Post-wildfire erosion: Soil hydrophobicity in Colorado soils Meredith Albright Soil Geography December 13, 2007 GEOG 5401 – Fall 2007 – Univ of Colorado,

Post-wildfire erosion:Soil hydrophobicity

in Colorado soils

Meredith AlbrightSoil Geography

December 13, 2007GEOG 5401 – Fall 2007 – Univ of Colorado, Boulder

Definition • When a drop of water is placed

on soil and it will not penetrate

Formation• Vaporizes hydrophobic

compounds in the litter and humus

• Compounds move downward (temperature gradient)

• Condense on cool soil particles • Form a hydrophobic coating

(Lewis et al., 2006, DeBano, 1966, Letey, 2001)

djdjsjfdksdskjkjdslkljdskjflPost-fire hydrophobic

layer and erosionUnburned vegetated

landscape, high infiltration

Importance of hydrophobic soils

• Ecological Importance– Considered to be the primary cause of

post-fire erosion in many regions (DeBano, 1981)

• Social importance – Water sources susceptible to pollution

(Benavides-Solorio and MacDonald, 2001)

– Hazard to property and lives

Specific questions about Colorado:

• What determines hydrophobic soil formation?• Do hydrophobic soils increase post-fire

erosion?

Determinants of hydrophobicity in Colorado

• Fire severity (DeBano, 1981, Huffman et al., 2001, Lewis et al., 2006)

• Soil texture (Lewis et al., 2006, Huffman et al., 2001)

• Soil moisture (MacDonald and Huffman, 2004)

Fire severity and strength of hydrophobicity

• Higher severity fire, stronger hydrophobicity (Huffman et al., 2001, Lewis et al., 2006)

• More vaporization and condensation of hydrophobic compounds

• High variability/uncertainty• Theoretical consequences: Higher severity

fires, stronger hydrophobicity, less infiltration and greater erosion

Fire severity and depth of hydrophobic layer

• Higher severity fires, deeper hydrophobic layer• Temperature gradient• High variability and uncertainty in studies• Theoretical consequences: Higher severity

fires, more available erosive material

Low severity and unburned soils are hydrophobic at surface (Huffman, et al., 2001)

Moderate to high severity fires soils are hydrophobic at 0, 3 and 6 cm in depth (Huffman, et al., 2001)

Soil Texture

• Higher sand percentage, stronger hydrophobicity (Huffman et al., 2001)

• Lower specific surface than fine soils• Inconsistent results (Robichaud and Hungerford, 2000)

ClaySand

Soil Moisture

• Inverse relationship between hydrophobicity and soil moisture (Benavides-Solorio and MacDonald, 2001)

• Threshold exists where hydrophobicity disappears– Increases with increased severity

(MacDonald and Huffman, 2004; Huffman et al., 2001)

(MacDonald and Huffman, 2004)

High severity Moderate severity

UnburnedLow severity

Why inconsistency?

• Methods of measuring hydrophobicity– Water drop penetration time (WDPT)– Critical surface tension (CST)– Other soil properties affect infiltration

• Regional variation– Complex environmental interactions– Additional region-specific determinants

Specific questions:

• What determines hydrophobic soil formation?• Do hydrophobic soils increase post-fire

erosion?

What is causing post-fire erosion?

• Study investigated causes of post-fire erosion– Found no correlation between

hydrophobicity and sediment yields– Higher severity fire, higher sediment

yield– Inverse relationship between %

ground cover and sediment yield– Increasing slope increases sediment

yield (in all burn-types)

(Benavides-Solorio and MacDonald, 2001)

High severity

Moderate severity

Unburned

What is causing the post-fire erosion?Vegetation loss or hydrophobic soil?

• High severity fires result in higher erosion rates (Benavides-Solorio and MacDonald, 2001)

– Clear pattern between ground cover and erosion (R2 = 0.81)

• Need for studies determining the effects of hydrophobicity on erosion– Hydrophobic soils present in unburned and burned

soils (Martin and Moody, 2001)

• Determinants of hydrophobic layer: fire severity, texture, and soil moisture– High variation/uncertainty

• Methods

• Regional variation

• Vegetation loss likely explains post-fire erosion– Hydrophobic soils may contribute– Erosion mitigation: maximize groundcover

• Future research– Region-specific studies to understand local

soil dynamics– Controlled studies to determine contribution of

hydrophobicity on sediment yields

Conclusions

References• Benavides-Solorio, J., L.H. MacDonald, 2001. Post-fire runoff and erosion from simulated rainfall

on small plots, Colorado Front Range. Hydrological Processes 15: 2931-2952.• DeBano L.F., 1981. Water repellent soils: a state-of-the-art. Gen. Tech. Rep. PSW-46, Pacific

Southwest Forest and Range Experiment Station, Forest Service, US Department of Agriculture, Berkeley, CA.

• Huffman, E.L., L.H. MacDonald, and J.D. Stednick, 2001. Strength and persistence of fire-induced soil hydrophobicity under ponderosa and lodgepole pine, Colorado Front Range. Hydrological Processes 15: 2877-2892.

• Letey, J., 2001. Causes and consequences of fire-induced soil water repellency. Hydrological Processes 15: 2867-2875.

• Lewis, S.A. J.Q Wu, P.R. Robichaud, 2006. Assessing burn severity and comparing soil water repellency, Hayman Fire, Colorado. Hydrological Processes 20: 1-16.

• MacDonald, L.H., E.L. Huffman, 2004. Post-fire soil water repellency: persistence and soil moisture thresholds. Soil Science Society of America Journal 68: 1729-1734.

• Martin, D., J. Moody 2001. Comparison of soil infiltration rates in burned and unburned mountainous watersheds. Hydrological Processes 15: 2893-2903.

• Moody, J and D. Martin 2001. Initial hydrologic and geomorphic response following a wildfire in the Colorado Front Range. Earth Surf Processes landforms 26: 1049-1070

• Robichaud, P.R., Hungerford, P.D., 2000. Water repellency by laboratory burning of four northern Rocky Mountain forest soils. Journal of Hydrology 231-232: 207-219.