Watershed Responses to Fire

1
Watershed Responses to Fire

• James M. Vose
• USDA Forest Service
• Southern Research Station
• Coweeta Hydrologic Laboratory

2
(No Transcript)
3
Focus on Water Quality
4
(No Transcript)
5
Key Components of Watershed Processes
INPUTS precipitation snow VEGETATION-SOIL-WATER
INTERACTIONS evapotranspiration water
yield overland flow - erosion storage
filtering - nutrients OUTPUTS streamflow groundw
ater
6
Vegetation
outputs
inputs
inputs
outputs
Litter Layer
Roots Soil
Microbes
7
Watershed responses depend on fire severity and
intensity!
wildfire gt site preparation gt stand replacement gt
understory
8
Today we will answer these questions

• What are the factors that control watershed
  response?
• How do you evaluate post-fire changes in
  watershed response factors?
• How do erosion processes,runoff, water quality
  change after a fire?
• How are aquatic organisms impacted by post-fire
  water flow and water quality changes?

9
Watershed Response Factors

• Loss of the forest floor (duff or humus and
  litter)
• Overstory mortality vegetation response
• Infiltration rates
• Rainfall intensity
• Topography

10
Factors Controlling Watershed Responses
Vegetation
outputs
inputs
inputs
outputs
Forest Floor
Roots Soil
Microbes
Infiltration rate
11
Loss of the Forest Floor

• The most important factor influencing watershed
  response
• Loss is directly proportional to fire severity
• Duff (humus) and Litter
• Provides soil cover
• Acts as a sponge
• Enhances infiltration

12
(Clinton et al. 1998)
100
litter
Mass Reduction ()
50
duff
0
400
200
50
Mean Peak Flame Temperature (oC)
13
Infiltration Rates

• Collapse of soil structure and reduction in soil
  porosity
• Ash and charcoal residues clog pores
• Hydrophobic soils (western U.S.)
• Raindrop splash (compaction and further loss of
  soil porosity)

14
Raindrop splash on soil surface
15
Rainfall Intensity

• In areas where thunderstorms are common runoff is
  strongly correlated with rainfall intensity
• Thunderstorm rainfall is not uniform
• Topography strongly influences thunderstorm
  rainfall

16
Vegetation Mortality

• Nutrient and water uptake
• Soil stability
• Litter source for forest floor replenishment

17
Today we will answer these questions

• What are the factors that influence watershed
  response?
• How do you evaluate changes in watershed response
  factors?
• How do erosion processes,runoff, water quality
  change after a fire?
• How are aquatic organisms impacted by post-fire
  water flow and water quality changes?

18
Watershed Response Evaluation Techniques

• Helicopter or fixed wing reconnaissance
• Remotely sensed images
• Field validation
• Soil cover
• Infiltration
• Overland Flow
• Modeling

19
(No Transcript)
20
(No Transcript)
21
Burned Area Reflectance Classification (BARC)

• BARC maps are made by comparing satellite near
  infrared reflectance values to satellite mid
  infrared reflectance values
• The relationship between these bands prior to the
  fire and after the fire are measured.
• The areas where that relationship has changed the
  most are the ones that are most likely to be
  severely burned. The areas where that
  relationship has changed little are likely to be
  unburned or very lightly burned.
• http//www.fs.fed.us/eng/rsac/baer/barc.html
• www.fs.fed.us/eng/rsac/baer/barc.html

22
(No Transcript)
23
(No Transcript)
24
Field Validation

• Soil cover and canopy condition
• Infiltration rate/overland flow

25
(No Transcript)
26
Line Intercept rock, vegetation, bare soil,
litter, humus, wood
27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
(No Transcript)
31
Measuring infiltration rates with infiltrometer
32
(No Transcript)
33
Modeling

• Flow, erosion, sediment delivery, nutrients, and
  debris flow modeling
• Watershed response map drives all models

34
Mountain
Stream NO3
Piedmont
35
Today we will answer these questions

• What are the factors that influence watershed
  response?
• How do you evaluate changes in watershed response
  factors?
• How do erosion processes,runoff, water quality
  change after a fire?
• How are aquatic organisms impacted by post-fire
  water flow and water quality changes?

36
Erosion Processes

• Surface erosion
• Ash wash
• Surface filling
• Rill formation
• Landslides
• Debris flows

37
Surface Erosion
Surface erosion is defined as the movement of
individual soil particles by a force, and is
initiated by the planar removal of material from
the soil surface (sheet erosion) or by
concentrated removal of material in a downslope
direction (rill erosion)
38
Surface Erosion Factors

• Susceptibility of the soil to detachment
• Magnitude of external forces (raindrop impact or
  overland flow)
• Amount of protection provided by material that
  reduces the magnitude of the external force

39
Overland Flow

• Overland flow occurs as a result of rainfall in
  excess of soil infiltration capacity and the
  storage capacity of depressions.
• On the unburned forest floor, overland flow
  follows a myriad of interlinking flow paths that
  constantly change as organic material (litter and
  duff layers) and inorganic material (rock) are
  encountered.
• Consumption of the forest floor by fire alters
  the path of overland flow.

40
Ash Wash and Surface Filling

• Ash-wash
• Ash is mobilized and transported off-site
• Surface Filling
• Surface depressions by are filled by sheet
  erosion

41
Ash wash
42
Ash wash and deposition
43
Rill Formation

• Requires high intensity storm
• Increases total channel length in watershed
• Decreases time of concentration
• Can increase probability of debris flows

44
(No Transcript)
45
Rill formation after ash has washed away
46
Landslides

• Removal of trees influences slide initiation
• Changes in watershed hydrology (immediately after
  the fire)
• Loss of shear strength provided by roots (takes
  several years to develop)
• Shear strength loss influences shallow,
  high-velocity slides

47
Debris flow source area unconsolidated material
stored in channel
48
Debris Flows

• Initiated by overland flow in channels
• Channels with unconsolidated material stored in
  the channel
• Initiated from rills and gullies on the hillslope
• Requires high intensity rainfall or rain-on-snow
  event

49
Debris flow initiation from rills and gullies on
a hillslope
50
Effect of Fire on Sediment Transport

• Post Fire Increase
• (kg/ha/yr)
• North Carolina 1,250
• Central Arizona 200,000
• Southern California 50,000

51
(No Transcript)
52
No Movement of Charcoal and Sediment Offsite
Why not ?
Duff layer Coarse woody debris Intact Riparian
Zone
53
Runoff

• Removal of soil cover decreases surface storage
  of water
• Increases concentration of water
• Increases runoff velocity and erosive energy
• Increases extent of overland flow
• Moderate and high watershed response areas can
  produce significant increases in runoff
• Dependent upon duration, intensity, and spatial
  extent of rainfall events

54
(Helvey 1980)
70
post-fire
Annual Runoff Burned Watershed (cm)
pre-fire
10
10 70
100
Annual Runoff Control (cm)
55
Peak Flows

• Relative Increase
• 10 to 400 times
• Values
• 100 cfs to 20,000 cfs
• Primary effect in smaller watersheds - lt 5,000
  acres

56
Water Quality Parameters Most Affected by Fire

• Short term
• Temperature
• Dissolved oxygen
• Turbidity and Total Suspended Solids (TSS)
• Nitrate
• Phosphorous
• Total Organic Carbon
• Manganese

• Long term
• Turbidity and TSS
• Nitrate
• Total Organic Carbon
• Mercury

57
Post-Fire Chemistry Factors

• Fuel characteristics
• Species
• Decay status
• Geologic/soil characteristics
• Alteration of detachment characteristics

58
Export of Chemicals From the Watershed

• Floatable organics including ash, charcoal and
  coarse woody debris
• Dissolved phase
• Particulate phase
• Suspended sediment
• Bedload

59
(No Transcript)
60
Temperature

• Fire and debris flows
• Loss of stream shading
• Solar radiation increased temperature

61
Today we will answer these questions

• What are the factors that influence watershed
  response?
• How do you evaluate changes in watershed response
  factors?
• How do erosion processes,runoff, water quality
  change after a fire?
• How are aquatic organisms impacted by post-fire
  water flow and water quality changes?

62
Fisheries

• Habitat Formation
• Woody Debris
• Sediment
• Complexity

• Stream Productivity
• Decreased Canopy
• Increased Sunlight
• Nutrient influx

63

• Habitat Degradation
• Pool filling
• Fine sediment in spawning gravel

64

• Water Quality Impacts
• Ash toxicity
• Lethal suspended sediment concentrations

65
Today we will answer these questions

• What are the factors that influence watershed
  response?
• How do you evaluate changes in watershed response
  factors?
• How do erosion processes,runoff, water quality
  change after a fire?
• How are aquatic organisms impacted by post-fire
  water flow and water quality changes?

66
Key Points

• Watershed response tied to fire severity and
  intensity
• Keeping an intact forest floor keeps sediment on
  site
• Rapid vegetation recovery keeps nutrients on site
• Aquatic responses tied to sediment and nutrients

67
QUESTIONS?