Effects of Wildfires on Runoff and Erosion - Lee H. MacDonald Watershed Science Program Colorado State University, Fort Collins, CO
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Effects of Wildfires on Runoff and Erosion
Lee H. MacDonald
Watershed Science Program
Colorado State University, Fort Collins, CO
Contributors • Tedd Huffman (M.S., 2002); • Juan Benavides-Solorio (Ph.D., 2003); • Joe Wagenbrenner (M.S., 2003); • Matt Kunze (M.S., 2003); • Zamir Libohova (M.S., 2004); • Jay Pietraszek (M.S., 2006); • Daniella Rough (M.S., 2007); • Duncan Eccleston (M.S., 2008); • Keelin Schaffrath (M.S., 2009); • Darren Hughes (M.S., 2010); • Ethan Brown (M.S., 2009); • Dr. John Stednick; • Isaac Larsen (Research Assistant, 2004-2007); • Sergio Alegre (visiting Ph.D. student, 2010).
Why the concern about wildfires?
Hayman Fire, Colorado: August 2004
Channel incision from a 20 mm/hr rain event after
the Cerro Grande Fire near Los Alamos, NM
Photo by John Moody, USGS
Alluvial fan from Saloon Gulch extending into
the South Platte River, Summer 2004
(2 years after burning!)
Post-fire Hydrology
Objectives
1. Provide a process-based understanding of the effects
of wild and prescribed fires on soils, runoff, and
erosion;
2. Evaluate the relative importance of different
controlling factors on post-fire erosion rates;
3. Determine the rate of recovery to pre-fire conditions;
4. Discuss how post-fire processes and recovery vary
with increasing scale, and put the effects of wildfire
in a broader context.
Post-fire Effects Vary with Burn Severity
1) High severity: complete consumption of organic
horizon and alteration of the structure or color of the
underlying mineral soil; loss of aggregates
(“pulverization”):
2) Moderate severity: consumption of litter layer but
no visible alteration of the surface of the mineral soil;
3) Low severity: only partial consumption of the
surface litter.
Severity is not equal to intensity (heat loss per unit width
per unit time), but severity and intensity often
assumed to be closely correlated;Why the sharp increase in runoff and
erosion after some high-severity wildfires?
1. Loss of canopy decreases interception and
evapotranspiration, increasing runoff;
2. Loss of litter decreases interception and exposes soil to
raindrop impacts (increased erodibility) and sealing;
3. Loss of soil organic matter disaggregates or pulverizes
the soil, and this increases soil erodibility;
4. Increase in soil water repellency can decrease
infiltration and increase surface runoff;
5. Loss of litter decreases surface roughness and
increases runoff velocities, increasing erosion;
Effects are synergistic, but which is most important?Soil Water Repellency
Fire-induced soil water repellency
(DeBano, 1981)Methods of Analysis
Water drop penetration time (WDPT):
• Apply drops at different depths, beginning at mineral soil
surface;
• Indefinite waiting time;
• Assesses persistence of soil water repellency.
Critical surface tension test (CST):
• Apply 5 drops of de-ionized water;
• If 4 of 5 drops are not absorbed within 5 seconds, test
solutions with progressively higher ethanol concentrations
(increasing ethanol concentrations decrease surface tension);
• Critical surface tension (CST) is the tension of the first
solution that is readily absorbed into the soil (“strength”).Critical surface tension in wild and
prescribed fires: High-severity sites
(bottom two sites are prescribed fires)
80
Critical surface tension
70
(dynes cm -1)
60 Crosier M tn.
Hi M eadows
50
B obcat
40 Lower Flowers
Dadd B ennett
30
0 3 6 9 12 15 18
Huffman et al., 2001 Depth (cm)Critical surface tension in wild and prescribed
fires: Moderate-severity sites
(bottom two sites are prescribed fires)
80
70
Critical surface tension
(dynes cm -1)
60 Hi M eado ws
Cro sier M tn.
50
B o bcat
Lo wer Flo wers
40
Dadd B ennett
30
0 3 6 9 12 15 18
Huffman et al., 2001 Depth (cm )Critical surface tension in wild and prescribed
fires: Low severity sites
(bottom two sites are prescribed fires)
80
Critical surface tension
70
Hi M eadows
(dynes cm -1)
60 B obcat
Crosier M tn.
50
Lower Flowers
40
Dadd B ennett
30
0 3 6 9 12 15 18
Huffman et al., 2001 Depth (cm )Median soil water repellency over time,
Star Fire, Tahoe National Forest
75
Median critical surface tension (dynes cm -1)
70
65
60
55
50
Unburned
45
Summer 2002 (10 months)
40 Summer 2003 (22 months)
Summer 2004 (34 months)
35
30
0 2.5 5 7.5 10 12.5
Depth (cm)
E. Chase, M.S.. thesis, Colorado State Univ., 2006Mean soil water repellency by depth:
Unburned vs. burned sites, summer 2002
Hayman fire (n=36)
80
Schoonover fire (n=18)
Critical surface tension (dynes cm-1 )
Trumbull: unburned
70 (n=39)
60
50
40
30
0 3 6 9 12
Depth (cm)
D. Rough, 2007Soil water repellency from 2002-2004:
Upper Saloon Gulch, Hayman fire
80
75
Critical surface tension (dynes cm -1)
70
65
60
55
2004
50
2003
45
2002
40 No water repellency
35
30
0 3 6 9 12
Depth (cm)
D. Rough, 2007Spatial variability in soil water repellency:
Plot H1, high severity, Hayman fire
14
12 Moles of
4 ethanol
10
per liter
3
Meters
8
2
6 1
4 0.4
0
2
0
0 2 4 6 8 10 12 14
Meters
Woods et al. 2007, GeomorphologySummary: Soil Water Repellency • Soils in unburned areas usually water repellent; • Fire-induced water repellency is usually shallow (maximum of 9 cm); • May be stronger in prescribed fires due to higher fuel loadings and slower rate of fire spread; • Very high spatial variability; • Relatively rapid recovery (≤ 2 years); • Not present under wet conditions (~10-35 percent soil moisture), depending on fire severity; • CST faster and more consistent than WDPT.
Supporting Data
Three papers on my web site (type “Lee MacDonald” into
google):
1. 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”,
Hydro. Proc. 15: 2877-2892.
2. MacDonald, L.H., and E.L. Huffman, 2004. “Persistence and
soil moisture thresholds”, Soil Sci. Soc. Am. J. 68: 1729-1724;
3. Doerr, S.H., R.H. Shakesby, and L.H. MacDonald, 2009.
“Soil water repellency: a key factor in post-fire erosion?” In
Restoration Strategies after Forest Fires, edited by A. Cerda
and P.R. Robichaud, Science Publishers, Inc., Enfield, NH.Sediment Production at the
Hillslope ScaleTotal plot years of data by treatment
Untreated
High severity 319
Moderate severity 55
Low severity 34
Treated (all high severity)
Seeding and scarification with seeding 36
Straw mulch and straw mulch with seeding 60
Contour-felled logs 44
Ground-applied hydromulch 20
Aerially-applied hydromulch 20
Polyacrylamide 12
Total 600Sediment yields by fire severity and season:
First two years after burning (Colorado)
10.0
Summer Rainfall (Jun-Oct)
Sediment yield (Mg ha-1)
8.0 Snowmelt (Nov-May)
6.0
4.0
2.0
0.0
High Moderate Low
Fire severityRole of surface cover, recovery over time, and rainfall intensity
Sediment production: Summer 2001 (before Hayman fire)
1.20
1.00
Sediment (kg/m 2)
0.80
0.60
0.40
0.20
0.00
1 2 3 4 5 6 7 8 9 10
Pairs of sedim ent fences (n = 20)Mean percent ground cover in Upper Saloon Gulch in
2001 (prior to burning) and 2002 (after the Hayman fire)
10 0
2001
90
2002
80
Ground cover (%)
70
60
50
40
30
20
10
0
1\ 8 1\ 9 1\ 12 1\ 13 1\ 14 1\ 15 1\ 16 1\ 17 1\ 18 1\ 19 1\ 2 0 1\ 2 1 1\ 2 2 1\ 2 3 1\ 2 4 1\ 2 5 1\ 2 6 1\ 2 7 1\ 2 8 1\ 2 9
SwaleSediment from 11 mm of precipitation in
45 minutes on 21 July 2002Sediment production after Hayman fire:
21 July 2002 storm (11 mm in 45 minutes)
1.40
Control
1.20 "Treated" (not
implemented)
1.00
Sediment (kg/m 2)
0.80
0.60
0.40
0.20
0.00
1 2 3 4 5 6 7 8 9 10
PairsVegetation recovery over time
Bobcat fire, sediment fence #9
Year 2000, 15 days after fire Year 2001
96% Bare soil 69% Bare soil
Year 2002 Year 2003
17% Bare soil 12% Bare soilPercent bare soil vs. time since burning
100 High severity y = -26.09Ln(x) + 70.03
R2 = 0.63
Moderate severity y = -21.86Ln(x) + 45.75
80 R2 = 0.60
Low severity y = -10.44Ln(x) + 26.21
R2 = 0.49
Percent bare soil
60
40
20
0
0 1 2 3 4 5 6 7 8 9 10
Time since burning (years)Sediment yield vs. percent bare soil
50
0.095x
Sediment yield (Mg ha-1 yr-1)
y = 0.0029e
p < 0.0001
40 R2 = 0.61
n=345
30
20
10
0
0 20 40 60 80 100
Percent bare soilEvent-based sediment production vs. I30:
High-severity wildfires
1-3 years after burning
10 4-5 years after burning 2
R = 0.62
Mean sediment production (Mg ha -1)
8
6
4
2
R2 = 0.50
0
0 10 20 30 40 50
-1
30-minute maximum intensity (mm hr )Sediment production over time: Pendola fire, Eldorado N.F.
Post-fire erosion vs. percent bare soil:
Pendola fire, Eldorado N.F.
18
1999-2000
16 2000-2001
) -1 yr-1
2001-2002
14
12
10
8 y=0.12900.0577x
6 R2=0.77
p=0.01
4
2
0
Sediment
0 10 20 30 40 50 60 70 80 90
Percent Bare Soil (%)Is all this sediment coming from:
(a) rainsplash and sheetwash on
the hillslopes; or
(b) rill, gully, and channel erosion?Upper Saloon Gulch: 10 July 2002 17 mm rain in 2 hours
Sediment yields from swales vs.
planar hillslopes in 2001: Bobcat fire
Sediment production (kg m -2)
1.4
1.2
1
0.8 R2 = 0.57
p = 0.01
0.6
0.4
0.2 R2 = 0.53
p = 0.007
0
0 50 100 150 200 250
Erosivity (MJ mm ha -1 h-1)
Planar hillslopes 2001 Swales 2001Measuring rill erosion, Hayman fire
Rill erosion in Swale 4: Storm on 21 August 2003
0 0
-5 -5
2
-10 1 -10
-15 -15
cm
cm
-20 -20
-25 -25
15-Aug-02
-30 -30
23-Aug-02 15-Aug-02
-35 -35
23-Aug-02
-40
-40
0 10 20 30 40 50 60 70 80 890 mm
100 rainfall 0 10 20 30 40 50 60 70 80 90 100
cm I30 = 15.6 mm/hr cm
27.4 MJ mm/ha yr
0 0
-5 3 4
-10
-10
cm
cm
-20
-15
-30
15-Aug-02
-20 15-Aug-02
23-Aug-02
23-Aug-02
-25
-40
0 10 20 30 40 50 60 70 80 90 100 0 10 20 30 40 50 60 70 80 90 100
cm cmEstimated sediment from rill erosion vs.
measured sediment: Hayman wildfire
3000
Estimated sediment from rill incision (kg)
26-Jun-03 14-Jun-03 20-Jul-03
12-Aug-03 6-Sep-03 16-Jun-04
2500 28-Jun-04 28-Jul-04 8-Sep-04
2000
1:1
1500
1000
500
0
0 500 1000 1500 2000 2500 3000
Measured sediment in fence (kg)Inferred sources of runoff and erosion • About 80% of the sediment is coming from rilling on the hillslopes; • These and other data indicate that the post-fire runoff is coming from the hillslopes, but most of the post-fire sediment is coming from incision due to concentrated flows (rill, gully, and channel erosion); • See also Moody and Martin, 2001; 2009.
Controls on Post-fire Erosion • Erosion rates most strongly related to percent bare soil, which is primarily a function of fire severity and time since burning; • For a given percent cover and slope, rainfall intensity is the dominant control, and erosion increases non- linearly with rainfall intensity or erosivity; • Soil water repellency can help reduce infiltration after burning, but the rapid decay and spatial variability suggests it is not the dominant control; • Soil type is generally a third-order control, after cover and rainfall intensity; • Rainfall simulations and other work suggest that post- fire soil sealing is limiting infiltration (SSSAJ, 2009).
Runoff and Water Quality at
Catchment ScaleSaloon Gulch and Brush Creek: A Paired Watershed
Study to Investigate the Effects of ThinningStream reaches: Summer 2001 Saloon Gulch Brush Creek
Saloon Gulch flume before Hayman fire
Saloon Gulch flume after first post-fire rainstorm
Saloon Gulch flume cleaned out after
first post-fire rainstormSaloon Gulch flume after second
post-fire rainstormLower Brush Creek: Upstream of flume
Since runoff rates decline within 2-4 years after burning,
how long will it take to transport the excess sediment
out of this channel?Bobcat fire, 8 years later: How long until this becomes a forest again?
Hayman fire, 7 years later: How long until this becomes a forest again?
Hayman fire, seven years later: How long until this stops eroding and degrading water quality?
Buffalo Creek fire, 2009 (13 years after burning)
Hypothetical erosion rates over
time from different sources
Wildfires
10
Roads
Thinning
Background
1
Erosion
.1
.01
TimeConclusions: Part 2 • High-severity fires can dramatically increase runoff and erosion rates in headwater areas; • Large sediment deposits in lower-gradient channels can result in long-term degradation of aquatic habitat; • For more information, see my web site (type “Lee MacDonald” into google).
Questions?
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