Post-fire Recovery and Restoration: Soils, Runoff and Erosion

99  Download (0)

Full text

(1)

Post-fire Recovery and Restoration:

Soils, Runoff and Erosion

Lee H. MacDonald

Watershed Science Program

Dept. of Ecosystem Science and Sustainability Colorado State University

(2)

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.

(3)
(4)
(5)

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?

(6)

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

(7)

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).

(8)

More students to do the hard work: Sarah

Schmeer (Watershed) and Dan Brogran (Civil

Engineering)

(9)

Key Observations about the High Park Fire

(10)

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 Park

(11)

Key Observations about the High Park Fire

• This patchiness relatively typical;

• Amount and distribution of burn severity has

important implications for runoff, erosion,

(12)

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;

(13)

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!

(14)
(15)

Newly-installed sediment fence to

measure sediment production

(16)
(17)
(18)
(19)
(20)
(21)
(22)

At Skin Gulch, a 7-ft culvert no

longer big enough ….

(23)
(24)

• Cross-section at

Skin Gulch,

(25)

Accumulation of woody debris and

sediment, Skin Gulch cross-section

(26)

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

(27)
(28)
(29)

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;

(30)

Implications

• Relatively low amounts of high severity and

a wet July has led to rapid regrowth;

– Riparian areas; – Shrublands;

(31)

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 )

(32)

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

(33)

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 )

(34)

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.

(35)

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?

(36)

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

(37)

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;

(38)

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;

(39)

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;

(40)
(41)

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.

(42)

More information

• Type Lee MacDonald into google, and look

at the publications on my web site;

• 2008 Stream Notes provides a nice readable

summary;

(43)

Predictions

• Relatively rapid regrowth in shrublands

and areas burned at moderate severity;

• Winter rains and snowmelt will:

(44)

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 )

(45)

Predictions

• Relatively rapid regrowth in shrublands

and areas burned at moderate severity;

• Winter rains and snowmelt will:

– Not cause any significant hillslope erosion; – ;

(46)

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.

(47)
(48)
(49)

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 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 )

1-3 years after burning 4-5 years after burning

(50)

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)

* *

(51)

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 )

(52)

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

(53)
(54)

Alluvial fan from Saloon Gulch extending into the South Platte River, summer 2004

(55)

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

(56)

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 precipitation

Historic mean precipitation

Climate change can greatly affect the amount and severity of future wildfires!

(57)
(58)

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, 2004

(59)

0 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

(60)

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;

(61)

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

(62)

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 )

(63)

Slower recovery on high-severity sites,

Hayman-Schoonover wildfires

0 4 8 12 16 20 0-1 1-2 2-3 3-4 4-5

Time 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 )

(64)

Upper Saloon Gulch: 10 July 2002

(65)

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 )

(66)

Treatments after the Hayman fire: 2002

1. Scarifying and seeding;

2. Straw mulching with seed;

3. Hydromulching

– Ground based; – Aerial;

(67)
(68)

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

(69)

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 )

(70)
(71)
(72)

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 * * * * * * * * *

(73)

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

(74)

Intensity threshold: Raked swales

0 3 6 9 12 15 0 5 10 15 20 25 30 35

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 )

(75)

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

(76)

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

(77)

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.

(78)

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;

(79)

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;

(80)

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.

(81)

Even more information . . . .

• Numerous papers on my web site

(82)
(83)

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 )

(84)

Scale effects:

How do post-fire sediment yields

vary with contributing area?

(85)

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=0.16

(86)

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=0.07

Sediment yield versus contributing area for

convergent hillslopes: Bobcat fire

(87)

Contributing area versus sediment yield

R2 = 0.10 0.0001 0.001 0.01 0.1 1 10 100 1000

0.1 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 )

(88)

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

(89)

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

(90)
(91)

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

(92)

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

(93)

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:

(94)

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

(95)

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

(96)

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

(97)

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

(98)

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.

(99)

Figure

Updating...

References

Related subjects : Recovery and Restoration