116108109
A study of Soil Erosion and Water Balance on Vertisols
of the Eastern Darling Downs.
*
Location:
Greenmount (south of Toowoomba)
Starting Date: 1976 Proposed Completion Date: 2076
Project Leaders: David Freebairn
Graeme Wockner
Background:
On June 5 1827 the explorer Allan Cunningham first glimpsed the Darling Downs which he named in
honour of Governor Darling. Despite Cunningham's glowing description of the new country it was 13 years
(Cunningham died in 1839) before the first wave of settlement, headed by Patrick Leslie swept north to the
rich fertile plains. Leslie was after fresh pastures and a few months after his initial journey arrived with 6000
sheep. The land around Greenmount was well and truly used for grazing by 1860.
Around 1900 as the population increased intensive dairying was commenced along the main highways near
Toowoomba and Warwick to supply milk to those cities and Brisbane. The Greenmount property was a dairy
farm until 1987. It is now used for cropping (wheat, barley, sorghum) and improved pastures for beef cattle
The trial site at Greenmount was probably first cultivated sometime after World War II for fodder crops such
as oats and was not protected by contour banks until the trial was commenced in 1976.
Introduction:
Soil erosion is a severe problem on the black soils of the eastern Darling Downs in south-east Queensland.
The underlying cause is the combination of high intensity storms, the large areas of unprotected fallow and
the highly erodible nature of the soils. Surveys after two major storms on the Darling Downs in 1980 &
1981 pictorially documented the damage done to croplands both in the uplands and floodplains (QDPI,
1980; 1981). In extreme cases where soil was devoid of cover and no contour banks had been constructed,
up to 300 t /ha of soil movement was measured.
Fig 1 Aerial view of five contour bays at Greenmount site
As part of on-going efforts by the Queensland
Department of Natural Resources to minimise soil
erosion, a research program was started in 1976 to
measure the rates of soil erosion on the Darling
Downs and to determine how effective differing
soil management options were in reducing erosion
and improving water storage. Water stress
frequently limits crop yield and the obvious
solution to these problems is to promote infiltration
into the soil profile. A sloping (5-7%) black earth
at Greenmount on the eastern Darling Downs was
chosen for the original work and results for the
years 1976-1984 were presented by Freebairn and
Wockner (1986 a,b).
The soil at the Greenmount site (Irving clay
Ug5.15) was derived from basalt. It has a relatively high water holding capacity with 300 mm of plantavailable water able to be stored in the top 1.5 m of soil. The infiltration rate is very high when the soil is
dry and cracked. However, closure of the cracks through swelling and formation of surface seals due to
instability of the soil under rainfall reduces infiltration rates. Mean annual rainfall at Greenmount is 724 mm
with 66% falling in the period October-March inclusive. Summer rainfall is characterised by high-intensity
summer storms and less intense depression rainfall.
The coefficient of variation of annual rainfall is 20% and the average rainfall intensity is 11 mm per wet day.
Eighty-eight percent of the annual rainfall energy, as estimated by Wischmeier and Smith's (1978) erosion
index (EI30), occurs in the October-March period (Rosenthal and White, 1980)..
Annual soil
movement t/ha
80-380
Innisfail
40-227
1-148
1-53
1-36
Mackay
Nambour
Greenwood
Greenmount
0.03-7
0.2-2
Gunnedah
Wagga Wagga
Fig 2 Increasing soil erosion risk as we travel north into the
high intensity summer storm environment
Cropping systems often include regular periods of
fallow for soil water storage to provide a buffer
against dry periods during crop growth. After a
winter crop, soil is commonly dry and cracked.
During the summer fallow, soil moisture increases
and so does the susceptibility to runoff and soil
movement, especially from February to May.
Average annual evaporation (Class A pan) is 1870 mm
with daily evaporation reaching 12 mm on some
summer days. The mean summer maximum
temperature is 29.5o C (January) and the winter
minimum is 3.3o C (July). Frosts occur from May to
October and their occurrence determines planting dates
of winter cereals .
Experimental Format:
The aim of the project is to determine the effects of soil surface conditions on runoff and soil erosion. Soil
movement, runoff and moisture accumulation were measured within each of five contour bay catchments.
The rate of runoff was measured through flumes or weirs to determine water loss into the grassed waterway.
Contour bays between graded banks were chosen as units of study because, over much of the region, graded
banks have been constructed for erosion and runoff control.
Treatments 1 to 4 were applied after wheat harvest in Nov-Dec:
1. Bare fallow
2. Stubble incorporation
3. Stubble mulch
4. Zero tillage
5. Summer crop
On 8-10-86 a pasture was established at Greenmount to quantify the differences in runoff and soil loss
between cultivated fallow practices and a permanent pasture. This treatment replaced the summer crop.
Treatments were annually rotated to different bays in order to minimise the possibility of catchment
differences being interpreted as management effects. These treatments provided a range of soil surface
conditions during the summer when high intensity storms are common.
Results:
74 mm
Bare
56 mm
Incorporated
Mulch
53 mm
61 mm
No-Till
Summer Crop
50 mm
0
10
20
30
40
50
60
70
80
Since the trial was established at
Greenmount 14 years ago, runoff has
been recorded on 81 days, indicating
that rainfall often exceeds the capacity
of the soil to absorb water. Soil surface
cover, either in the form of crop residue
or growing crop, reduces runoff.
Runoff (mm)
Fig 3 Mean annual runoff for five management
options during the summer fallow period at
Greenmount (mean of 11 years 1978-89)
The annual soil movement for five surface
management treatments shows that
surface cover has a profound effect on soil
Fig 4 Mean annual soil movement for five management options during
the summer fallow period at Greenmount (mean of 11 years 1978-89)
erosion. With little surface cover, soil
loss rates average 30 to 50 t /ha/y.
This is equivalent to an annual loss of
3 to 5mm of topsoil.
Stubble
mulching using primarily sweep or
blade tillage and to a lesser extent
chisel cultivation reduced the average
annual soil movement by greater than
80% compared to bare fallow.
Summer crops provided little surface
cover early in the summer storm
period when the soil profile often
approached field capacity causing
considerable erosion before the crop canopy covered the soil.
Most of the soil moved in an event is retained in
the contour bank channel. A portion of material
deposited in the channel is re-entrained by
channel flow and deposited further along the
channel while the smaller and lighter suspended
material is carried into the waterway.
Evaporation from the soil accounted for 44-88%
of rainfall during summer fallows. We found that
stubble did not reduce evaporation over the fallow
period .
Fig 5 Water balance for a tillage treatment
during the summer fallow period
The average of eight years runoff and soil water
data demonstrated that both zero tillage and stubble
mulch were the most efficient means of storing
fallow rainfall with similar fallow efficiencies in both treatments. Yields were similar for the bare,
incorporated and zero tillage treatments while the stubble mulch treatment yielded 6% more grain.
Future Direction:
If this trial had been carried out for a
typical funding period of four
years, erosion rates of 14
t/ha/y would have been
measured in one period and
78 t/ha/y in another. These
results underscore the need for
long term experimentation when
erosion events are episodic.
Fig 6 Annual soil movement at Greenmount.. Means for the period 1976-91
(measured) and 1915-85 (predicted) using the PERFECT model are also shown
Soil properties of Irving clay at Greenmount experimental site
Depth
(cm)
pH
0_10
10_30
30_60
60_90
90_120
7.97
8.17
8.40
8.60
8.65
Electrical
Conductivity
ms/cm
0.15
0.13
0.18
0.19
0.30
0.31
Cation exch
Particle size distribution (% of total)
capacity
Coarse
Fine
Silt
Clay
(m.e./100g)
sand
sand
68.5
65.5
63.0
60.0
59.5
3.5
5.2
6.5
8.0
7.3
12.8
12.8
12.3
13.3
14.0
20.7
18.5
17.2
19.2
19.3
62.2
62.2
63.7
59.8
60.0
Moisture holding
capacity (% g/g)
min
max
12.5
29.6
29.1
28.1
27.1
Bulk
density
(g/cm3)
49.7
50.7
47.8
44.7
41.1
1.00
1.17
1.26
1.31
1.34
REFERENCES
Freebairn, D.M., and Wockner, G.H., (1986a). A study of soil erosion on vertisols of the eastern Darling Downs, Queensland. 1.
Effect of surface conditions on soil movement within contour bay catchments. Aust. J. Soil Res., 24, 135-158.
Freebairn, D.M., and Wockner, G.H., (1986b). A study of soil erosion on vertisols of the eastern Darling Downs, Queensland.11. The
effect of soil, rainfall and flow conditions on suspended sediment losses. Aust.J.Soil Res., 24, 159-172.
Freebairn, D.M., Hancock, N.H., and Lott, S.C., (1987). Soil evaporation studies using shallow weighing lysimeters : Techniques and
preliminary results. Trans. Mech. Engn., Australia, Vol ME12, 67 - 72.
Littleboy, M., Silburn, D.M., Freebairn, D.M., Woodruff, D.R., and Hammer, G.L., (1989). PERFECT. A computer simulation
model of Productivity Erosion Runoff Functions to Evaluate Conservation Techniques. Queensland Department of Primary
Industries Bulletin QB89005., pp.119.
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