Published in Australian Geographer (2003) 34(2), 147-175
Fire severity, water repellency characteristics and hydrogeomorphological changes following the Christmas 2001 Sydney forest fires R.A. SHAKESBY1, C. J. CHAFER2, S.H.DOERR1, W.H.BLAKE1, P.WALLBRINK4, G.S. HUMPHREYS3 & B.A. HARRINGTON3 1 University of Wales Swansea, 2Sydney Catchment Authority, 3Macquarie University, 4CSIRO Land & Water. Correspondence: Dr Richard A. Shakesby, Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, United Kingdom; e-mail:
[email protected]. Tel. 00-44-1792 295236; fax. 00-44-1792 295955. ABSTRACT Soil water repellency can enhance overland flow and erosion and may be altered by fire. The Christmas 2001 bushfires near Sydney allowed investigation of the relationship between fire severity, water repellency and hydrogeomorphological changes. For two sub-catchments with differences in fire severities in Nattai National Park, south-west of Sydney, this paper considers: (1) the links between fire severity based on SPOT image analysis and ground observation of fire severity and repellency; (2) the textural and organic/minerogenic characteristics of eroded sediment; and (3) erodibility, erosion and deposition of soils in both catchments. Ground surveys show that image analysis reflects well the degree of vegetation consumption by fire, but cannot adequately predict the degree of ground litter consumption, associated soil heating and repellency effects. Fire had varying effects on repellency leaving it unchanged, destroying it or enhancing it depending on the soil temperature reached. The main post-fire hydrogeomorphological changes have been widespread erosion and colluvial and alluvial deposition of topsoil in footslope locations and river systems, but only localised redistribution of the highly erodible, repellent sandy subsurface layer. The fire did not trigger major geomorphological change in the study area, but fires probably cause important topsoil and nutrient depletion and may also affect water quality. KEY WORDS Eucalypt; Nattai; forest fire; fire severity; satellite image analysis; soil erosion; soil water repellency; soil hydrophobicity.
Introduction Most workers have concluded that forest fires increase overland flow and erosion substantially relative to unburnt forest land (e.g. Sartz 1953; Swanson 1981; Morris & Moses 1987; Scott & Van Wyk 1990; Shakesby et al. 1993; Andreu et al. 1996; Inbar et al. 1997; Robichaud & Brown 1999). This increase in geomorphic activity has frequently been associated with the development or intensification of water repellency in the soil during burning (e.g. DeBano 1981; Wells 1987). Hydrophobic organic compounds present in the soil and litter are volatilised and condense onto soil particles during the passage of a fire. Where soil temperatures exceed 300-400ºC, it has been suggested that water repellency can be destroyed, while for soil layers heated to below these temperatures, it has been suggested that repellency may be intensified (DeBano 1981). It has, however, been argued that the destruction of the vegetation cover and litter cover, and, where applicable, the exposure of a naturally water repellent topsoil, could be just as, if not more important than repellency changes (Shakesby et al. 2000). Modest as well as large post-fire soil losses have been reported. On the one hand, Krammes & Osborn (1969) reported losses from erosion plots of as much as 346 t ha-1 (equivalent to about 35 mm of ground lowering) in the first year after prescribed fire on slopes with repellent chaparral soils in southern California. Shakesby et al. (2002) reported up to 18 mm of ground surface lowering on eucalypt-forested slopes in Portugal in the first year after wildfire. On the other hand, Booker et al. (1993) recorded less than 1 mm of ground lowering on plots in 290 days after fire on repellent soil in California, and Ferguson (1957) noted plot losses of only 0.086 t ha-1 (equivalent to about 0.009 mm of ground lowering 18 months after prescribed burning of loblolly pine (Pinus estimata) in east Texas. In Spain, Cerdà (1998) maintained that immediate post-fire overland flow and erosion rates were negligible due to the high infiltration rates of
the dry and ash-covered soils and Germanoski & Miller (1995) found that post-fire geomorphic instability in Nevada was spatially heterogeneous with substantial erosion confined largely to channels developed in pronounced concave hillslope sections compared with more stable, gently concave, straight or convex sections. In Australia, an accepted view is that soil erosion rates are considerably enhanced after fire (Boughton 1970; Good 1973), but the opportunities to investigate whether this is correct have been few (cf. Leitch et al. 1983; Atkinson 1984). According to Prosser & Williams (1998), the situation is rarely attained when severe fire, requiring drought conditions, is followed closely by conditions likely to lead to high post-fire erosion (high-magnitude storms). Following the 1994 fires around Sydney, these authors found that, for a moderately severe fire, burning produced localised redistribution of sediment but little if any sediment was delivered out of the catchment because of the low rainfall input and hence low runoff. For fires in the same year, Zierholz et al. (1995, p. 36) similarly concluded that “no intense degradation was observed on undisturbed areas” (i.e. areas burnt but unaffected by human impact). The Christmas 2001 bushfires in the Sydney region were characterised by extensive and in places severe fires brought about by hot and dry pre-fire conditions. They were followed shortly afterwards by some substantial rainstorms. These conditions provided an opportunity to examine the premise that intense degradation may result from the combination of widespread severe fire and substantial post-fire rainfall events. As part of a larger project considering the impact of bushfire severity on soil water repellency and its erosional consequences, this paper draws on immediate preand post-fire satellite image analysis and on ground measurements made during May 2002. The overall aims of the paper are to examine the links between fire severity and soil water repellency, to investigate the erosional consequences of post-fire storms in areas of differing fire severity (and therefore repellency) and to assess the hydrogeomorphological consequences of fire-induced erosion cycles. Specifically, the paper reports on: (1) the correspondence between fire severity as revealed by normalised difference vegetation index analysis (NDVI) of SPOT satellite images and by ground assessment of vegetation and litter consumption and soil water repellency; (2) the particle size characteristics and proportions of minerogenic and organic matter in different size fractions in post-fire colluvial and alluvial deposits; (3) detailed measurements of ground surface characteristics relating to erodibility, soil loss and soil redistribution in high and low fire severity catchments (e.g. percentage covers of rock and leaf litter, soil pedestal height, shear strength of the surface soil, litter dam depths and coverage); and (4) implications for the short- and longer-term fire-induced erosional impacts of fire. Study area Following reconnaissance in the area affected by the Christmas 2001 bushfires, the Blue Gum Creek catchment in Nattai National Park (c. 85 km south-west of Sydney; 150º29.5’E, 34º13.3’S) was chosen as the study area (Figure 1). It was chosen because fire severity varied and the area is protected from human-induced post-fire disturbance by jurisdiction from the Sydney Catchment Authority (SCA) and New South Wales National Parks and Wildlife Service (NPWS). Two small study catchments were selected on the western side of Blue Gum Creek (Figure 2): catchment H (63 ha in extent), characterised mainly by moderate to extreme fire severity according to remote sensing imagery analysis (see below) and catchment L (89 ha), characterised mainly by low to moderate fire severity. In addition, two ancillary study sites not burnt in the 2001 fires were selected in ridge-top positions 2 km southeast and 11 km north of catchment H. These sites were similar in soil and vegetation characteristics to the corresponding slope positions in catchments H and L and thus provided good analogues of pre-fire conditions. Blue Gum Creek is a tributary of the Little and Nattai Rivers (Figure 1; Plate 1). The underlying bedrock is Hawkesbury Sandstone in the uplifted south-west sector of the Permo-Triassic Sydney Basin. Stream incision has produced a series of canyons and gorges with watershed ridges and gently-sloping plateaux denoting more resistant sandstone beds. In the study reach, ridge tops typically lie close to c. 500 m a.s.l., while the valley floor extends to below 300 m. From a geomorphological point of view, valley-side slopes can be divided into four slope units. Gentle ridge-top slopes (2-10º); steep bedrock exposures and cliffs (70-90º) in the upper mid-slope section; rockfall- and talusstrewn slopes (15-35º) in the lower mid-slope section; and foot-slope zones (5-10º), which continue to the stream or, as in the study reach, merge with a distinct valley floor (Bishop et al. 1980). Figure 3 depicts schematically some of the post-fire micro-topographic forms developed on the valley-side slopes. On ridge tops and in foot-slope positions in particular but also in places elsewhere where slope angles are sufficiently low (10 mm up to the end of April 2002, with a maximum daily total of 63 mm recorded at The Causeway on February 5 during 7-11 consecutively wet days. These rainfall events triggered sizeable runoff events (Figure 5) recorded at the SCA gauging stations on the Little River downstream of Blue Gum Creek and at the Causeway on the higher order Nattai River, located upstream of the Little River confluence. During the period February 4-11, maximum daily discharges of 249.7 and 427.7 Ml day-1 were recorded for Little River and Nattai River respectively indicating a regional runoff response to rainfall. Substantial subsequent runoff events were recorded at both stations between early February and July, but the magnitudes of daily discharge peaks varied between the two rivers suggesting runoff responses to localized rainstorms. Comparison of the Nattai River discharge data with a flow duration curve (Figure 6) based on daily discharge records from October 1988 to June 2002 at the Causeway demonstrates that the magnitude of these events recorded at the higher order Nattai River was not extreme. It must be noted, however, that over this period only 15 % of daily discharge peaks were >1000 Ml and no daily discharge peaks were >5000 Ml during the summer months (December to February). Methods Fire severity assessments from remotely sensed images and field observations To understand the spatial variations in bushfire impact on the forest vegetation communities, a series of post-fire analyses was undertaken by the SCA using multi-temporal satellite imagery (Jakubauskas et al. 1990; White et al. 1996; Viedma et al. 1997) and field-based measurements. Two SPOT satellite images were obtained for the study region relating to the pre-fire (November 11 2001) and immediate post-fire (January 8 2002) periods. By the latter date, no vegetation regrowth or substantial rainfall had occurred. Climatic conditions were similar for both image dates, obviating the need for computational correction of the normalised difference vegetation index (NDVI) for the two images. The NDVI was calculated and a difference image of the two dates computed providing direct comparison of burnt and unburnt states, which allowed calculation of burn severity with minimal field checking. From these and preexisting data on the pre-fire fuel load (i.e. above-ground vegetation), the amount of fuel available and burnt in the study area and fire severities were estimated (Table 1). Field-based post-fire analyses consisted of sampling 325 GPS-located sites spread over a large part of the entire burnt area in the Sydney region and fire damage to the vegetation was assessed according to the descriptions in Table 1. The site differential NDVIs were extracted into a geographic information system (GIS) and comparisons made using the classification accuracy assessment of Congalton (1991), which indicated a classification accuracy of >87 %. A six-class fire severity scheme (unburnt, low, moderate, high, very high, extreme) was derived based on the energy rating in Table 1.
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For the Blue Gum Creek area, detailed analysis was carried out of, first, the relationship between fire severity based on the remotely-sensed images and slope position and aspect. Using a simple five-class slope classification (footslope, lower-mid-slope, mid-slope, upper-mid-slope and plateau/ridge) and a three-class aspect model (north-easterly (0135º), north-westerly (225-360º) and southerly (135-225º)) derived from the GIS, NDVI values were derived for 30 random points extracted from each of the 15 classes to analyse statistically the variations in fire severity in general, and in order to determine whether fire on north-westerly facing slopes was more severe than on southerly and north-easterly slopes (using a two-factor ANOVA test). Second, an assessment was made at each repellency site as to whether or not the pre-fire litter had been destroyed by fire. As far as possible, any post-fire litter was excluded from the analysis. Soil water repellency measurements Water repellency determinations for burnt soils were carried out at 227 sites in catchments H and L. On steep slopes, the ash layer and burnt soil had generally been eroded so that the post-fire soil surface could not be sampled. Only sites on low-angle slopes with an undisturbed ash layer and/or clear protection from erosion downslope of obstacles (e.g. logs and rock outcrops) (cf. Leitch et al. 1983) were selected on ridge tops and in foot-slope zones. Comparable sites for sampling unburnt ‘control’ soils were only found on ridge tops in the study area. At each site (c. 4-6 m2 in extent), repellency was measured using the Water Drop Penetration Time (WDPT) technique (Letey 1969; Doerr 1998) at three points at least 0.3 m apart on (1) burnt mineral soil below any loose ash, and (2) the underlying uppermost unburnt soil (denoted by a paler colour). At each point, 3 drops of water were placed onto both soil layers in turn. Where all drops infiltrated in < 5 s, the soil was classed as wettable (Letey 1969). Where all drop penetration times exceeded 5 s, the soil was classed as water repellent. At the few sites where the three drops did not respond in a similar fashion and where soils were excessively damp, the results were excluded from the analysis. Samples of both soil layers were taken. At unburnt sites, all soils were moist beneath the thick litter so that laboratory repellency testing was carried out on air-dried samples (see below). Heating experiments were undertaken on three long unburnt (> 10 years) and three burnt soil samples to examine whether the reported 300-400ºC temperature threshold, above which repellency is destroyed (DeBano 1981), applies to the study area soils. Air-dried samples (5 g) were placed in crucibles and subjected to set temperatures in a pre-heated furnace for 30 minutes and, after 24 hours conditioning at 20ºC and 45-55 % relative humidity, tested for water repellency (Doerr et al.2002). Ground surface changes To assess the micro-scale impacts of fire on erodibility and on soil erosion and deposition in catchments H and L, measurements were carried out within quadrats 40 x 40 cm in size at 1 m intervals along three parallel 5 m-long downslope-aligned transects in representative ridge top, upper mid-slope, lower-mid-slope and foot-slope positions. The following observations and measurements were made for each quadrat: (1) percentage of bedrock cover; (2) percentage area characterised by newly deposited sediment; (3) percentage area affected by faunal activity (ant and termite mounds, and mammal and lyrebird scrapes); (4) the heights of litter dams compared with the soil level on the downslope side; (5) the heights of soil pedestals capped by stones and roots, and the heights of exposed roots above the underlying soil surface; and (6) shear strength of the surface soil. The latter was measured using a pocket vane tester (or torvane) (Brunori et al. 1989; Zimbone et al. 1996). Although significantly correlated with soil moisture for comparatively cohesive soils, shear strength has an extremely weak, non-significant correlation for sandy soils. Soil moisture variations during the measurement period can thus be regarded as having a minimal effect on shear strength (Zimbone et al. 1996). Particle size distribution and organic content of deposited sediments Analysis of the particle size distribution and organic content was undertaken on post-fire eroded deposits of bulked samples of sediment and ash from two key locations: (1) a low-energy depositional fan on the alluvial valley floor opposite catchment H (sample DF) (Plate 5), and (2) a sediment drape (sample SD) near the confluence of the Nattai and Little Rivers (Plate 6). Sample SD was first placed in a sonic bath for 10 minutes prior to separation to allow dispersal of the sediment. Wet sieving of each sample was then carried out to separate it into the following size fractions: >2000, 500-2000, 250-500, 125-250 and 63-125 µm. For material
