CSIRO PUBLISHING
www.publish.csiro.au/journals/ijwf
International Journal of Wildland Fire, 2004, 13, 157–163
Heating effects on water repellency in Australian eucalypt forest
soils and their value in estimating wildfire soil temperatures
Stefan H. DoerrA,H , William H. BlakeB , Richard A. ShakesbyA,C , Frank StagnittiD ,
Saskia H. VuurensE , Geoff S. HumphreysF and Peter WallbrinkG
A Department
of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, United Kingdom.
of Geography, University of Plymouth, Drake Circus, Plymouth PL4 8AA, United Kingdom.
Telephone: +44 (0)1752 233069; fax: +44 (0)1752 233054; email: [email protected]
CTelephone: +44 (0)1792 2955236; fax: +44 (0)1792 295955; email: [email protected]
D School of Ecology and Environment, Deakin University, Warrnambool Campus,
PO Box 423, Vic. 3280, Australia. Telephone +61 3 5663 3535;
fax: +61 3 556 33143; email: [email protected]
E Wageningen University, Department of Environmental Science, Nieuwe Kanaal 11, 6709 PA Wageningen,
The Netherlands. Telephone: +31 (0) 317 489 111; fax: +31 (0) 317 48 39 99;
F Department of Physical Geography, Macquarie University, North Ryde, Sydney,
NSW 2109, Australia. Telephone: +61 2 9850 7990; fax: +61 2 9850 8420;
email: [email protected]
G CSIRO Land & Water, GPO Box 1666, Canberra, ACT 2601, Australia.
Telephone: +61 2 6246 5823; fax: +61 2 6246 5800; email: [email protected]
H Corresponding author. Telephone: +44 1792 295147; fax: +44 1792 295955;
email: [email protected]
B School
Abstract. Wildfires can induce or enhance soil water repellency under a range of vegetation communities. According to mainly USA-based laboratory studies, repellency is eliminated at a maximum soil temperature (T ) of
280–400◦ C. Knowledge of T reached during a wildfire is important in evaluating post-fire soil physical properties, fertility and seedbed status. T is, however, notoriously difficult to ascertain retrospectively and often based
on indicative observations with a large potential error. Soils under fire-prone Australian eucalypt forests tend to be
water repellent when dry or moderately moist even if long unburnt. This study aims to quantify the temperature
of water repellency destruction for Australian topsoil material sampled under three sites with contrasting eucalypt
cover (Eucalyptus sieberi, E. ovata and E. baxteri). Soil water repellency was present prior to heating in all samples, increased during heating, but was abruptly eliminated at a specific T between 260 and 340◦ C. Elimination
temperature varied somewhat between samples, but was found to be dependent on heating duration, with longest
duration resulting in lowest elimination temperature. Results suggest that post-fire water repellency may be used as
an aid in hindcasting soil temperature reached during the passage of a fire within repellency-prone environments.
Additional keywords: hydrophobicity; Eucalyptus; Australia; fire intensity; fire severity; soil ecology; soil
hydrology.
Introduction
The heat generated during a fire can have a significant effect
on soil and the post-fire recovery of the ecosystem. For example, biological disruptions begin in the 40–70◦ C temperature
range with microbes starting to be affected between 50 and
120◦ C (Neary et al. 1999) while, for some seedbeds, dormancy is broken only if 80◦ C is exceeded (Bradstock and
Auld 1995). Physicochemical changes occur at higher temperatures with considerable distillation of organic matter
© IAWF 2004
taking place between 200 and 315◦ C. Nutrient volatilisation begins between 200 and 400◦ C (Neary et al. 1999),
and changes to the particle size distribution towards the sand
fraction (i.e. aggregation) begin beyond 220◦ C (Giovannini
1994). Knowledge of the soil temperature reached during a
fire is therefore of importance in evaluating post-fire soil
fertility and physical properties as well as seedbed status (Burrows 1999). Apart from prescribed fires, for which
temperature information can be obtained with pre-installed
10.1071/WF03051
1049-8001/04/020157
158
probes, soil temperatures reached during a fire are notoriously
difficult to ascertain. For example knowledge of the commonly calculated intensity I (kW m−1 ) of a fire, as defined
by Byram (1959), gives little information with respect to how
much of this energy has been released into the soil (Hartford
and Frandsen 1992). Soil temperatures can be estimated only
with a large margin of error using indicative and somewhat
subjective factors such as ash colour (e.g. Bentley and Fenner
1958) and the nature of the remains of the ground fuel consumed (e.g. Chandler et al. 1983; Moreno and Oechel 1989).
Furthermore, such estimates do not provide a sound basis
for comparison between observers and events (Wight and
Bradstock 1999). Considering the effects of fire on soil properties, Chandler et al. (1983) differentiated between lightly
burned areas with temperatures at the soil surface above
250◦ C and exceeding 100◦ C at 1–2 cm depth; moderate burns
with surface temperatures of 300–400◦ C and 200–300◦ C at
1 cm depth; and severely burned areas with 500–750◦ C at the
surface and more than 300◦ C at 3 cm depth.
A characteristic that may provide specific information
on the temperature reached during a fire is the wettability
of the soil. It has long been established that, in many forest and shrubland environments, water repellency may be
induced or where already present enhanced after fire and,
where heating has been excessive, water repellency in the surface layer may in turn be eliminated (see reviews by DeBano
2000; Doerr et al. 2000). These effects have been studied
in detail for slightly water-repellent Californian chaparral
soil by DeBano and Krammes (1966), who heated samples
for periods between 5 and 20 min. They found that heating
to more than 175◦ C caused little alteration in repellency,
between 175–200◦ C repellency increased considerably, but
it became destroyed on heating to between 280 and 400◦ C.
A range of other, also northern USA-based, studies generally agree that repellency is lost when soils are heated to this
temperature range (Savage 1974; Scholl 1975; DeBano et al.
1976; Robichaud and Hungerford 2000). For four unspecified Japanese brown forest soils, Nakaya (1982) found that
repellency increased to some extent between 105 and 200◦ C,
increased steeply between 200 and 250◦ C and declined above
250◦ C. Increases in repellency associated with heating have
been attributed to the polymerisation of organic molecules
into more hydrophobic ones (Giovannini and Lucchesi 1983),
the improved bonding of such substances to soil grains
(Savage 1974), and the melting and redistribution of waxes
from interstitial organic matter onto soil aggregates and mineral grains (Franco et al. 2000).The reduction and elimination
of repellency near or above 300◦ C is likely to be caused
by the volatilisation and combustion of organic compounds
(DeBano et al. 1976; Chandler et al. 1983).
The differences in thresholds between studies may be a
result of differences in measuring methods, heating duration,
types of organic matter, and differences in initial moisture and
textural characteristics, and in the soils used. For instance, the
S. H. Doerr et al.
cooling effect of evaporating moisture will prevent the soil
temperature from exceeding 100◦ C until much of the soil
water has been removed. Once a soil is dry, the thermal conductivity, and thus the time until the whole soil sample has
reached the ambient temperature, can vary more than 3-fold
between sand and clay (Chandler et al. 1983). Also longer
heating duration provides more total energy which may, for
example, allow enhanced migration, redistribution or structural change for the hydrophobic compounds present in the
soil organic matter. This may explain the finding that water
repellency increased at lower temperatures for longer heating
duration reported by DeBano and Krammes (1966). Comparison of data obtained in different studies remains problematic,
since there is usually limited information available on the
intrinsic thermal properties of the soils investigated as well
as sample surface area and volume. Thus it is often difficult
to ascertain how well the exposure temperature under experimental conditions actually reflects the maximum temperature
reached within the soil sample.
To the authors’ knowledge, no studies have been conducted to date on heating effects on water repellency for
Australian soils, despite (i) fires being very common in many
eucalypt forest environments; (ii) soil water repellency having often been noted after burning (e.g. O’Loughlin et al.
1982; Mitchell and Humphreys 1987; Zierholz et al. 1995);
and (iii) soils under some eucalypt species exhibiting some
of the most severe repellency levels reported in the literature even at long-unburnt locations (Burch et al. 1989;
Crockford et al. 1991; Doerr et al. 1998; Scott 2000). The
latter fact is not widely acknowledged in post-fire studies
and it is often assumed, but rarely demonstrated, that water
repellency noted after a fire has actually been induced during
the burn (Shakesby et al. 2000).
This study (i) examines the effects of different heat
treatments (250–400◦ C, 5–40 min heating duration) on the
repellency of soil samples from fire-prone Australian eucalypt environments and (ii) discusses the use of the presence
or absence of water repellency after a wildfire as an objective
and easy-to-use indicator of the soil temperature reached at
the soil surface or at a given depth during burning.
Materials and methods
Three bulk surface soil samples (∼5 kg each) were collected
under long-unburnt (more than 10 years) eucalypt stands in
south-eastern Australia on, respectively, a ridge-top position
in the Nattai National Park in New South Wales (sample
NT), a north-facing slope in the northern-most part of the
Grampians National Park in Victoria (sample GP) and flat
terrain south of Mortlake in Victoria (sample ML) (Fig. 1).
Samples consisted of mineral soil material, taken at 0–2.5 cm
depth pooled from at least five areas more than 2 m apart. Litter cover at each area was 2–5 cm for all sites. Further site
and soil characteristics are given in Table 1. Sample material
from each sampling site was air-dried at room temperature
Water repellency and soil temperature
159
(∼20◦ C) to equilibrium weight and passed through a 2 mm
sieve before further analysis. Each of the bulk samples was
then homogenised by hand mixing. For each experimental
treatment, a total volume of 20 cm3 of soil, amalgamated
from separate areas of the bulk soil sample, was spread into
an aluminium dish with an area of 16 × 11 cm, resulting in a
soil layer of 1–2 mm thickness. This was preferred rather than
placing samples into porcelain crucibles, since the shallow
depth and large surface area of the soil material placed into
the dishes used here allowed sample material to be heated far
more thoroughly than would otherwise have been achieved.
Heat treatments were carried out in a pre-heated muffle furnace with an internal thermometer (B & L Bartlow,
Model WIT, 2.4 kW), using heating durations of 5, 10, 20 and
40 min and temperatures ranging from 250 to 400◦ C at 10◦ C
intervals. Exploratory experiments showed that for a selected
temperature setting, the actual temperature inside the furnace
was held constant to within ±10◦ C. Given the thin layer of
NEW SOUTH WALES
Blue Mountains
Sydney
Nattai National Park
NT
Grampians
National Park
GP
N
VICTORIA
Melbourne
ML
200 km
Fig. 1.
Location of sampling sites in south-eastern Australia.
Table 1.
Code
Location
soil used in the experiments here, soils were expected to have
reached nearly the temperature obtained in the furnace. Rates
of heating were expected to vary little between samples due to
their similar thermal diffusivity values (Table 1). These were
determined using a Decagon KD2 thermal meter and allowed
calculation of the final temperature reached in the centre of
the soil layer. Thus, based on an average thermal diffusivity
D of ∼0.12 × 10−3 mm2 s−1 for the soils investigated here
(Table 1) and an equal heat input from both the top and the
bottom of the soil layer, the approximate final temperature
T (◦ C) in the centre of the soil layer was derived using the
following equation (Koorevaar et al. 1983):
x
T = Tb + (Ta − Tb)erfc √
,
(1)
2 Dt
where Ta = initial soil temperature, Tb = oven temperature,
t = heating duration (s), erfc = complementary error function (dimensionless), and x = average depth to centre of soil
layer (mm).
Thus, for example, the predicted temperature reached in
the centre of the soil layer (at ∼0.8 mm depth) for the shortest
heating duration (300 s) would have been 233◦ C and 372◦ C
for furnace temperatures of 250◦ C and 400◦ C respectively,
and for the longest heating duration (2400 s), and 244◦ C and
390◦ C for furnace temperatures of 250◦ C and 400◦ C respectively. Given that the predicted temperatures relate to the
centre of the thin soil layer (i.e. the outer parts of the soil
layer will have heated more rapidly than the centre), these
predictions suggest that samples would have reached overall
temperatures not substantially different from those applied.
After heating, the soil sample material from each of the
temperature intervals and heating periods was distributed
over three separate aluminium cups (∼5 cm diameter). Samples were then allowed to equilibrate to the ambient atmospheric conditions of the laboratory (∼22◦ C and a relative
humidity RH of 40–60%) for at least 1 week prior to being
tested for repellency. This long equilibration procedure to
Sample characteristics and water repellency levels (WDPT and associated severity rating)
See Table 2 for repellency levels of air-dry samples prior to treatments
WGS
coordinates
Mean
annual
rainfall
(mm)
Dominant tree
species
Texture
(USDA)A
Total
organic
carbon
(%)A
Thermal
diffusivity
D (mm2 s−1 )A
WDPT (s)
Repellency
rating
GP
Grampians
National Park,
Victoria
142◦ 23 E,
36◦ 59.5 S
∼650
Eucalyptus baxteri
Sand
4.1
0.11 (±0.01)
16200B
(±3600)
Extreme
NT
Nattai National
Park, NSW
150◦ 29.5 E,
34◦ 13.3 S
∼950
Eucalyptus sieberi,
Corymbia gummifera
Sand
4.9
0.12 (±0.01)
360C
(±60)
Strong
ML
Mortlake Region
Victoria
142◦ 43.3 E,
38◦ 24.1 S
∼700
Eucalyptus ovata
Sandy
loam
4.0
0.14 (±0.00)
120C
(±20)
Strong
A Mean
of three subsamples.
B Median
of three subsamples. Value to nearest 1800 s.
C Median
of three subsamples. Value to nearest 20 s.
160
S. H. Doerr et al.
atmospheric conditions was adopted here since it has been
demonstrated that changes in atmospheric humidity near saturation RH can affect sample repellency (Doerr et al. 2002)
and that heat-induced changes in repellency may take several
days until they are fully equilibrated to ambient atmospheric
conditions (Doerr et al., unpublished data).
Samples were then tested for water repellency using the
Water Drop Penetration Time (WDPT) method (Letey 1969),
which is used widely as a simple indicator for determining
the persistence of water repellency, a measure that is also
broadly related to the severity of repellency (Doerr 1998).
This involved placing three droplets (80 µL) of distilled
water onto separate positions on the surface of each sample and the time needed for the water droplet to infiltrate was
recorded. When a water droplet infiltrated within 5 s the soil
was considered wettable; if more time was required, specific
repellency class intervals and associated ratings were used
(Table 2). Penetration times for samples that exhibited repellency (WDPT >5 s) were recorded in 20 s intervals for the
first 600 s, and thereafter every 30 min (1800 s). Repellency
values reported here are the median WDPT class of three samples, which is derived from the median WDPT class of three
droplets for each sample. WDPT tests where terminated after
5 h. If drops had not penetrated by that time, samples were
assigned a WDPT of >18 000 s. To explore the reproducibility of the heating results, the 20-min heating procedure was
repeated for all temperature settings on sample GP. WDPT
classes obtained with each repeat treatment were identical to
those obtained in the original experiment.
Results and discussion
Effects of temperatures and heating duration
All three samples investigated here exhibited natural water
repellency when air dry with WDPT values of 350 s (NT),
100 s (ML) and 13 500 s (GP), which equate to strong repellency for the first two, and extreme repellency for the latter
(Table 2). This confirms the notion that soils under some
eucalypt species, such as those investigated here, can be
water repellent even when long-unburnt (Burch et al. 1989;
Crockford et al. 1991; Doerr et al. 1998; Scott 2000).
Water repellency levels (WDPT values) for all treatments are
presented in Table 3. For all soils and heating durations, heating resulted in a considerable increase in repellency. Thus
WDPT values increased to >18 000 s (extreme repellency)
for all samples, despite different textural properties, vegetation cover and pre-heating repellency levels. At higher
temperatures, this was then followed by the rather abrupt and
complete elimination of repellency.
Table 2. WDPT class increments used (seconds) and corresponding descriptive repellency rating
WDPT
classes (s)
≤5
>5, 20,
40, 60
80–600B
>600–
3600C
>3600C
Repellency
ratingA
Wettable
Slight
Strong
Severe
Extreme
AAfter
Bisdom et al. (1993). B Measured here in 20 s intervals.
here in 1800 s intervals until 18 000 s was reached.
C Measured
Table 3. Pre- and post-heating WDPT classes for all treatments (temperature T ±10◦ C) and samples
Median of three tests on each of three subsamples. WDPT values for repellent samples were recorded in 20 s class intervals for the first 600 s, and
thereafter every 30 min (1800 s; see also Table 2). The first line (22◦ C) refers to repellency levels prior to heating. Empty cells indicate that no
measurements were carried out and ‘0’ indicates complete elimination of repellency
T (◦ C)
WDPT class (s)
5 min
GP
22
250
260
270
280
290
300
310
320
330
340
350
360
370
380
390
400
16200
>18000
>18000
>18000
>18000
16200
>18000
>18000
>18000
>18000
0
0
0
0
0
0
0
NT
10 min
ML
360
120
12600
1800
>18000
1800
18000
1800
>18000 >18000
>18000 >18000
>18000 >18000
>18000
0
>18000
0
>18000
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
GP
16200
>18000
>18000
>18000
>18000
14400
1800
0
0
0
0
0
0
0
0
0
0
NT
20 min
ML
360
120
>18000
1800
>18000
1800
>18000 >18000
>18000 >18000
>18000 >18000
>18000
0
16200
0
>18000
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
GP
16200
>18000
>18000
18000
0
0
0
NT
40 min
ML
360
120
>18000
16200
>18000 >18000
>18000
0
>18000
0
>18000
0
0
0
0
0
0
0
0
0
0
0
0
0
GP
NT
16200
360
>18000 >18000
>18000
18000
>18000
0
0
0
0
0
0
0
ML
120
1800
0
0
0
0
0
Water repellency and soil temperature
161
T (ºC)
This is consistent with the patterns reported in previous
studies carried out in the USA (e.g. DeBano and Krammes
1966; Savage 1974; Scholl 1975) and Japan (Nakaya 1982)
and indicates that (i) water repellency can be enhanced during
the passage of a fire even when no further input of volatilised
organic compounds from the litter is available, and (ii) excessive soil heating eliminates water repellency. For the shortest
heating duration of 5 min, all samples approached their maximum measured repellency between 250 and 280◦ C, while
repellency was eliminated between 310 and 340◦ C. For the
10-min heating duration, increases in repellency were similar, but its elimination occurred between 290 and 330◦ C.
For 40 min of heating, destruction temperature was lowered
further to 260–300◦ C.
The destruction temperature of repellency is lowered by
similar temperature steps with increasing heating duration
(±10◦ C) for all three samples (Fig. 2). Given that water repellency in soils is thought to be generally caused by an outer
layer of aliphatic hydrocarbon chains protruding from soil
particles (see reviews of DeBano 2000; Doerr et al. 2000),
it is likely that these compounds undergo similar changes
at similar temperature thresholds in soils. Thus it is suggested here that differences in the temperature threshold
at which repellency is destroyed for different durations of
heating indicate that sufficient volatilisation and/or combustion of organic matter and thus the elimination of repellency
may already occur at a temperatures of around 260◦ C, but
only if this temperature is sustained for a sufficiently long
period. For shorter heating periods such as 5, 10 or 20 min,
somewhat higher temperatures may be required to achieve
sufficient volatilisation or oxidation of these compounds for
water repellency to be eliminated. This bears implications on
the focus on maximum soil temperatures determined in experimental burns using thermosensitive paints or thermocouples
for evaluating effects on soil wettability and potentially also
other physical or chemical soil parameters. Even where similar maximum soil temperatures have been recorded, different
350
340
330
320
310
300
290
280
270
260
250
GP
NT
ML
0
5
10
15
20 25 30 35
Heating time (min)
40
45
50
Fig. 2. Temperature at which complete elimination of water repellency [i.e. reduction of Water Drop Penetration Time (WDPT) to ≤5 s]
occurred for different durations of heating in three initially repellent
surface soil samples taken from different eucalypt stands (GP, NT
and ML).
heating durations may have induced profound differences in
heat-sensitive soil properties, the spatial distribution of which
in turn may have profound effects on runoff generation and
vegetation recovery (see reviews of DeBano 2000; Doerr
et al. 2000). Whilst temperature–duration data have been
obtained in a number of studies using experimental burns
(e.g. Bradstock and Auld 1995; Giovannini and Lucchesi
1997; Mataix-Solera 1999), these burns have generally had
to be restricted to burn intensities far below those attained
by wildfires occurring after prolonged drought periods. The
challenge remains for future research to obtain temperature–
duration data under the more extreme heating conditions
associated with wildfires.
Water repellency as an indicator of soil temperature
reached during burning
During wildfires, heat production can be intense, but is generally of short duration and only a small fraction of the heat
produced is transferred to the soil (Hartford and Frandsen
1992). Furthermore, excess soil moisture, where present,
has to be evaporated before the soil temperature can exceed
100◦ C. Thus, except where smouldering combustion occurs,
or where slash deposits or other large accumulations of
organic material are consumed, periods during which even
surface soils are heated to much above 250◦ C are relatively
short. For example, for a slow-moving prescribed fire in a
Eucalyptus pauciflora forest, Raison et al. (1986) found that
soil temperatures exceeding 250◦ C were sustained for less
than 40 min at 2 cm depth, and during a prescribed burn in
mallee vegetation (E. dumosa, E. socialis, E. gracilis and
E. foecunda), soil heating to more than 120◦ C occurred only
in the upper 2 cm and lasted more than 30 min (Bradstock
et al. 1992). In experimental studies in the Mediterranean,
surface soil heating to more than 250◦ C was confined to less
than 10 min for both slow-moving, severe (Giovannini and
Lucchesi 1997) and fast-moving scrub fires (Mataix-Solera
1999). The heating durations of 5–40 min used in this study
are therefore considered to encompass the maximum period
for which soil temperature remains higher than 250◦ C for the
most common types of forest fire.
The results of this study do not demonstrate unequivocally
that the destruction temperatures for water repellency determined in the laboratory for selected air-dry soils and without
the flaming combustion of a litter layer reflect real-world
destruction temperature thresholds under wildfire conditions.
Given, however, the common general chemical nature of
hydrophobic compounds in soils (see reviews by DeBano
2000; Doerr et al. 2000) and the fact that destruction temperatures reported in previous studies fall in the general range
obtained here, we consider it highly likely that the overall temperature threshold for repellency destruction observed here
applies to many fire-prone eucalypt forests in Australia and
probably also to other soil/vegetation zones. Thus, the presence or absence of water repellency after a fire may provide
162
an easy-to-use indicator of soil temperature reached during
burning in repellency-prone eucalypt forests and potentially
also other environments.
In terms of practical applicability, a burn intensity classification used in the USA (USDA 2000) already uses the
presence of ‘medium’ to ‘high’ water repellency in the topsoil as an indicator of ‘high fire intensity’, whereas low
or absent repellency indicates ‘low fire intensity’. This is,
however, potentially misleading since this present study, as
well as previous USA-based studies, indicate that during
the most intense fires, which are often but by no means
always associated with a high heat input into the soil (Oliveira
et al. 1994), repellency may be either elevated or entirely
eliminated in the topsoil depending on whether the critical
temperature–duration conditions for repellency destruction
have been reached. Rather than indicating burn intensity, the
presence of water repellency would indicate that the temperature in the soil has not exceeded 300–350◦ C for more than
5 min and 250–290◦ C for more than 40 min∗ . In contrast,
provided water is present in lower soil layers or in adjacent
unburnt or lighter-burnt areas with similar soil and vegetation
characteristics, the absence of repellency suggests that soil
temperatures will have exceeded 300–350◦ C for more than
5 min and 250–290◦ C for more than 40 min∗ (Fig. 2). Based
on equation (1) the knowledge of the elimination temperature
of repellency, and of the maximum depth at which this temperature was reached, allows calculation of the approximate
soil temperatures for any given soil depth, provided the residence time of the fire front and the thermal diffusivity (D) of
the soil can be established.
A simple field procedure for examining the presence of
water repellency at any given soil depth could involve the
careful removal of any remaining duff and ash layer until the
mineral soil surface or any chosen depth is exposed and testing for repellency by applying drops of distilled water to the
soil surface. If drops do not penetrate within 5 s, the soil can
be classed as repellent. If the soil is wettable, i.e. drop penetration occurs within 5 s, the potential of this soil to have been
repellent has to be established by demonstrating the presence
of repellency in the subsoil or in adjacent areas of comparable duff, vegetation and soil characteristics (Shakesby et al.
2003). Tests should be conducted under relatively dry soil
conditions in order to avoid any complicating soil moisture
effects on the presence of repellency (see review by Doerr
et al. 2000).
Conclusions
In this study, the water repellency characteristics of three contrasting soils taken from under Eucalyptus sieberi, E. ovata
and E. baxteri in south-eastern Australia were investigated
before and after selected heat treatments encompassing a
∗To
allow for potential experimental error, ±10◦ C have been added to the
experimentally derived temperatures.
S. H. Doerr et al.
wide range of temperature–duration conditions that may
occur during a wildfire. It is demonstrated that water repellency was already present in the soil prior to heating,
increased during heating, and was abruptly eliminated at
specific temperatures between 260 and 340◦ C. The temperature threshold for repellency destruction decreased with
increased heating duration and, taking potential experimental error into account, ranged from 300–350◦ C for 5 min
of heating to 250–290◦ C for 40 min of heating. This confirms that not only maximum soil temperature reached, but
also heating duration, is critical in determining changes to
soil water repellency characteristics. Results suggest that for
repellency-prone environments, the presence or absence of
post-fire water repellency may be used as a simple semiquantitative indicator to determine whether or not the above
given temperature thresholds have been exceeded at a given
soil depth during the passage of a fire.
Acknowledgements
This study was supported by a NERC Research Grant
(NER/A/S/2002/00143), the NERC Geophysical Equipment
Pool, NERC Advanced Fellowship (NER/J/S/2002/00662)
and Australian Research Council International Linkage
Scheme Grant LX0211202. Julia Schneider kindly carried
out some initial exploratory heating experiments. We are
grateful to staff at the Warragamba Catchment Office, Sydney
Catchment Authority, and particularly Glen Capararo, for
logistical support during sample collection in Nattai National
Park. We also thank Rob Bryant of the Department of Chemical Engineering, University of Wales Swansea, for his advice
regarding heat transfer processes in soils.
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