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

Transcription

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.

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4 Post-fire Hydrology

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 40 O N Lower Flowers Hourglass Big Elk Dadd Bennett Hewlett Gulch Bobcat Crosier Mt. Bear Tracks Denver Hayman Schoonover 4 O 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, ); Sergio Alegre (Visiting Ph.D. student, 2010).

8 More students to do the hard work: Sarah Schmeer (Watershed) and Dan Brogran (Civil Engineering) Thanks to private landowners who allow access!

9 Key Observations about the High Park Fire Relatively small area burned at high severity;

10 Fire severity map (CSU version) Gordon Creek Cache la Poudre River Poudre Park Hill Gulch Skin Gulch Bennett Creek Lewstone Creek Lewstone Creek Pendergrass Creek Redstone Creek Redstone Creek Laurence Creek Stove Prairie Creek

11 Key Observations about the High Park Fire This patchiness relatively typical; Amount and distribution of burn severity has important implications for runoff, erosion, and delivery to the Poudre River;

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 Hillslope component

15 Newly-installed sediment fence to measure sediment production

16 Later that afternoon in a rainstorm...

17 600 kg of sediment from 0.75 inches of rain!

18 At Hill Gulch in a storm...

19 And downstream at Highway 14

20 Using high water marks to estimate peak flows

21 Threat of yet another flood!

22 At Skin Gulch, a 7-ft culvert no longer big enough.

23 Flow across Highway 14 and into the Poudre

24 Cross-section at Skin Gulch, pre-flood

25 Accumulation of woody debris and sediment, Skin Gulch cross-section

26 Cross-section 100 m upstream over time Skin Gulch at Stove Prairie and Highway Z (m ) 1 7/4/2012 7/22/2012 8/5/ Distance (m)

27 Channel cross-section change

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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; Most sites burned at moderate severity;

31 Sediment yield vs. percent bare soil Sediment yield (Mg ha -1 yr -1 ) y = e 0.095x p < R 2 = 0.61 n= Percent bare soil

32 Percent bare soil vs. time since burning 100 High severity y = Ln(x) Percent bare soil Moderate severity Low severity R 2 = 0.63 y = Ln(x) R 2 = 0.60 y = Ln(x) R 2 = Time since burning (years)

33 Annual sediment yields vs. time since burning: High severity sites 40.0 Sediment yield (Mg ha -1 ) Big Elk Hayman Schoonover Hewlett Gulch Bobcat Lower Flowers Crosier Mountain Bear Tracks Hourglass Median value Time since burning (years)

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 downstream;

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; Poudre River should slowly clean itself;

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 Good cover, but all leafy spurge (Hill Gulch)

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: Not cause any significant hillslope erosion;

44 Sediment vs. I 30 : Recently burned, high severity wildfires 10-1 ) Mean sediment production (Mg ha minute maximum intensity (mm hr -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; ; Why is this important?

46 Collecting Data at Different Spatial Scales Point scale: soil water repellency; Small plot scale (1 m 2 ): 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 km 2 ): Measuring runoff, suspended sediment yields, water quality, and channel morphology.

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48 Our cover data

49 Event-based sediment production vs. I 30 : High-severity wildfires Mean sediment production (Mg ha -1 ) years after burning 4-5 years after burning R 2 = 0.50 R 2 = minute maximum intensity (mm hr -1 )

50 Mean sediment yields on control and mulched plots: Bobcat fire, Sediment yield (Mg ha -1 ) * * * * Control Old mulch New mulch * = Significant treatment effect (p<0.05)

51 Sediment yields for controls and straw mulch 20 with seeding: Hayman fire, Sediment yield (Mg ha -1 ) * * Control Straw mulch with seeding * = Significant treatment effect (p<0.05)

52 Sediment yields by fire severity and season: First two years after burning 10.0 Sediment yield (Mg ha -1 ) Summer Rainfall (Jun-Oct) Snowmelt (Nov-May) 0.0 High Moderate Low Fire severity

53 Hayman Fire, Colorado: August 2004

54 Alluvial fan from Saloon Gulch extending into the South Platte River, summer 2004 (two years after the 2002 Hayman fire)

55 Causes of Post-fire Runoff and Erosion Soil water repellency Loss of surface cover Loss of organic matter Loss of plant canopy Reduced roughness Increased velocity Soil sealing Rainsplash Increased soil erodibility Reduced Interception and transpiraton Increased runoff (peak flows, water yield) Increased surface erosion (rainsplash, sheetwash, rilling, gullying)

56 Area Burned and Precipitation, Km2 burned Annual precipitation 70 Historic mean precipitation Wildfire (km 2 ) Annual precipitation (cm) Years Climate change can greatly affect the amount and severity of future wildfires!

57 Soil Water Repellency

58 Soil water repellency over time: Bobcat fire 80 a) 0 mo (n = 45 at each depth) 80 b) 3 mo (n = 45 at each depth) Critical surface tension (dynes cm -1 ) Unburned 50 Low severity 40 Moderate severity High severity Depth (cm) Critical surface tension (dynes cm -1 ) Unburned Low severity Moderate severity High severity Depth (cm) Critical surface tension (dynes cm -1 ) c) 12 mo (n = 45 at each depth) Unburned Low severity Moderate severity High severity Depth (cm) MacDonald and Huffman, 2004

59 Spatial variability in soil water repellency: Plot H1, high severity, Hayman fire 14 Meters Moles of ethanol per liter Meters Woods et al. 2007, Geomorphology

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; CST faster and more consistent than WDPT.

61 Year 2000, 15 days after fire 96% Bare soil Vegetation recovery over time Bobcat fire, sediment fence #9 Year % Bare soil Year % Bare soil Year % Bare soil

62 Rainfall erosivity versus mean sediment yields: Five storms on three areas, Hayman fire, Rainfall erosivity (MJ mm ha -1 h -1 ) Sediment yield Mg ha -1 ) Jun Jun Jul Aug-03 6-Sep-03 USG South (n=3) USG North (n=8) USG East (n=5)

63 Slower recovery on high-severity sites, Hayman-Schoonover wildfires 20 Mean sediment production (Mg ha -1 ) Time since burning (years)

64 Upper Saloon Gulch: 10 July mm rain in 2 hours

65 Sediment yields from swales vs. planar hillslopes in 2001: Bobcat fire Sediment production (kg m -2 ) Erosivity (MJ mm ha -1 h -1 ) Planar hillslopes 2001 Swales 2001 R 2 = 0.57 p = 0.01 R 2 = 0.53 p = 0.007

66 Treatments after the Hayman fire: Scarifying and seeding; 2. Straw mulching with seed; 3. Hydromulching Ground based; Aerial; 4. Polyacrylamide (soil binding agent).

67 Seeding

68 Percent surface cover for controls and seeding: Bobcat fire, Percent Cover Fall 2000 Spring 2001 Fall 2001 Spring Fall 2002 Spring 2003 Fall 2003 Control Seeding No significant treatment effect

69 Sediment yields for controls and seeding: Bobcat fire, Sediment yield (Mg ha -1 ) Control Seeding No significant treatment effect

70 Mulching

71 Video Clip: Mulching

72 Percent surface cover on control and mulched plots: Bobcat fire, Percent Cover * * * * * * * * * Fall 2000 Spring 2001 Fall 2001 Spring Fall 2002 Control Old mulch New mulch Spring 2003 Fall 2003 * = Significant treatment effect (p<0.05)

73 Runoff and sediment yields from rainfall simulations on control and straw mulch plots: Hayman fire, 2003 Runoff / Rainfall Ratio Sediment Yield Runoff / rainfall ratio (%) % mean 45% mean Sediment production (g/hr) g mean 86 g mean Controls Dry Mulch Controls Dry Mulch

74 Intensity threshold: Raked swales 15-1 ) Mean sediment production (Mg ha minute maximum intensity (mm hr -1 )

75 Event-based sediment production vs. I 30 : Burned and raked swales 15 Mean sediment production (Mg ha -1 ) Burned swales Raked swales minute maximum intensity (mm hr -1 )

76 Causes of Post-fire Runoff and Erosion Soil water repellency Loss of surface cover Loss of organic matter Loss of plant canopy Reduced roughness Increased velocity Soil sealing Rainsplash Increased soil erodibility Reduced Interception and transpiraton Increased runoff (peak flows, water yield) Increased surface erosion (rainsplash, sheetwash, rilling, gullying)

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 (type Lee MacDonald, Colorado into google).

82 Questions?

83 Soil water repellency and sediment yields Mean sediment yield (Mg ha -1 ) over time: Hayman fire 12 Sediment yield 0 Soil water repellency at 0 cm 10 No water repellency Time since burning (years) Mean critical surface tension (dynes cm -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 100 Sediment yield (Mg ha Year 2 Year 3 Year 4 p=0.72 p=0.49 p= Contributing area (m 2 )

86 Sediment yield versus contributing area for convergent hillslopes: Bobcat fire Sediment yield (Mg ha ) Year 2 Year 3 Year 4 p< p=0.07 p< Contributing area (m 2 )

87 Contributing area versus sediment yield Sediment yield (Mg ha -1, Mg ha -1 yr -1, Mg ha -1 h -1 ) R 2 = Spain: High severity 0.01 Spain: Low severity US: High severity US: Moderate severity US: Low severity Best fit line E+07 1E+08 1E+09 Contributing area (m 2 )

88 Percent bare soil on control and mulched plots: Bobcat fire, Percent bare soil Fall 2000 Spring 2001 Fall 2001 Spring 2002 Fall 2002 Spring 2003 Fall 2003 Control Old Mulch New Mulch

89 Mean sediment yields on control and mulched plots: Bobcat fire, Sediment yield (kg ha -1 ) Control Old Mulch New Mulch

90 Questions?

91 Sediment yield versus contributing area for planar 100 hillslopes: Hayman fire 10 Sediment yield (Mg ha -1 ) p=0.04 p=0.05 p<0.01 Year 2 Year 3 Year Contributing area (m 2 )

92 Sediment yield versus contributing area for planar hillslopes: Bobcat fire Sediment yield (Mg ha ) p<0.01 p<0.01 p=0.5 Year 2 Year 3 Year Contributing area (m 2 )

93 Sediment yield versus contributing area for mulched convergent hillslopes and watersheds: Hayman fire ) 10 Sediment yield (Mg ha Year 2 Year 3 p=0.6 p=0.5 Year Contributing area (m 2 ) p=0.06

94 Mean sediment production by fire severity 2500 and year: Bobcat Fire Sediment yield (g) High Moderate Low / Unburned Fire severity

95 Sediment yields from unburned plots and burned untreated plots, Hayman fire Sediment yield (g) Unburned 2003 burned untreated 2004 burned untreated

96 Sediment yield vs. percent bare soil for rainfall simulations, Bobcat Fire 3000 Sediment yield (g) High severity Moderate severity Low severity/unburned R 2 = 0.72 p < % 20% 40% 60% 80% 100% Bare soil

97 Percent bare soil and sediment yields for three areas in the Bobcat Fire (all high severity; bars indicate one standard deviation) 100 a Percent bare soil Sediment yield (Mg ha -1 ) Bobcat Gulch Jug Gulch Green Ridge 0 Bobcat Gulch Jug Gulch Green Ridge

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 Cross-section slightly upstream