The Geomorphology of Fire: The Effects of Fire on SoilErosion and Hydrologic Response.

Wildfires transform the landscape long after they die. Geomorphologistsstudy the effects of fire on the landscape by measuring soil erosion andhydrologic response. Results from the field show that post-fire responsesvary depending on geology, soil type, vegeatation, topography, climate, andland use of a given area. Geomorphologists are particularly interested in theeffects of fire in Mediterranean climatic regions because fires often occurduring long, dry summers and burn the highly flammable vegetation. Therainy season, often characterized by high intensity storms, follows andrainfall strikes the burned landscape. This provides hydrologic energy thatcan lead to landslides, debris flows and flooding. Policy makers, thus, oftenrespond to fires with expensive mitigation in order to protect human life andproperty. A review of the literature on the geomorphic effects of fire on soilerosion and hydrologic response provides evidence that due to the variedresponse of different watersheds, policy-makers must base post-firedecisions and actions on area-specific research by geomorphologists.

Terri FashingJanuary 30, 2004Geography 810

Professor Jerry Davis

Introduction

After the embers die and the smoldering subsides, wildfires continue to transform

the landscape. Geomorphologists have documented such transformations by studying the

effects of fire on soil erosion and hydrologic responses within watersheds. Increased

storm flow, overland flow, soil erosion, and debris flows are among the indirect effects of

fire (Scott 1993). The intensity and severity of these post-fire responses vary depending

on the geology, soil type, vegetation, topography, climate, and land use of a particular

area (Booker, Dietrich, and Collins 1993, Sevink et al. 1989, Scott and VanWyk 1990).

The often dramatic post-fire responses in Mediterranean climatic regions capture the

interest of geomorphologists. The hot, dry Mediterranean summers and highly flammable

vegetation create landscapes ready to ignite. Cool, wet winters, sometimes characterized

by high-intensity storms, follow, providing the hydrologic energy that can lead to

landslides, debris flows, and flooding. Thus, policy-makers often respond to fire with

expensive mitigation procedures in an effort to prevent destructive mass movements. The

following is a review of the literature on post-fire soil erosion, and hydrologic response in

Mediterranean type climatic regions. I conclude that due to the variable response of

watersheds to fire in Mediterranean regions, policy-makers must consult research specific

to a given area before initiating post-fire mitigation procedures.

Early Studies in Post-Fire Erosion

In the literature, workers recognized the relationship between fires and floods by

1930, with Eaton (1935) providing the distinction that the sometimes catastrophic post-

fire floods in Southern California are really debris flows (Wells 1987). Eaton (1935:

1303) cautioned that the foothill areas of Los Angeles County were subject to flash floods

and debris flows “particularly following water-shed denudation by fires” (Figure 1). He

explained that steep slopes, high intensity storms, and unstable soil and geology all

contribute to such post-fire responses. He theorized that fire-related debris flows may

occur when denuded hillsides are first saturated with rainwater and then give way to a

high intensity storm. Cannon (2001: 321) refers to this mechanism as “infiltration-

triggered soil slips”.

Sampson (1944)

gave an alternate

explanation for increased

soil erosion after chaparral

burning in California,

although the erosion events

he described were not

referred to as debris flows.

He reviewed a 1941 study

by Rowe that attributed

post-fire increases in

erosion and runoff to a

decrease in infiltration

capacity (Sampson 1944).

In his own study,

Sampson (1944) looked at

the effects of burning on the

Figure 1. La Crescenta-Montrose debris flow, January 1, 1934.This post-fire debris flow killed 80 people and destroyed 488homes. The shaded areas in this graphic shows where an affectedneighborhood was covered by the flow. (From Eaton 1935: 1328.)

infiltration capacity of chaparral soils. In the late summers of 1935 and 1936 three sets of

burned and unburned plots were studied, with each pair defined by uniform soil and

vegetation characteristics. In each set the infiltration capacity of the soil in the burned

plot was less than it was in the unburned plot, and the differences were significant at the

five percent level (Sampson 1944). The vegetation and soil types studied were chamise

growing in Aiken clay loam, chaparral growing in Melborne stony loam, and woodland-

chaparral growing in Sites clay loam (Sampson 1944). The main conclusion of this study

was that the decreased infiltration capacity of burned soil would “favor greater surface

runoff and increases [in] soil erosion” (Sampson 1944).

Fire and Water Repellent Soils

Sampson’s (1944) conclusions, and those of his contemporaries, were missing a

critical perspective: the effects of fire on particular soil properties that contribute to a

decrease in infiltration capacity (Debano 2000a). Before the 1960s, workers attributed the

increased runoff to post-fire surfaces devoid of protective plant covering and to ash-filled

soil pores (Debano 2000a). During the early 1960s it was determined that fires form or

help to intensify a water repellent layer in the soil that resists infiltration (Debano 2000a).

In a historical overview of water repellency in soils, Debano (2000b) explains that

inquiries into water repellent soils and their diminished capacity for absorbing water

began before the 20th century. In 1910 Schreiner and Shorey presented their findings on

organic substances found in California soil that were responsible for impeding water

infiltration (as reviewed by Debano, 2000b). During the 1960s, workers found that fire-

induced water repellency (otherwise referred to as hydrophobicity) “dramatically reduced

infiltration, created overland flow, and as a result, accelerated erosion” (Debano 2000a).

Water repellency in soils does occur without the introduction of fire (Debano

1981). Four factors influence the development and severity of hydrophobic soils: organic

matter, soil texture, fire intensity, and soil water (Debano 1981). The most critical

parameters are organic matter and soil texture, but fire can increase the severity of a

water repellent layer in susceptible soils (Debano 2000a).

In an earlier review, Debano (1981) presents the results of many studies on water

repellency in soils. Workers discovered that irreversible drying of organic matter in peat

soils induced water repellency (Debano 1981). It was also found that sand and coarse

grained soils leach and become coated by organic substances from plant litter. Laboratory

studies showed that

heating soil containing

organic water resistant

material increased the

water repellency of the

soil (Debano 1981).

When fire occurs

in the field, heat from the

burning litter layer

vaporizes organic

substances (Debano

2000a). These substances

Figure 2. (A) Water repellent layer just below the soil surface. (B)Heat from fire vaporizes hydrophobic substances and they move downin the soil. (C) Water repellent layer remains after fire below awettable surface layer (From Debano 2000a: 197).

move down along a steep temperature gradient and condense, forming a water repellent

layer below the surface (Debano 2000a). Figure 2 shows an idealized sketch of the

process.

Further studies revealed a more complex relationship between soil water

repellency and soil temperature (Debano 2000a). Workers found that soils heated to less

than approximately 175°C underwent negligible changes in water repellency. Soils

heated between 175°C and 200°C produced high water repellency. In soils heated

between 280°C and 400°C, the organic substances responsible for water repellency were

destroyed (Debano 2000a). Debano (1981) found high variability in the water repellent

layer created during fire due to differences in fire behavior as well as soil characteristics.

Post-Fire Water Repellent Soil and Erosion

Wells’ (1987) findings in the field supported Debano’s conclusions about post-

fire water repellent soils. According to Wells (1987), these water repellent soils

contribute to debris flows that often occur on smaller watersheds during small, early

storms in southern California. During the fall and winter of 1984-1985, Wells (1987)

prescribe-burned four watersheds in the San Dimas Experimental Forest northeast of Los

Angeles, California. The geology of these watersheds is mainly Mesozoic granite and a

complex of Precambrian igneous and metamorphic rocks which weather at a fast rate and

are poorly consolidated (Wells 1981). The soils are shallow, show little profile

development, and are composed of coarse grains which, as noted above, tend to attract

the organic substances that contribute to water repellency (Wells 1981).

After the prescribed burns, Wells (1987) inspected the mouths of all four

watersheds after major storms for debris flow deposits. Table 1 shows the results of this

procedure. From these data Wells (1987) detected a relationship between peak storm

intensity and debris flow initiation. However, it appears that later in the rainy season

greater peak storm intensity was required to initiate these flows. Wells (1987) reasoned

that sediment supplies were removed by debris flows during the first storms, therefore

more intense storms were required to entrain soil later.

During the early storms, rill networks formed quickly (Figure 3). These networks

probably supplied the sediment for the debris flows. Wells (1987) attributes the formation

of rill networks to post-fire water-repellent soils (Figure 4). The wettable surface above

the hydrophobic layer becomes saturated with rainwater that cannot percolate down.

Eventually the water pressure in the pores under the soil layer causes the soil to move

downslope (Wells 1981).

Table 1. Results of debris flow counts after the first 12 post-fire storms. Debris flows occurredmore consistently during the first storms. It took a storm of extreme magnitude to induce fourdebris flows a little later in the season, on December 20 (From Wells 1987: 107).

Figure 4. Rill Formation: (A) surface soil is saturated; (B) area between wettable andwater repellent layer fails; (C) surface layer erodes, exposing hydrophobic layer torain; (D) water repellent layer begins to erode due to raindrop splash and water flow;(E) wettable layer below removed hydrophobic layer now begins to become saturated;and (F) rill is thus formed and begins to direct some water flow (From Debano 2000a:202.) (This figure was adapted by Debano from Wells 1981).

Figure 3. Rill network streaks the prescribe-burned hillside after early storms.The longest rill is 40 m. (From Wells 1987: 113).

Wells (1987) also observed evidence

of another type of post-fire erosion: “dry

ravel”. This refers to the “active downslope

sediment movement and channel filling by

the dry, unconsolidated flow of surface

material” after a fire (Wells 1987: 107)

(Figure 5). Dry ravel produces small amounts

of sediment at a high frequency. This process

ultimately delivers large quantities of

sediment to streambeds where it can be

moved by sufficient rainfall flows (Well

1987).

Recent Studies on the Geomorphic Effects of Fire

From the late 1980’s to the present there have been many studies on the effects of

fire on soil erosion and hydrologic processes in Mediterranean climates. Sevink et al.

(1989) studied the relationship between humus form development and hillslope runoff,

and showed differential effects of fire on soil erosion in different types of humus form.

In their study area in northeast Spain, Sevink et al. (1989) explain that mor type

humus form is characterized by a well-developed but shallow ectorganic horizon (L, F,

H) that ends abruptly above a water repellent mineral A or E horizon (Sevink et al. 1989).

Figure 5. Dry Ravel photographed 10 minutes afterprescribed burn took place. From Wells (1987: 108).

The moder type humus form exhibits a gradual transition between the ectorganic layer

and the Ah horizon and it has a well-developed root structure, high soil moisture, and

high macro-porosity (Sevink et al. 1989). They found an increase in post-fire hillslope

runoff on slopes with both mor and moder type humus forms; however, the effect was

pronounced with the former type and very limited with the latter (Sevink et al. 1989).

Scott and VanWyk (1990) looked at how a wildfire affected soil wettability,

erosion, and hydrology in the first post-fire year. The fire decimated a pine plantation in a

gauged watershed in the Jonkershoek State Forest, South Africa. The authors

hypothesized that after the first rains, fire-induced water repellency would cause an

increase in overland flow, stream flow, and soil erosion (Scott and VanWyk 1990).

Their study area comprised two gauged, adjacent watersheds: one burned by the

wildfire, and one unburned control (Scott and VanWyk 1990). The soils are characterized

as sandy to silty loams with high gravel and rock content, providing favorable infiltration

conditions. Eighty percent of the burned catchment was affected by the fire. The

vegetation of both watersheds included Pinus radiata plantations and native fynbos

(sclerophyllous scrub). The watersheds were calibrated against each other using pre-fire

(pre-treatment) hydrological data (Scott and VanWyk 1990).

Using the water drop penetration time (WDPT) and the critical surface tension

(CST) methods described by Debano (1981), Scott and VanWyk (1990) investigated the

degree and depth of water repellency in soils based on vegetation type and fuel-load.

Soils under the pine plantation exhibited more pronounced repellencies that extended

deeper than those under the native fynbos vegetation (Figure 6). However, where fuel

loads were high due to slash piles, regardless of vegetation type, surface repellency was

destroyed and the hydrophobic layer was more pronounced deeper in the soil. Root

channels, rocks, and severely burned patches resulted in discontinuous hydrophobicity

over large areas (Scott and VanWyk 1990). This observation of patchy hydrophobicity

corroborates Debano’s (1981) finding of high variability in the water repellent layer.

Scott and VanWyk (1990) also found that there were statistically significant increases in

monthly suspended sediment yields from the burned watershed but not from the control

watershed. The authors explained that low intensity storms led to overland flow and soil

erosion due to fire-induced water repellency. In addition, they found a significant

steepening of the hydrograph in the burned watershed. They conclude that the severity of

fire-induced water repellency was related to vegetation type and fire intensity (Scott and

VanWyk 1990). They also state that while water repellent patches were not continuous it

is likely that infiltration decreased enough to cause saturated overland flow.

Figure 6. This figure classifies 12 samples taken at each depth as repellent, somewhat repellent,and wettable. (From Scott and VanWyk 1990: 246.)

In another study, Imeson et al. (1992) showed that while fire does have a dramatic

effect on the landscape, the spatial pattern of runoff and infiltration does not change. In

the study, the researchers performed 150 simulated rainfall experiments in the Selva

region of Catalonia to study the effects of wildfires and soil hydrophobicity on soil

properties, soil erosion, and organic material loss. Workers in this study compared plots

that had been destroyed by fire, either partially or completely, to vegetated plots that had

not undergone a fire in at least 50 years. Most of their 20 sampling sites were on the

uppermost portion of slopes or on crests. Parent material of granite or granodiorite was

noted at most of the sites.

Referring to Sevink et al. (1989), Imeson et al. (1992) looked mainly at slopes

characterized by a mor type profile because of the likelihood of a more pronounced post-

fire response. Their data revealed the expected increase in post-fire hillslope runoff on

most of their plots. They concluded, however, that large spatial discontinuities in water

infiltration remain unchanged on burned and unburned slopes and are spatially structured

according to vegetation patterns.

In a study in Israel, Inbar et al. (1998) looked at the effects of a September 1989

fire that covered 4 km_ in a Mediterranean forest area on Mt. Carmel near the town of

Haifa. Three weeks after the fire workers established two sites differentiated by exposure

to the sun (northern and southern). Each site had one unburned plot, and 3-5 burned plots.

From 1989 through 1992 they measured vegetation cover and they collected runoff and

sediment after flow events. They also measured rainfall after storms using nearby rain

gauges and an electronic rain recorder (Inbar et al. 1998).

The study area is characterized by steep slopes underlain by fine-grained shallow

soils and sedimentary rocks (mainly chalk, limestone and dolomite) (Inbar et al. 1998). A

Mediterranean evergreen sclerophyll forest type dominates the area, including Pinus

halepensis and Quercus calliprinos.

Inbar et al. (1998) explain that in the first winter rain season (1989/1990) after the

fire, there was a great increase in runoff and sediment yield in comparison to unburned

plots (Table 2, Figure 7). The total rainfall for the year was very close to the annual

average. During the second year of study, runoff and sediment yields decreased

substantially in the burned plots, but the total rainfall for the year was only 68 percent of

the annual average (Inbar et al. 1998). In the third post-fire wet season (1991/1992)

rainfall was extremely high, breaking records in the Carmel area and ultimately reaching

165 percent of the annual average (Inbar et al. 1998). This increase in rainfall from the

previous year produced higher sediment yields in some of the burned plots, however, the

total sediment yield was lower than it was during the second post-fire wet season (Figure

8). The control plots lost almost no sediment during the three years after the fire (Inbar et

al. 1998).

Inbar et al. (1998) link the decrease in runoff and erosion to the re-growth of

vegetation. They state that “vegetational cover was the major factor affecting runoff: sites

with a dense vegetational cover showed a more rapid decrease in runoff and sediment

yield than sites with less vegetational cover” (Inbar et al. 1998:27). They completely

discount the idea that hydrophobic soils, in this case, had anything to do with post-fire

erosion and post-fire landscape recovery. They reason that because the soil in the study

has little organic content, vegetation must be the controlling factor with regard to erosion.

They also noted that slope aspect and exposure did not have much of an effect on erosion

in the experimental plots. However, excessive erosion occurred only on the south-facing

burned plots during the first year after the fire.

aControl-non burnt plots.bLow magnitude fire.

Plot No. Sediment Yield g/m_ Drainage1989-90 1990-91 1991-92 Area Exposure

1 202 23 7.8 400 N2 18 2 15 200 N3 50 4.5 11 200 NS

ite 1

4a 0.001 0.01 0.002 200 N11 42 25 5 200 S12 875 12 12 200 S13 440 7 12 200 S14 515 100 11 200 S15b 3 0.1 2 100 S

Site

2

16a 0.02 0.05 0.04 200 S

Table 2. Sediment yield from experimental plots from 1989-1992 (Adapted from Inbar et al.1998: 28).

Post-Fire Sediment Yields

0

100

200

300

400

500

600

700

800

900

1 2 3 4a 11 12 13 14 15b 16a

Plot

Sed

imen

t Yie

ld g

/m2

1989-901990-911991-92

Figure 7. Sediment yields in the years after the fire on Mt. Carmel in Israel. Runoff in the first wet seasonafter the fire entrained substantially more sediment than runoff in the second and third wet seasons. Plots 1-4 were on north-facing slopes and plots 11-16 were on south-facing slopes. Plots 4a and 16a were unburnedcontrol plots and plot 15b was in an area that experienced low magnitude fire. The sediment yields were toolow in the unburned plots to be visible on this graph. (Adapted from Inbar et al. 1998: 28).

WetSeason

Figure 8. Total sediment yield from all experimental plots displayed by year. This figureshows that despite exceptionally high rainfall during the 1991/1992 wet season, totalsediment yield was less than the previous, low rainfall year (Adapted from Inbar et al.

1998: 28).

Total Sediment Yield By Wet Season

0

500

1000

1500

2000

2500

1989-90 1990-91 1991-92

Wet Season

Sed

imen

t g/m

2

Policy Implications of Post-Fire Erosion Studies

While much can be gained from examining the general geomorphic effects of fire

in Mediterranean type climatic regions, Booker et al. (1993) show the hazards of ignoring

the diversity of responses within these regions. They examine some of the geomorphic

effects of and the human responses to the 1991 Tunnel Fire of Oakland, California. The

authors explain that on October 20, 1991 the Tunnel Fire engulfed the Oakland and

Berkeley hills. Eighteen hundred acres, including urban land and wildlands, burned,

leaving steep, exposed slopes. The Soil Conservation Service estimated that soil

hydrophobicity would be relatively high in the burned area, and they characterized soils

there as highly erodible. With this setting in mind and with knowledge of the destructive

debris flows that follow wildfires in southern California, policymakers and land managers

in Oakland took comprehensive actions to prevent excessive erosion in the burned area

(Booker et al. 1993).

Booker et al. (1993) examined Oakland’s post-fire response by measuring winter

runoff and erosion during the rainy winter after the fire and they conducted simulated

rainfall experiments during the summer following the fire (1992). Their field and

literature research revealed several results. First, geology, topography, geomorphology,

climate, and historical records should be consulted before assuming a debris flow, or

“fire-flood” sequence, will follow a wildfire (Table 3) (Booker et al. 1993). After the

Tunnel Fire, land managers used soil erosion indices calibrated to southern California,

despite Oakland’s differences in the above-mentioned factors. The authors recommended

that local empirically derived indices be used instead. Second, as evidenced by aerial

photographs taken after the fire, reseeding and copious applications of hydromulch may

be counterproductive on wildlands without much ground disturbance: treated slopes were

sparsely vegetated and untreated slopes were more densely vegetated. Third, land

managers should avoid erosion control measures that increase infiltration on slopes prone

to infiltration-triggered landslides. The authors note that landslide danger in the Oakland

Hills is significant and not necessarily related to fire (Table 4). Fourth, gophers caused

the largest natural slope response after the fire, dispelling the idea that soil

hydrophobicity is the leading cause of post-fire soil erosion (Figure 9). Finally, the

evidence suggests that fire suppression and post-fire reconstruction of urban areas

produced the most erosion after the Tunnel Fire (Figure 10). The authors stress that

reassessment of erosion control spending is needed to ensure cost-effectiveness.

Table 3. Comparison of watershed parameters between southern California and the Oakland Hills.From Booker et al. (1993: 161).

PARAMETER COASTAL SOUTHERNCALIFORNIA “FIRE-

FLOOD” WATERSHEDS

OAKLAND HILLS FIREAREA WATERSHEDS

Max. Relief 9,184 feet (2,800 m) 1,100 feet (335 m)Slope Ave: 65%; Max: > 100%

(Wells 1981)Ave: 35%; Max: 90%

Watershed area 1 km2 – 13 km2 (0.6 – 7.8square miles) (Wells, 1981;Taylor, 1983)

<2km2 (1.2 square miles)

Soils Coarse (granitic, sandy)soils, shallow no soil profiledevelopment (Wells, 1981)

Loams, shallow, moderate to welldeveloped profiles (Welch, 1981)

Background erosion rate 1.4 – 2.3 mm/year (0.06 –0.09 inch per year) (Wells,1981; Scott and Williams,1978)

0.08 mm/year (0.003 inch/year)(Reneau, 1988)

Rainfall intensity 25 mm/hr (1 inch/hr) 2-10-year return interval (PhilHolland, Santa BarbaraCounty Flood Controldistrict, oral communication,1993)

25 mm/hr (1 inch/hr) 100 yearreturn interval (Rantz, 1971)

Table 4. Grasses that grew from seeds in the hydromulch application allowed higher infiltrationthan non-treated or unburned areas. Because many slopes in the Oakland Hills are prone tolandslides, the higher infiltration on some of these slopes could have led to soils saturated enoughto produce debris flows. Thus, the hydromulch treatment appeared counterproductive. FromBooker et al. (1993: 165).

Figure 9. Graph shows that soil disturbance due to gophers was significantly higher at test plotsthan soil disturbance due to winter storms. From Booker et al. (1993: 167).

Figure 10. Post-fire erosion control methods were not employed at post-fire construction sites.This photo shows rill and gully formation due to post-fire construction. From Booker et al. (1993:p. 169).

Debris Flows Defined

Referring to Booker et al. (1993), Cannon (2001) notes the considerable public

expense associated with debris-flow hazard mitigation. She explains that policy makers

and citizens share the expectation that steep, burned watersheds will lead to debris flows.

Thus, despite the evidence that there is a range of responses in watersheds to wildfires,

hillslope rehabilition and retention and diversion structure construction tend to be

employed after a fire without regard for the unique conditions of the watershed (Cannon

2001).

Cannon’s (2001) paper explores how and when debris flows occur on recently

burned watersheds. She explains that because of the destructive nature of debris flows

and the expense involved in preventing them, it is critical that geomorphologists better

understand them.

Cannon (2001) conducted fieldwork in California, New Mexico, and Colorado in

order to better define fire-induced debris flows and the conditions that precede them. She

assessed the effects of lithology, water repellency, basin configuration, and burn extent

and she studied debris flow initiation processes. She looked at the debris-flow response in

95 recently burned watersheds (Cannon 2001). Eighty-six of these were in the southern

California Mediterranean region.

Critical to her study and results was that she separated debris flows into two

classes. Type 1 debris flows are characterized by poorly sorted materials (up to boulder-

sized) in levee and lobe forms with “significant relief and sharp, well-defined

boundaries” (Cannon 2001: 326). This type can be destructive and dangerous. Well

sorted sand and gravel-sized material in an ash and charcoal-rich matrix define Type 2

debris flows (Cannon 2001: 328). Due to the smaller grain-size found in Type 2 debris

flows they were found to be much less destructive.

The results of Cannon’s (2001) study showed that only 37 of the 95 burned

watersheds produced debris flows and only 23 of them were characterized as Type 1. The

Sedimentary rock types (marine and nonmarine sediments and Quaternary landslides)

were noted as the parent material in most of the watersheds that produced Type 1 debris

flows. Type 1 debris flows originated in watersheds that ranged in size from 0.02 km2 to

10 km2. Relief ratios ranged from 0.07 to 0.6 (Cannon 2001: 336). Interestingly, Cannon

(2001) also found that Type 1 debris flows are sometimes initiated by infiltration-

triggered soil slip. Because hillslope rehabilitation has been based on the idea that

hydrophobic soils induce debris flows due to progressive bulking of surface runoff,

efforts are often made to increase infiltration. Cannon’s (2001) results suggest that this

could be a detrimental approach.

Cannon (2001) also found that Type 1 debris flows occurred in watersheds that

ranged in burn extent from 5 to 100 percent. And crucial to the study of fire-related

erosion, she determined that it is 6 times more likely for a Type 1 debris flow to initiate

in a watershed lacking hydrophobic soils than from a watershed with extensive

hydrophobic soils (Cannon 2001).

Type 2 debris flows, Cannon (2001) explains, originated only within watersheds

that are underlain by crystalline rock types. In addition, only runoff-dominated processes

produced Type 2 debris flows. Water-repellency was noted 16 times more often than not

in the watersheds that produced Type 2 debris flows (Cannon 2001). Watersheds ranged

in size from 0.02 km2 to 2 km2 and relief ratios ranged from 0.8 to 0.35. Burn extent

ranged from 8 to 100 percent. These results show that geology is a critical factor that can

be used to evaluate the likely post-fire response within a basin. It is also clear that water-

repellent soils may not be the precursor to devastating debris flows as previously thought.

Conclusion

The effects of fire on soil erosion and hydrologic response are not easy to predict.

We know that the removal of vegetation and exposure of bare soil will inevitably lead to

increased soil erosion and runoff. As shown here, the severity of the post-fire response,

however, depends on the intensity and severity of the fire, and on the geology, soil type,

vegetation, topography, climate, and land use of a particular area (Booker, Dietrich, and

Collins 1993, Sevink et al. 1989, Scott and VanWyk 1990). And while the Mediterranean

wet winters provide hydrologic energy that can lead to landslides, debris flows, and

flooding, not all watersheds respond so dramatically. Therefore, before spending public

funds on expensive mitigation, policy-makers must study the existing literature on post-

fire responses in their area or in areas with very similar geology, lithology, and other

relevant factors.

References

Booker, F. A., W. E. Dietrich, and L. M. Collins. 1993. Runoff and erosion after the Oakland firestorm: expectations and observations. California Geology 46(6): 159-173.

Cannon, S. H. 2001. Debris-flow generation from recently burned watersheds. Environmental Engineering Geoscience 7(4): 321-341.

DeBano, Leonard F. 1981. Water repellent soils: a state-of-the-art. Gen. Tech. Rep. PSW-46. Berkeley, CA: Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture; 21 p.

Debano, L. F. 2000a. The role of fire and soil heating on water repellency in wildland environments: a review. Journal of Hydrology 231-232: 195-206.

Debano, L. F. 2000b. Water repellency in soils: a historical overview. Journal of Hydrology 231-232: 4-32.

Imeson, A. C., J. M. Verstraten, E. J. van Mulligen, and J. Sevink. 1992. The effects of fire and water repellency on infiltration and runoff under Mediterranean type forest. Catena 19: 345-361.

Inbar, M. M. Tamir, and L. Wittenberg. 1998. Runoff and erosion processes after a forest fire in Mount Carmel, a Mediterranean area. Geomorphology 24: 17-33.

Sampson, A. W. 1944. Effect of Chaparral Burning on soil erosion and on soil moisture relations. Ecology 21(2): 171-191.

Sevink, J., A. C. Imeson, and J. M. Verstraten. 1989. Humus form development and hillslope runoff, and the effects of fire and management, under Mediterranean forest in NE Spain. Catena 16(4-5): 461-475.

Scott, D. F. 1993. The hydrological effects of fire in South African mountain catchments.Journal of Hydrology 150: 409-432.

Scott, D. F. and D. B. VanWyk. 1990. The effects of wildfire on soil wettability and hydrological behavior of an afforested catchment. Journal of Hydrology 121: 239-256.

Wells, W. G. II. 1981. Erosion and sediment transport in Pacific Rim steeplands. I.A.H.S. Publication No. 132 Chiristchurch, New Zealand.

Wells, W. G. II. 1987. The effects of fire on the generation of debris flows in southern California. Geological Society of America Reviews in Engineering Geology 7: 105-114.