian deflation in
less than 100 years time.
But what initiated such changes? Can the 'blame' be laid solely at the hard hooves and
sharp teeth of the millions of sheep that occupied this area between 1864 and 1900? Were
Aboriginal occupation or secular climatic change also in part responsible for initiating such
a dramatic change in the erosional balance?
The role of altered climatic regime in initiating this current phase of erosion is difficult
to examine. Temperature and rainfall records in the region are few and incomplete, and do
not extend beyond the time of introduction of exotic grazers when the land was first settled
by Europeans in the 1860s. Analysis of rainfall records kept since that time is limited and
somewhat equivocal, with Wasson & Galloway (1986) claiming a persistence of aboveaverage rainfall since 1940, and Doran & McGilchrist (1983) unable to find statistical
evidence for any such trend. Thus, we are forced to rely on indirect evidence for change in
range condition over the last 120 years.
Evidence about the condition of the range at the time of European settlement is sparse,
and difficult to interpret in geomorphological terms. Charles Sturt, the English explorer
who passed along the Barrier Ranges in the vicinity of the study area in 1844-46, was a
poor record-keeper, and his landscape descriptions are often difficult to interpret (see
Sturt, 1849). However, the sense that one gets from this and other descriptions,
particularly from sketches and paintings by the men who came after Sturt (such as Ludwig
Becker in 1860), is of a landscape of fairly continuous vegetation cover, with an even
scatter of trees, particularly Mulga, Belah and Callitris. Of greater significance, however,
is the description of the grasses and other groundcover plants which no longer exist over
much of the range. Their presence in the vegetation community was possibly a function of
Aboriginal burning ('fire stick farming' - - Jones, 1969). It is believed that the Kooris
regularly fired the range as part of their 'housekeeping', in order to improve its
attractiveness to kangaroos and other native grazers which they hunted. The removal of
EROSION RATES IN ARID RANGELANDS
185
the continuous groundcover coincides with the introduction of exotic grazers, particularly
sheep, whose numbers reached a peak of almost 8 million west of the Darling River in
N S W in 1894 (Mabbutt, 1973).
The removal of the groundcover increased runoff on the slopes, and the erosivity of
flows along the creeklines. Valley floors were widened and incised, and the waterholes
along the main channels, which had provided a reliable source of water for substantial
Aboriginal populations for an indeterminate amount of time, were destroyed. Such
incision lead to surface stripping over the valley floors, such as described above for
Homestead Creek, and the consequent degradation of significant areas of land. Headward
retreat of gullies now threatens roads and watering points in the Homestead Creek
catchment, and results in the destruction of hundreds of hectares of once productive
rangeland.
But can anything be done to control the problem and reclaim the land? A number of
techniques for reclamation of scalded (i.e. severely stripped) land have been given trials
throughout the arid rangelands of Austalia (see, for example, Cunningham, 1987). The
most successful to date, called 'waterponding', involves the creation of low earth banks
behind which water can pond when it rains. The ponded water infiltrates slowly into the
bare soil, breaking down the harsh structure and improving its moisture-holding capacity.
Vegetation naturally recolonizes as the ponds dry out.
This technique has been successfully applied over large areas of flat alluvial land
adjacent to the large river systems of the Darling, Lachlan and Murrumbidgee in western
NSW, and in Western Australia. However, the incised nature of the main channel of
Homestead Creek, and the close-gullying of the land adjacent to the channel (Fig. 4) means
that there is too great a hydraulic slope and too restricted an area of flat land on which to
establish the ponds in this area.
It is more likely that the stripping will be self-limiting, but not until up to halfa million
tonnes of soil is lost. It is promoted by the marked texture contrast between the A and B
horizons of the soils developed in the valley floor alluvium, as previously described. The
more erodible topsoil is easily removed by sheetwash and deflation. However, on the
slopes beyond the valley fill, gradational profiles predominate and, as long as current
conservative stocking rates are maintained (around 1 dry sheep equivalent [dse] to 6" 1 ha,
compared with 1 dse to 3" 1 ha in 1894 - - Mabbutt, 1973), these soils should be more
resistant to erosion. However, the problem of gully network expansion, which from the
data presented here is rapid indeed, still remains.
The severe surface stripping described in this paper is not confined to just one grazing
property, but is widespread throughout the Australian arid zone. The rates of erosion
presented in Table 1 are comparable with the highest quoted in the literature. Thus, it
would appear that excessive stocking rates in the late nineteenth century may have lead to
the destruction of millions of hextares of formerly relatively stable land surface, much of
which cannot now be successfully rehabilitated.
This project was financed by the School of Geography at the University of NSW and a Macquarie
University Research Grant. The permission of the University of NSW to use facilities at Fowlers Gap
Arid Zone Research Station is gratefully acknowledged. Field assistance was provided by Bruce
Smart, John Jansen, Sandra Riddell and Andrea Zambolt. John Cleasby kindly drew the figures.
Thanks to Peter Mitchell for on-site discussions, and review of a draft of this paper, and to an
anonymous referee for helpful comments.
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