Post-wildfire erosion: Soil hydrophobicity in Colorado soils

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

2. 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)

3. Unburned vegetated landscape, high infiltration Post-fire hydrophobic layer and erosion djdjsjfdksdskjkjdslkljdskjfl

4. 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

5. Specific questions about Colorado: • What determines hydrophobic soil formation? • Do hydrophobic soils increase post-fire erosion?

6. 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)

7. 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

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

9. Soil Texture • Higher sand percentage, stronger hydrophobicity (Huffman et al., 2001) • Lower specific surface than fine soils • Inconsistent results(Robichaud and Hungerford, 2000) Sand Clay

10. High severity Moderate severity Low severity Unburned 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)

11. 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

12. Specific questions: • What determines hydrophobic soil formation? • Do hydrophobic soils increase post-fire erosion?

13. High severity Moderate severity Unburned 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)

14. 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)

15. Conclusions • 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

16. 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.