Deriving hillslope sediment budgets in wildfire-affected forests using fallout radionuclide tracers

Geomorphology 104 (2009) 105–116

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Deriving hillslope sediment budgets in wildfire-affected forests using fallout radionuclide tracers W.H. Blake a,b,⁎, P.J. Wallbrink c, S.N. Wilkinson c, G.S. Humphreys e, S.H. Doerr d, R.A. Shakesby d, K.M. Tomkins e a

School of Geography, University of Plymouth, Plymouth PL4 8AA, UK Consolidated Radioisotope Facility (CORiF), University of Plymouth PL4 8AA, UK CSIRO Land and Water, P.O. Box 1666, Acton, ACT 2601, Australia d Department of Geography, School of the Environment and Society, Swansea University, Swansea SA2 8PP, UK e Department of Physical Geography, Macquarie University, Sydney, Australia b c

a r t i c l e

i n f o

Article history: Received 15 August 2007 Received in revised form 15 August 2008 Accepted 18 August 2008 Available online 27 August 2008 Keywords: Wildfire Catchment management Phosphorus Environmental radioactivity Water quality Bioturbation

a b s t r a c t Information on post-fire sediment and nutrient redistribution is required to underpin post-fire catchment management decisions. Fallout radionuclide budgets (210Pbxs, 137Cs and 7Be) were derived to quantify soil redistribution and sediment yield in forested terrain following a moderately severe wildfire in a small (89 ha) water supply catchment in SE Australia. Application of these techniques in burnt terrain requires careful consideration of the partitioning of radionuclides between organic and mineral soil components. Beryllium-7 and 210Pbxs were shown to be closely associated with ash, litter and soil organic matter whereas 137Cs was more closely associated with subsurface coarse mineral soil. Comparison of the three tracer budgets indicated that the dominant sediment source areas were ridgetops and steep valley sideslopes, from which burnt surface material was conveyed to the stream network via pre-existing gullies. Erosion was predominantly driven by sheetwash, enhanced by soil water repellency, and modified by bioturbation which both supplies subsurface sediment and provides sinks for erosive overland flow. Footslope and riparian zones were not important sediment source areas. The estimated event-based (wildfire and subsequent rainfall) sediment yield is 58 ± 25 t km− 2, based on fallout 7Be measurements. The upper estimate of total particulate phosphorus yield (0.70 kg ha− 1) is more than 10 times that at equivalent unburnt sites. This illustrates that, soon after fire, burnt eucalypt forest can produce nutrient loads similar to those of agricultural catchments. The tracer budgets indicate that wildfire is an important control on sediment and phosphorus inputs to the stream network over the decadal timeframe and the pulsed nature of this release is an important concern for water quality management. © 2008 Elsevier B.V. All rights reserved.

1. Introduction It is widely accepted that significant rainfall after wildfire in forested terrain leads to enhanced soil erosion rates (Lane et al., 2006; Renau et al., 2007) although the specific erosional responses can vary markedly (Shakesby and Doerr, 2006). The combustion of litter and the surface horizons of soils often produces nutrient-rich erodible ash, organic matter and mineral material, which are readily exported from burnt slopes to impact on adjacent streams and rivers and negatively affect water quality in aquatic ecosystems (Reiman and Clayton, 1997; Bowman and Boggs, 2006). Quantifying these off-slope impacts, however, remains a challenge at the whole slope or catchment scale. A wide range of hillslope erosion rates have been reported for wildfire⁎ Corresponding author. School of Geography, University of Plymouth, Plymouth PL4 8AA, UK. E-mail address: [email protected] (W.H. Blake). 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2008.08.004

affected landscapes in Mediterranean Europe, the USA and Australia with post-fire increases of up to two orders of magnitude. Much of this previous erosion work, however, has been based on small-scale ground-height change measurements or plot-scale runoff experiments (Shakesby and Doerr, 2006). Consequently, it is difficult to upscale data to slope unit or first-order catchment scales or deriving slope unit sediment budgets that also quantify the storage of eroded material within the hillslope unit. The latter, in particular, is important information for the management of downstream water resources. Stream gauging studies provide more spatially-representative information on total sediment yields from small catchments (e.g. Lane et al., 2006) although such studies require a considerable investment of resources and labour, are difficult to set up in time for opportunistic wildfire studies and do not explicitly reveal channel storage or sediment deposition terms. Where continuous water and sediment flux data are available for the catchment outlet before and after wildfire, additional plot-scale investigation of runoff generation

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dynamics can be used to develop knowledge of key erosion processes and sediment-contributing areas within the hillslope unit (Sheridan et al., 2007) providing a comprehensive picture of sediment generation processes and yields required to underpin post-fire catchment management decisions. As a complement to instrumental records, the tracer-based sediment budget approach also offers a means of assessing soil redistribution rates within, and yields from, the landscape together with information on dominant sediment source areas (Wallbrink et al., 2002). Since sediment budgets quantify both gross erosion and deposition, data can be used to derive the hillslope sediment delivery ratio, i.e. the proportion of eroded material that reaches the stream (Walling, 1983), which is also useful for assessing the downstream consequences of soil erosion. Fallout radionuclide tracer techniques using excess (i.e. fallout) 210Pb (210Pbxs) (half-life, 22.26 years), 137Cs (half-life, 30.17 years) and 7Be (half-life, 53.3 days) can be used to construct hillslope sediment budgets and quantify sediment delivery ratios over a range of timescales. This approach avoids problems with upscaling both plot-scale erosion data and point-scale measurements, and obviates the need for long-term monitoring programmes. However, there are important trade-offs with respect to spatial and temporal resolution of data as illustrated in this study. The fallout radionuclide approach is described in detail by Ritchie and McHenry (1990) with respect to 137Cs, and can also be adapted for 210Pbxs and 7Be. In brief, both natural and artificial radionuclides present in the atmosphere are delivered to the soil surface by wet and dry precipitation. Once in contact with the soil, they rapidly adhere to mineral and organic particles. Subsequent redistribution of soil material is hence reflected by redistribution of radioactivity (Ritchie and McHenry, 1990). The technique relies upon several key assumptions all of which must be considered in the interpretation of derived sediment budget data: (i) the initial deposition of radionuclides across the study area is broadly uniform and largely delivered by wet fallout; (ii) there is an exponential decrease of radionuclide mass concentration and inventory with depth down an undisturbed soil profile; (iii) radionuclides remain attached to particulate material during erosion and deposition processes (i.e. particulate binding is irreversible); (iv) radionuclides leave their

point of deposition only in association with soil particles; (v) radionuclide properties are not altered as a function of physical transport; and (vi) radionuclide inventories can be readily measured (Ritchie and McHenry, 1990). Application of radionuclides 210Pbxs, 137Cs and 7Be to determining hillslope sediment budgets varies amongst different environments as regards its complexity. Open arable and pastoral land-uses offer the most straightforward environments for application of the techniques, permitting erosion and deposition data to be derived at a high spatial resolution (e.g. Owens et al., 1997; Walling et al., 2000; Blake et al., 2002; Wilson et al., 2003; Walling et al., 2003). Logged and undisturbed forested terrain offer more challenging environments owing to (i) spatial heterogeneity in fallout distribution, due to canopy interception, and (ii) partitioning of radionuclides between the organic litter layer and mineral soil such that significant proportions of fallout radionuclides may be associated with forest litter. The latter depends on fallout dynamics and forest disturbance history. The problem of heterogeneity has been successfully overcome by adopting spatially-integrated sampling approaches to derive landscape unitbased sediment budgets (e.g. Wallbrink and Croke, 2002; Wallbrink et al., 2002; Belyaev et al., 2005) or careful selection of the sampling window for using shorter-lived 7Be to estimate post-logging erosion rates at a higher spatial resolution (e.g. Schuller et al., 2006). The problem of partitioning requires detailed assessment of radionuclide distribution in forest soil and careful interpretation of fallout radionuclide budgets as regards organic and/or mineral redistribution. Burnt forested terrain presents additional challenges because of combustion of the litter layer and O-horizon. Recent work (Harden et al., 2004; Owens et al., 2006; Blake et al., 2006a) illustrates that the stable element content of surface soil can be enhanced due to mineralisation of canopy foliage and litter and the inclusion of derived ash into the soil matrix. There remain, however, unresolved issues regarding radionuclide partitioning between organic litter material and mineral soil and the fate of these radionuclides following wildfire. Against this background, this study aims to derive sediment budget information for a small (89 ha) wildfire-affected forested catchment in southeast Australia using fallout radioisotope tracer techniques as well as process information on the behaviour of

Fig. 1. Study area location (after Shakesby et al., 2003).

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radionuclides in soils after fires. This provides valuable information on sediment and nutrient yield from burnt forested landscapes as well as improvements on how fallout radionuclide tracer techniques can be applied in wildfire-affected terrain. 2. Regional setting 2.1. Study catchment The 89-ha study catchment, a first-order tributary of Blue Gum Creek (150° 29.5′ E, 34° 13.3′ S), is located in the Nattai National Park, New South Wales, Australia, approximately 100 km southwest of Sydney (Fig. 1). Blue Gum Creek (catchment area 44.6 km2) is a tributary of Little River (catchment area 139 km2), which flows into the Nattai River (catchment area 701 km2) which in turn flows into to Lake Burragorang, Sydney's principal water supply reservoir (Fig. 2). As a water supply catchment area, the study region has restricted access and is managed as a conservation zone. It has a complete forest cover apart from a management road network. Vegetation comprises native eucalypt woodland/forest. The climate is humid temperate with an average annual rainfall of 840 mm (SCA, 2007). Like most eucalypt forest, the Nattai National Park is very wildfire-prone. The catchment terrain is typical of the region, comprising densely forested gorges incised into Hawkesbury Sandstone in the uplifted western sector of the Permo-Triassic Sydney Basin. Ridgetops are close to 500 m above sea level with valley floors below 300 m. The area's steep topography results from relatively rapid fluvial incision of the eastern Australian continental margin over the last 100 million years (van der Beek et al., 1999). Soil texture ranges between sandy loams and loamy sands with a thin (10–20 mm) organic-rich horizon at the surface, beneath the litter layer (Tomkins et al., 2004). All surface soils exhibit a background water repellency such that they are repellent both pre- and post-wildfire except where heating during burning has exceeded the threshold temperature for repellency destruction (N300 °C; Doerr et al., 2004) in the surface layer. Development of a surface wettable layer has been implicated as a key controlling factor

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of post-fire runoff and sediment generation whereby if there is sufficient rain to saturate it, extensive ‘rafting’ of the scorched surface soil by overland flow can occur (Doerr et al., 2006). Recent research has also illustrated the importance of faunal activity in disturbing the soil surface, particularly the role of ants in bringing subsurface mineral soil to the surface following wildfire in ridgetop and footslope zones through creation of subsurface tunnel networks (Shakesby et al., 2006, 2007). The Blue Gum Creek area was burnt extensively in 1968 and 1994 and later during major wildfires that affected ca. 250,000 ha of forested land west of Sydney in late December 2001 and early January 2002. During the latter fires, the study catchment experienced moderate to high burn severities defined by Chafer et al. (2004, page 230) respectively as “all ground and shrub vegetation consumed by fire” and “all ground and shrub vegetation consumed by fire and lower tree canopy scorched” using satellite data validated by ground observations, reflecting estimated fire intensities of 500–7000 kW m− 1 (Byram, 1959) (i.e. mid-range for the area affected by the Nattai wildfire). The 2001/2002 fires were followed by intermittent rainstorms from late January to April 2002 (prior to the commencement of this study in May 2002) with a rainfall maximum of 63 mm in one event during early February and up to 13 days when rainfall exceeded 10 mm. These events triggered notable overland flow events delivering large quantities of eroded ash, charcoal, organic matter and mineral matter to the stream channels prompting a comprehensive research programme into the hydrogeomorphological consequences of wildfire (Humphreys et al., 2003; Shakesby et al., 2003; Doerr et al., 2004; Wallbrink et al., 2005; Blake et al., 2006a,b; Doerr et al., 2006; Shakesby et al., 2006; Blake et al., 2007; Shakesby et al., 2007; Tomkins et al., 2007, 2008). 2.2. Fallout radionuclide behaviour in temperate eucalypt forest Previous studies of fallout radionuclides in southeast Australia (Wallbrink and Murray, 1996; Wallbrink et al., 1997) have illustrated that up to 30% and 95% respectively of total inventories (activity per

Fig. 2. Stream network and study catchment.

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Fig. 3. Typical depth profiles for

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210

Pbxs, 137Cs and 7Be in eucalypt forest soils (note different vertical scales) after Wallbrink and Murray (1996) and Wallbrink et al. (1997).

unit area) of 210Pbxs and 7Be are stored in surface leaf litter in southeast Australian eucalypt forest, reflecting the continuous delivery of these natural fallout radionuclides to the forest floor. The lower proportion of longer-lived 210Pbxs in surface litter of eucalypt forests reflects the transfer of this fallout radionuclide from litter to the soil profile beneath via organic matter decay and bioturbation processes. Owing to its short half-life and the time lag in physical migration, negligible 7Be is transferred below the litter layer. In contrast, very low levels of 137Cs are measured in modern leaf litter. This largely reflects the production and delivery dynamics of 137Cs (i.e. non-continuous delivery). Bomb-derived 137Cs reaching forest floor litter during the 1950s and 1960s would have been transferred into the soil profile through bioturbation of decayed litter over the 50 years since atmospheric weapons testing ceased. The concentration of 137Cs in leaf litter would have been diluted through time with litter replenishment by new vegetation growth. These processes lead to distinctive depth-profile distributions for 210Pbxs, 137Cs and 7Be in undisturbed eucalypt forest environments (Fig. 3) which have been

shown to be typical of eucalypt forest soils in southeast Australia (Wallbrink and Murray, 1996; Wallbrink et al., 1997, 2002). 3. Materials and methods 3.1. Overview Assessment of soil redistribution between defined geomorphic landscape units of the Blue Gum Creek tributary subcatchment was undertaken using a tracer-based (210Pbxs, 137Cs and 7Be) sediment budget approach (cf Appleby et al., 2003). This approach is based on a comparison between the total activity of each tracer within constituent landscape units (Wallbrink et al., 2002). Areal activity of an undisturbed reference area is used to represent the radionuclide inventory of the whole study catchment prior to soil redistribution. Comparison of post-erosion with pre-erosion activities allows identification of soil loss or gain within each landscape unit. If there are no net losses of material from the study catchment, the sum of tracer activities in the

Fig. 4. Depth profiles for (a) 210Pb (where dotted line is ‘total’ and solid line is ‘excess’) and (b) 137Cs at the reference site. Note different vertical scales. Error bars are uncertainty due to gamma counting statistics.

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Fig. 5. Modelled growth of 7Be inventory and associated rainfall during the period leading up to wildfire and rainstorm events (dashed line represents a 5 half-life inventory build up period). See text for further details.

units should equal the calculated initial value. Any system losses are evident as tracer deficit in the total budget. Tracer values in each land unit show the location of losses and/or gains within the system which are convertible to soil losses and gains using established inventory conversion models (Walling and He, 1999; Walling et al., 2007). The different temporal resolutions of the three radionuclides must be taken into account. Redistribution of 7Be with a half-life of 53.3 days is hypothesised to represent sediment redistribution soon after the wildfire (i.e. approximately 3 half-lives). Redistribution of 137 Cs reflects sediment redistribution over the past 50 years (i.e. since weapons testing began), and for 210Pbxs we use a period of 60 years representing approximately 3 half-lives. Hence, in this study, we take sediment budgets derived from 137Cs and 210Pbxs to represent the integration of all sediment redistribution (both fire-induced and background) that has taken place over these timeframes. 3.2. Defining constituent landscape slope units During May 2002, the study catchment was divided into 5 landscape units based on observations by Shakesby et al. (2003) and additional field observations specific to the study catchment: (1) ridgetop (slopes b11°), (2) sideslopes (N11°), (3) a long-established depositional fan, (4) a footslope and riparian zone, and (5) a contemporary outwash fan. The surface areas of the major landscape units were calculated using 1:25,000 topographic maps and aerial photography. The long-established fan and contemporary outwash fan were surveyed on the ground.

Table 1a Distribution of

3.3. Determining

210

Pbxs, 137Cs and 7Be reference inventories

Reference zones, experiencing neither apparent net loss to nor gain of material from the surrounding landscape, were selected along a level plateau area (Buxton Plateau, Fig. 2), i.e. with no upslope contributing area, to quantify the pre-erosion reference inventory of the study catchment. The plateau area had been burnt at the same severity as the other landscape units. The burnt soil surface was observed to be intact, although there was potential for minor localised redistribution of ash from combusted leaf litter within the site by wind action and rainsplash. In this respect, the sampling strategy was designed so that the degree of spatial variability within the site arising from such processes could be quantified. It is also acknowledged that there is an inevitable degree of internal variability within any reference site and indeed a forested reference site in particular (Wallbrink and Murray, 1996) will exhibit enhanced variability between individual cores due to interception of fallout by tree canopies. Hence, 20 individual section cores (0.008 m2) were collected at 500 m intervals along the length of Buxton Plateau using a hand auger. All cores were sectioned at depth increments of 0–20 mm, 20–50 mm, 50–100 mm and 50–300 mm in order to compare the radionuclide depth profiles in this study site with those shown in Fig. 3. Since water repellency measurements suggest surface soil temperatures during burning of ~300 °C (Doerr et al., 2004), it is conceivable that some vaporisation of 137Cs (i.e. inventory loss additional to soil erosion) could have occurred. This, however, is not considered to be a major concern since (i) the reference site was burnt

210

Pbxs between the different landscape units showing changes following wildfire and an erosion event

Landscape unit

Area (m2)

% of total catchment area

210

Pbxs inventory (Bq m− 2)

CV

Total activity (MBq)

Total reference activity (MBq)

Difference (MBq)

% Loss/gain of total inventory

Ridgetop Sideslopes (N11°) Long-established fan Footslope and riparian zone Contemporary outwash fan

172,102 679,371 13,934

19.3 76.1 1.6

1482 ± 38 1309 ± 45 1915 ± 113

0.17 0.26 0.14

255 ± 7 889 ± 30 27 ± 2

331 ± 9 1307 ± 45 27 ± 2

−76 ± 11 − 417 ± 54 −0.1 ± 2.2

−4 + 6 −24 + 15 No change

19,967

2.2

1588 ± 163

0.24

32 ± 3

38 ± 4

−7 ± 5

−0.4 + 3

7063

0.8

3565 ± 105

0.11

25 ± 1

14 ± 0

12 ± 1

0.7 + 1

110 Table 1b Distribution of

W.H. Blake et al. / Geomorphology 104 (2009) 105–116

137

Cs between the different landscape units showing changes following wildfire and an erosion event

Landscape unit

Area (m2)

% of total catchment area

137

Cs inventory (Bq m− 2)

CV

Total activity (MBq)

Total reference activity (MBq)

Difference (MBq)

% Loss/gain of total inventory

Ridgetop Sideslopes (N11°) Long-established fan Footslope and riparian zone Contemporary outwash fan

172,102 679,371 13,934 19,967 7063

19.3 76.1 1.6 2.2 0.8

398 ± 10 491 ± 9 444 ± 13 367 ± 17 988 ± 11

0.19 0.24 0.05 0.02 0.10

69 ± 2 334 ± 6 6±0 7±0 7±0

85 ± 2 335 ± 6 7±0 10 ± 0 3±0

−16 ± 3 − 1.6 ± 8.4 −0.7 ± 0.3 −2.5 ± 0.6 3.5 ± 0.1

−4 ± 1 −0.4 ± 8 −0.2 ± 1 −0.5 ± 3 0.8 ± 1

to the same extent as the landscape unit sites, and (ii) the bulk of the 137 Cs inventory is stored beneath the surface and hence insulated from the highest temperatures (see Section 2.2 and Figs. 3 and 4). Soil samples were dried at 40 °C and weighed. They were ashed at 450 °C and reweighed to determine mass loss on ignition (LOI) then ground in a rock mill to a fine powder. As these are acid soils with negligible carbonate content, the LOI approximates to organic material. The powder was cast in a polyester resin matrix for analysis by gamma spectrometry at CSIRO Land and Water following methods described by Murray et al. (1987) and Wallbrink et al. (2002). Independent checks on detector calibrations were undertaken via International Atomic Energy Authority (IAEA) interlaboratory comparisons. 3.4. Determining

210

Pbxs, 137Cs and 7Be inventories for landscape units

The approach for characterising the spatially-averaged radionuclide activity of each landscape unit followed that of Wallbrink et al. (2002), which was designed to address problems with smallscale spatial heterogeneity in radionuclide deposition beneath woodland and forest canopies. A total of 45 soil core samples (0.008 m2) were collected from each landscape unit using a hand auger at 2 m intervals along 3 representative 30 m transects. Each core was sectioned into three subsamples at depths 0–20 mm, 20–50 mm and 50–300 mm. Samples were weighed in the field and bulked together in sets of 15 for each transect, thoroughly mixed in the field and subsampled. This approach allowed many cores to be taken from each location, improving confidence that the distributions of 210Pbxs, 137 Cs and 7Be had been adequately sampled without unduly increasing analytical demand. A limitation with the approach is the lack of detailed information on intra-unit core variability, although this was not a concern for tracer budgeting purposes, which requires only the spatially-representative mean inventories of the landscape units with some quantification of internal variability. 3.5. Sampling and processing of material to determine particle size associations and the partitioning of 210Pbxs, 137Cs and 7Be between organic and mineral material Conversion of radionuclide inventories to mass depths (kg m− 2) of soil loss and gain also requires consideration of particle size selectivity and selective sorption of radionuclides by the finer fraction of the soil (Walling and He, 1999). Furthermore, in forested terrain, particular

consideration needs to be given to the partitioning of radionuclides between the organic and mineral material owing to surface litter layers intercepting a large proportion of fallout. To address these questions, a 10 kg bulk sample was collected from the contemporary outwash fan deposit within the study area to determine the particle size and partitioning relationships of the fallout radionuclides in remobilised material. The mixed sample was divided into 5 kg subsamples one of which was slaked in water, subjected to ultrasonication and mechanically agitated through sieves with apertures of 500, 250, 125 and 63 µm. The b63 µm fraction was further separated into 63–40, 40–20, 20–10 and b10 µm fractions by settling (Shakesby et al., 2003). Each sample was dried at 40 °C prior to preparation for gamma analysis as described earlier. The 5 kg second subsample was slaked in water and the organic component floated off. The latter was analysed separately and the remaining mineral component separated and sieved as described earlier. The mineral and organic components of the sample were difficult to disaggregate as noted by Blake et al. (2007), who demonstrated the formation of tightly-bound organic–mineral soil aggregates in burnt surface soil, and hence the separation of these components was not complete. 3.6. Converting radionuclide budgets into sediment and sedimentassociated nutrient budgets The spatially-averaged 210Pbxs and 137Cs inventories for each landscape unit were converted into soil losses/gains (kg m− 2) using the diffusion and migration conversion models described in detail by Walling et al. (2007). These models account for the time-dependent behaviour of radionuclide fallout input and its subsequent redistribution in the soil profile through site-specific parameters: diffusion and migration coefficients and relaxation depth factor (describing profile shape) and, for 137Cs only, annual fallout flux. Diffusion coefficient, downward migration rates and profile shape factors were approximated for eucalypt forest soil from the concentration depth profiles shown in Fig. 3 (corroborated by measured depth profiles shown in Fig. 4). To convert 7Be inventories into soil losses/gains, a profile distribution model was applied (Blake et al., 1999; Schuller et al., 2006). All conversion models were run using software provided by the University of Exeter, UK (Walling et al., 2007). Because of the significant proportion of the 7Be and 210Pbxs inventories associated with mineralised surface litter, profile shape factors (h0) were adjusted to reflect the concentration effect litter combustion has on surface

Table 1c Distribution of 7Be between the different landscape units showing changes following wildfire and an erosion event Landscape unit

Area (m2)

% of total catchment area

7 Be inventory (Bq m− 2)

CV

Total activity (MBq)

Total reference activity (MBq)

Difference (MBq)

% Loss/gain of total inventory

Ridgetop Sideslopes (N11°) Long-established fan Footslope and riparian zone Contemporary outwash fan

172,102 679,371 13,934 19,967

19.3 76.1 1.6 2.2

128 ± 39 175 ± 34 171 ± 48 400 ± 32

0.37 0.25 0.21 0.54

22 ± 7 119 ± 23 2±1 8±1

46 ± 14 180 ± 35 4±1 5±1

− 24 ± 16 −61 ± 42 −1 ± 1 3±1

−10 ± 2 −26 ± 5 −0.6 ± 1 1.1 ± 3

7063

0.8

324 ± 13

0.30

2±0

2±0

0.4 ± 0

0.2 ± 4

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Table 2 Affinity of 210Pbxs, 137Cs and 7Be for different size classes and organic material in material deposited in the contemporary outwash fan Sample

Size fractions (μm)

N 2000 2000–500 250–500 125–250 63–125 40–63 20–40 10–20 b 10 Total ‘Organic’ matter N 2000 floated off from bulk 2000–500 250–500 125–250 63–125 40–63 20–40 10–20 b 10 Total Bulk sediment

210

210 Pbxs Pbxs total (Bq kg− 1) activity (Bq)

Dry weight (g)

Organic weight (g)

LOI (%)

676 3706 2330 803 252 175 133 86 212 8373 373 331 24 19 14 25 27 22 32 868

350 278 123 82 58 46 43 32 69 1082 312 225 16 11 8 8 10 8 10 609

52 8 5 10 23 26 32 38 32

228 ± 3 41 ± 1 34 ± 2 88 ± 2 197 ± 3 340 ± 4 408 ± 4 554 ± 9 440 ± 4

84 68 64 56 53 33 37 38 32

1033 ± 14 1023 ± 10 1022 ± 15 775 ± 12 773 ± 15 429 ± 10 497 ± 8 612 ± 9 548 ± 10

154 ± 2 152 ± 4 80 ± 4 71 ± 2 50 ± 1 60 ± 1 55 ± 1 48 ± 1 93 ± 1 763 ± 15 385 ± 5 338 ± 3 25 ± 1 15 ± 0.2 11 ± 0.2 11 ± 0.3 14 ± 0.2 14 ± 0.2 18 ± 0.3 830 ± 10

trace element concentrations (cf. Harden et al., 2004) i.e. the litter layer in Fig. 3 was collapsed to a mass depth representing the remnant ash veneer following surface burning. 3.7. Geochemical analysis The outwash fan bulk sample was also analysed for total phosphorus (TP) content (as % P2O5) by X-ray fluorescence using a Phillips PW 1480 wavelength dispersive XRF at CSIRO Land and Water following standard approaches (Norrish and Hutton, 1969; Norrish and Chappell, 1977). Percentage weight data were converted to parts per million elemental P (mg kg− 1). 4. Results 4.1. Radionuclide activities in the different landscape units The mean reference inventories for 210Pbxs, 137Cs and 7Be were calculated as 1923 ± 171, 493 ± 34 and 266 ± 33 Bq m− 2, respectively, where the error value represents the standard error of the mean from the 20 individual reference cores. Spatial variability within the reference site is low considering the potential for internal heterogeneity discussed in Section 3.3 (see also Smith et al., 1997). The inventory depth profiles averaged across the 20 sectioned cores (Fig. 4) show similar shapes (bearing in mind the coarser resolution) to those from the literature (Fig. 3). The measured 137Cs inventory is not dissimilar with the approximation of 600 Bq m− 2 (decay-corrected to the sampling date) generated by the fallout deposition model described by Walling et al. (2007), based on Sarmiento and Gwinn (1986) and Agudo (1998). The measured 210Pbxs inventory also compares favourably with an approximation of 1400 Bq m− 2 based on estimated annual flux data for eastern Australia (Pfitzner et al., 2004). The measured 7Be inventory is almost identical to the modelled one of 264 Bq m− 2 derived using the rainfall-inventory algorithm developed for southeast Australia by Wallbrink and Murray (1994). The excellent fit between the measured and modelled inventories permitted rainfall data to be used to illustrate the growth of the 7Be inventory for 10 half-lives prior to soil sampling (Fig. 5). The pattern of inventory development has important implications for the interpretation of 7Be-derived estimates of sediment yield. Table 1a shows the measured 210Pbxs inventories for each landscape unit of the study area. Tables 1b and 1c show the same

210

Pbxs (%)

137

137 Cs Cs total 137Cs (Bq kg− 1) activity (%) (Bq)

20 ± 0.2 20 ± 0.5 11 ± 0.5 9.3 ± 0.2 6.5 ± 0.1 7.8 ± 0.1 7.1 ± 0.1 6.3 ± 0.1 12 ± 0.1

9 ± 0.3 3 ± 0.1 3 ± 0.2 7 ± 0.2 17 ± 0.3 37 ± 0.5 40 ± 0.5 50 ± 1.0 54 ± 0.6

46 ± 1 41 ± 0.4 3.0 ± 0 1.8 ± 0 1.3 ± 0 1.3 ± 0 1.6 ± 0 1.6 ± 0 2.1 ± 0

37 ± 0.9 50 ± 0.8 50 ± 1 42 ± 1 48 ± 1 39 ± 1 44 ± 1 55 ± 1 62 ± 1

6.0 ± 0.2 9.7 ± 0.3 6.4 ± 0.4 5.9 ± 0.2 4.3 ± 0.1 6.4 ± 0.1 5.3 ± 0.1 4.3 ± 0.1 12 ± 0.1 59.7 ± 1.5 14 ± 0.3 17 ± 0.3 1.2 ± 0.0 0.8 ± 0.0 0.7 ± 0.0 1.0 ± 0 1.2 ± 0 1.2 ± 0 2.0 ± 0 39 ± 0.6

7 7 Be Be total (Bq kg− 1) activity (Bq)

10 ± 0.3 44 ± 3 16 ± 0.6 6±1 11 ± 0.6 6±1 9.8 ± 0.3 14 ± 1 7.3 ± 0.1 32 ± 2 11 ± 0.1 66 ± 3 8.8 ± 0.1 83 ± 4 7.2 ± 0.1 125 ± 7 20 ± 0.2 107 ± 5 36 ± 1 43 ± 1 3.1 ± 0.1 2.1 ± 0.1 1.8 ± 0.1 2.5 ± 0.1 3.1 ± 0.1 3.2 ± 0.1 5.1 ± 0.1

145 ± 22 119 ± 11 155 ± 10 145 ± 15 155 ± 18 77 ± 19 101 ± 12 89 ± 14 111 ± 18

30 ± 2 22 ± 3 13 ± 3 11 ± 1 8±1 12 ± 0.5 11 ± 0.5 11 ± 1 23 ± 1 139 ± 12 54 ± 8 39 ± 4 3.8 ± 0.2 2.8 ± 0.3 2.2 ± 0.3 1.9 ± 0.5 2.8 ± 0.3 2.0 ± 0.3 3.5 ± 0.6 112 ± 14

7

Be (%)

21 ± 2 15 ± 2 10 ± 2 8±1 6 ± 0.4 8 ± 0.4 8 ± 0.4 8 ± 0.5 16 ± 1 48 ± 7 35 ± 3 3.4 ± 0.2 2.5 ± 0.3 2.0 ± 0.2 1.7 ± 0.4 2.5 ± 0.3 1.8 ± 0.3 3.2 ± 0.5

information for 137Cs and 7Be respectively. The coefficient of variation (CV) is also represented in Tables 1a–c to provide an indication of intra-unit spatial variability. With respect to 210Pbxs, all landscape units show inventories lower than the reference one except for the contemporary outwash fan which has the highest inventory of 3565 ± 105 Bq m− 2. Greatest spatial variability is observed within the sideslope and riparian zones, and least variability within the fan deposits. All 137Cs inventories except for the outwash fan are lower than the reference. Greatest variability occurs in the ridgetop and sideslope areas, with low spatial variability in the fan and riparian zones. For 7Be inventories, the footslope/ riparian region also show gains in inventory values although all 7Be inventories are more spatially-variable. The ridgetop unit shows losses for all three radionuclides with the export of 4, 4 and 10% of the total inventories of 210Pbxs, 137Cs and 7Be, respectively. The sideslopes show similar losses of 210Pbxs (24%) and 7 Be (26%) contrasting with a minor loss of only 0.4% of the total 137Cs inventory. The amounts within each landscape unit can also be summed and then compared to the reference amount to derive an overall system loss. All radionuclide budgets showed a net loss of activity from the study catchment when compared with the reference, although losses of 137Cs were notably lower: 210Pbxs loss = 28.5 ± 4%, 137 Cs = 4 ± 1% and 7Be = 35 ± 6%. 4.2. Particle size separation and mineral-organic fractionation experiment The data from the particle size separation experiment are given in Table 2. The incomplete separation of the sediment into mineral and organic components affected particle separation into distinct size

Table 3 Models and shape factors used to convert inventories into erosion and deposition rates Radionuclide Conversion model

210

Pbxs

137

Cs

7

Be

a

Diffusion and migration model Diffusion and migration model Profile distribution model

See text for further details.

Migration Relaxation depth Diffusion coefficient (D) coefficient (V) (h0) (kg m− 2) (kg2 m− 4 yr− 1) (kg m− 2 yr− 1) 7.9 14.3 a

0.2

1

0.02

44

1.9

–

–

112

W.H. Blake et al. / Geomorphology 104 (2009) 105–116

Fig. 6. Schematic annual average sediment budgets derived from the three tracer budgets. Thickness of arrows is approximately proportional to total sediment mass.

relatively high concentrations of 210Pbxs, and 7Be consistent with their attachment to surface litter, noting the high % LOI in this fraction (52%). Regression analysis of radionuclide concentration and LOI% indicates positive significant (p b 0.05) relationships for 210Pbxs (r2 = 0.50) and 7Be (r2 = 0.45) but no significant relationship for 137Cs, indicating the association of the former two radionuclides with surface organics (e.g. litter and O-horizon).

classes by settling. Within the context of this limitation, Table 2 indicates the partitioning of 210Pbxs, 137Cs and 7Be between (i) different particle size classes and (ii) the mineral and organic fraction of the sediment. With regard to the latter, LOI data for the floated ‘organic’ component show that significant proportions of mineral matter are associated with the extracted organics, presumably reflecting organic–mineral aggregation processes as discussed elsewhere (Shakesby et al., 2003; Blake et al., 2007). The coarsest fraction (N2 mm) has the greatest LOI value at 84% which decreases with decreasing particle size to 32% in the b10 μm fraction. The b10 and 10–20 μm fractions of both the ‘organic’ and ‘bulk’ material components have the same LOI values (32% and 38%, respectively) illustrating the close association of burnt organic and mineral material and their similar fluvial sediment behaviour. The preservation of organic–mineral bonds after rigorous sample treatment in the laboratory (i.e. ultra sonication and wet sieving) illustrates the robust nature of organic– mineral association in the fine fraction of the burnt surface material. The radionuclide concentrations of the coarse ‘organic’ fraction are relatively high at 1033 ± 14, 37 ± 0.9 and 145 ± 22 Bq kg− 1 for 210Pbxs, 137 Cs and 7Be, respectively, but the relationship between each radionuclide concentration and particle size differs. Excess 210Pb concentrations decline with decreasing particle size reflecting the decrease in LOI% (r2 = 0.89, p b 0.05). Contrasting behaviour is seen with 137Cs where the relationship carries an r2 value of −0.15 and is not significant (p = 0.3). Beryllium-7 shows a similar relationship to 210 Pb with a positive, statistically significant though weaker, correlation between LOI% and concentration (r2 = 0.51, p b 0.05). The relationship between particle size class and radionuclide concentration within the b2 mm component of the bulk sample largely conforms to those reported by others (He and Owens, 1995; Wallbrink et al., 1999) with an increase in concentration with decreasing particle size. However, both 210Pbxs, and 7Be show a slight decrease in the b10 μm class whereas 137Cs shows a progressive increase, in line with its reported affinity for clay minerals. The coarse sediment fraction (N2 mm) shows

The parameters used for each inventory conversion model are given in Table 3. It is noteworthy that the value for the relaxation depth (h0) for 7Be is somewhat lower than that reported for other environments (e.g. Blake et al., 1999 report 5.4 kg m− 2 for an arable field). This is due to the retention of 7Be by surface leaf litter, which is subsequently converted to a thin surface ash layer by burning, giving a very shallow depth profile. Particle size enrichment factors (defined as enrichment of the radionuclide concentration of the b63 μm sediment fraction relative to the bulk concentration) of 1.6, 2.4 and 1.8 were applied for 210Pbxs, 137Cs and 7Be, respectively. The estimated sediment budgets for the three radioisotopes are shown in Tables 4a–c with summary information provided in Table 5. Schematic diagrams of sediment losses, gains and output are presented in Fig. 6 (as annual average figures for the longer-lived radioisotope budgets). The 7Be budget reflects material redistribution during the ca. 3 months since fire and prior to sample collection, with the 2001/2002 wildfire event and subsequent rainstorms being the major disturbances. The sediment yield for this post-fire period is estimated as 58 ± 25 t km− 2 assuming that each major rainfall event contributed to material redistribution (see discussion in Section 5.2 linked to Fig. 5). The sideslopes are the primary source of material (38 ± 20 t for the event) with the ridgetop being a secondary contributor (17 ± 9 t for the event). The footslope/riparian and contemporary fan zones both show net accumulation of material giving an overall sediment delivery ratio of 99%.

Table 4a Sediment budget derived from 210Pbxs measurements showing material losses and gains for different landscape units of the study area, predominantly over the past 3 halflives (~ 60 years) of 210Pb (where errors represent propagated counting statistics only)

Table 4b Sediment budget derived from 137Cs measurements showing material losses and gains for different landscape units of the study area since weapons testing commenced in the 1950s (where errors represent propagated counting statistics only)

Landscape unit

% of total catchment area

Total material loss/gain (t)

Areal material loss (t ha− 1)

Landscape unit

% of total catchment area

Total material loss/gain (t)

Areal material loss (t ha− 1)

Ridgetop Sideslopes (b11°) Long-established fan Footslope and riparian zone Contemporary outwash fan

19.3 76.1 1.6 2.2 0.8

−688 ± 172 −3396 ± 680 −14 ± 14 −60 ± 40 182 ± 23

− 40 ± 10 − 50 ± 10 − 10 ± 10 − 30 ± 20 258 ± 32

Ridgetop Sideslopes (b 11°) Long-established fan Footslope and riparian zone Contemporary outwash fan

19.3 76.1 1.6 2.2 0.8

−826 ± 69 −272 ± 272 −33 ± 11 −120 ± 16 415 ± 9

−48 ± 4 −4 ± 4 − 24 ± 8 −60 ± 8 587 ± 12.8

4.3. Hillslope sediment budgets

W.H. Blake et al. / Geomorphology 104 (2009) 105–116

113

Table 4c Sediment budget derived from 7Be measurements showing material losses and gains for different landscape units of the study area following the 2001/2002 wildfire and subsequent rainfall (where errors represent propagated counting statistics only)

5. Discussion

Landscape unit

% of total catchment area

Total material loss/gain (t)

Areal material loss (t ha− 1)

Ridgetop Sideslopes (b11°) Long-established fan Footslope and riparian zone Contemporary outwash fan

19.3 76.1 1.6 2.2

− 17 ± 9 −38 ± 20 − 1 ± 0.6 4 ± 0.7

−1 ± 0.5 −0.6 ± 0.3 −0.6 ± 0.4 1.8 ± 0.4

0.8

0.6 ± 0.1

0.8 ± 0.2

All three tracers point to the ridgetop and sideslope units as the major sources of sediment delivered from the hillslope to the stream, as is also shown by tracer-based evaluation of sediment sources within this system using mineral magnetics (Blake et al., 2006b) and fallout radionuclides (Wilkinson et al., submitted for publication). This contrasts with recent observations of post-fire sediment dynamics within wetter (1978 mm yr− 1 rainfall) eucalypt forest in Victoria, Australia (Sheridan et al., 2007), some 500 km south of the study area. Here, the main source areas for delivered sediment are riparian zones where sediment is entrained by saturation overland flow that develops in riparian zones during major rainstorm events. This fundamental difference in runoff and sediment generation between drier temperate and wetter montane eucalypt forest may be linked to several features, including: (i) the lower infiltration capacity of the dry forest soil owing to perennial subsurface soil water repellency (Doerr et al., 2006); and (ii) other hillslope hydrological differences linked to climate, elevation and bedrock/soil types (quartz diorite and gneiss with pedal loams in the Victorian study area). In addition, bioturbation by ants causes spatial variability in infiltration capacity in the present study site (Shakesby et al., 2006). Evidence of soil faunal activity by ants is pervasive in the ridgetop zone where it may cause increases in localised infiltration. This activity is largely absent on sideslopes, although mammal and bird disturbance (e.g. lyrebirds and bandicoots) of the surface is more pronounced here. Ant faunal activity is, however, most pronounced in footslope zones where it provides an extensive macropore network of infiltration sinks. These have been implicated in reducing overland flow within this landscape unit (Shakesby et al., 2007) and go towards explaining the low soil redistribution rates recorded in the footslope zone, despite similar slope angle and water repellency status to the eroded ridgetop. This implies that material exported from the study area is most probably transferred from ridgetop and sideslope units via the established ephemeral gully network connecting the upper slopes to the riparian zone. The differences between the 7Be- and 210Pbxs-derived sediment budgets and the 137Cs-derived sediment budget, in terms of proportions of material redistributed from different units, offer insight into sediment mobilisation processes. The main control over these differences is the partitioning of the radionuclides between the organic-rich litter layer and O-horizon and the subsoil. The low levels of 137Cs in litter and deeper penetration of the depth profile would suggest that the 137Cs budget is likely to be more sensitive to sub Ohorizon mineral soil erosion rather than the surface-held 7Be and 210 Pbxs which reflects remobilisation of litter, ash and the burnt Ohorizon (Wallbrink et al., 2005), as illustrated by their relationship to TOC. Data from the 7Be and 210Pbxs budgets are hence more comparable, in terms of setting the contemporary wildfire (7Be) in

The 210Pbxs-based sediment budget integrates all soil redistribution over the past ~60 years (i.e. 3 half-lives of 210Pb during which ~70% of the present day inventory accumulated) and indicates that the greatest material losses within the system (3396±680 t) are from the sideslope landscape unit. Similar to the 7Be budget, the ridgetop is the second most important in terms of sediment production with a total loss of 688 ± 172 t. There was negligible net material loss from the long-established fan, minor loss from the footslope and riparian zone (60 ± 40 t) and some deposition in the contemporary fan zone (182 ± 23 t). The budget suggests a high overall sediment delivery ratio of 96%. The sediment yield for the 3 half-life period (~60 years) is 4468 ± 790 t km− 2, which equates to an annual average sediment yield of 75 ± 13 t km− 2 yr− 1. In contrast to evidence from 210Pbxs and 7Be, the 137Cs-based budget indicates that the greatest material losses within the system are from the ridgetop landscape unit (826 ± 69 t over the 50-year period since weapons testing commenced) with notably lower losses from the sideslopes (272 ± 272 t). Both the long-established fan and the footslope and riparian areas have more modest losses (33 ± 11 and 120 ± 16 t, respectively). With 415 ± 9 t of deposition in the contemporary outwash fan unit, the 137Cs-based sediment budget indicates a sediment delivery ratio of 67%, notably lower than for 210 Pbxs and 7Be, reflecting localised storage of coarser mineral material within the hillslope (Shakesby et al., 2006). The areal sediment yield for the 50 year period is 940 ± 316 t km− 2 which equates to an annual average sediment yield of 19 ± 6 t km− 2 yr− 1. This is of a similar order to the long-term sediment yield estimates of 12.1 ± 8 t km− 2 yr− 1 for the Nattai river basin derived by Tomkins et al. (2007). 4.4. Sediment-associated phosphorus budgets The affinity of TP for different particle size fractions in the contemporary outwash fan bulk sediment sample is given in Table 6. Progressive increases in TP concentration are seen with decreasing particle size as reported elsewhere (Owens and Walling, 2002). The bulk material has a TP concentration of 172 mg kg− 1 which, when compared to the concentration of the b63 μm fraction (851 mg kg− 1), indicates an enrichment factor of 4.95 in the silt and clay fraction. Applying the b63 μm TP concentration to the sediment yield data from the 7Be sediment budget (i.e. reflecting the impacts of the most recent wildfire and rainfall disturbance event) gives a sediment-associated TP yield of 0.49 ± 0.21 kg ha− 1 from the study catchment, i.e. a maximum estimated yield of 0.70 kg ha− 1.

Table 5 Sediment yield from each radionuclide budget (where errors represent propagated counting statistics only) 7

Sediment delivery ratio (%) Total sediment yield (t km− 2) Annual average sediment yield (t km− 2 yr− 1)

Be budget

99 58 ± 25 Na

137

210

67 940 ± 316 19 ± 6

96 4468 ± 790 75 ± 13

Cs budget

Pbxs budget

5.1. Sediment source, processes and pathways

Table 6 Affinity of TP for different size classes in the bulk sediment sample from the contemporary outwash fan Sample

Size fractions (μm)

Dry weight (g)

Organic weight (g)

LOI (%)

TP (mg kg− 1)

Bulk sediment

N2000 2000–500 250–500 125–250 63–125 40–63 20–40 10–20 b10

676 3706 2330 803 252 175 133 86 212

350 278 123 82 58 46 43 32 69

52 8 5 10 23 26 32 38 32

218 87 87 175 436 655 742 611 1178

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W.H. Blake et al. / Geomorphology 104 (2009) 105–116

an annual-average context (210Pbxs), whilst comparison with the 137Cs budget offers insight into erosion processes within each landscape unit and potential underestimation of material losses by the more surface-bound tracers. This was most evident in the sideslope redistribution data where the relatively high loss of 7Be and 210Pb from sideslopes but minimal loss of 137Cs suggests that only a thin veneer of ash and burnt litter has been removed from this unit with subsurface material remaining relatively undisturbed. This accords with the conceptual model for sediment mobilisation suggested by Doerr et al. (2006), which describes the removal of the thin wettable burnt surface soil layer from above the strongly water repellent mineral soil with low infiltration capacity during prolonged rainfall. Similar mechanisms have been invoked for burnt soil in the United States (Gabet, 2003). Hence, material exported from this unit largely comprises burnt litter, ash and associated surface mineral material. 5.2. The role of bioturbation in sediment supply and transport The major discrepancy between the three sets of tracers is the different estimations of soil loss from the sideslope landscape unit. The differences between the 210Pbxs and 137Cs-derived erosion rates for the ridgetop unit (Tables 4a and b) imply a preferential loss of 137 Cs-tagged material (i.e. subsurface material relative to 210Pbxs and 7 Be-tagged surface material). Direct comparison of the 137Cs and 210 Pbxs data would indicate a larger loss of subsurface material over the 50-year period (48 ± 4 t ha− 1) than of surface material over the 60year period (40 ± 10 t ha− 1). The obvious explanation for this anomaly is rilling causing erosion below the burnt surface layer and additional losses of 137Cs relative to 210Pbxs (and 7Be) (cf. Wallbrink and Murray, 1996) implying underestimation of erosion by 210Pbxs. However, we found little sign of rilling. Alternatively, ant bioturbation brings significant quantities of subsurface material to the surface, which is then available for subsequent entrainment and transport from the local area. Shakesby et al. (2007) found that ant turnover in the severely burnt ridgetop zones (145 g m− 2 yr− 1) is higher than on the upper and middle sideslopes (65 and 4 g m− 2 yr− 1, respectively). Thus removal of 137Cs-rich ant mound material could augment the reduction in 137Cs inventory relative to that of 7Be and 210Pbxs on ridgetops. The lack of ant bioturbation on the steep sideslopes is reflected in the very low 137Cs losses from here where sheetwash of the O-horizon is the dominant sediment transport process and bioturbation is restricted to surface scrapes by birds and mammals. Although subsoil ant turnover is greatest in the lower slope and footslope zones for severely burnt units (593 and 781 g m− 2 yr− 1, Shakesby et al., 2007), the greatly enhanced macropore networks, and hence low overland flow generation in these zones, led to relatively little export of material. The 137Cs-based estimated footslope losses are still twice those estimated by the 210Pbxs budget. This further supports ant mound bioturbation as a mechanism for enhanced 137Cs losses linked to the redistribution of ant mound material. Thus, the data suggest that 7Be reflects loss of the burnt O-horizon, ash and litter due to its short half-life and retention on organic materials, whereas the 137Cs-based sediment budget shows loss of the underlying mineral soil, largely as a result of ant mounding bringing erodible subsurface material to the surface. The 210Pbxs budget, given its intermediate depth penetration provides a more balanced indication of overall system losses, integrated over the past 60 years (i.e. three half-lives of 210Pb). 5.3. Erosion rates and catchment sediment yields following the 2001– 2002 wildfire The 7Be-derived sediment budget data (Tables 4a, b, c and 6; Fig. 6) quantify post-fire sediment redistribution resulting from the main rainfall events in early February and late March 2002. The temporal

pattern of 7Be inventory growth is paramount when interpreting the budget data since the influence of the earlier rainstorm event on the measured inventories will have decayed between erosion and sampling (Blake et al., 1999). Fig. 5 illustrates that prior to the early February wet period, 30% of the later reference inventory was in place compared with 74% following the second wet period. Therefore, if the second period of rainfall had caused no soil redistribution, the inventory changes subsequently measured in May would have led to underestimation of soil erosion since there would be overprinting of decayed 7Be redistribution patterns over the catchment. It is assumed, however, that potential for soil mobilisation would have increased following the first event (cf. Doerr et al., 2006). The method implicitly assumes that the processes driving erosion were largely consistent during this period as were the resulting patterns of soil redistribution. These assumptions accord with field observations and analysis of stream sediment that indirectly suggest continued mobilisation of hillslope material up to a year after the wildfire (Wilkinson et al., 2007). This emphasises the importance of considering the timing of 7 Be fallout (via rainfall), erosion initiation and subsequent soil sampling (Blake et al., 1999; Walling et al., 2000). Prosser (1990) estimated long-term annual erosion rates of 1 t km− 2 under unburnt Australian eucalypt forest. The sediment (organic and mineral material) yield for the study catchment (89 ha), as estimated using the 7Be budget for the 2001/2002 wildfire and subsequent rainfall events (58 ± 25 t km− 2), is well above this long-term annual rate. However, it is less than 296 t km− 2 reported by Lane et al. (2006) for the outlet of a small (136 ha) wet eucalypt-forested mountain catchment (although this estimate included bed material). Recent analysis of river gauge data to derive a catchment sediment yield for Lake Burragorang indicates average annual hillslope sediment yields of 40 t km− 2 yr− 1 for the Little River catchment area (Rustomji et al., 2008; Rustomji and Wilkinson, 2008). These suggest the 7Be rates for the post-fire ‘event’ scenario alone in the Blue Gum Creek subcatchment are close to the annual rate for this system. Rustomji and Wilkinson (2008) suggest that the relatively high annual average erosion rates within this system, compared to those reported by Prosser (1990), are due to the area's steep topography. The short-term hillslope sediment yield estimate derived from the 7 Be tracer budget is somewhat lower than those of 500–10,000 t km− 2 derived from point-scale erosion bridge data (Shakesby et al., 2006). This has been taken into consideration in the spatial-integration approach of the tracer budget technique. The discrepancy, however, may also relate to losses of subsoil which are not tagged by 7Be (i.e. bioturbated material as discussed above and localised wash around trees and boulders). Hence the 7Be budget represents a good estimate of burnt O-horizon and litter material but a conservative estimate of overall loss from these slopes. Comparison of the 7Be data with the annual average rate derived from 210Pbxs (75 ± 13 t km− 2 yr− 1) suggests that wildfire is an important disturbance agent leading to delivery of surface slope material to the stream network. Comparison with the 137Cs-derived annual average rates indicates that ant bioturbation is critical for sediment generation after fire (cf. Dragovich and Morris, 2002; Shakesby et al., 2007; Tomkins et al., 2007, 2008). 5.4. Catchment phosphorus yields From water quality and forest soil health perspectives, the post-fire hillslope particulate phosphorus yield (0.49±0.21 kg ha− 1) is highly relevant to catchment and forest management yet few comparable data exist. Young et al. (1996) reported a typical TP yield for small (10–1000 ha) forested southeast Australian catchments of 0.06 kg ha− 1 yr− 1, which is less than one tenth the upper estimated limit over a 3 month period from the present study. Young et al. (1996) also reported TP yields for improved pasture (0.3 kg ha− 1 yr− 1), unimproved pasture (0.07 kg ha− 1 yr− 1) and urban catchments (1.0 kg ha− 1 yr− 1) suggesting that the study catchment

W.H. Blake et al. / Geomorphology 104 (2009) 105–116

behaves more like an urban environment than a pristine forest. The postfire TP yield reported here is also considerably higher than that reported by Townsend and Douglas (2000). They observed no significant difference between phosphorus yields from experimentally-burnt (0.03 kg ha− 1 yr− 1) and unburnt (0.04 kg ha− 1 yr− 1) mixed eucalypt forest catchments in a tropical savannah (northern Australia), despite significant increases in suspended sediment load. The low TP yields in this case were attributed to generally low fertility of the soil and could also conceivably relate to contrasting sediment sources and overland flow processes linked to the relatively low-intensity fire (Townsend and Douglas, 2000). The post-fire TP yield for the Blue Gum Creek tributary catchment, however, is closer to annual yields reported for scrub (0.8 kg ha− 1 yr− 1) and grassland (0.9 kg ha− 1 yr− 1) affected by fire (Gabet et al., 2005). Blake et al. (2008) report similar hillslope yields for the burnt Evrotas river basin following the Greek 2007 wildfires. It is also noteworthy that the post-fire TP yield is comparable to those reported for intensively farmed arable land, i.e. 0.8 kg ha− 1 yr− 1 (Foster et al., 2003) and 1.4 kg ha− 1 yr− 1 (Walling et al., 1997). This is significant in view of the pristine nature of the study water supply catchments. Wildfire is an important control on phosphorus inputs to the stream network in this temperate, eucalypt catchment and the pulsed nature of TP release following wildfire is an important consideration for water quality management. 6. Conclusions Tracer budgets using the fallout radionuclides 210Pbxs, 137Cs and 7Be have been used to construct sediment budgets for a small wildfireaffected eucalypt forest catchment near Sydney, Australia. Comparison of the budgets and data describing partitioning of radionuclides within the soil indicates that fire and bioturbation exert important controls on radionuclide concentration depth profiles and the construction and interpretation of sediment budget estimates. Partitioning of radionuclides between organic matter and mineral soil requires careful attention in forested terrain, especially following wildfire. However, the combination of tracers and their contrasting spatial and temporal distribution has been shown to provide useful information on sediment generation processes and sources within the landscape. Our dataset suggests that the dominant post-fire sediment sources in the study catchment were ridgetop and steep sideslope units, where a thin wettable surface layer of ash, burnt litter and soil was transported by surface wash over the perennially water repellent subsurface mineral soil during sufficiently prolonged rainfall. Further, in the ridgetop zone, ant mounding provided subsurface mineral soil for subsequent entrainment and transport. Surface erosion may have been enhanced by bird and mammal scrapes. Footslope and riparian zones were not significant sediment sources because of extensive ant bioturbation providing a large macropore network which acted as a sink for overland flow. This contrasts with recent observations by other workers in wet eucalypt forest where saturation overland flow in the riparian zone is the main process for sediment generation. Sediment delivery ratios of 96–99% from 7Be and 210Pbxs indicate that the bulk of eroded organicrich material reached the stream network. The 137Cs-derived sediment delivery ratio of 67% suggests that a greater proportion of denser eroded mineral subsoil is retained on-slope and in the valley floor. The results confirm the important role of bioturbation in sediment erosion, transport and redistribution processes in this environment supported by direct observation and field measurements. The 2001 wildfire event led to temporally-discrete inputs of particulate-associated phosphorus into the low-order stream network with similar areal yields to those reported for urban and agricultural catchments. This pulsed nature of phosphorus inputs to the stream network has important implications for post-fire water quality management strategies. Furthermore, medium-term storage of nutrient-rich material within the channel network means that wildfire-

115

related water quality problems may persist for longer than currently acknowledged. Acknowledgements This research was made possible by funding from NERC (Urgency Grant NER/A/S/2002/00143 and Advanced Fellowship NER/J/S/ 200200662 to S.H.D.) and through the Sydney Catchment Authority (SCA) Collaborative Research Project Grants (#91001289 to P.J.W. and 2003/28 to G.S.H.). The late Geoff Humphreys was assisted at Macquarie University (MU) through various research grants. The following people are acknowledged for kind assistance in this work: SCA staff at the Warragamba Office, in particular Glen Capararo for logistical assistance, Chris Chafer and James Ray for accessing data; Danny Hunt and Chris Leslie of CSIRO Land &Water who helped with fieldwork and radionuclide analysis; Russell Field (MU) for assistance with fieldwork logistics; Suzannah Blake and Ben Harrington for assistance with sample collection and processing; and Jamie Quinn (UoP) for drawing the illustrations. Prof. Des Walling (Exeter) kindly provided software for conversion of inventory data. Gary Hancock and Peter Hairsine provided useful comments on early drafts and we are grateful to three anonymous reviewers and Andy Plater for thorough and constructive reviews that greatly improved the manuscript. References Agudo, E.G., 1998. Global distribution of 137Cs inputs for soil erosion and sedimentation studies. In: International Atomic Energy Agency (Ed.), Use of 137Cs in the Study of Soil Erosion and Sedimentation. IAEA-TECDOC-1028, Vienna, Austria, pp. 117–121. Appleby, P.G., Haworth, E.Y., Michel, H., Short, D.B., Laptev, G., Piliposian, G.T., 2003. The transport and mass balance of fallout radionuclides in Blelham Tarn, Cumbria (UK). Journal of Paleolimnology 29, 459–473. Belyaev, V., Wallbrink, P.J., Golosov, V., Sidorchuk, A.S., Murray, A.S., 2005. A comparison of methods for evaluating soil redistribution in the severely eroded Stavropol region, southern European Russia. 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