Environmental Geology (1994) 23:125-133 �9 Springer-Verlag 1994

D. S. Gillieson - J. A. Cochrane - A. Murray

Surface hydrology and soil movement in an arid karst: the Nullarbor Plain, Australia

Received: 18 August 1993 / Accepted: 13 September June 1993

Abstract The Nullarbor Plain is the largest karst area in Australia (220,000 km 2) and one of the largest in the world. Its climate is arid (K6ppen BWk and BWh), and the surface relief is tess than 10 m. The landscape is divided into exten- sive closed karstic depressions separated by low rocky ridges, and the dominant vegetation is chenopod shrub- land. The extent and severity of soil degradation has been assessed using remote sensing. GPS rectified images from 1972-1973, 1979, 1983, 1988, and 1991 have been com- pared for two sites on the Nullarbor. Over the t9 years the total extent of bare soil has reduced significantly, but some areas around water points have degraded and there is some disturbance due to fossorial wombats. Sheet-flow processes occur during intense rainfall events, which hap- pen two or three times per decade. Runoffonly occurs after the 10- or 50-year return frequency events, and at these times turbid water ponds in depressions and enters caves. Surface soil sorptivity and hydraulic conductivity differ markedly between ridges and depressions; the ridges are clearly zones of groundwater recharge, while ponding is evident in most depressions. Sorptivity is influenced by the extent and nature of ground cover and cryptogam crusts on the soil. The landscape has been divided into classes on the basis of vegetation type and percentage of foliage cover. In each class the activities of fallout radionuclides will be determined by high resolution 7 spectroscopy. Preliminary results suggest that in undisturbed sites little sediment movement has occurred over the time scale of cesium-137 (the last 35 years) but that the landscape has been well sorted over a much longer time scale. Future work will investigate disturbed sites to estimate relative soil loss during the pastoral period.

D, S. Gillieson ([7::~1) �9 J. A. Cochrane Departmenl of Geography & Oceanography, University College, University of New South Wales. Canberra. A.C.T. 2601, Australia

A. Murray CSIRO Division of Water Resources. GPO Box t666, Canberra, A.C.T. 2601. Australia

Key words Karst - - Soil movement - - Hydrology Australia

Introduction

Less is known about karst (limestone landscapes) in deserts than anywhere else in the world (Jennings 1983). This is despite their extent in North Africa, the southwestern United States, Central Asia and Australia. Regionally the desert karsts are vitally important for water supply and for nomadic pastoralism. Theory and imperfect knowledge have combined to suggest three limiting factors in the dominant solutionat process--low and highly variable rainfall, high rates of evaporation, and little biogenic car- bon dioxide in the soil. The episodic activity of water in this landscape is hard to observe and harder to measure. Sheetwash processes do occur during rare intense rainfall events and move some sediment underground. Wind ero- sion is an intermittently and seasonally active process whose operation depends on reduction in vegetation cover (grazing or fire). It is thus very difficult to estimate "nat- ural" rates of geomorphic processes and therefore even harder to evaluate perceived accelerated processes leading to land degradation.

The Nullarbor Plain, about 220,000 km z in area, is the largest area of exposed limestone in Australia and is one of the largest desert karst areas in the world. The develop- ment of plans of management for several conservation reserves has been aided by biological surveys and archaeo- logical and karst inventories prepared by consultants for the two state governments involved (Davey 1978; McKenzie and Robinson 1987; Cane and Gara t989). De- spite this wealth of information, the past and present rates of geomorphological processes on the Nullarbor are poorly understood. The broad regional framework of its geomorphology was described by Lowry and Jennings (1974), and brief visits by other researchers have added a little to this (Grodzicki 1985; Spate and others t984). Its climate is arid (K6ppen BWk and BWh) and the surface

126

relief is less than 10 m. The landscape is divided into extensive closed karstic depressions separated by low rocky ridges, and the dominant vegetation is chenopod shrubland (chiefly species of AtripIex and Maireana). On the southern fringe of the plain are stands of myall (Acacia papryocarpa) and mallee (Eucalyptus gracilis and E. diver- sifolia) woodland. Wind erosion is most noticeable around stock watering points where grazing and trampling by sheep and rabbits bares the soil. Sheet-flow processes have been observed by speleologists but are rare.

The overall purpose of this study has been to develop a regional overview of rates of past and present soil move- ment on the Nullarbor landscape. It is clear that wide- spread soil erosion has occurred on the Nullarbor since the introduction of pastoralism in the 1870s, but it has not been quantified in any systematic way (Gillieson 1993). Remotely sensed data were first used to define the extent and severity of soil degradation (in particular near stock watering points) over the last two decades. Secondly, the activity levels of lithogenic and fallout radionuclides in topsoils have been analyzed to determine areas of soil erosion, stable soils, and soil accumulation. The answers will significantly advance the understanding of the rates of erosion and renewal of the natural resources of the Nullar- bor Plain. What erosional processes are active on this arid

karst? How often are these processes active? How does soil movement relate to vegetation cover and land use?

Monitoring land cover change

Two properties were sampled, Mundrabilla Station (387,000 ha) in Western Australia and the former Koo- nalda Station in Nullarbor National Park (590,000 ha), South Australia (Fig. 1). The use of data from Landsat Thematic Mapper (TM) and Multi Spectral Scanner (MSS) has enabled identification of areas of bare soil and regener- ation on georeferenced and rectified images from 1972 and 1973, 1979, 1983, 1988, and 1991. The overall accuracy of this rectification is + 42 m. These scenes have been overlain as difference images of the red spectral band to allow estimation of landscape change over the last 20 years (Figs. 2 and 3). Since red visible light is absorbed by healthy green vegetation, low reflectances (black) indicate good vegetation cover and high reflectances (white) indicate bare ground. Initial study suggests that soil degradation has decreased over the last decade and that most cover change occurred in the droughts of the early 1970s and 1980s (Gillieson and others 1992).

i

1250

Margin ol

Nu l l a rbo r kars[

PLUMRIE}GE S . " j~ ' ] I LAKES N(R.: i/(:':.: :

300 ~ i f

)

/ f " I

Rawlinna

,-Balladonia

< i

Cocklebiddy"

t ~ 100 kiiametres

I

. : . . " : u AMED.:....::I < f " : CONSERVAT ON PARK::: "~ �9 < . .

-'_Z:.t--:.....,.~; .: - . . : . . . ." . : x~ .. . ~ : . . . . . . z : : . . : MARALINGATJURATJA

: , : . . ' '>----.:--:4"~"r~.k~ ' ::: r ^ND e . . . . . . . ~ , O L ~ .. , -. GREAT VICTORIA �9 .~. ~ . :~,.

" ' : D E S E R T N R " : . . i . . . ":",, : : : . . . : . . . . . . . . . . 1 . . . : - . ",, - , I 7 �9 " . . ' �9 " : " - . - " " - - - , MARALINGA

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Abrakur r ie Cave O Bunda C[ilfs Eucia Nullarbor ot Bigh;

Roe Pl&ins

N.R.

GREAT AUSTRALIAN BIGHT L O C A T I O N c[7

Fig. 1. Locat ion of s tudy sites on the Nu l l a rbo r Pla in

127

Fig. 2. Difference images for the period 1973-1991 for Mundrabilla. Area of each scene is 57,600 ha

Mundrabilla Station, Western Australia

On Mundrabilla between 1973 and 1983, the 1974-1975 summer fires principally affected the ridges in the north- west of the property, with revegetation occurring in the dune swales. Revegetation occurred in the far south myall/ mallee woodland complex (Fig. 2) above the escarpment, and in the northeast, directly south of the waterpoint. Between 1983 and 1991 obvious regeneration of vegetation occurred in the north and east of the property, while the home paddocks showed continued reduction in cover. This may indicate good management of the property by spelling paddocks before total loss of cover occurs, especially dur- ing a period when a number of droughts occurred (1972- 1973, 1983, 1988). Extensive bare soil remained around the principal water points in the west and southeast. It is possible to see the development and partial regeneration of bare soil patches around bores that are initiated by overgrazing over a few months and regenerate over five to ten years. It is also possible to note the regeneration of grazed linear depressions in the northeast following heavy rainfall. The overall picture is of stability, except for the continued use of certain waterpoints, perhaps due to their proximity to the homestead and their reliability during drought. The development and regeneration of the linear

features in the north may be exhibiting natural variability in seasonal growth patterns and native herbivore grazing (kangaroos and wombats).

Koonalda Station, South Australia

On Koonalda Station in 1972 wombat disturbance was evident in the northwest part of the image (Fig. 3) with bare soil/reduced land cover clearly evident in the Koonalda Cave paddock. Overall, the image from this date shows heavy use of the stations' paddocks, particularily close in to the homestead. When assessing the difference between 1972 and 1979, the overall revegetation and recovery of the station can be seen. Drought conditions in 1972 and previ- ous high stock numbers may have accounted for the fire scars and bare soil evident in the early image. It appears that high stocking rates could account for the increases in land cover losses around the immediate homestead in the ensuing years to 1979 as rainfall increased, with revegeta- tion of outlying paddocks from good rainfall. Stock were relocated from the more northerly paddocks to those closer to the homestead and south of the old highway. In the years between 1979 and 1983, stock numbers on Koo- nalda remained reasonably high, and drought in 1982 would have led to the development of excessive loss of cover where stock were held. Little change is evident be- tween 1983 and 1988: the general decline in stock numbers,

128

Fig. 3. Difference image for the period 1972-1991 for Koonalda. Area of each scene is 57,600 ha

coupled with lower rainfall, may have accounted for the overall stability of the area. In 1991 there was an increase in the number of individual disturbed patches, presumably due to increased wombat and rabbit activity, since de- stocking of the station in 1988 when it became part of the Nutlarbor National Park. Bare areas around waterpoints regenerated to some degree since 1988 as rainfall for those intervening years was good, and this would have hastened the regrowth of vegetation.

The presence of the hairy-nosed wombat (Lasiorhinus latifrons) can be detected on satellite imagery due to the destruction of vegetation in areas occupied by its bur- rows (Loftier and Margules 1980). These sedentary animals graze in expanding circles away from their warrens causing large areas of bare ground. The size of the grazing circle is determined by the quality and quantity of forage available (St. John and Saunders 1989), and the area covered by a colony may therefore extend from approximately 100 m z to 1 km 2. The dietary requirements of the wombat are critical to recruitment and survival and, according to St. John and Saunders (1989), a minimum of two years of effective rainfall and generally three years are needed be- fore a significant recruitment of the adult population can occur. Drought therefore governs the size and status of the

population. In the past, farmers have been able to obtain approval for control of wombats, although by law they are a protected species on nonaboriginal lands. This, and sev- eral years of good rainfall between 1988 and 1991, may account for the apparent rise in the concentration of wom- bat warrens in the later images if the changeover from lessee to national parks meant total protection of wom- bats, that is, no control of the animals by the pastoralist. Range extension is limited by increasing aridity in the north and sand dunes in the east. Bareness evident in the northeast of the images is thus unlikely to be due to wom- bat disturbance, but rather to increasing dryness of the environment.

Runoff potential

Within each landscape type, soil water infiltration has been measured using a disk permeameter. The principal param- eters used have been sorptivity and steady-state infiltra- tion. Both parameters are significantly higher (P = 0.05) on ridges at Mundrabilla when compared to swales. This does not hold for the lower reliefofKoonalda. At both sites there are significant differences (P = 0.05) between the water infiltration in grazed grasslands and myall or mallee communities on ridges. At Mundrabilla there are also

129

Table 1. Runoff probabilities and areas for I0- and 50-year rain- fall recurrence intervals for Mundrabilla and Koonalda stations, Nullarbor Plain

10 yr 50 yr

Probability Area (ha) % Area (ha) %

Mundrabilla 0-0.09 56698 98.4 22307 38.7

0.10-0.49 0 0 34391 59.7 0.50-0.89 287 0.5 0 0 0.90-1.00 615 I 902 1.6

Total 57600 Koonalda

0-0.09 47030 81,6 47030 81.6 0.10-0.49 8905 15,5 0 0 0.50-0.89 0 0 8905 15.5 0.90-1.00 1665 2.9 1665 2.9

Total 57600

significant differences (P = 0.05) between open chenopod and myall communities. These results imply that most water infiltrates readily into the ridges to recharge ground- water, while clay soil drainage is slow in the closed depres- sions. If one hour rainfall intensities are overlain on the steady-state infiltration data for Mundrabilla, significant runoff will only occur in the heavily grazed grassland and open chenopod communities for the 10 and 50 year return frequencies (Table 1, Fig. 4). At Koonalda, runoff will only occur for the 50-year frequency in grazed grass/chenopod communities (Table 1, Fig. 4). At these times runoff will probably only be generated from less than 3% of the karst landscape, but significant volumes of turbid water flow underground to form "creams," freshwater lenses that sit on top of the brackish groundwater. The probability of runoff has been estimated using data from Figure 4 in the GIS package IDRISI. For each vegetation type, the stan-

Fig. 4. Steady-state water infiltration by vegetation type for Mundrabilla and Koonalda, with 1-h intensities of rainfall for varying recurrence intervals

z: E g 2: s p-

I- _J

z

uJ i,-

I-. o3

,,r uJ p.

150

100

50

MUNDRABILLA

w i t h r a i n f a l l i n t e n s i t y ( ram/h)

2 2 2 2 2 2 2 2 3 4 4 6 6 6 6 6 6 6 6 8 8 8 8 8 9 9 9 9

VEGETATION TYPE

2 Heavily grazed grassland 3 Grazed grass/chenopod 4 Grassed claypan 5 Grassland 6 Open chenopod 8 Open ridge complex 9 Myall woodland

50 year 1 hour {ntensity

10 year 5 year

1 year

E

z 0 p-

rt- I-

z

f- 1-- 03

LU p- 03

KOONALDA

w i t h r a i n f a l l i n t e n s i t y ( m m / h )

152 /.t0 20048?

iii!ili

U i 5 0 i[i i :

))) :" ................ . . . . . . '

III Ht : ,ilHlili o i I: i!] lii:l fi =I{::il I ..... . . . . . ., ,,

2 2 3 3 3 3 3 3 3 4 5 5 6 6 6 8 8 8

1 Bare soil 2 Heavi ly grazed grassland

3 Grazed grass /chenepod 4 Chenopod ridge 5 Chenopod swale 6 Myali woodland

8 Maltee woodland

50 year 1 hour in tensi ty

10 year 5 year

1 year

VEGETATION TYPE

t30

plant regeneration. During individual events, rainfall varies from 25 to 48 ram/d, and water ponds in the lowest parts of the karst depressions for several days.

Soil surface condition and hydrology

Statistical relationships between vegetation properties, soil surface condition, and water infiltration have also been developed (Table 2). These relationships suggest that leaf litter, soil surface cryptogams, associated crusts, and asso- ciated microtopography are of crucial importance in pre- serving hilMope hydrology and in reducing erosion by both water and wind. Soil texture does not seem to be significant per se, nor does slope. These soil crusts are readily disrupted and destroyed by sheep and rabbits, and when loose will be deflated in the strongest gusts. At water points, the soil crusts are absent and the ground surface is a hard, compacted soil with scattered annual herbs and halophytic annual shrubs such as Sarcocornia sp. This surface is relatively impermeable and sheet flow will readily occur, transporting soil fines and dung underground. Re- habilitation of such areas is a slow process that entails ripping and replanting with saltbushes. Recovery time will probably be measured in decades.

Fig. 5a,b. Probability of runoff for Koonalda Station for 1-h inten- sities in the a I in 10- and b 1 in 50-year rainfall events. White = probabiIity greater than 0.9, gray = probability greater than 0.5

dard error of the mean has been used to determine the probability of whether the steady-state infiltration rate is less than the one-hourly rainfall intensity for different re- currence intervals. These estimates are given spatially in Figure 5 for Koonalda Station only.

On MundrabiUa sheetwash has been observed in dolines during deep rain depressions that derive from trop- ical cyclones crossing the northwest coast; these events occur two or three times per decade (Fig. 6) and are impor- tant factors in both sediment redistribution and perennial

Radionuclides and surface process rates

The fundamental unit in this arid karst landscape is a closed depression or donga, which can be up to 2 km long and up to 500 m wide. These dongas are aligned in rows, and the whole landscape is comprised of these rows. Thus the basic sampling unit for the landscape is the donga. The Nutlarbor has been divided into landscape patches, each of which is made up of dongas homogeneous in terms of their vegetation structure, terrain, and surface soil condi- tion. Replicated sampling within patch types should allow us to estimate mean soil loss or gain with error terms from the activity levels of lithogenic and fallout radionuclides (226Ra, 232Th, and 137Cs) in topsoil (0-10 cm). Within each landscape patch type, soil has been sampled along environmental gradients that relate to down- and across- wind directions and are also located in erosion, transport, and deposition zones of erosion cells. At each sampling point, three replicates for radionuclide analysis are taken and are located in the open and under shrubs to sample aeolian accumulation as well.

The activity levels of cesium-137 in the soils at relatively undisturbed sites are high for the rainfall range (120-250 ram) with a mean activity of 495 Bq/m 2. There are signifi~ cant differences between erosional and depositional sites (one-way ANOVA DF = 1,32; F = 4.842, P = 0.0351), with sites in claypans having higher activities, and between sites under saltbushes and sites in the open spaces be- tween bushes (one-way ANOVA DF = t,32; F = 8.977, P = 0.0052). From Fig. 7, there is clearly enhancement of

; I- . . . . . . . . . . . . . . . . "1 - - - -~

i %0~ 1~oo ,,oo ~,oo ~i,oo

1937 - 194910o,

20 o,

30 ~ .

160 ~ 40 ~

.~oo .~~~ -~ 1ooo.1ooO,ooi

o,

.40~ ~ ; 160o~O~ 110 ~ 120 ~ 130 ~ 140 ~ 5 o ]

.............. :71 . . . . . . . . . . . .

11oo 12o ~ 130o 14oo so o , 1 6 0 . . . i

Fig. 6. Paths of cyclones in Western Australia, 1937-1989, indicating that rain depressions reach the Nullarbor Plain two or three times per decade

131

10 ~ , " / , _ . ~ A E ~ - . . x ' a 195o-1959100]

~0o ~ ~ ~oo

;4oo" ~ 4bo~ i 11oo lZOO 13oo .... 14.____oo ,.~1~0o___1oo%, j

00 { i " 3oo

I4~176 120 ~ 130 ~ 140 ~ 50 ~ 160 ~ 1 7 6

CYCLONE TRACKS OVER

WESTERN AUSTRALIA

1937 - 1989

D a t a f r o m B u r e a u o f M e t e o r o l o g y

0 1000 km

132

Table 2. Relationships between water infiltration and soil surface condition for Mundrabilla and Koonalda sites, Nullarbor Plain

Significant variables DEPCRUST is presence ofa depositional crust (4 classes, 1 m 2

quadrat) MICROTOP is soil microtopography (5 classes, 1 m 2 quadrat) TREELITTER is presence of tree leaf litter (4 classes, 1 m 2

quadrat) INSTABILITY is surface soil instability (breakage and water

stability, 5 classes, 1 m 2 quadrat) CRUSTNATURE is crust coherence (7 classes, 1 m 2 quadrat) %COVER is projected foliage cover of shrubs and herbs (50-m

transect) Mundrabilla

Sorptivity (mm/h u2) -- 37.66 - 8.95 DEPCRUST + 4.94 MICROTOP + 15.17 TREELITTER (r 2 = 0.638, DF = 20, P = 0.001)

Steady-state infiltration (mm/hr) = 66.23 - 18.55 DEPCRUST + 5.65 MICROTOP + 31.02 TREELITTER (r 2 = 0.679, DF = 20, P = 0.001)

Koonalda Sorptivity (mm/h w2) = -4.39 + 34.68 STABILITY

- 8.66 CRUSTNATURE + 0.65 %COVER (r'- = 0.476, DF = 15, P = 0.05)

Steady-state infiltration (ram/h) = - 148.72 + 122.42 STABILITY - 14.44 CRUSTNATURE + 1.16 %COVER (r 2 = 0.520, DF = 15, P = 0.05)

Site type-Mundrabi[ta

750- I _ 700-

E .. ..~ .... ~,

iiiiiiiiiiiii ii ~' 300- u 250-

2oo-iiiiiiiiiiiiiiiiiiiiiil " 150-

0 in o?en un-der in ~pen under

sa[tbush sG[tbush CtGypon Ridge

Fig. 7. Mean cesium-137 activities for landscape units and sample locations in undisturbed chenopod shrubland sites, Mundrabilla station, Nullarbor Plain. Error bars indicate one standard error, number of samples = 34

- ] e ~ v

E 20"

r

r 1[?

Under saitbush - c[aypan

o In open - ciaypan

'~'~ Under saitbush - ridge

"~ in o p e n - ridge o

0 10 20 30 L0 50 60 2 3 2 - T h o r i u m (Bq /kg )

Fig. 8. Radium-226 plotted against Thorium-232 for undisturbed chenopod shrubland sites, Mundrabilla station, Nullarbor Plain. Number of samples = 34

the restricted oppor tuni t ies for water t ranspor t in this landscape.

Thus it can be inferred that undis turbed sites in this landscape show little evidence of recent sediment move- men t but that on a longer time scale t ranspor t processes have produced well-sorted sediments. The most likely geo- morphic process is aeol ian transport . The next phase of this work will be to examine disturbed sites and to assess total soil loss for the range of modified vegetat ion types and for the bare soil areas a round water points.

Acknowledgments This research has been supported by an ARC Small Grant and an UNSW Special Research Grant in 1991-1993. Permission to undertake the work was given by the Department of Conservation and Land Management, Western Australia (Permit No. NE000618) and the South Australian National Parks and Wild- life Service (Permit No. A01312). We are grateful to John Watson of CALM and Ross Allen of SA NPWS for their help and support at all stages. Permission to work on Mundrabilla and useful advice was readily given by Bob and Lucy Egglington. Assistance with field and laboratory work was provided by Ray Stanton, Peter Palmer, and Wayne Cook at the Defence Force Academy and Peter Wall- brink at CSIRO: we are grateful to them all.

cesium-137 under the sal tbush by t rapping of aeol ian ma- terial. There is clearly some var ia t ion in cesium-137 activi- ties in the landscape, bu t this may reflect pat terns of fallout ra ther than pat terns of redis t r ibut ion by geomorphic pro- cesses within the cesium-137 time scale of abou t 35 years.

In contrast , there is a very good relat ionship (r = 0.958, P = 0.0001) between the l i thogenic radionucl ides radium- 226 and thor ium-232 (Fig. 8). These radionucl ides are re- leased by rock weathering and tend to be associated with the clay coatings on soil particles. Both c laypan and ridge sites fall on the same regression line and are clearly sepa- rated. This suggests that the soils have been well sorted by t ranspor t processes in the past, p robab ly by wind, given

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133

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Spate AP, Gillieson DS, and Jennings JN (1984) Anomalous red sands in certain Nullarbor Plain caves and dolines. Proceedings, 2nd Australian New Zealand Geomorphology Conference, Bro- ken Hill, N.S.W., July 1984: pp 93-97