TABLE OF CONTENTS

SOIL PHYSICAL PROPERTIES OF THREE GEOMORPHIC ZONES IN A SEMI-ARID MULGA WOODLAND

-.

R.S.B. Greene

ABSTRACT

I INTRODUCTION

METHODS

The Study Site

Soil Samplina and Analvsis

Determination of Soil ~hvsical Properties

Chemical Analysis

Measurement of Soil Hydraulic Properties

Measurement of Soil-Water Content follow in^ Rain . Preparation of Samples for Micromorpholoeical and SEM Observation

RESULTS

Aeeregate stabilitv (wet-sieving).

Water Characteristics

Soil Hvdraulic Pro~erties

Soil-Water Content Measured Following Rainfall

Micromorpholo~y

Scanning Electron Microsco~y

DISCUSSION

COSCLUSIONS

REFERENCES

1 SOIL PHYSICAL PROPERTIES OF THREE GEOMORPHIC ZONES I N A SEFII-ARID

2 MULGA WOODLAND

3 R.S.B. Greene

CSIRO,

D i v i s i o n of W i l d l i f e and Ecology,

PO Box 8 4 ,

Lyneham ACT 2602

ABSTRACT

S o i l p h y s i c a l p r o p e r t i e s were measured i n t h r e e contiguous

geomorphic zones of a p a t t e r n e d sequence of a l t e r n a t i n g groves and

i n t e r g r o v e s i n a semi-arid mulga (Acacia aneura) woodland: ( 1 ) a

runoff zone of s toney , s e v e r e l y sea l ed , s u r f a c e s o i l , (2) an

i n t e r c e p t i o n zone a t t h e bottom of t h e runoff zone, and a d j o i n i n g ( 3 )

a mnon zone o f mulga groves. . I n f i l t r a t i o n was measured i n t h e f i e l d under u n s a t u r a t e d and

s a t u r a t e d cond i t ions us ing a d i s c permeameter a t w a t e r supply

p o t e n t i a l s of -40 and +10 mm r e s p e c t i v e l y . Under u n s a t u r a t e d f low

cond i t ions , t h e r e were no s i g n i f i c a n t d i f f e r e n c e s i n s o r p t i v i t y ,

i n f i l t r a t i o n r a t e and h y d r a u l i c c o n d u c t i v i t y between t h e t h r e e zones.

However, under s a t u r a t e d flow cond i t ions , t h e s o i l s i n t h e mulga

groves had i n f i l t r a t i o n r a t e s 5-10 t imes h ighe r t h a n t h e s o i l s i n t h e

23 runoff and i n t e r c e p t i o n zones. This d i f f e r e n c e was exp la ined by t h e

2 4 presence of s t a b l e macropores >0.75 rmn diameter i n t h e mulga grove

2 5 s o i l s . Surface s o i l (0-10 mm) aggrega te s from mulga groves were a l s o

2 6 p a r t i c u l a r l y s t a b l e t o r a p i d wet t ing , measured by wet-s ieving.

27 Volumetric water con ten t s (measured o v e r a range of m a t r i c

28 p o t e n t i a l s from 0 t o - 5.0 kPa) of t h e 0-50 mm l a y e r of s o i l from t h e

mulga grove and in tercept ion . .zone were s i g n i f i c a n t l y (p=0.05) higher

than t h e 0-50 mm l a y e r from t h e runoff zone. ~ i c r o m o r ~ h o l o ~ i c a l and

scanning e l e c t r o n microscope (SEM) examination i n d i c a t e t h a t t h e s o i l -.

s u r f a c e from t h e mulga groves and i n t e r c e p t i o n z o n e s had a more porous

s t r u c t u r e than t h e runoff zones. Measurement o f so i l -water con ten t

fo l lowing a major r a i n f a l l even t i n d i c a t e d t h a t wa te r had flowed o f f

the runoff zanes and accumulated i n t h e mulga 'groves.

These f i n d i n g s he lp t o e x p l a i n t h e h i g h e r herbage production

t h a t occurs i n t h e mulga grove and i n t e r c e p t i o n zone compared wi th t h e

runoff zone fo l lowing adequate r a i n f a l l . They a l s o r e v e a l p a r t of the !

d e l i c a t e ba lance of r a i n f a l l r e d i s t r i b u t i o n i n grove/ in tergrove a reas

and t h e p o t e n t i a l f o r management t o a l t e r t h i s ba lance .

INTRODUCTION

Mulga (Acacia aneura) woodlands a r e widely d i s t r i b u t e d i n a r i d

and semi-arid A u s t r a l i a (Neldner 1986) and a r e e s t ima ted t o occupy

1,500,000 km '. (Johnson and Burrows 1981). They a r e important a reas

f o r p a s t o r a l product ion o f wool and beef .

Mulga woodlands occur mainly on i n f e r t i l e r ed e a r t h s o i l s

(S tace e t a1. 1968) wi th f l a t t o s l i g h t l y undu la t ing topography. The

topography f r e q u e n t l y r e s u l t s i n a d i s t i n c t v e g e t a t i o n p a t t e r n , with

the mulga occur r ing a s groves i n s l i g h t l y lower a r e a s t h a t r e c e i v e

runoff water from i n t e r g r o v e a r e a s . I n c e n t r a l and western A u s t r a l i a ,

t h e mulga groves occur a s d i s c r e t e bands on t h e downslope s i d e o f

' r i s e r s ' o r on 'convex s l o p e breaks ' (Mabbutt and Fanning 1987). Near

Louth, New South Wales, a d i f f e r e n t p a t t e r n i n g o f mulga occurs , with

mulga groves on d i s t i n c t ' s t e p s ' or ' f l a t s ' i n t h e landscape (Tongway

and Ludwig 1990).

Rainfall redi.stribution is an important factor in determining

herbage productivity in arid and semi-arid systems. Noy-Meir 11985)

argued on theoretical grounds that redistribution of rainfall, whereby - .

-. - water 'and nutrients are concentrated in a manner, would

significantly enhance herbage production, especially after high

intensity, short duration, rainfall events. In mulga woodland

landscapes, groves of mulga are the runon zones where highest herbage I

production occurs (A.D. Wilson; personal communication).. Tongway et

al. (1989) also found that throughout mulga landscapes there are small

patches, usually associated with mulga logs, that have higher

infiltration rates and nutrient status than surrounding soil. The

nunibers and biomass of plant species on.mulga log mounds were also

greater than on the adjacent soil.

Red earth soils are susceptible to degradation . by overgrazing (Greene and Tongway 1989). Grazing degrades soil physical properties

by removing vegetation cover and physically impacting the soil (Thurow

et al. 1988). Bridge et al. (1983) considered that removal of the

protective vegetation cover on red earth soils by overgrazing, exposes

the soil surface to raindrop splash and results in the formation of

surface seals. Such changes can have a pronounced effect on the

hydrologic characteristics of rangeland soils (Warren et al. 1986).

Even though the redistribution of rainfall in the

grove-intergrove system in a mulga community has been extensively

investigated (Slatyer 1961, Winkworth 1970), little is known about the

soil processes controlling the infiltration and redistribution of

rainfall into the different zones. The aims of this study were to

i n v e s t i g a t e t h e s o i l phys ica l :p roper t i e s of t h e grove- in tergrove zones

and how d i f f e r e n c e s i n s o i l p h y s i c a l p r o p e r t i e s c o n t r o l t h e

i n f i l t r a t i o n and r e d i s t r i b u t i o n of r a i n f a l l . A s i t e o f p r i s t i n e mulga .. .

woodland was s e l e c t e d f o r t h e s tudy.

METHODS

The Studv S i t e

The s tudy was conducted on "Lake Mere" sta:ion (145 9 54' E.,

30 17' S.) , 3 5 h n o r t h o f Louth, N.S.W., and comprised an a r e a

1,250m by 1,600m of p r i s t i n e mulga woodland. The a r e a was subsequent ly

used f o r a sheep and kangaroo g raz ing s tudy. Tongway and Ludwig (1990) !

described a p a t t e r n e d sequence o f t h r e e geomorphic zones, each w i t h a

d i s t i n c t i v e vege ta t ion , on t h e s tudy s i t e . The zones a re : ( i ) a

runoff s lope of E r a g r o s t i s er iopoda savanna ( runoff zone), ( i i ) a

runon zone of Monachather paradoxa savanna a t t h e t o e o f the runoff

s lope ( i n t e r c e p t i o n zone) and, ( i i i ) a runon zone o f Acacia aneura . woodland (mulga g rove) .

The s i t e was s i t u a t e d on the Landsdowne l a n d system ( S o i l

Conservation Se rv ice , N.S.W. 1983), and comprised low undu la t ing stony

r idges o f mainly sedimentary rock, with a d e n d r i t i c dra inage system.

The s o i l was c l a s s i f i e d as a massive r ed e a r t h Gn 2.12 (Northcote e t

d l . 1975) c o n s i s t i n g o f a red-brown loamy s u r f a c e s o i l over a massive

red t e x t u r e B hor izon. Some a n a l y t i c a l d a t a f o r t h e upper 0.75111 of a

t y p i c a l p r o f i l e i n a mulga grove ad jacen t t o t h e s t u d y s i t e a r e given

i n Table 1.

S o i l Sampline and Ana lvs i s •

S o i l samples f o r aggregate s t a b i l i t y de terminat ions and

chemical a n a l y s i s were taken from 1 2 l i n e t r a n s e c t s ( c a . 100 m long)

d i s t r i b u t e d ac ross t h e s tudy s i t e . Each t r a n s e c t was p a r a l l e l t o the

direction of overland water f'low and included the three geomorphic

zones described by Tongway and Ludwig (1990).

Three soil samples of the 0-lcm layer (each sample consisting -

of two bulked cores) within each zone were collected at random along

each of the line transects , giving a total of 36 replications.

Following air-drying, the soil samples were sieved into two size

I fractions of 2-10 mm and <2 mm.

Determination of Soil Phvsical Properties

Aggregate stability of soil samples (2-10 mm size fraction)

was determined by using the wet-sieving method (Kemper and Rosenau

1986). Air-dried soil (20g) was transferred to the uppermost of a set

of four sieves (with openings of 2.0, 1.0, 0.5 and 0.25 mm diameter).

The height of the 2.0mm sieve was adjusted so that immediately the

aggregates were added, they were completely immersed in distilled

water and underwent rapid wetting. The soil was then wet-sieved for 5

min (150 oscillations), with the water level adjusted so that the

aggregates on the 2.0mm sieve were just submerged at the highest point

of the oscillation. The soil remaining on each sieve was dried at

105OC and weighed.

The wet sieving results are expressed as a percentage of

water-stable aggregates >2, 2-1, 1-0.5, 0.5 - 0.25 and <0.25 rmn

diameter. The wet-sieving results were also expressed as a

mean-weight-diameter (MWD) value (Kemper and Rosenau 1986).

Water characteristics were measured on undisturbed cores (70

mm dim. and 50 mm depth) of surface soil. Three cores were taken at

random from each of the three zones in two line transects, giving a

total of six replications per zone. The cores were wet up on ceramic

plates and then drained to the following values of matric potential:

-0.3, -1.0, -3.0 and -5.0 kPa., The vo lumet r i c mois tu re con ten t of the I

s o i l a t each m a t r i c p o t e n t i a l was determined from t h e mass of t h e co re

a t t h a t ma t r i c p o t e n t i a l and t h e oven-dried (105OC) mass of t h e core. -

T o t a l p o r o s i t y c a l c u l a t e d from bulk d e n s i t y (Loveday 1974) w a s used

f o r t h e volumetr ic mois tu re c o n t e n t a t s a t u r a t i o n (0 kPa). The amount

of volumetr ic wa te r con ten t dra ined from t h e c o r e s i n t h e m a t r i c

I p o t e n t i a l range o f 0 t o -0.3 kPa and -0.3 t o -3.0 H a was $ l s o

c a l c u l a t e d .

Bulk d e n s i t y va lues were a l s o determined a t va r ious he igh t s i n

t h e s o i l p r o f i l e by t ak ing undis turbed c o r e s (70 m diam. and 50 m

depth) and drying them a t 105 C.

S o i l s u r f a c e r e s i s t a n c e measurements were taken wi th in each o f

t h e 12 l i n e t r a n s e c t s by us ing a P r o c t o r penet rometer ; su r face

c racks , clods o r p l a n t r e s i d u e s were avoided. Three readings were

taken a t random w i t h i n each zone, g i v i n g a t o t a l o f 36 r e p l i c a t i o n s . .

The maximum f o r c e r equ i red t o push a 65 m2 c i r c u l a r f l a t top

v e r t i c a l l y i n t o a depth o f 12.5 nun was recorded. The corresponding

r e s i s t a n c e (MPa) was then c a l c u l a t e d . The m o i s t u r e content o f t h e 0-10

nun s o i l l aye r , immediately a d j a c e n t t o where t h e penetrometer

measurement was made, was a l s o determined c o n c u r r e n t l y with t h e s o i l

s u r f a c e r e s i s t a n c e measurements.

Chemical Analvsis

Chemical analyses were c a r r i e d o u t by t h e fo l lowing methods:

( i ) pH and e l e c t r i c a l c o n d u c t i v i t y (EC) - 1:s so i l -wa te r suspension

shaken f o r l h .

( i i ) Organic carbon - Walkley-Black wet combustion (Colwell 1969).

( i i i ) T o t a l n i t rogen - Kje ldah l d i g e s t i o n (Twine and Williams 1967).

Measurement of Soil Hvdraulic '.Properties

Infiltration measurements were made in the field using disc

permeameters (200 mm diameter) that supplied water at a potential of - . -- - . . - -

either +I0 o; -40mm (~erroux and White 1988). Within each of the 12

line transects, three measurements of infiltration at each supply

potentials were made at random within each zone. The water used for

' the measurements had an electrical conductivity of 0.07 dS m-'. In the

mulga groves, the Ao layer of litter (mainly leaves from Acacia

aneura) was carefully removed before carrying out the measurements. At

a supply potential of -40mm, a layer of diatomacous earth,

approximately 5mm thick, was used to provide a contact with the soil.

Initial and final water contents were also detem&ed. Immediately

adjacent to the area where the infiltration measurements were carried

out, the bulk density of the soil was measured with . . undisturbed cores

of 70 mm diam. and 20 mm depth. At both water supply potentials,

sorptivity, infiltration rate and hydraulic conductivity were

calculated according to the method of White (1988).

Measurement of Soil-Water Content Followinz Rain

Over the period 1-2 March, 1987, 37.5 mm of rain fell at the

site. Following the rainfall event, the gravimetric soil-water content

profile was measured in each of the three geomorphic zones in three

transects. These values were converted to volumetric soil-water

content using values of bulk density measured down the profile (Table

1). Soil-water contents were measured to approximately 300 mi in the

runoff slopes and interception zones (the depth of wetting) and

approximately 400 mm in the mulga groves. The soil-water content

obtained in the lower limit of drying under rangeland conditions

(Johns 1984) was then s u b t r a c t e d from each p r o f i l e . These va lues were

16.4 mm and 25.4 mm f o r t h e 0-300 mm and 0-400 mm d e p t h s r e spec t ive ly .

This gave t h e amount of water i n each zone r e s u l t i n g from the . -

r a i n f a l l .

Preuara t ion of Samples f o r Mic romor~ho loe ica l and SEM Observat ion

Dup l i ca te v e r t i c a l samples 160 x 90 x 50 mm depth were I

c o l l e c t e d i n t i n s from t h e t h r e e geomorphic zones f o r th inLsec t ion ing .

Samples were a i r - d r i e d (40 C ) , impregnated w i t h p o l y e s t e r con ta in ing

a f l u o r e s c e n t dye ( V r i t e x OB) and th in-sec t ioned by s tandard

procedures (Brewer 1976).

Undisturbed p i e c e s ( ca . 30 mm diameter , 10 mm depth) o f the

s o i l su r face from each o f t h e t h r e e geomorphic zones were c a r e f u l l y

removed with a s p a t u l a and t r anspor t ed t o t h e l a b o r a t o r y . A f t e r

a i r -dry ing , t h e samples were broken i n t o s m a l l e r p i e c e s and glued onto

an SEM s tub , w i t h t h e s o i l s u r f a c e f ac ing upwards. The . samples were

vacuum coated w i t h go ld and examined wi th a Cambridge SlSO SPI a t a

range o f magnif ica t ions .

RESULTS

A ~ a r e z a t e S t a b i l i t v (wet-sievinpj .

Fig.1 shows t h e t h a t t h e r e i s a marked d i f f e r e n c e i n t h e

d i s t r i b u t i o n o f wa te r - s t ab le aggregates between t h e t h r e e zones. Af te r

wet s i ev ing , t h e s o i l aggregates mainly occur as p a r t i c l e s >2.0 mm and

p a r t i c l e s ~ 0 . 2 5 mm. The mulga grove s o i l s have more water -s table

aggregates >2.0 mm than t h e i n t e r c e p t i o n zone o r t h e e r o s i o n s l o p e

s o i l s and correspondingly l e s s <0.25 mm aggregates .

MWD f o r each o f t h e t h r e e zones, along wi th s o i l su r face

r e s i s t a n c e , bulk d e n s i t y , o rgan ic carbon percentage and t o t a l n i t rogen

percentage a r e a l s o g iven i n Table 2. Even though t h e r e was a

significant difference in bull; density between the three zones, there

was no significant difference in soil surface resistance.

Water Characteristics . - . . .- -

At all iitric potential values from 0 to -5.0 kPa , the soil

from the mulga groves and interception zones had a significantly

higher (P=0.05) volumetric water content than the soil from the runoff

slopes (Table 3). Also, in the matric potential range -0.3 to -3.0

kPa, the amount of volumetric water content drained from the mulga

grove and interception zone soils was significantly higher (P=0.05)

than the soil from the runoff slope (Table 4).

Soil Hvdraulic Properties

Table 5 shows the sorptivity, infiltration rate and hydraulic

conductivity values at supply potentials of + l o and -40 nnn for the

three zones. Under ponded infiltration (+I0 mm), there are significant

differences in the hydraulic properties between the th~ee zones. This

particularly occurs when log values are compared. For example, the

sorptivity of the runoff slope is significantly lower than the

interception zone and mulga grove. Also, the infiltration rates of the

three zones is in the order: runoff slope < interception zone C mulga

grove. The hydraulic conductivities of both the runoff slope and

interception zone are significantly lower than the mulga grove.

However, at -40 nnn supply potential, there are no significant

differences in the hydraulic properties between the zones. The

infiltration rate of all zones is approximately half that of the

runoff slope measured under ponded conditions.

Soil-Water Content Measured Followine Rainfall

Table 6 shows that the 37.5 mm rainfall event resulted in a

major redistribution of water between the three zones. Of the 37.5 m,

15.7 mm (42%) occurred i n t h e runoff s lope , 33.7 mm (90%) i n the

i n t e r c e p t i o n zone, and 51.6 mm (138%) i n t h e mulga grove.

Micromorphological examination o f t h e t h r e e zones showed

marked d i f f e r e n c e s i n p o r o s i t y . S o i l from t h e runoff s lope e x h i b i t s a

d iscont inuous s e a l wi th a smooth su r face l a y e r con ta in ing no l a r g e

I pores and absence o f b i o l o g i c a l a c t i v i t y (Figure 2a) . )The f i n e

h o r i z o n t a l c racking i s probably an a r t i f a c t produced by sampling. Even

though t h e i n t e r c e p t i o n zone s o i l a l s o conta ined a smooth surface ,

t h e r e was evidence of c racks , 1-2 rmn diameter i n t o t h e s u b s o i l (Figure j

2b). A t a depth o f 2-3 cm, t h e s u b s o i l c o n t a i n s burrows, i s we l l

s t r u c t u r e d and s t r o n g l y b io tu rba ted . The mulga grove s o i l c o n s i s t s of

an upper l a y e r , s e v e r a l m i l l i m e t r e s th i ck , c o n s i s t i n g predominantly o f

organic l i t t e r , wi th channels , 1-2 mm d iameter , i n t o t h e s u b s o i l

(Figure 2c) .

S c a n n i n ~ E lec t ron Microscoov

The SEM micrographs i n d i c a t e d a marked d i f f e r e n c e between the

su r faces of t h e t h r e e zones (Fig.3) . The s u r f a c e of t h e runoff zone

appeared t o have s l aked i n t o i n d i v i d u a l sand and s i l t p a r t i c l e s ,

forming a dense s t r u c t u r e wi thout o r g a n i c d e b r i s (F ig . 3a ) . However,

i n t h e i n t e r c e p t i o n zone, t h e su r face has a more porous s t r u c t u r e and

the re was evidence of aggregat ion between sand, s i l t and c l a y

p a r t i c l e s (Fig. 3b) . There was a l s o a n abundance o f o rgan ic d e b r i s and

r o o t l e t s a s s o c i a t e d wi th t h e aggregates . The mulga grove s o i l s a l s o

had a w e l l aggregated s t r u c t u r e with abundant o rgan ic d e b r i s , and

l a r g e pores, >I00 um diameter , were v i s i b l e (Fig. 3 c ) .

DISCUSSION

The i n f i l t r a t i o n a n d ' r e d i s t r i b u t i o n o f r a i n f a l l i n a mulga

woodland community w i l l be c o n t r o l l e d by b o t h t h e geomorphic p a t t e r n

of t h e landscape and t h e s o i l p h y s i c a l p r o p e r t i e s of t h e runoff/runon - . -- . . . . ..

zones: Even though previous i ~ o r k by S l a t y e r (1961) had suggested t h a t

t h e i n f i l t r a t i o n and r e d i s t r i b u t i o n of r a i n f a l l i n t o grove/ in tergrove

a r e a s was l a r g e l y due t o landscape p a t t e r n , v e r y l i t t l e a t t e n t i o n was

g i v e h t o t h e r o l e of s o i l p h y s i c a l p r o p e r t i e s . The measurements

desc r ibed i n t h i s paper i n d i c a t e t h a t t h e s u r f a c e s o i l s i n t h e t h r e e

zones have marked d i f f e r e n c e s i n t h e i r p h y s i c a l p r o p e r t i e s

( p a r t i c u l a r l y s t r u c t u r a l s t a b i l i t y and i n f i l t r a t i o n r a t e s ) and t h a t

t h e s e d i f f e r e n c e s had a l a r g e e f f e c t on t h e r e d i s t r i b u t i o n of

r a i n f a l l .

Di f ferences i n s o i l phys ica l p r o p e r t i e s w i l l be l a r g e l y

determined by t h e amount of v e g e t a t i o n cove r i n t h e d i f f e r e n t zones.

More v e g e t a t i o n cover occurs i n t h e i n t e r c e p t i o n zones and mulga

groves than i n runoff zones, p a r t i c u l a r l y i n y e a r s of low annual

r a i n f a l l (A.D. Wilson; pe r sona l communication). Vegeta t ion cover i s

impor tant because n o t only does i t p r o t e c t t h e s o i l s u r f a c e from

r a i n s p l a s h and s u r f a c e c r u s t i n g e f f e c t s (Eps te in and Grant 1973), bu t

it a l s o inc reases t h e s o i l o rgan ic m a t t e r c o n t e n t . Table 2 shows t h a t

t h e mulga grove s o i l s have h ighe r o rgan ic carbon (and t o t a l n i t rogen)

l e v e l s than t h e runoff s lope and i n t e r c e p t i o n zone s o i l s . S o i l s with

h i g h e r organic m a t t e r content u s u a l l y c o n t a i n more s t a b l e aggregates

( T i s d a l l and Oades 1982). Th i s i s because s o i l po lysaccha r ides and

humic subs tances i n s o i l o rgan ic m a t t e r a r e absorbed by s o i l mineral

p a r t i c l e s and a s s i s t w i th t h e formation and s t a b i l i z a t i o n of

aggrega te s .

Thus t h e mulga grove s o i l s have h i g h e r aggrega te s t a b i l i t i e s

1 than t h e runoff s lope and i n t e r c e p t i o n zone s o i l s (Table 2 ) . Also, t h e

wet-sieving r e s u l t s (F igure 1) demonstrate t h a t t h e s o i l s from t h e

runoff s l o p e s and i n t e r c e p t i o n zones s l a k e more r e a d i l y i n t o c0.25 mm

diameter microaggregates than t h e s o i l s from t h e mulga groves. S laking

i s t h e p rocess by which u n s t a b l e s o i l macroaggregates >0.25 mn

diameter breakdown i n water i n t o microaggregates 4 . 2 5 mn d i m e t e r

(Oades 1984). I n a d d i t i o n t o s l a k i n g dur ing r a ins to rms , unktable s o i l s 1

can a l s o undergo d i s p e r s i o n . D i spe r s ion is de f ined a s t h e

d i saggrega t ion i n wa te r of microaggregates i n t o s m a l l e r u n i t s , o f t e n

d i s c r e t e c l a y p a r t i c l e s , <2.0 um d iamete r . Greene and Tongway (1989)

have shown t h a t red e a r t h s o i l s undergo d i s p e r s i o n when subjec ted t o

mechanical s t r e s s , e .g. by r a ind rop s p l a s h .

Therefore , when t h e u n s t a b l e s o i l s i n t h e runoff s lope and

i n t e r c e p t i o n zone a r e wet up du r ing r a i n , they breakdown not only

because they have l e s s cover, b u t because they a r e more s t r u c t u r a l l y

uns table . The rearrangement o f s o i l p a r t i c l e s a t t h e Surface r e s u l t i n g

from s l a k i n g and d i s p e r s i o n o f u n s t a b l e aggrega te s causes s o i l pores

t o become blocked and t h e i n f i l t r a t i o n r a t e t o decrease . Dexter

(1988), cons idered t h a t i t i s impor tant t h a t t h e r e is always <15% of

c0.25 mm p a r t i c l e s which can b lock pores . Williams and Bone11 (1988),

found t h a t where macropores were produced by t h e a c t i v i t i e s of s o i l

fauna, such a s a n t s and t e r m i t e s , t h e openings o f such channels a r e

temporar i ly o r permanently blocked by inwashing o f sp lashed m a t e r i a l

from ra ind rop impact.

Therefore, t h e tendency f o r t h e runoff zone and i n t e r c e p t i o n

zone s o i l s t o s l a k e more r e a d i l y i n t o ~ 0 . 2 5 urn p a r t i c l e s compared wi th

t h e mulga grove s o i l s , would p a r t l y account f o r t h e l o w s a t u r a t e d

i n f i l t r a t i o n r a t e s measured i n t h e s e zones. The micromorphological and

SEM examinat ion o f t h e t h r e e s o i l s a l s o show t h e more porous n a t u r e of

mulga grove s o i l compared w i t h t h a t of t h e runoff and i n t e r c e p t i o n

zone s o i l s . - . --

I n - a d d i t i o n t o promoting s t r u c t u r a l s t a b i l i t y , h i g h e r

i n f i l t r a t i o n r a t e s are u s u a l l y a s s o c i a t e d w i t h v e g e t a t i o n cover ,

p a r t i c u l a r l y t h a t due t o shrubs . Scho l t e (1989) measured t h e

i n f l i l t r a t i o n r a t e s o f Chromic Luviso ls i n a number of a r e a s w i t h

va r ious v e g e t a t i o n covers and showed a c l e a r r e l a t i o n s h i p o f

i n f i l t r a t i o n r a t e w i t h v e g e t a t i o n cover and biomass. The i n f i l t r a t i o n

r a t e measured under t h e shrub Boscia co r i aceae was approximately 20

times g r e a t e r t h a n t h a t measured on a sea l ed s u r f a c e s o i l . Other

workers (Lyford and Qashu 1969, Blackburn 1975) 'have a l i o shown t h a t

the i n f i l t r a t i o n r a t e measured near t h e stems of p l a n t s was h ighe r

than t h e a r e a between p l a n t s . Johnson and Gordon (1988) used a

r a i n f a l l s i m u l a t o r on a sagebrush (Artemisia t r i d e n t a t a ) dominated

site t o show t h a t t h e i n t e r s p a c e a r e a s produced 2.5 t imes a s much

runoff a s from shrub canopy zones. I n t h e c u r r e n t work,the

i n f i l t r a t i o n r a t e s measured under t h e shrubs Acacia aneura were

approximately 10 t imes h igher than t h e i n f i l t r a t i o n r a t e s measured i n

the runoff zones.

The i n f i l t r a t i o n measurements a t s a t u r a t e d (+ lo rn ) and

unsa tu ra t ed (-40mm) supply p o t e n t i a l s , show t h e importance o f pores

>0.75 mm diameter i n conduct ing water i n t o t h e s u b s o i l i n t h e mulga

groves . Emerson e t a1 . (1986) considered t h a t h igh i n f i l t r a t i o n r a t e s

would be l a r g e l y determined by t h e presence of a cont inuous network o f

macropores, approximately 1.0 mm diameter , which a r e s t a b l e t o we t t ing

and can conduct wa te r i n t o t h e subso i l . White (1988) has sugges ted

t h a t i t was t h e s e l a r g e pores which tend t o c o l l a p s e f i r s t under

compaction o r r a i n f a l l . A t a , s u p p l y p o t e n t i a l of -40mm, when pores

>0.75 mm d iameter are n o t c o n t r i b u t i n g to w a t e r flow; t h e s u r f a c e

s o i l s i n t h e t h r e e zones have s i m i l a r i n f i l t r a t i o n r a t e s . Kowever, t h e

-. g r e a t l y enhanced i n £ i l t r a t i o n r a t e s (under ponded cond i t ions ) i n -the

mulga grove soils i n d i c a t e t h e r o l e o f po res >0.75 mm d iameter i n

conducting water . This s i z e pore i s very prominent i n t h e mulga grove

s o i l s and t o a l e s s e r e x t e n t i n t h e i n t e r c e p t i o n zond s o i l s . , I

These macropores could be formed by s o i l fauna, p l a n t roo t s o r

c o n s i s t o f c r a c k s and f i s s u r e s due t o shr inkage o f t h e soil (Beven and

Germann 1982, Moore e t a l . 1986). There i s more oppor tun i ty f o r t h e 1

formation of such macropores i n the mulga grove s o i l s , where roo t and

faunal a c t i v i t y i s h ighe r than i n t h e runoff and i n t e r c e p t i o n

zone s o i l s . S c h o l t e (1989) found t h a t high i n f i l t r a t i o n r a t e s of

Chromic Luvisols measured under shrubs was due t o t e r m i t e a c t i v i t y .

Athough t e r m i t e s and a n t s may e i t h e r impede o r f a c i l i t a t e t h e flow of

water through t h e s o i l (Lobry de Bruyn 1990) , work on o t h e r massive

red e a r t h s o i l s (Greene and Tongway 1989, Greene e t a l . 1990), s i m i l a r

t o t h e Lake Mere s o i l s , i n d i c a t e t h a t h igher i n f i l t r a t i o n r a t e s

coinc ide with t h e presence of p r e f e r e n c i a l f low p a t h s caused by fauna l

a c t i v i t y .

The g r e a t e r volumetr ic moisture c o n t e n t s i n t h e m a t r i c

p o t e n t i a l range 0 t o -0.5 kPa of t h e mulga grove and i n t e r c e p t i o n zone

s o i l s compared w i t h t h e runoff s lope s o i l s (Table 3 ) i s f u r t h e r

evidence t h a t they con ta in more macropores and hence can conduct water

a t a g r e a t e r r a t e . The g r e a t e r amount o f volumetr ic mois ture content

drained by t h e mulga grove s o i l s i n the ma t r i c p o t e n t i a l range of -0.3

t o -3.0 kPa ( e q u i v a l e n t t o pores of 1.0 and 0.1 mm diameter ,

r e spec t ive ly ) a l s o shows t h a t pores of 1 .0 mm diameter a r e more

prominant i n t h e mulga g r o v e . s o i l s t han i n t h e i n t e r c e p t i o n zone and

runoff zone s o i l s (Table 4) .

The h i g h e r i n f i l t r a t i o n r a t e s o f t h e s o i l s i n t h e mulga groves --

and i n t e r c e p t i o n zones l a r g e l y determine how r a i n i s r e d i s t r i b u t e d i n

t h e d i f f e r e n t zones. S l a t y e r (1961) a l s o demonstrated t h a t bes ides t h e

advantage of runoff water , t h e s o i l s i n t h e groves have h igher

i n f i l t r a t i o n r a t e s t h a n t h e i n t e r g r o v e s o i l s . It bas been suggested I

t h a t , i n t h e m a j o r i t y of r a i n f a l l s , t h e i n t e r g r o v e and i t s downslope

grove c o n s t i t u t e a c losed system, w i t h no n e t runoff through t h e t i e r

(Mabbutt and Fanning 1987). The sampling f o r so i l -water i n t h e t h r e e

zones a f t e r t h e r a i n event o f 37.5 mm showed t h a t t h e r e had been a

major r e d i s t r i b u t i o n of water . The h i g h e r i n f i l t r a t i o n r a t e s i n t h e

mulga grove s o i l s r e s u l t e d i n more wa te r e n t r y i n t o t h e s u b s o i l than

i n t h e runoff and i n t e r c e p t i o n zone s o i l s .

CONCLUSIONS .

Mulga grove s o i l s were found t o c o n t a i n s t a b l e macropores

>0.75 mm diameter , bu t t hese were l a r g e l y a b s e n t i n t h e runoff a r e a s .

The s t a b i l i t y o f t h e s e macropores was l a r g e l y due t o t h e vege ta t ion

cover i n t h e mulga groves. They caused t h e i n f i l t r a t i o n r a t e t o be 10

times h igher i n t h e runon a r e a s than t h e runof f a r e a s . They t h e r e f o r e

l a r g e l y c o n t r i b u t e t o t h e r e d i s t r i b u t i o n of wa te r from t h e runoff zone

t o the mulga g roves t h a t occur s dur ing major r a i n f a l l events .

Rangeland management p r a c t i c e s , such a s p re sc r ibed burning o r

overgrazing, t h a t markedly a l t e r t h e v e g e t a t i o n cover and hence t h e

s o i l phys ica l p r o p e r t i e s of t h e d i f f e r e n t zones, have t h e p o t e n t i a l t o

a l t e r t h e r a i n f a l l r e d i s t r i b u t i o n i n g rove / in t e rg rove a reas . Fu r the r

s t u d i e s a r e p r e s e n t l y underway t o q u a n t i f y t h e e f f e c t s of d i f f e r e n t

grazing i n t e n s i t i e s on vege ta t ion cover and wa te r r e d i s t r i b u t i o n .

ACKNOWLEDGEMENTS

The t e c h n i c a l a s s i s t a n c e o f M r G.R. Sawte l l i s g r a t e f u l l y

acknowledged. D r C.J . Char t r e s and M r I. S a l i n s of the Div i s ion of .

S o i l s a s s i s t e d w i t h t h e microkorphology and D r J.J. ~ i i i a r , Bendigo

College of Advanced Education, V i c t o r i a , a s s i s t e d i n the p repa ra t ion

and i n t e r p r e t a t i o n o f t h e SEM micrographs. Drs I. White and M . J . Su l ly

! of t h e Centre o f Environmental Mechanics provided va luable aflvice and

t r a i n i n g wi th t h e d i s c permeameters. Comments on a n e a r l i e r d r a f t by

Drs K.R.J . Smettem, K.C. Hodgkinson and B.H. Mckenzie were g r a t e f u l l y

apprec ia ted . The typ ing of t h e manuscript was e x p e r t l y c a r r i e d out by

Robyn Quinnell .

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FIGURE .'CAPTIONS:

Fig I . Distribution of water-stable aggregates obtained after wet - . - - -. -

sieving. Bars represent standard errors of the mean.

Fig 2. Photomicrographs of thin-sections from the three geomorphic

zones.

! t (a) Runoff zone (b) Interception zone (c) Mulga grove

Fig 3. SEM micrographs of the soil surface from the three

geomorphic zones.

(a) Runoff zone

(b) Interception zone

(c ) Mulga grove. Note the large pore'>100 diameter (arrow)

Table 1: SOME PHYSICAL AND CHEMICAL PROPERTIES OF A TYPICAL R]~D EARTH PROFILE

AT THE LAKE MERE EXPERIMENTAL SITE . .

DEPTH PH EC BULK DENSITY PARTICLE SIZE ANALYSIS (%)

(cm) (1:5) (1:5) 0% m-3) <.ooz .doz i .02- .2-

(yscm-') mm . 0 2 m . 2 m 2.0m

Table 2: AGGREGATE STABILITY (MUD), , SOIL SURFACE RESISTANCE BULK DENSITY,

ORGANIC CARBON ( % ) AND TOTAL NITROGEN ( % ) FOR THE THRICE

GEOMORF'HIC ZONES AT LAKE MERE. - .

- ~- . - .. . - - - .

Each measurement i s a mean o f t h i r t y s i x r e p l i c a t e s . Means fol lowed by t h e

same l e t t e r a r e n o t s i g n i f i c a n t l y d i f f e r e n t a t P=0.05

I

I

S o i l Proper ty Runoff Zone I n t e r c e p t i o n Zone Mulga Zone

Mean weight d i m . 305.7=

S o i l sur face r e s i s t . (MPa) 3.9Ga

Bulk d e n s i t y (Mg m-3) 1 . 4nb

(0-2cm)

Organic Carbon (%) 0.67'

T o t a l n i t rogen (%) 0.08'

Table 3. Volumetric wa te r c o n t e n t s of t h e t h r e e zones a t a range of

m a t r i c p o t e n t i a l s

- . . . -. Means fo~llowed by t h e same let ter a r e -not s & n i f i c a n t l y d i f f e r e n t a t P = O . O ~

Matr ic P o t e n t i a l (yo) Volumetric water c o n t e n t (cm3 cmk3)

@Pa) Runoff Zone I n t e r c e p t i o n Zone Mulga Grove

Table 4. Volumetric water content drained in the matric potential range of 0

to -0.3 kPa and -0.3 to -3.0 kPa for the 0-50 mm depth of the three

zones. - -. - .

- . - . . . . - - - . .

Means followed by the same letter are not significantly different

Drainage Range Change in volumetric water content (cm3 crnF3)

(kpa) Runoff zone Interception Zone Mulga Grove

Figure 1 . Distribution of water stable aggregates obtained after wet sieving

c' . - 0 ' 0) 8 0 0 Runoff Slope - Intercept. Zone 0 - BEE! Mulga Grove 0 u

x V

60 . 0 aJ

-C/

o cn a~ 40 L 0

2 - 20 _D 0

4-J

v, L a, Y

n 0

I Size of Aggregates (mrn)

. .. ~ . . .~ ~ .. ... .

Figure 2a. U 10 m

Figure 2b. U 10 m

Figure 2 c . U 10 m

Figure 3a.

F igu re 3b.

F igure 3 c .