EFFECT OF VEGETATIVE ASH ON INFILTRATION RATES AFTER FOREST WILDFIRE IN THE
NORTHERN ROCKY MOUNTAIN REGION, U.S.A.
Scott W. Woods and Victoria Balfour
The University of Montana, Department of Ecosystem and Conservation Sciences, 32 Campus Drive,
Missoula MT 59812 USA. e-mail: scott.woods@cfc.umt.edu
Increases in runoff have been widely documented after forest wildfires, particularly in the first
year after the fire and in areas burned at high severity (DeBano, 1981; Shakesby et al. 2000; Huffman
et al. 2001; Moody and Martin, 2001; Benavides–Solorio and MacDonald, 2001). These increases are
usually attributed to the development of hydrophobic soils and the loss of the surface litter layer and
other vegetation. Another process that may significantly reduce infiltration capacity after wildfire is
surface sealing by vegetative ash. Surface sealing has been observed in bare agricultural soils, where
the beating action of raindrops causes a loss of soil aggregation, providing a source of fine sediment
that seals the surface pores (Rohdenburg et al., 1990). A similar phenomenon may occur after
wildfires, with the exception that vegetative ash rather than disaggregated mineral soil provides the
fine grained material that blocks soil pores. This inference is supported by a study that concluded that
the penetration of ash into surface pores reduced infiltration rates after fire in a Scottish heathland
(Mallik, 1984). The effectiveness of ash in reducing infiltration rates may be accentuated by wetting
induced swelling; volume increases of up to 13% have been noted after ash generated by burning
lodgepole pine (Pinus contorta) was wetted (Etiegni and Campbell, 1991). Until now, the phenomenon
of surface sealing after wildfire, and the role of vegetative ash in its development, has not been
systematically evaluated.
In 2005 we began a study to 1) document the existence of the ash sealing phenomenon, and
2) understand the factors controlling its occurrence, such as ash thickness, rainfall intensity and soil
texture. Fourteen bounded 0.5 m2 runoff plots were established within areas burned by a recent
(August 2005) forest wildfire in western Montana, USA. The plots were laid out in pairs in areas where
the fire had produced ash at the soil surface. Ash was retained in one of the plots and removed from
the other plot using a fine brush. The plots were then subjected to simulated rainfall at a nominal rate
of 75 mm hr-1 for one hour using a Norton rainfall simulator. The actual rainfall intensity was measured
in a calibration pan placed over the plot for 5 minutes prior to the start of each simulation. Volumetric
runoff samples were collected every 1 minute for the first 10 minutes and every 2 minutes thereafter in
1 L bottles. Runoff samples were returned to the lab where the volume of runoff was recorded. The
samples were then gravity filtered through a 0.45 μm filter paper, and the filtered sediment was dried
and weighed.
5
Ash plot
No ash plot
-1
Runoff rate (cm hr )
4
3
2
1
0
0
10
20
30
40
50
60
Time (minutes)
Figure 1. Runoff hydrographs for an ash plot and a no-ash plot, showing the difference in hydrologic
response to rainfall.
The mean total runoff from the ash plots (1.3 cm) was significantly less than the mean total
runoff produced from the no-ash plots (2.9 cm) and there was a similar difference in terms of the peak
runoff rates. This suggests that, contrary to the ash-sealing hypothesis, the ash on the surface acted
as a storage reservoir for rainfall, retaining a portion of the total runoff rather than causing reduced
infiltration. However, while the runoff from the no-ash plots reached a more or less constant rate
before the end of the 1-hour simulations, the runoff rate from the ash plots continued to increase
throughout the simulation (Figure 1). The continuous decline in the infiltration rate in the ash plots
could be due to progressive sealing of the soil surface as ash was washed into the soil pores. Further
rainfall on the study plots may continue to wash ash into the soil pores, with a consequent decline in
the infiltration rate. Additional rainfall simulation experiments are necessary to determine whether this
is the case, and these will be conducted after the spring snowmelt in May 2006.
In addition, we will conduct manipulative field experiments aimed at addressing objective 2 of
our study, which is to determine the effect of ash thickness, soil texture and rainfall intensity on the
development of the surface sealing phenomenon. These experiments will involve conducting rainfall
simulations in areas burned in a series of small (< 1 ha) prescribed fires at the Lubrecht Experimental
Forest in western Montana. We will manipulate fuel loads within the burned area to obtain differing
amounts of ash on the surface. Additional experiments will be performed by “making” ash and
applying it to different soil textures in unburned and burned plots. Finally, we are developing analytical
techniques for determining the effect of ash on the soil physical properties such as porosity, water
retention and hydraulic conductivity. The results of these experiments will improve our understanding
of the role of vegetative ash in post-fire runoff processes.
References
Benavides – Solorio J. and L.H. MacDonald, 2001. Post fire runoff and erosion from simulated rainfall
on small plots, Colorado Front Range. Hydrological Processes 15:2931-2952.
DeBano L.F., 1981. Water repellent soils: a state of the art. General Technical Report PSW-46.
Pacific Southwest Forest and Range Experiment Station, USDA Forest Service, Berkeley, California.
Etiegni, L., and Campbell, A.G., 1991, Physical and chemical characteristics of wood ash:
Bioresource Technology, v. 37.
Huffman E.L., L.H. MacDonald and J.D. Stednick, 2001. Strength and persistence of fire induced soil
hydrophobicity under ponderosa and lodgepole pine, Colorado Front Range. Hydrological Processes
15(15):2877-2892
Mallik A.U., C.H. Gimingham and A.A. Rahman, 1984. Ecological effects of heather burning 1. Water
infiltration, moisture retention and porosity of surface soil. Journal of Ecology 72:767-776.
Moody J.A. and D. A. Martin, 2001. Initial hydrologic and geomorphic response following a wildfire in
the Colorado Front Range. Earth Surface Processes and Landforms, 26:1049-1070.
Rohdenburg H, S. Assouline and Y. Mualem, 1990. Rainfall Induced Soil Seal: (A) A Critical Review
of Observations and Models. Catena, Vol. 17, No. 2, p 185-203.
Shakesby R.A., S.H. Doerr and R.P.D. Walsh, 2000. The erosional impact of soil hydrophobicity:
current problems and future research directions. Journal of Hydrology 231-232:178-191.