Post-fire Recovery and Restoration:
Soils, Runoff and Erosion
Lee H. MacDonald
Watershed Science Program
Dept. of Ecosystem Science and Sustainability Colorado State University
Objectives
1. Provide a process-based summary of how fires in the Colorado Front Range affect soils, runoff, and erosion:
At different spatial scales; Recovery over time;
2. Present data and observations from the High Park fire;
3. Use this knowledge and understanding to predict future recovery and potential for rehabilitation.
Post-fire Hydrology
• Loss of vegetative canopy and surface cover;
• Shift in runoff processes from sub-surface stormflow to infiltration-excess (Horton) overland flow;
• LARGE increases in peak flows and erosion rates; • Downstream sedimentation;
• Degradation in water quality (turbidity, suspended sediment, nitrate, manganese, dissolved organic carbon), and aquatic habitat.
What are the primary causes of post-fire runoff
and erosion, and what can we do to reduce them?
4O 40O N Denver Bobcat Crosier Mt. Lower Flowers Bear Tracks Hourglass Dadd Bennett Hewlett Gulch Hayman Schoonover Big Elk
Study sites in the Colorado
Front Range
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); • Ethan Brown (M.S., 2009); • Darren Hughes (M.S., 2010?); • Dr. John Stednick;• Isaac Larsen (Research assistant, 2004-2007); • Sergio Alegre (Visiting Ph.D. student, 2010).
More students to do the hard work: Sarah
Schmeer (Watershed) and Dan Brogran (Civil
Engineering)
Key Observations about the High Park Fire
Fire severity map (CSU version)
Hill Gulch Skin Gulch Cache la Pou dre River Pe nd erg ras s C ree k Lewstone Creek Laurence Creek Reds ton e C reek Benn ett C reek Go rdo n C ree k S to v e P ra ir ie C re e k Lewstone Creek R ed sto ne C re ek Poudre ParkKey Observations about the High Park Fire
• This patchiness relatively typical;
• Amount and distribution of burn severity has
important implications for runoff, erosion,
Key Observations about the High Park Fire
• Burned relatively long and late, as only
contained on 1 July;
• First major storm occurred on 6 July, so little
opportunity to collect baseline (pre-storm)
data;
• Relatively wet July led to lots of runoff and
erosion;
• Second largest fire in Colorado history, plus
proximity to CSU, led to an interdisciplinary
research proposal;
RAPID Grant from NSF
• Detailed imagery and topography of the burned area;
• Physical science component:
– Sediment fences to measure hillslope-scale sediment production;
– Channel cross-sections and longitudinal profiles to track incision and deposition;
– Use measured sediment production rates and detailed imagery to predict watershed-scale sediment production; – Use channel data to roughly estimate the proportion of
sediment being delivered to the Cache la Poudre River;
• Now have about 20 sediment fences and 8 rain gages installed, but relatively dry August!
Newly-installed sediment fence to
measure sediment production
At Skin Gulch, a 7-ft culvert no
longer big enough ….
• Cross-section at
Skin Gulch,
Accumulation of woody debris and
sediment, Skin Gulch cross-section
Skin Gulch at Stove Prairie and Highway 14 0 1 1 2 2 3 -2 3 8 13 18 23 28 Distance (m) Z ( m ) 7/4/2012 7/22/2012 8/5/2012
Key Observations about the High Park Fire
• Very large increase in peak flows;
• Undersized culverts at risk, particularly on
private roads;
• Large amounts of sediment deposited in
main channels;
• Believe that most of the sediment is NOT
reaching the Poudre River;
• Ash and fine sediment deposited in the
Poudre River, primarily along channel
margins;
Implications
• Relatively low amounts of high severity and
a wet July has led to rapid regrowth;
– Riparian areas; – Shrublands;
y = 0.0029e0.095x p < 0.0001 R2 = 0.61 n=345 0 10 20 30 40 50 0 20 40 60 80 100
Sediment yield vs. percent bare soil
Percent bare soil
S e d im e n t y ie ld ( M g h a -1 y r -1 )
y = -26.09Ln(x) + 70.03 R2 = 0.63 y = -21.86Ln(x) + 45.75 R2 = 0.60 y = -10.44Ln(x) + 26.21 R2 = 0.49 0 20 40 60 80 100 0 1 2 3 4 5 6 7 8 9 10 High severity Moderate severity Low severity
Time since burning (years)
P e rc e n t b a re s o il
Annual sediment yields vs. time since burning:
High severity sites
0.0 10.0 20.0 30.0 40.0 0 2 4 6 8 10
Time since burning (years)
Big Elk Hayman Schoonover Hewlett Gulch Bobcat Lower Flowers Crosier Mountain Bear Tracks Hourglass Median value S e d im e n t y ie ld ( M g h a -1 )
Hierarchy of controls on post-fire
runoff and erosion
• Percent ground cover;
• Rainfall intensity (8-10 mm/hr
sufficient to generate surface runoff
and erosion);
• Soil:
– Moisture;
– Water repellency;
– Soil type.
Predictions: Now to summer 2013
• Relatively rapid regrowth in shrublands
and areas burned at moderate severity;
• Nearly at the end of the summer
thunderstorm season, so probably will
not see flooding like we saw in July
(whew!);
• Fall rains and winter-spring snowmelt
should?
Predictions
• Fall rains and winter-spring snowmelt
should:
– Not cause any significant hillslope erosion;
– May cause some continuing channel incision in areas that have large sediment deposits, but most of this will still not reach the Poudre River;
– Mobilize much of the ash and fine sediment that is currently stored along the channel margins in the Poudre River and transport it further
Predictions: Summer 2013?
• Most of the ash and readily available fine
sediment has already washed off the slopes
and been delivered to the Poudre River, so
next summer the water will not be nearly as
black;
• Hillslope erosion rates, given similar rainfall
events, will be much less due to the
reduction in percent bare soil;
• Tributary channels will continue to incise
and slowly stabilize;
Recovery: What to do?
• Selective mulching can help in summer
2013:
– Focus on high severity areas that still have relatively little regrowth;
– Even less of the sediment generated from the hillslopes and channels will reach the Poudre River;
– Focus on those tributaries that burned at high severity in their lower sections;
Recovery: What to do?
• Consider planting trees in the areas where
natural seeding is unlikely;
– Litter is arguably the most important source of ground cover, particularly in poorer sites that cannot support a dense ground cover;
• Big problem with invasive species in some
areas;
Recovery: What to do?
• If we get an extreme storm event, all
these predictions go out the window!
– In Hill Gulch, the 1976 Big Thompson storm has had a larger effect on the channels than the High Park fire.
More information
• Type Lee MacDonald into google, and look
at the publications on my web site;
• 2008 Stream Notes provides a nice readable
summary;
Predictions
• Relatively rapid regrowth in shrublands
and areas burned at moderate severity;
• Winter rains and snowmelt will:
Sediment vs. I
30:
Recently burned, high severity wildfires
0 2 4 6 8 10 0 10 20 30 40 50
30-minute maximum intensity (mm hr-1)
M e a n s e d im e n t p ro d u c ti o n ( M g h a -1 )
Predictions
• Relatively rapid regrowth in shrublands
and areas burned at moderate severity;
• Winter rains and snowmelt will:
– Not cause any significant hillslope erosion; – ;
Collecting Data at Different Spatial Scales
• Point scale: soil water repellency; • Small plot scale (1 m2):
– Runoff and sediment yields from rainfall simulations;
• Hillslope scale (0.01 to 1 ha):
– Sediment production from planar hillslopes and swales (zero-order catchments) using sediment fences;
– Using replicated hillslopes (swales) to compare different rehabilitation methods against untreated controls;
• Small catchment scale (0.04 to 6 km2):
– Measuring runoff, suspended sediment yields, water quality, and channel morphology.
Event-based sediment production vs. I
30:
High-severity wildfires
R2 = 0.62 R2 = 0.50 0 2 4 6 8 10 0 10 20 30 40 5030-minute maximum intensity (mm hr-1)
M e a n s e d im e n t p ro d u c ti o n ( M g h a -1 )
1-3 years after burning 4-5 years after burning
Mean sediment yields on control and
mulched plots: Bobcat fire, 2000-2003
0 2 4 6 8 10 12 14 16 18 1
Control Old mulch New mulch
2000 2001 2002 2003 S e d im e n t y ie ld ( M g h a -1 )
* = Significant treatment effect (p<0.05)
* *
Sediment yields for controls and straw mulch
with seeding: Hayman fire, 2002-2006
0 4 8 12 16 20 1
Control Straw mulch with seeding
2002 2003 2004 2005 2006 * * S e d im e n t y ie ld ( M g h a -1 )
0.0 2.0 4.0 6.0 8.0 10.0
High Moderate Low
Fire severity S e d im e n t y ie ld ( M g h a
-1 ) Summer Rainfall (Jun-Oct)
Snowmelt (Nov-May)
Sediment yields by fire severity and season:
First two years after burning
Alluvial fan from Saloon Gulch extending into the South Platte River, summer 2004
Causes of Post-fire Runoff and Erosion Soil water repellency Increased runoff (peak flows, water yield) Increased surface erosion (rainsplash, sheetwash, rilling, gullying) Loss of organic matter Loss of surface cover Soil sealing Reduced roughness Increased velocity Rainsplash Increased soil erodibility Reduced Interception and transpiraton Loss of plant canopy
Area Burned and Precipitation, 1994-2003
0 500 1000 1500 2000 2500 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 Years W il d fi re ( k m 2 ) 0 10 20 30 40 50 60 70 A n n u a l p re c ip it a ti o n ( c m ) Km2 burned Annual precipitationHistoric mean precipitation
Climate change can greatly affect the amount and severity of future wildfires!
Soil water repellency over time: Bobcat fire
30 40 50 60 70 80 0 3 6 9 12 15 Depth (cm ) C ri ti c a l s u rf a c e t e n s io n (d y n e s c m -1) Unburned Low severity Moderate severity High severity a) 0 mo (n = 45 at each depth) 30 40 50 60 70 80 0 3 6 9 12 15 Depth (cm) C ri ti c a l s u rf a c e t e n s io n (d y n e s c m -1) Unburned Low severity Moderate severity High severity b) 3 mo (n = 45 at each depth) 30 40 50 60 70 80 0 3 6 9 12 15 Depth (cm ) C ri ti c a l s u rf a c e t e n s io n (d y n e s c m -1) Unburned Low severity Moderate severity High severity c) 12 mo (n = 45 at each depth) MacDonald and Huffman, 20040 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 0.4 1 2 3 4 Meters Me te rs
Spatial variability in soil water repellency: Plot H1, high severity, Hayman fire
Moles of ethanol per liter
Summary: Soil Water Repellency
• Surface in unburned areas naturally water repellent, but less subsurface water repellency;
• Fire-induced water repellency is usually shallow (maximum of 9 cm);
• Usually strongest in high and moderate severity fires;
• 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;
Year 2001
69% Bare soil
Year 2002
17% Bare soil
Year 2000, 15 days after fire 96% Bare soil
Vegetation recovery over time
Bobcat fire, sediment fence #9
Year 2003
Rainfall erosivity versus mean sediment yields:
Five storms on three areas, Hayman fire, 2003
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0 20.0
10-Jun-03 25-Jun-03 20-Jul-03 11-Aug-03 6-Sep-03
0 100 200 300 400 500 600 700
USG South (n=3) USG North (n=8) USG East (n=5)
S e d im e n t y ie ld M g h a -1 ) R a in fa ll e ro s iv it y (M J m m h a -1 h -1 )
Slower recovery on high-severity sites,
Hayman-Schoonover wildfires
0 4 8 12 16 20 0-1 1-2 2-3 3-4 4-5Time since burning (years)
M e a n s e d im e n t p ro d u c ti o n ( M g h a -1 )
Upper Saloon Gulch: 10 July 2002
Sediment yields from swales vs.
planar hillslopes in 2001: Bobcat fire
R2 = 0.57 p = 0.01 R2 = 0.53 p = 0.007 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 50 100 150 200 250 Erosivity (MJ mm ha-1 h-1) S e d im e n t p ro d u c ti o n ( k g m -2 )
Treatments after the Hayman fire: 2002
1. Scarifying and seeding;
2. Straw mulching with seed;
3. Hydromulching
– Ground based; – Aerial;
0 10 20 30 40 50 60 70 80 90 100 1 P e rc e n t C o v e r Control Seeding Fall 2000 Spring 2001 Spring 2002 Fall 2002 Spring 2003 Fall 2001 Fall 2003
Percent surface cover for controls and seeding:
Bobcat fire, 2000-2003
0 5 10 15 20 25 1 Control Seeding 2000 2001 2002 2003
Sediment yields for controls and seeding:
Bobcat fire, 2000-2003
S e d im e n t y ie ld ( M g h a -1 )Percent surface cover on control and
mulched plots: Bobcat fire, 2000-2003
0 10 20 30 40 50 60 70 80 90 100 1 P e rc e n t C o v e r
Control Old mulch New mulch
Fall 2000 Spring 2001 Spring 2002 Fall 2002 Spring 2003 Fall 2001 Fall 2003 * * * * * * * * *
Runoff and sediment yields from rainfall simulations on control and straw mulch plots: Hayman fire, 2003
Runoff / Rainfall Ratio
0 10 20 30 40 50 60 70 80 90 100 1 2 3 4 5 20 7 8 13
Controls Dry Mulch
R u n o ff / r a in fa ll ra ti o ( % ). 43% mean 45% mean Sediment Yield 0 100 200 300 400 500 600 1 2 3 4 5 20 7 8 13
Controls Dry Mulch
S e d im e n t p ro d u c ti o n ( g /h r) 364 g mean 86 g mean
Intensity threshold: Raked swales
0 3 6 9 12 15 0 5 10 15 20 25 30 3530-minute maximum intensity (mm hr-1)
M e a n s e d im e n t p ro d u c ti o n ( M g h a -1 )
Event-based sediment production vs.
I
30: Burned and raked swales
0 3 6 9 12 15 0 10 20 30 40 50
30-minute maximum intensity (mm hr-1)
M e a n s e d im e n t p ro d u c ti o n ( M g h a -1 ) Burned swales Raked swales
Causes of Post-fire Runoff and Erosion Soil water repellency Increased runoff (peak flows, water yield) Increased surface erosion (rainsplash, sheetwash, rilling, gullying) Loss of organic matter Loss of surface cover Soil sealing Reduced roughness Increased velocity Rainsplash Increased soil erodibility Reduced Interception and transpiraton Loss of plant canopy
Lessons for Future Studies
• High variability between sites necessitates replication; • Replication at plot and hillslope scale feasible, while
replication at the catchment scale is costly and sites are difficult to find and replicate;
• One can never have too many rain gages, as the variability in rainfall can easily confound the results;
• Detailed site measurements are needed to interpret and
explain the results (how do you understand the processes if you only measure the outputs?);
• Few studies are lucky enough to capture the largest storm events that may be of most interest, so the effects of these events have the greatest uncertainty.
Conclusions (1)
• High-severity wildfires increase runoff and
sediment production rates by several orders of magnitude;
• Sediment production rates from high-severity sites are nearly an order of magnitude higher than sites burned at moderate or low severity;
• Sediment production rates are highest in the first two summers after burning, and rapidly decline to near-background levels except in sites with
exceptionally coarse soils with poor growing conditions;
Conclusions (2)
• Percent ground cover is the most important control on post-fire erosion rates;
• Rainfall erosivity, topographic convergence, and soil texture are secondary controls on post-fire erosion; • Rill erosion in convergent areas is the dominant
erosion process rather than sheetwash on hillslopes; • Soil water repellency is too short-lived to account for
the observed increases in sediment production; • We believe that soil sealing on bare soil is the
primary cause of high post-fire runoff and erosion rates;
Conclusions (3)
• Erosion prediction models are not very accurate for individual sites, but can provide a first-order
estimate of average sediment yields;
• Seeding and scarification do not increase ground
cover or reduce erosion rates. Mulching is the most effective post-fire rehabilitation technique because it immediately provides ground cover and prevents
soil sealing, but does not increase regrowth rates; • Large amounts of sediment are deposited in
downstream areas, and downstream channels are
likely to take decades or even centuries to recover to pre-fire conditions.
Even more information . . . .
• Numerous papers on my web site
Soil water repellency and sediment yields
over time: Hayman fire
0 2 4 6 8 10 12 1 2 3 4 5 0 10 20 30 40 50 60 70 80 Sediment yield
Soil water repellency at 0 cm No water repellency M e a n s e d im e n t y ie ld ( M g h a -1 ) M e a n c ri ti c a l s u rf a c e t e n s io n ( d y n e s c m -1 )
Scale effects:
How do post-fire sediment yields
vary with contributing area?
Sediment yield versus contributing area for
convergent hillslopes and watersheds: Hayman and
Schoonover fires
0.001 0.01 0.1 1 10 100 10 100 1000 10000 100000 Contributing area (m2) Year 2 Year 3 Year 4 S e d im e n t y ie ld ( M g h a - p=0.72 p=0.49 p=0.160.001 0.01 0.1 1 10 100 10 100 1000 10000 100000 Contributing area (m2) Year 2 Year 3 Year 4 S e d im e n t y ie ld ( M g h a -1 ) p<0.01 p<0.01 p=0.07
Sediment yield versus contributing area for
convergent hillslopes: Bobcat fire
Contributing area versus sediment yield
R2 = 0.10 0.0001 0.001 0.01 0.1 1 10 100 10000.1 1 10 100 1000 10000 100000 1000000 1E+07 1E+08 1E+09
Spain: High severity Spain: Low severity US: High severity US: Moderate severity US: Low severity Best fit line
Contributing area (m2) S e d im e n t y ie ld ( M g h a -1 , M g h a -1 y r -1 , M g h a -1 h -1 )
Percent bare soil on control and mulched
plots: Bobcat fire, 2000-2003
0 10 20 30 40 50 60 70 80 90
Control Old Mulch New Mulch
P e rc e n t b a re s o il Fall 2000 Spring 2001 Fall 2001 Spring 2002 Fall 2002 Spring 2003 Fall 2003
Mean sediment yields on control and
mulched plots: Bobcat fire, 2000-2003
0 2000 4000 6000 8000 10000 12000 14000 16000
Control Old Mulch New Mulch
S e d im e n t y ie ld ( k g h a -1 ) 2000 2001 2002 2003
0.001 0.01 0.1 1 10 100 10 100 1000 10000 100000 Contributing area (m2) Year 2 Year 3 Year 4 S e d im e n t y ie ld ( M g h a -1 ) p<0.01 p=0.05 p=0.04
Sediment yield versus contributing area for planar
hillslopes: Hayman fire
0.001 0.01 0.1 1 10 100 10 100 1000 10000 100000 Contributing area (m2) Year 2 Year 3 Year 4 S e d im e n t y ie ld ( M g h a -1 ) p<0.01 p<0.01 p=0.5
Sediment yield versus contributing area for planar
hillslopes: Bobcat fire
0.001 0.01 0.1 1 10 100 100 1000 10000 100000 Contributing area (m2) Year 2 Year 3 Year 4 S e d im e n t y ie ld ( M g h a -1 ) p=0.5 p=0.6 p=0.06
Sediment yield versus contributing area for
mulched convergent hillslopes and watersheds:
Mean sediment production by fire severity
and year: Bobcat Fire 2000-03
0 500 1000 1500 2000 2500
High Moderate Low / Unburned
Fire severity S e d im e n t y ie ld ( g ) 2000 2001 2002 2003
Sediment yields from unburned plots and
burned untreated plots, Hayman fire
0 50 100 150 200 250 300 S e d im e n t y ie ld ( g ) Unburned 2003 burned untreated 2004 burned untreated
Sediment yield vs. percent bare soil for rainfall simulations, Bobcat Fire
R2 = 0.72 p < 0.0001 0 500 1000 1500 2000 2500 3000 0% 20% 40% 60% 80% 100% Bare soil S e d im e n t y ie ld ( g ) High severity Moderate severity Low severity/Unburned
Percent bare soil and sediment yields
for three areas in the Bobcat Fire
(all high severity; bars indicate one standard deviation)
a 0 20 40 60 80 100
Bobcat Gulch Jug Gulch Green Ridge
P e rc e n t b a re s o il 2000 2001 2002 2003 S e d im e n t y ie ld ( M g h a -1 ) 0 5 10 15 20 25 30
Bobcat Gulch Jug Gulch
2000 2001 2002 2003
Hydrology of Unburned Forests
• Typically coarser-textured soils (loam, or sandy loam);
• Generally good ground cover (usually ≥ 80%); • Storm runoff generated primarily by subsurface
stormflow;
• Low peak flows from all but highest-magnitude storm events;
• Very low mean erosion rates (<0.1 t ha-1 yr-1); • Clean, high quality water.