Watershed Response to Fire Christine May Earth & Planetary Science

Severe fires can result in accelerated erosion by: 1.) removing the forest canopy and litter layer, exposing mineral soil to the direct impact of rainfall 2.) heating and combusting soil organic matter 3.) burning logs that trap soil on steep slopes or store sediment in stream channels 4.) reducing the root strength of the soil

Rain Splash Fine sediment or ash dislodged by rain splash can clog soil pores causing surface sealing.

Severe fires can result in accelerated erosion by: 1.) removing the forest canopy and litter layer, exposing mineral soil to the direct impact of rainfall 2.) heating and combusting soil organic matter 3.) burning logs that trap soil on steep slopes or store sediment in stream channels 4.) reducing the root strength of the soil

Rainfall Infiltration Subsurface flow Overland flow

Infiltration Rates Infiltration = the movement of water across the soil surface Influenced by Soil texture Ambient moisture

Time Infiltration Rate (mm/min) clay

Time Infiltration Rate (mm/min) clay sand

Overland Flow Surface erosion requires overland flow, which occurs when 1.) the rainfall rate exceeds the infiltration rate of the soil surface, or 2.) the soil is saturated

> <15 Runoff power index, slope (m/m) x relief: POSTFIRE REVEGETATION and RILL EROSION IN DEBRIS-FLOW BASINS

Hydrophobic Soils

A water-repellent layer of soil that prevents infiltration below that layer. Derived from plant material burned during a hot fire. The hydrophobic compounds (hydrocarbons) penetrate the soil surface as a gas and solidify after cooling, forming a waxy coating. Sandy soils with large pore spaces and areas with thick litter accumulations that experience very hot fires are especially susceptible.

Hydrophobic Soils (con’t) The thickness and continuity of hydrophobic layers varies, as does their persistence. Recovery: plant roots, soil microorganisms, and soil fauna help break up the hydrophobic layer. Negative feedback: reduced infiltration will decrease the amount of water available for plant growth and biological activity in the soil.

Field Tests Sprinkler experiments Infiltrometer Analysis: compare infiltration rates with rainfall rates from local raingages

Rehabilitation Measures On-site: revegetation –grass seeding – pitfalls? –straw mulch Off-site: sediment retention devices –straw bale check dams –directional log felling –sediment retention ponds

Photos by F.J. Swanson

[debris flow video]

Debris Flows Two initiation mechanisms: 1.) runoff-initiated - driven by low soil infiltration rates and the bulking of sediment detached by overland flow and surface erosion 2.) landslide-initiated - driven by soil saturation (requires high infiltration rates and low rooting strength)

Rill and channel erosion (1988 fire, 1989 storm, Yellowstone) Loss of root strength, saturation-failure of colluvium (1989 fire, 1997 storm, Idaho) Initiation of events through runoff and sediment bulking, early post-fire

Copyright © Tom Black 2002

1989 debris flow-dominated event, NE Yellowstone

Initiation of events through loss of root strength, saturation and failure of colluvium (1989 fire, 1997 storm, central Idaho)

Years after forest removal / fire Relative root reinforcement From Ziemer 1981 Decay of dead roots

Years after forest removal / fire Relative root reinforcement From Ziemer 1981 Decay of dead roots Live roots

Years after forest removal / fire Relative root reinforcement From Ziemer 1981 Decay of dead roots Live roots 5 – 15 yrs

Probability of Occurrence Depends upon post-fire storm characteristics and the spatial pattern of high severity fire patches.

Temporal Patterns

Time since fire Likelihood of large-scale erosion Runoff-Dominated Surface Erosion Two-Phase Erosional Response

Time since fire Likelihood of large-scale erosion Runoff-Dominated Surface Erosion Saturation-Induced Slope Failures Two-Phase Erosional Response

Time since fire Likelihood of large-scale erosion Runoff-Dominated Surface Erosion Saturation-Induced Slope Failures Two-Phase Erosional Response 0 – 5 yrs 5 – 15 yrs

Post-Fire Rehabilitation Efforts for Debris Flows Suggestions from the class…

0 to 30 yrs30 to 60 yrs 60 to 90 yrs > 90 yrs Time Since the Previous Debris Flow

Salvage Logging Dead wood can be an important element in sediment storage on steep hillslopes and in stream channels. Soil disturbance by logging operations and road construction can further accelerate erosion.

Short-term Patterns

Long-term Patterns

Event Reconstructions Studies that attempt to decipher long-term correlations among climate, fire, and erosion and their effects on landscape evolution using a variety of dating methods and evidence for past erosional events.

Copyright © Ron Dorn 2002

1989 debris flow older fan sediments 1988 charred litter layer (burned soil surface) ‘Fire-related debris flows’

Yellowstone lodgepole: large, severe stand- replacing fires, RI yr

Onset of the Little Ice Age (1200 AD) Meyer et al. 1992

Warmer millennial-scale periods

Cooler millennial-scale periods (terraces?)

Idaho batholith estimated mean sediment yields over different timescales (log scale)

Suspended sediment and bedload transport Watershed 3, HJA t/km 2 /yr Suspended sediment Bedload Roads Patch cut Floods and debris flows

Questions?

Questions for the class: Is there evidence that fires preferentially travel through or burn hotter in steep, narrow valleys compared to planar hillslopes?

Questions for the class: How can information about erosion-prone areas be incorporated into: 1.) planning fuels treatment projects? 2.) wildfire management?

Central Idaho ponderosa: presettlement regime of light surface fires, RI 5-30 yr

Northern Hemisphere tree-ring temperature reconstruction (from Esper et al., 2002) Mann-Bradley-Hughes (1999) Esper et al. (2002) multiproxy tree-rings “Medieval Warm Period”“Little Ice Age” Severe fire, large debris flows both areas Frequent light fires Idaho; few fires Yellowstone Year AD

Conclusions Herbaceous revegetation reduces probability of post-fire runoff-generated events (maximum probability of occurrence 1-3+ yr after fire?) Later postfire saturation-induced failures –root strength control (maximum probability 4-10 yr after fire?) Geomorphic response to fire is transient but produces transient to persistent stream habitat alteration Climate is a strong control on fire regimes and associated geomorphic response, both in space and time, therefore… Fire-induced sedimentation is strongly episodic and variable at ~1000 yr timescales