Synergistic effects of water repellency and macropore ...

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Jun 28, 2010 - in north-east Victoria, Australia (see map in Sheridan et al., 2007). ...... We gratefully thank John Costenaro, Gabi Szegedy, Chris. Sherwin and ...
HYDROLOGICAL PROCESSES Hydrol. Process. 24, 2871– 2887 (2010) Published online 28 June 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/hyp.7701

AUTHORS COPY

Synergistic effects of water repellency and macropore flow on the hydraulic conductivity of a burned forest soil, south-east Australia Petter Nyman,* Gary Sheridan and Patrick N. J. Lane Department of Forest and Ecosystem Science, The University of Melbourne, Parkville, Victoria 3010, Australia

Abstract: Research shows that water repellency is a key hydraulic property that results in reduced infiltration rates in burned soils. However, more work is required in order to link the hydrological behaviour of water repellent soils to observed runoff responses at the plot and hillslope scale. This study used 5 M ethanol and water in disc infiltrometers to quantify the role of macropore flow and water repellency on spatial and temporal infiltration patterns in a burned soil at plot ( 0Ð5 mm), comprising just 5Ð5% of the soil volume, contributed to 70% and 95%, respectively, of the field-saturated and ponded hydraulic conductivity (Kp ). Because flow occurred almost entirely via macropores in non-repellent areas, this meant that less than 2Ð5% of the soil surface effectively contributed to infiltration. The hydraulic conductivity increased by a factor of up to 2Ð5 as the hydraulic head increased from 0 to 5 mm. Due to the synergistic effect of macropore flow and water repellency, the coefficient of variation (CV) in Kp was three times higher in the water-repellent soil (CV D 175%) than under the simulated non-repellent conditions (CV D 66%). The high spatial variability in Kp would act to reduce the effective infiltration rate during runoff generation at plot scale. Ponding, which tend to increase with increasing scale, activates flow through macropores and would raise the effective infiltration rates at larger scales. Field experiments designed to provide representative measurements of infiltration after fire in these systems must therefore consider both the inherent variability in hydraulic conductivity and the variability in infiltration caused by interactions between surface runoff and hydraulic conductivity. Copyright  2010 John Wiley & Sons, Ltd. KEY WORDS

hydraulic conductivity; water repellency; hydrophobicity; macropores; wildfire; spatial variability

Received 11 June 2009; Accepted 18 March 2010

INTRODUCTION Wildfire causes significant changes in the hydrological behaviour and erosion processes of forested catchments (Wondzell and King, 2003; Shakesby and Doerr, 2006). Elevated erosion rates and increased sediment yields have been documented in burnt hillslopes and catchments for 2–4 years following fire (Imeson et al., 1992; Scott, 1993; Inbar et al., 1998; Gabet, 2003; Lane et al., 2006b; Reneau et al., 2007; Sheridan et al., 2007; Pierson et al., 2008; Smith and Dragovich, 2008). Increased erosion after fire has been attributed to a number of factors relating to both erodibility of source material and the hydrological mechanisms by which material is mobilized and redistributed (Shakesby and Doerr, 2006). Due to the availability of large volumes of ash and loose soil in burned areas, erodibility is very high, and erosion rates are primarily transport limited. The effect of fire on hillslope hydrology is related to both infiltration and runoff processes. Infiltration is reduced through fire-induced water repellency (Doerr

* Correspondence to: Petter Nyman, Department of Forest and Ecosystem Science, The University of Melbourne, Parkville, Victoria 3010, Australia. E-mail: [email protected] Copyright  2010 John Wiley & Sons, Ltd.

et al., 2004, 2006), breakdown of soil structure (Giovannini and Lucchesi, 1983; Neary et al., 1999) and sealing of macropores by ash and fine sediment (Onda et al., 2008). At the same time low vegetation cover, lack of woody debris and hydraulic smoothing by eroded ash can result in low rainfall interception and high runoff velocity (Lavee et al., 1995). The net effect on hillslope hydrology is that flood peaks and surface erosion rates in burned areas are up to two orders of magnitude higher than in comparable unburned areas in the first year after fire (Prosser and Williams, 1998; Moody and Martin, 2001; Pierson et al., 2008). The time for recovery to pre-fire conditions depends primarily on the initial fire severity and the rate of vegetation recovery which is a function of the prevailing climate. Water repellency has been identified as the key hydraulic property resulting in reduced infiltration rates of burned soils at the plot scale (Prosser and Williams, 1998; Martin and Moody, 2001; Doerr et al., 2003; LeightonBoyce et al., 2007; Sheridan et al., 2007). Water-repellent soils are generated from soil heating when organic vapours disperse through the porous soil and condense as a hydrophobic coating on soil particles (DeBano, 2000). In south-east Australian forests, water-repellent soils are common in the absence of fire and have been

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found to oscillate with seasonal changes in climate (King, 1981; Burch et al., 1989; Crockford et al., 1991; Doerr et al., 2006). Sheridan et al. (2007) found that wildfire perturbed the natural oscillations and generated waterrepellent soils that persisted in the first winter following the fire during which unburned soils were found to be mostly non-repellent. While water repellency has been shown to reduce infiltration rates and produce infiltration excess runoff at the plot scale, the impact on hillslope and catchment processes depends on the spatial distribution of water repellency and the distribution of pathways for infiltration (Burch et al., 1989; Ritsema and Dekker, 1995; Ritsema and Dekker, 1996; Shakesby et al., 2000). There has been an exponential increase in research articles focusing on water repellency and its hydrological implications (Dekker et al., 2005). However, as a spatially and temporally distributed soil variable in hydrological models, water repellency remains largely unaccounted for. Water repellency affects the matric potential of porous soil by increasing the contact angle between the solid–liquid interface (Bauters et al., 2000). During infiltration this results in a non-uniform wetting behaviour where a large proportion of the porous soil is non-wettable or non-conducting (Dekker and Ritsema, 1995; Bauters et al., 1998; Dekker and Ritsema, 2000; Wang et al., 2000b; Moody et al., 2009). The wettable and non-wettable domains of the soil can be separated into hydrologically active and non-active regions (Liu et al., 2005). This pattern contrasts with the commonly observed relationship between soil moisture and matric potential. In water-repellent soils, the reduction in hydrologically active pores results in lower observed flow rates even though the soil structure and porosity remain unchanged. While the underlying flow potential of the soil is well represented by parameters of hydraulic conductivity and traditional matrix flow theory, the effective flow through the porous soil is reduced. The dominant role of macropore flow in forest soil has been used to explain the low impact of water repellency on runoff at the hillslope and catchment scales (Burch et al., 1989; Doerr et al., 2003; Sheridan et al., 2007). Infiltration through macropores is gravity driven and triggered by ponding and given unlimited supply, macropores have capacity for high velocity flow (Beven and Germann, 1982; Moore et al., 1986; Watson and Luxmoore, 1986, 1988; Luxmoore et al., 1990). Due to the high proportion of non-infiltrating area in water repellent soils, ponded infiltration and macropore flow is expected to occur more readily than in non-repellent soils. The low areal contribution to infiltration and the high relative contribution of macropore flow and preferential pathways can result in extreme variability in the spatial distribution of infiltration capacities. Erosion models that fail to account for runon–runoff processes and the discontinuous infiltration potential in these systems are unlikely to produce a valid output (Smith and Hebbert, 1979; Hawkins and Cundy, 1987; Wood et al., 1988; Woolhiser et al., 1996). Copyright  2010 John Wiley & Sons, Ltd.

Spatial variability in hydraulic conductivity due to water repellency and macropore flow may explain the high variability and apparent scale dependency of postfire runoff and erosion rates (Imeson et al., 1992; Scott, 1993; Kutiel et al., 1995; Cerd`a et al., 1998; BenavidesSolorio and MacDonald, 2001; Doerr et al., 2003; Sheridan et al., 2007; Woods et al., 2007). Representation of spatial variability and its effects is thus especially important in runoff and erosion models of burned areas (Kinner and Moody, 2010). While recent approaches provide a basis for representing infiltration, runoff and runon interactions on heterogeneous hillslopes (Morbidelli, 2006; Nahar, 2004; Jones, 2009), these must be complemented by empirical research that examines infiltration processes in different soil systems and quantifies patterns and variability in key parameters. While ethanol solutions have been used widely as a point measurement of strength and spatial distribution of water repellent soils (Letey et al., 1962; King, 1981; Doerr et al., 1998; Letey et al., 2000; Woods et al., 2007), only a small number of previous studies have demonstrated the use of ethanol solutions to examine infiltration processes at larger scales (Tillman et al., 1989; Feng et al., 2001; Hallett et al., 2004; Miyata et al., 2007). Miyata et al. (2007) measured effect of water repellency on runoff generation from 2 m2 plots using 5 M ethanol. Tillman et al. (1989) measured sorptivity using both 95% ethanol and water in a sorptivity tube and found that the method was successful at isolating the effects of water repellency on infiltration processes. Hallett et al. (2004) used pure ethanol and a miniaturized tension infiltrometer to measure the effect of water repellency on variability in soil sorptivity at the millimetre scale. This study used 5 M ethanol in tension and ponded infiltrometers in order to examine the effect of water repellency on spatial and temporal patterns of variability in hydraulic conductivity at the plot ( 0, flow is initiated in large structural features of the soils such as cracks and large macropores, and the maximum conducting pore size (rmax ) is equal to the maximum pore size of the soil (rn ). In combination with infiltration data that give Q at a range of hz , the model of flow in a distribution of pores given by Equation (4) allows for detailed examination of infiltration processes and the spatial contribution of different pore size classes to the hydraulic conductivity of the soil. Study site The study was conducted within a 9 m2 plot at an elevation of 1360 m.s.l. (36° 480 N, 147° 110 E) in a gently sloping section of the Slippery Rock Catchment in north-east Victoria, Australia (see map in Sheridan et al., 2007). The study site was burned at very high severity (100% crown removal) in a bushfire in January 2007 but was unaffected by a wildfire that burned large parts of the Slipper Rock catchment in 2003. The area consists of wet sclerophyll forest and is dominated by pure stands of Alpine Ash (Eucalyptus delegatiensis) with sparsely distributed Mountain Gum (Eucalyptus darlympeana). Summer precipitation in the study area occurs primarily as high-intensity convective storms. At elevations above 1100 m, winter precipitation sometimes falls as snow. The mean annual precipitation at Bogong Village (1939–2001), which lies 1Ð5 km from the study catchment at an elevation of 718 m.s.l. is 1805 mm, of which 70% falls in the months from May to October. A mean annual precipitation of 1836 mm was measured in the Slippery Rock Catchment over a 13-year monitoring period prior to the wildfire in 2007 (Lane et al., 2008). The mean daily maximum temperature is 17Ð4 ° C and the daily mean minimum temperature is 5Ð8 ° C. The underlying geology is gneiss and schist derived from regionally metamorphosed sedimentary rocks. The Hydrol. Process. 24, 2871– 2887 (2010)

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soil consists of a friable and highly structured clayloam A-horizon overlaying a deep (10 s and 2 M ethanol in 10 s for 2 M ethanol to absorb, the soil was classified as highly repellent. Gravimetric water content (GWC) was determined from soil samples collected at depths of 0–5 cm from sixteen 10 ð 10 cm2 quadrats located adjacent to the plot boundary. Infiltration rates were measured using tension and disc infiltrometers (disc radius D 5 cm) which were developed in accordance with the designs described by Perroux and White (1988). Infiltration measurements obtained in March were used in the analysis of spatial patterns of variability, whereas measurements in September were used primarily to detect temporal change and therefore had less spatial replication. In March, three sampling locations were randomly assigned to coordinates within each of nine adjacent 1 m2 blocks within the plot (Figure 1). The total sample size was 29 because two blocks were assigned with four sampling location instead of three. Despite an average slope of less than 10° , some areas of the plot were too steep to conduct measurements using the infiltrometers. As a result, some sampling locations within blocks had to be selected based on the practical considerations rather than through random spatial allocation. Large rocks (>5 cm) embedded in the soil further prevented unbiased representation of the plot. If the randomization resulted in sampling locations that were located in close proximity to one another, flow measurements were unlikely to be independent due to the lateral flow of water and 5 M ethanol in surface and subsurface soils. In this study, 25 cm was considered to be adequate minimum sample spacing. In September, 12 sampling locations were strategically dispersed across the plot in order to maximize spatial coverage while ensuring that measurements were conducted within areas that had not been disturbed during previous experimentation (Figure 1). Steady-state flow rates were measured at hydraulic heads (hz ) of 75, 55, 25, 5, 0 and 5 mm in an increasing sequence (75 < 55 < 25 mm, etc.). Hydrol. Process. 24, 2871– 2887 (2010)

AUTHORS COPY THE EFFECT OF MACROPORE FLOW AND WATER REPELLENCY ON INFILTRATION Paired measurements were conducted at each location using water first, then 5 M ethanol, as the infiltration fluid. The sampling locations were prepared by carefully driving a thin-walled steel cylinder into the soil to depth of 4 cm. Vaseline was applied to the inside of the cylinder in order to prevent preferential flow along the perimeter. The top 10–30 mm of ash was carefully removed from all sampling locations and the vegetation was trimmed level with the soil surface. Cheese cloth was placed on the soil surface followed by a 5-mm layer of damp contact sand (saturated hydraulic conductivity D 750 mm h1 ) which was compacted firmly to achieve good contact between the supply disc and the underlying soil. After obtaining a sequence of steady unsaturated flow rates for each hz , the contact material and the cheese cloth were removed. The disc infiltrometer was then used to measure ponded flow directly on the soil surface. After conducting all measurements with water, the soil was left to drain before repeating the procedure with 5 M ethanol. The sand and cheese cloth used for water and 5 M ethanol were differentiated and damped with the respective liquids in order to minimize potential crossover effects caused by the moisture contained in the contact material. The steady-state flow rate (Q) was determined for all hz for each location based on the cumulative infiltration plots which show when flow had reached steady state. The rate, Q, was converted to estimates of unsaturated hydraulic conductivity, Kh, using the procedure devised by Reynolds and Elrick (1991). Steady rate at two hydraulic heads (hz1 , hz2 ) was used to estimate one value for Khz , where hz is the average hydraulic head of the flow rates used in the calculation. The field-saturated flow (Kfs ) and ponded flow (Kp ) (hz D 0 mm and hz D 5 mm, respectively) were calculated directly from the steady infiltration rate. These values assume that the flow was determined by properties of the upper soil within the depth of the inserted ring and that gravity-driven flow is the dominant process. Similarly to the procedure followed by Feng et al. (2002), the steady-state flow rate across all tensions was assumed to be proportional to the liquid density and inversely proportional to the liquid viscosity. Flow rates for 5 M ethanol could then be converted to hydraulic conductivity by multiplying by its viscosity and dividing by the density which provided values that were directly comparable to water. See Table I for values of physical properties of water and 5 M ethanol used in the analysis. The steady infiltration rate was also measured as a function of positive hydraulic heads (hz ) between Table I. Physical properties of water and 5 M ethanol

Contact angle, a Density, pa Viscosity, b a,b

Unit

Water

Degrees kg m3 kg m1 s1

Unknown 997 0.001

Values at 25 ° C (Lide, 1999).

Copyright  2010 John Wiley & Sons, Ltd.

5

M

ethanol

0 950 0Ð0024

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5 and 60 mm using a single-ring infiltrometer on flat surfaces adjacent to the main study plot. The aim was to quantify the change in steady infiltration rate over a wider range of ponded depths than what was achievable using the ponded infiltrometer that was set to a fixed depth. These measurements were obtained in September, and only water was used in the infiltrations. The ring infiltrometer was 300 mm in diameter and was inserted 4 cm into the soil after applying vaseline to the inside of the ring. Water was added and held at a constant depth by carefully pouring water from a measuring cylinder. The water was poured onto a piece of cheese cloth dish placed on the soil surface in order to avoid unnecessary disturbance caused by the force of the water. The hydraulic conductivity was determined from the steady-state infiltration rate measured at hz of 5, 10, 40 and 60 mm. The steady-state infiltration rate was then compared with hydraulic conductivity values obtained by correcting for the positive hydraulic head using the Ge shape factor proposed by Reynolds and Elrick (1990). The effective pore size distribution The relationship between hydraulic conductivity (K) and hydraulic head (hz ) was used to derive a function that quantifies the contribution of different pore size classes to the field-saturated hydraulic conductivity (Kfs ) of the soil. The analysis was adopted from the procedure outlined in Moore et al. (1986) which used a theoretical model of flow through a porous medium as per Equations (1) and (3). First, the Gardner’s rational power model (Gardner, 1965) was fitted to the K  hz data using the Levenberg–Marquardt nonlinear estimation procedure. The model represents the relationship between unsaturated hydraulic conductivity (K) and hydraulic head (hz ) through the equation: Kfs 1 C hz /Hc ˇ where Hc and ˇ are fitted parameters

Khz  D

5

By assuming a zero contact angle for 5 M ethanol and expressing rmax and hz in millimetres, the capillary equation in Equation (3) reduced to: 14Ð918 ð cos˛w  for water, hz 7Ð157 rmax D for 5 M ethanol hz

rmax D

and 6

The contact angle for water (˛w ) was unknown. By substituting hydraulic head (hz ) in with the pore radius (rmax ) in Equation (6), the unsaturated hydraulic conductivity was expressed as the cumulative distribution of K as a function of the maximum conducting pore radius: 1 Krel rmax  D 7 1 C Hb /rmax ˇ Hydrol. Process. 24, 2871– 2887 (2010)

AUTHORS COPY P. NYMAN, G. SHERIDAN AND P. N. J. LANE

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ð cos ˛w or where Krel rmax  D KK and Hb D 14Ð918 H fs c for water or 5 M ethanol, respectively. Hb D 7Ð157 Hc The function Krel rmax  represents the relative hydraulic conductivity for effective pores of radius less than rmax . Differentiating Krel rmax  with respect to rmax generated the hydraulic conductivity density function of effective pore sizes (effective pore size distribution): krel rmax  D

ˇ/Hb Hb /rmax ˇC1 [1 C Hb /rmax ˇ ]2

8

This function has the properties of a probability density function in that  rDrn krel rmax dr D 1 and, rD0 rmax



krel rdr D Krel rmax 

9

rD0

The area under the graph between any given pore size interval represents the proportional contribution of that pore size class to total saturated conductivity. When hz ½ 0 and for ˛w < 90, the effective pore size distribution krel r must be the same for water and 5 M ethanol. This is assuming that the relative contribution of different pore size classes to flow is not affected by the different physical properties of water and 5 M ethanol. The effective pore size distribution of water was obtained by adjusting the contact angle (˛w ) so that the distribution corresponded with that measured for 5 M ethanol where the contact angle (˛e ) was assumed to be 0. The contact angle of water (˛w ) was treated as a fitted parameter when optimizing the effective pore size distribution of water to fit with the true effective pore size distribution of 5 M ethanol obtained in Equation (8). Porosity distribution The pore size distribution of soil samples was determined in the laboratory from 18 intact cylindrical cores

obtained from the study plot in March 2007. The cores were sampled from within a subset of locations where infiltration was measured with two cores obtained form each of the nine quadrats within the plot (Figure 1). The sample cylinders were 56 mm deep and 73 mm in diameter. A core sampler was driven into the soil enough to fill the core cylinder but without compressing the sample. After carefully removing the cylinder from the sampler, the soil core was retained in the cylinder and trimmed back in line with the rim. Cheese cloth was wrapped around the base of the core sample to avoid soil loss during transport and handling. A tension table was prepared following the design by Topp et al. (1993). A slurry of diatomaceous earth and water was poured into the tension table to form a tension medium with a depth of 10 cm. The soil cores were embedded in the diatomaceous earth to within 1 cm of the cylinder rim (Figure 2). Water was gradually added through the hanging column to the tension table until the water was level with the soil surface. At the onset of the experiment the method assumes that all available pores are saturated, so in order to overcome the water repellency the cores had to be subjected to an extended period of wetting. The cores were covered with plastic wrap, and the tension table was covered with a lid in order to minimize evaporation and maximize the air moisture surrounding the soil cores. A sample of six highly repellent cores acted as indicators of water repellency. A small amount of soil from each core was removed once a week and measured for GWC. After 4 weeks there was no further increase in the GWC of the soil, and the cores were therefore considered to be fully saturated. One soil core was disturbed during the preparation of the tension table and was therefore excluded from the experiment. The soil cores were drained at hydraulic heads (hz ) of 5, 10 20, 30, 50, 75 and 100 mm by lowering the hanging water column and fixing the constant head burette at the desired height using a clamp and stand (Figure 1). The hydraulic head (hz )

Figure 2. Tension table design and method for measuring gravimetric water content of soil Copyright  2010 John Wiley & Sons, Ltd.

Hydrol. Process. 24, 2871– 2887 (2010)

AUTHORS COPY THE EFFECT OF MACROPORE FLOW AND WATER REPELLENCY ON INFILTRATION was measured as the height difference between the water level in the burette and the midpoint of the 1 cm deep sub-sample removed from the cores. The porosity of sub-samples from the cores was measured gravimetrically after equilibrium was reached for the set suction. Equilibrium was assumed when no drainage was observed within a 30-min period. Using a sharp cutting blade and a small spatula, a small amount ( rn lim Cr D 0 and, lim Cr D 1, and rn is r!0 r ! rn the maximum pore radius of the soil. The parameters a and b were optimized for data from individual cores using the Levenberg–Marquardt algorithm for nonlinear least square estimation.

where

RESULTS Water repellency and hydraulic conductivity The soil was water repellent in both March and September as indicated by the point measurements from the plot (Figure 3). Wettable ash was present in both March and September (Figure 3). In March (summer), the mineral soil surface was water repellent or highly repellent in 28 out of the 30 sampled points. The occurrence of water repellency diminished at a soil depth of 5 cm. In September (winter), it was difficult to differentiate between ash and the topsoil due to vertical

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mixing caused by redistribution of surface material and bioturbation. The ash/topsoil and mineral soil surface was mainly non-repellent. The soil at 2 cm was water repellent or highly water repellent in 26 out of the 30 sampled points. The GWC at soil depth of 0–5 cm was significantly lower in March (mean D 0Ð09, coefficient of variation D 6%) than September (mean D 0Ð36, coefficient of variation D 28%). The study used 5 M ethanol to represent infiltration under non-repellent conditions because previous water repellency measurements in the same catchment indicate that this concentration would be of sufficiently low surface tension to infiltrate the water repellent soil (Sheridan et al., 2007). The surface tension of ethanol solutions decreases exponentially with increasing concentration of ethanol, and the curve starts to asymptote at concentrations above 5 M. Therefore, it was assumed that this concentration would be sufficient to completely overcome water repellency. However, when examining the soil profile following infiltration experiments, there was evidence to suggest that flow was diverted to inter-aggregate pores between cemented, dry and highly water repellent aggregates. After prolonged wetting and under fully saturated conditions, it is likely that the soil matrix would be conducting more water than under the conditions represented by this study. This is supported by the observation that at high tensions in summer, the flow of 5 M ethanol increased instead of decreasing towards a steady state. Despite these observations, given the highly macroporous soil and the high field-saturated hydraulic conductivity (Kfs ), it is assumed that intra-aggregate flow is not important during infiltration excess runoff generation in this type of forest soil. The ponded hydraulic conductivity measured on water repellent soil under a 5 mm pressure head in September using 5 M ethanol was 1071 mm h1 (SD D 567). In the same catchment, Sheridan et al. (2007) used a ring infiltrometer and measured the hydraulic conductivity of a non-repellent soil to be 1409 mm h1 (SD D 1008). These distributions are not significantly different (ttest, t D 1Ð93, d.f. D 22, p < 0Ð05) and suggest that 5 M ethanol provides an accurate representation of the infiltration expected to occur in non-repellent soils at the study site. For the purpose of further interpretation, it

Figure 3. Water repellency measured by water drop penetration time method for (A) ash, the mineral soil surface and at a depth of 5 cm in March (summer) and for (B) ash, the mineral soil surface and at a depth of 2 cm in September (winter) Copyright  2010 John Wiley & Sons, Ltd.

Hydrol. Process. 24, 2871– 2887 (2010)

AUTHORS COPY P. NYMAN, G. SHERIDAN AND P. N. J. LANE

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Table II. The mean, coefficient of variation and distribution of hydraulic conductivity of 5 M ethanol (non-repellent) and water (repellent) at hydraulic head (hz ) of 0 mm (field saturated flow) and 5 mm (ponded flow). The table provides values to show the proportional increase in hydraulic conductivity between March (summer) and September (winter) and from field saturated flow to ponded flow Hydraulic Conductivity, K (mm h1 ) MARCH (2 months since fire)

SEPTEMBER (6 months since fire)

5 M ETHANOL

N

Mean

CV

Distribution

N

Mean

CV

Distribution

Proportional increase (summer to winter)

Field Saturated Flow, Kfs

29

229

70

12

552

61

29

680

66

12

1075

53

Lognormal (x, 6.15, 0.59) Lognormal (x, 6.78, 0.75)

1Ð4

Ponded Flow, Kp

Lognormal (x, 5.15, 0.83) Lognormal (x, 6.23, 0.87)

Proportional increase (Kfs to Kp )

2

0Ð58

0Ð9

WATER

Na

Mean

CV

Distribution

Na

Mean

CV

Distribution

Proportional increase (summer to winter)

Field Saturated Flow, Kfs

28

64

88

10

217

55

28

118

175

10

548

67

Lognormal (x, 5.28, 0.44) Lognormal (x, 6.13, 0.62)

2Ð4

Ponded Flow, Kp

Lognormal (x, 3.72,1.08) Exponential (x, 0.012)

Proportional increase (Kfs to Kp )

0Ð8

3Ð6

1Ð5

is assumed that 5 M ethanol represents flow under nonrepellent conditions Table II summarizes the field-saturated hydraulic conductivity (Kfs ) and ponded hydraulic conductivity (Kp ) of water and 5 M ethanol measured in March and September. At field saturation and ponding, K is independent of the capillary equilibrium. Therefore, the values for K (5 M ethanol) and K (water) could be compared directly. The data in Table II support the following key points: 1. The field-saturated hydraulic conductivity (Kfs ) measured using 5 M ethanol was higher than Kfs of water by a factor of 3Ð5 and 2Ð5 for March and September, respectively. Because the hydraulic head (hz ) is 0 and large macropores are excluded, the difference in Kfs of 5 M ethanol and water can be viewed as a direct function of the area that is repellent and thus not contributing to infiltration. The proportional area in which infiltration occurred was expressed as the ratio Kfs (water) : Kfs 5 M (ethanol), which was 0Ð28 and 0Ð39 for March and September, respectively. These figures translate to an area of approximately 70% and 60%, respectively, that was water repellent and inactive in the infiltration process (Figure 4). The spatial distribution of this proportional area within the disc area is unknown. However, distinct patches of wet soil were observed in the majority of cases, suggesting that infiltration did not occur evenly across the sampled area. In March, the effect of the 5 M ethanol treatment was much stronger for the ponded hydraulic conductivity, Kp (factor increase D 5Ð7) than for Kfs (factor increase D 3Ð5). Copyright  2010 John Wiley & Sons, Ltd.

Figure 4. A schematic representation of the effect of water repellency and macropore flow on the ponded hydraulic conductivity, Kp . The area of the repellent regions relative to the non-repellent region represents the proportional reduction flow due to water repellency. The length of the arrow is proportional to the relative increase in flow due to ponding

2. The field-saturated conductivity (Kfs ) and Kp for both water and 5 M ethanol increased from March to September. The effect of water repellency on infiltration is represented by the difference in the seasonal response patterns of the two fluids. For water, the proportional increase from March to September was 2Ð4 and 3Ð6 for Kfs and Kp , respectively. For 5 M ethanol, the proportional increase over the same period was 1Ð4 and 0Ð68 for Kfs and Kp , respectively. If 5 M ethanol is assumed to isolate for the effect of water repellency, the temporal change in infiltration from March to September represents the (i) recovery from fire-related impacts other than water repellency and/or Hydrol. Process. 24, 2871– 2887 (2010)

AUTHORS COPY THE EFFECT OF MACROPORE FLOW AND WATER REPELLENCY ON INFILTRATION

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Figure 5. Semivariogram of field-saturated (hz D 0) and ponded (hz D 5) hydraulic conductivity for 5 M ethanol (A and C) and water (B and D) in March, 2 months following wildfire. The analysis was performed on log-transformed data

(ii) seasonal changes in hydraulic properties other than water repellency. 3. The ponded hydraulic conductivity (Kp ) was always greater than Kfs for both 5 M ethanol and water. The increase from Kfs to Kp relative to Kfs is assumed to represent the proportional increase in flow due to unlimited gravity-driven flow in macropores (0Ð5 mm  r < 1Ð5 mm) and large macropores (r > 1Ð5 mm). In March, ponding resulted in a 200% and 80% increase in flow for 5 M ethanol and water, respectively. In September, 6 months after fire, the pattern was reversed with an increase of 90% and 150% for 5 M ethanol and water, respectively (Figure 4). 4. All K distributions were log-normally distributed with the exception of Kp (water) in March which was exponentially distributed. The distributions of K (water) in March had the lowest means but exhibited higher variability than all other K distributions. The variability of Kp (water) in March was extremely high (CV D 175%) and was up by a factor of 2 compared with Kfs (water) and Kp (5 M ethanol). 5. The relative increase in flow between Kp and Kfs was greater in March for 5 M ethanol (non-repellent) but for water (repellent) the increase in flow was greater in September (Figure 4). Only Kp (water) in March showed evidence of spatial correlation (Figure 5d). The lack of spatial correlation for Kfs (water) and Kp (5 M ethanol) suggests that when water repellency or macropore flow operate in isolation, the variability in K does not exhibit spatial structure at the support of the infiltrometer (10 cm diameter) and at scales ranging from the minimum sample spacing (25 cm) to the sampling extent (¾2 m) (Figure 5a–c). However, when the macropore flow and effect of water repellency operate in synergy, the net effect on infiltration results in extremely high spatial variability and spatial correlation Copyright  2010 John Wiley & Sons, Ltd.

at scales 40 mm where K started to asymptote at ¾500 mm h1 . Effective pore size distribution The hydraulic conductivity function Khz , in Figure 8, was obtained by averaging parameter values that were optimized in individual Khz  regressions at each sampling location. Gardner’s function (Equation (6)) accounted for between 45% and 99% of the variability in hydraulic conductivity (K) with changes in hydraulic head (hz ) for water. For 5 M ethanol the function accounted for between 73% and 99%. Due to the log-normal distribution of K, the regression residuals were negatively skewed around the fitted curve. However, the analysis focused on the relative flow Krel rmax , Copyright  2010 John Wiley & Sons, Ltd.

(K at hz , relative to Kfs ; Equation (8)), which was normally distributed around the fitted curve. The curves of unsaturated K show that the increase in flow occurs primarily for hydraulic heads between 10 and 5 mm. Direct comparison of Khz  for water and 5 M ethanol for hz < 0 is not meaningful due to the different physical properties resulting in different maximum conducting pore size (rmax ) for the two liquids at the same hydraulic head (hz ). The combined effect of season and time since fire on unsaturated hydraulic conductivity is displayed by the temporal changes within the respective fluids. The unsaturated flow increased from March to September for both water (Figure 8a and b) and 5 M ethanol (Figure 8c and d). Within the hydrologically active region where the contact angle (˛) is less than 90° , the relative contribution of different pore size classes to the field-saturated hydraulic conductivity (Kfs ) is equal for water and 5 M ethanol. Using the fitted parameter values in Figure 6 and solving for contact angle of water (˛w ) in Equation (8) resulted in an average effective contact angle of 58° in March and 61Ð5° in September. This is the contact angle of the soil which contributed to infiltration at hz  0. This contact angle was incorporated into the Krel r function and plotted as a density function of effective (hydrologically active) pores (Figure 8). The effective pore size distribution is shown for Kfs and Kp in March and September. The density function shows that macropores (r > 0Ð5 mm) account for approximately 70% and 80% of Kfs in March and September, respectively. While the unsaturated K of both water and 5 M ethanol increased from March to September (Figure 6), the increase in the relative contribution of smaller pores to the Kfs was offset by a 10% increase in macropore flow (Figure 9a). These results show that the relative contribution of the soil matrix to Kfs is very low ( 0Ð5 mm) and a large proportion of the density function (65–75%) fell outside the limits of the experimental data (r > 1Ð5 mm). The functions show that infiltration in a simulated non-repellent soil (5 M ethanol) versus repellent soil (water) responds differently to the effect of ponding. For water the relative contribution of large macropores (r > 1Ð5 mm) increased by 10% from March to September, whereas for 5 M ethanol the relative contribution of these pores decreased by 10%. This pattern is consistent with the patterns in the experimental data reported in Table II. Hydrol. Process. 24, 2871– 2887 (2010)

AUTHORS COPY THE EFFECT OF MACROPORE FLOW AND WATER REPELLENCY ON INFILTRATION

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Figure 8. Hydraulic conductivity Khz  for unsaturated (65 mm  hz < 0 mm), field-saturated (hz D 0 mm) and ponded flow (hz D 5 mm) for water in (A) March and (B) September; and 5 M ethanol in (C) March and (D) September. The unsaturated and field-saturated K was fitted with Gardner’s rational power model (Gardner, 1965)

Figure 9. The effective pore size distribution in March and September showing the relative contribution of pore size classes to field-saturated conductivity (A) and ponded flow (B). The effective pore size distribution for water was derived based on the contact angle estimated from Equation (8). The distributions are supported by data for pore radii between 0Ð1 and 1Ð5 mm

Porosity distribution The mean bulk gravimetric porosity of four sample cores was 0Ð78. The maximum gravimetric porosity measured in the tension table at hz D 5 mm was 0Ð72, whereas the mean porosity across all 17 samples at Copyright  2010 John Wiley & Sons, Ltd.

hz D 5 mm was 0Ð56. Overall, there was no detectable difference in GWC between hydraulic heads of 10 and 5 mm, due to large variability in porosity relative to the detectable difference in GWC. The small scale of sampling relative to the high spatial variability in macropore Hydrol. Process. 24, 2871– 2887 (2010)

AUTHORS COPY P. NYMAN, G. SHERIDAN AND P. N. J. LANE

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Figure 10. The cumulative relative porosity distribution, measured using tension table. Individual cores (n D 17) were treated as one distribution and fitted with a cumulative distribution function. The maximum pore radius ranged from 0Ð55 to 2Ð7 mm

distribution resulted in five cases where porosity estimates increased with decreasing hydraulic head (hz ) from 5 to 10 mm. The fitted relative porosity distribution function accounted for an average of 66% of the variability within 17 individual distributions (Figure 10). The coefficient of determination, r 2 , ranged from 0Ð26 to 0Ð97. When fitted to the entire data set, the percentage of variance accounted for by the function reduced to 28%. There was very high variability in the distribution of large macropores relative to the proportional contribution of these pores to the total porosity, and the coefficient of variation within pore size classes decreased exponentially with decreasing pore size classes (Table III). This trend is expected, given the small scale of sampling and the fact that smaller pore size classes are distributed more homogenously and are less dependent on the structural elements of the soil.

DISCUSSION Saturated hydraulic conductivity of wet eucalypt forests In the absence of fire, wet eucalyptus forests have high saturated hydraulic conductivity ranging from 100 to 2000 mm h1 depending on local conditions, the methodology used and the scale of measurement (Vertessy et al.,

1993; Davis et al., 1999; Lane et al., 2004, 2006a). In this study, the average ponded hydraulic conductivity (Kp ) measured in March, 1 month after the fire (118 mm h1 , coefficient of variation (CV) D 175%) and in September, 7 months after fire (548 mm h1 , CV D 67%) were 7 and 2Ð5 times lower than saturated hydraulic conductivity values reported in the same catchment for summer and winter in unburnt conditions (Sheridan et al., 2007). Rapid vegetation recovery, high levels of bioturbation and high macroporosity means that soils in wet eucalypt forest have the capacity for rapid recovery from fire and other disturbances (Lane et al., 2006b). In summer 1 year after a high-intensity fire in the Slipper Rock Catchment, the Ks (1350 mm h1 , CV D 66%) was not significantly different from unburnt soil (Sheridan et al., 2007) and more than an order magnitude higher than the Kp measured the first year after wildfire in this study. In combination, the studies show that the effect of fire on infiltration is most pronounced in the first summer and winter after fire and then declines rapidly, and that Ks returns to pre-fire conditions within 1 year. Water repellency, macroporosity and hydraulic conductivity Reduced water repellency on the mineral soil surface in September (6 months after fire) is probably linked to the increase in soil moisture content driven by higher rainfall, lower temperatures and snow during winter. Seasonal fluctuations in water repellency have been documented in the Slippery Rock Catchment (Sheridan et al., 2007) and elsewhere (Burch et al., 1989; Crockford et al., 1991; Doerr and Thomas, 2000; MacDonald and Huffman, 2004; Keizer et al., 2008). The persistence of water repellency at 2 cm depth in winter must be considered in combination with the low rainfall in 2007. In the first 6 months of the year, the catchment received 704 mm of precipitation, which is approximately 40% below the long-term average rainfall for this period. Given the close interrelationship between soil moisture and water repellency, it is possible that repellent soils were more widespread in September 2007 than what would be expected for burned soils under normal rainfall conditions in this environment. Water repellency in March and September, respectively, resulted in 70% and 60% reduction in fieldsaturated hydraulic conductivity (Kfs ) of the soil. In September, there was a 15% decrease in the effect of

Table III. Relative porosity, gravimetric porosity and associated variability for different functional pore size classes estimated form the tension table Pore functional class Large macropores Macropores Large mesopores Mesopores Small mesopores

Pore size class radius (mm)

Proportion of total porosity (%)

r > 1Ð5 0Ð50 < r  1Ð5 0Ð25 < r  0Ð5 0Ð05 < r  0Ð25 0Ð01 < r  0Ð05

0Ð2 7Ð4 9Ð3 34Ð1 31Ð5

Gravimetric porosity (%) 0Ð1 5Ð4 6Ð7 24Ð6 22Ð7

Coefficient of variation (%) 105 47 33 15 16

Pore size classes defined as per Luxmoore (1981). Copyright  2010 John Wiley & Sons, Ltd.

Hydrol. Process. 24, 2871– 2887 (2010)

AUTHORS COPY THE EFFECT OF MACROPORE FLOW AND WATER REPELLENCY ON INFILTRATION water repellency on Kfs even though the proportion of water repellent points remained the same. The reduced effect of water repellency on Kfs could be due to a lowering of the upper boundary of the water repellent soil layer within the profile. Feng et al. (2001) showed that a wettable soil overlying a water repellent soil resulted in a positive hydraulic head at the interface between the two soil layers. This can lead to preferential flow and hence reduced impact of water repellency on Kfs (Wang et al., 2000a). The lower impact of water repellency on flow in September compared with March could also be linked to the slight reduction in the strength of water repellency. A reduced strength of water repellency would result in higher sorptivity and higher Kfs due an increase in the proportion of effective pores (Moody et al., 2009). Finally, if the water repellent soil was distributed over a thinner layer in September, flow may have occurred more readily in preferential pathways through the repellent soil. The large increase (80–200%) in flow between Kfs and Kp and the rapid initial increase in infiltration with increased hydraulic head highlights the large potential for gravity-driven macropore flow under both repellent and non-repellent conditions when supplied by ponded surface water. For instance, macropores (r > 0Ð5 mm), which comprised 5Ð5% of the soil volume, contributed to approximately 70% and 95% of the flow through the soil for Kfs and Kp , respectively. These values are similar to the findings by Moore et al. (1986) and Watson and Luxmoore (1986) where macropores (r > 0Ð5 mm) accounted for between 63% and 73% of Kfs . Davis et al. (1999) reported that 83–95% of saturated flux through soil cores from wet eucalypt forest occurred in large macropores (r > 1Ð5 mm). Similarly, this study found that for Kp , a major proportion (65–76%) of flow occurred in large macropores (r > 1Ð5 mm) that make up 0Ð1% of the soil volume. However, the actual contribution by these large macropores pores is based on the assumption that the flow increase from Kfs to Kp is entirely due to the inclusion of new pores. This is unlikely and the increase in flow probably also represents the effect of unlimited supply, increased saturation and enhanced contribution by already active macropores (0Ð5 mm