Post-wildfire erosion: Soil hydrophobicity in Colorado soils Meredith Albright Soil Geography December 13, 2007 GEOG 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)

djdjsjfdksdskjkjdslkljdskjfl Post-fire hydrophobic layer and erosion Unburned 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 severityModerate 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 (R 2 = 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, Post-fire runoff and erosion from simulated rainfall on small plots, Colorado Front Range. Hydrological Processes 15: DeBano L.F., 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, Strength and persistence of fire- induced soil hydrophobicity under ponderosa and lodgepole pine, Colorado Front Range. Hydrological Processes 15: Letey, J., Causes and consequences of fire-induced soil water repellency. Hydrological Processes 15: Lewis, S.A. J.Q Wu, P.R. Robichaud, Assessing burn severity and comparing soil water repellency, Hayman Fire, Colorado. Hydrological Processes 20: MacDonald, L.H., E.L. Huffman, Post-fire soil water repellency: persistence and soil moisture thresholds. Soil Science Society of America Journal 68: Martin, D., J. Moody Comparison of soil infiltration rates in burned and unburned mountainous watersheds. Hydrological Processes 15: Moody, J and D. Martin Initial hydrologic and geomorphic response following a wildfire in the Colorado Front Range. Earth Surf Processes landforms 26: Robichaud, P.R., Hungerford, P.D., Water repellency by laboratory burning of four northern Rocky Mountain forest soils. Journal of Hydrology :