Post-wildfire erosion: Soil hydrophobicity in Colorado soils

1. Post-wildfire erosion:Soil hydrophobicity in Colorado soils Meredith Albright Soil Geography December 13, 2007 I am interested in how fire alters soil characteristics in Colorado. One of the most talked about post-fire effects on soil is soil-hydrophobicity which is thought to cause erosion which has widespread effects on ecosystems and communities. Middle picture:www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg&imgrefurl left picture: http://wwwbrr.cr.usgs.gov/projects/Burned_Watersheds/Depos_MiddleF.jpg Left picture: http://vulcan.wr.usgs.gov/Imgs/Jpg/Rainier/rainier_debris_flow.jpg I am interested in how fire alters soil characteristics in Colorado. One of the most talked about post-fire effects on soil is soil-hydrophobicity which is thought to cause erosion which has widespread effects on ecosystems and communities. Middle picture:www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg&imgrefurl left picture: http://wwwbrr.cr.usgs.gov/projects/Burned_Watersheds/Depos_MiddleF.jpg Left picture: http://vulcan.wr.usgs.gov/Imgs/Jpg/Rainier/rainier_debris_flow.jpg

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) Soil hydrophobocity or water repellency is when water does not infiltrate the soil within a short period of time. It occurs in both unburned and burned areas. I would like to focus on the fire-induced hydrophobicity for this presentation. During a wildfire, the heat from the flames creates a temperature gradient through the soil: hotter soil towards the surface, cooling with depth. The heat from the fire, vaporizes hydrophobic compounds, such as aliphatic hydrocarbons, that naturally occur in litter and humus. These vaporized compounds move deeper into the soil profile because of the temperature gradient. The compounds cool as they move downward and eventually condense on soil particles. Once enough hydrocarbons have condensed, a hydrophobic layer is created. The optimal conditions for the creation of the hydrocarbons is less than 280 degrees Celsius due to combustion of the compounds at higher temeratures. Soil hydrophobocity or water repellency is when water does not infiltrate the soil within a short period of time. It occurs in both unburned and burned areas. I would like to focus on the fire-induced hydrophobicity for this presentation. During a wildfire, the heat from the flames creates a temperature gradient through the soil: hotter soil towards the surface, cooling with depth. The heat from the fire, vaporizes hydrophobic compounds, such as aliphatic hydrocarbons, that naturally occur in litter and humus. These vaporized compounds move deeper into the soil profile because of the temperature gradient. The compounds cool as they move downward and eventually condense on soil particles. Once enough hydrocarbons have condensed, a hydrophobic layer is created. The optimal conditions for the creation of the hydrocarbons is less than 280 degrees Celsius due to combustion of the compounds at higher temeratures.

3. In a vegetated, pre-fire environment, there is generally high infiltration and minimal erosion. After a fire, vegetation is reduced and because soil particles are coated with hydrophobic compounds, infiltration is decreased at the hydrophobic layer. Researchers are concerned this reduction in infiltration (on right) increases run-off and erosion causing ecological and property damage. In a vegetated, pre-fire environment, there is generally high infiltration and minimal erosion. After a fire, vegetation is reduced and because soil particles are coated with hydrophobic compounds, infiltration is decreased at the hydrophobic layer. Researchers are concerned this reduction in infiltration (on right) increases run-off and erosion causing ecological and property damage.

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 Picture bottom: http://www.spanishforkriver.org/history/landslides/slides01.jpg Picture top: http://volcanoes.usgs.gov/ash/water/Johnston_dam_large.jpg Hydrophobic layers in regions are blamed for being the primary cause of post-fire erosion in many regions. Particularly, California, Spain, Australia and South Africa. Also, the erosion attributed to them is thought to cause water quality issues and large erosive events such as debris flows and mudslides that damage property and can prove to be fatal. Picture bottom: http://www.spanishforkriver.org/history/landslides/slides01.jpg Picture top: http://volcanoes.usgs.gov/ash/water/Johnston_dam_large.jpg Hydrophobic layers in regions are blamed for being the primary cause of post-fire erosion in many regions. Particularly, California, Spain, Australia and South Africa. Also, the erosion attributed to them is thought to cause water quality issues and large erosive events such as debris flows and mudslides that damage property and can prove to be fatal.

5. Specific questions about Colorado: What determines hydrophobic soil formation? Do hydrophobic soils increase post-fire erosion? Because there is so much regional variation in soils and fire, I wanted to specifically ask questions about the Colorado Front Range. First, what factors have researchers identified that determine the formation of a hydrophobic layer, and secondly, does the formation of a hydrophobic layer result in increased erosion? Picture, right: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg Picture, left: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpgBecause there is so much regional variation in soils and fire, I wanted to specifically ask questions about the Colorado Front Range. First, what factors have researchers identified that determine the formation of a hydrophobic layer, and secondly, does the formation of a hydrophobic layer result in increased erosion? Picture, right: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg Picture, left: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg

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) The three determinants I identified in several papers for Colorado specifically, are fire severity, soil texture, and soil moisture. There is often confusion about the meaning of the term �fire severity�. Fire severity is the effect of the fire on organisms, or the vegetative mortality. It is often used as a proxy for fire intensity, which is the energy released during the fire or the temperature. So fire severity, is really another way of saying ground cover left over after the fire. Severe fires result is high vegetative mortality and low-severity fires often burn only fine fuels such as grasses, shrubs, and small trees. Left picture: http://www.donpearman.com/fire/pix/hillsidehouses3burned.lg.jpeg Center picture: http://csfs.colostate.edu/library/photos/fire/rehab02.jpg Right picture: http://www.global-garden.com.au/burnley/images/Water_repellent_soil.jpg The three determinants I identified in several papers for Colorado specifically, are fire severity, soil texture, and soil moisture. There is often confusion about the meaning of the term �fire severity�. Fire severity is the effect of the fire on organisms, or the vegetative mortality. It is often used as a proxy for fire intensity, which is the energy released during the fire or the temperature. So fire severity, is really another way of saying ground cover left over after the fire. Severe fires result is high vegetative mortality and low-severity fires often burn only fine fuels such as grasses, shrubs, and small trees. Left picture: http://www.donpearman.com/fire/pix/hillsidehouses3burned.lg.jpeg Center picture: http://csfs.colostate.edu/library/photos/fire/rehab02.jpg Right picture: http://www.global-garden.com.au/burnley/images/Water_repellent_soil.jpg

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 A few studies for the area found the fire severity correlates with the strength of the hydrophobic layer. A strong hydrophobic layer results in less infiltration than a weak hydrophobic layer. This can be explained by a higher severity fire being able to vaporize more hydrophobic compounds resulting in more condensation at the bottom of the temperature gradient. This results in a more uniform and consistent hydrophobic layer, and therefore minimizing infiltration. Although these study identified this trend, there is variability within and among the sites. I will discuss potential explanation for this variation later in the presentation. However, if this trend is consistent for an area, the theoretical consequences are that higher severity fires will result in reduced infiltration, and potentially greater erosion. A few studies for the area found the fire severity correlates with the strength of the hydrophobic layer. A strong hydrophobic layer results in less infiltration than a weak hydrophobic layer. This can be explained by a higher severity fire being able to vaporize more hydrophobic compounds resulting in more condensation at the bottom of the temperature gradient. This results in a more uniform and consistent hydrophobic layer, and therefore minimizing infiltration. Although these study identified this trend, there is variability within and among the sites. I will discuss potential explanation for this variation later in the presentation. However, if this trend is consistent for an area, the theoretical consequences are that higher severity fires will result in reduced infiltration, and potentially 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 Fire severity has also been correlated with the depth of the hydrophobic layer. It was found that higher severity fires result in deeper hydrophobic layers. This can be explained by the temperature gradient as well. Since higher severity fires are usually hotter (more intense) than the hydrophobic compounds must move deeper into the soil profile to condense on the soil particles (illustrated on the right). The study identified after moderate or severe fires the hydrophobic layer was at the 0, 3, or 6 cm. Whereas with a low-severity fire (or an unburned area such as illustrated on the left), the temperature gradient is short, so the hydrophobic layer was most commonly identified on the surface of the soil. There was obvious variability, especially with the moderate to high severity fires, so this trend as well as the last doesn�t completely explain the depth of the hydrophobic layer. Fire severity has also been correlated with the depth of the hydrophobic layer. It was found that higher severity fires result in deeper hydrophobic layers. This can be explained by the temperature gradient as well. Since higher severity fires are usually hotter (more intense) than the hydrophobic compounds must move deeper into the soil profile to condense on the soil particles (illustrated on the right). The study identified after moderate or severe fires the hydrophobic layer was at the 0, 3, or 6 cm. Whereas with a low-severity fire (or an unburned area such as illustrated on the left), the temperature gradient is short, so the hydrophobic layer was most commonly identified on the surface of the soil. There was obvious variability, especially with the moderate to high severity fires, so this trend as well as the last doesn�t completely explain the depth of the hydrophobic layer.

9. Soil Texture Higher sand percentage, stronger hydrophobicity (Huffman et al., 2001) Lower specific surface than fine soils Inconsistent results (Robichaud and Hungerford, 2000) Soil texture has been found to effect the strength of hydrophobicity. Soil with higher percentages of sand result in the formation of a stronger hydrophobic layer. This can be explained by the relative surface areas of soil particles. One volume of sand has lower surface area than one volume of clay so it would take more hydrophobic compounds to coat the clay particles than it would to ciat the sand particles. The result is that soils with sand show greater effects of hydrophobicity than those with finer particles. This is a phenomenon that has been illustrated for many regions throughout the world, although within Colorado the studies show some inconsistency. Sand picture: http://www.shef.ac.uk/mecheng/tribology/research/projects/images/SAND1.GIF Clay picture: http://www.drainfieldrepair.com/images/clay.jpgSoil texture has been found to effect the strength of hydrophobicity. Soil with higher percentages of sand result in the formation of a stronger hydrophobic layer. This can be explained by the relative surface areas of soil particles. One volume of sand has lower surface area than one volume of clay so it would take more hydrophobic compounds to coat the clay particles than it would to ciat the sand particles. The result is that soils with sand show greater effects of hydrophobicity than those with finer particles. This is a phenomenon that has been illustrated for many regions throughout the world, although within Colorado the studies show some inconsistency. Sand picture: http://www.shef.ac.uk/mecheng/tribology/research/projects/images/SAND1.GIF Clay picture: http://www.drainfieldrepair.com/images/clay.jpg

10. 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) Lastly, soil moisture is an important component that can prevent the formation of a hydrophobic layer. Several studies have found an inverse relationship between the two, noting that increased soil moisture levels results in decreased hydrophobicity. Also, a threshold has been identified where at a certain soil moisture level result in the nullification of the hydrophobic layer. A trend has been found in some data where the threshold increases with increased fire severity. MacDonald and Huffman (2004) found High-moderate severity fires resulted in a threshold of approximately 28 while low severity was about 15, and unburned regions showed a threshold of 12. Lastly, soil moisture is an important component that can prevent the formation of a hydrophobic layer. Several studies have found an inverse relationship between the two, noting that increased soil moisture levels results in decreased hydrophobicity. Also, a threshold has been identified where at a certain soil moisture level result in the nullification of the hydrophobic layer. A trend has been found in some data where the threshold increases with increased fire severity. MacDonald and Huffman (2004) found High-moderate severity fires resulted in a threshold of approximately 28 while low severity was about 15, and unburned regions showed a threshold of 12.

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 In most of the data I presented so far, there has been a lot of inconsistency within sites, and among sites and regions. I feel this variation can be explained partially by two factors. First, the methods of measuring hydrophobicity may allow for some error. The WDPT measures the amount of time it takes for a drop of water to penetrate the soil at a particlar depth (often measured in seconds). The CST technique is the amount of time it takes for a certain concentation of water and ethanol to penetrate a soil at a particular depth. The depths measured most commonly throughout studies were 0, 3 and 6cm. However, these methods measure infiltration, not necessarily chemical hydrophobicity. So, other factors influencing infiltration may alter the results. Also, there is no visual way to identify hydrophobic soil, so if it exists between those measurements, data will be altered. Secondly, ecological data commonly has regional variation and that is apparent in soil hydrophobicity. There are complex interactions among environmental conditions that may vary spatially. Also, there are clearly determininants of hydrophobicity in addition to the three I listed that affect layer formation. Picture: http://ss.ffpri.affrc.go.jp/labs/dfse/SoilGeoCheLab/img/WaterRepellencySoil.jpg In most of the data I presented so far, there has been a lot of inconsistency within sites, and among sites and regions. I feel this variation can be explained partially by two factors. First, the methods of measuring hydrophobicity may allow for some error. The WDPT measures the amount of time it takes for a drop of water to penetrate the soil at a particlar depth (often measured in seconds). The CST technique is the amount of time it takes for a certain concentation of water and ethanol to penetrate a soil at a particular depth. The depths measured most commonly throughout studies were 0, 3 and 6cm. However, these methods measure infiltration, not necessarily chemical hydrophobicity. So, other factors influencing infiltration may alter the results. Also, there is no visual way to identify hydrophobic soil, so if it exists between those measurements, data will be altered. Secondly, ecological data commonly has regional variation and that is apparent in soil hydrophobicity. There are complex interactions among environmental conditions that may vary spatially. Also, there are clearly determininants of hydrophobicity in addition to the three I listed that affect layer formation. Picture: http://ss.ffpri.affrc.go.jp/labs/dfse/SoilGeoCheLab/img/WaterRepellencySoil.jpg

12. Specific questions: What determines hydrophobic soil formation? Do hydrophobic soils increase post-fire erosion? The second question I asked is �do hydrophobic soils increase post-fire erosion?�. Since it has been declared to be the primary cause of post-fire erosion, I wanted to identify studies that looked into the direct effects of hydrophobic soils. Picture, right: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg Picture, left: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpgThe second question I asked is �do hydrophobic soils increase post-fire erosion?�. Since it has been declared to be the primary cause of post-fire erosion, I wanted to identify studies that looked into the direct effects of hydrophobic soils. Picture, right: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg Picture, left: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg

13. 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), investigated the causes of post-fire erosion. They found no correlation between hydrophobicity and sediment yields (amount of erosive materials). However, they did identify a correlation between higher severity fire and higher sediment yields. As seen in the graphs on the right, higher severity fires resulted in five times more sediment yields than moderate severity fires (significant result) and moderate severity fires resulted in two times more sediments yield than unburned soils (not-significant). The study also found a relationship between bare soil and sediment yield which is another way of representing fire severity (since fire severity if amount of vegetation mortality). Also, not surprisingly, the study found that steeper slopes result in more sediment yield in all burn types. Clearly gravity is not escaped by these systems. Benavides-Solorio and MacDonald (2001), investigated the causes of post-fire erosion. They found no correlation between hydrophobicity and sediment yields (amount of erosive materials). However, they did identify a correlation between higher severity fire and higher sediment yields. As seen in the graphs on the right, higher severity fires resulted in five times more sediment yields than moderate severity fires (significant result) and moderate severity fires resulted in two times more sediments yield than unburned soils (not-significant). The study also found a relationship between bare soil and sediment yield which is another way of representing fire severity (since fire severity if amount of vegetation mortality). Also, not surprisingly, the study found that steeper slopes result in more sediment yield in all burn types. Clearly gravity is not escaped by these systems.

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) To reiterate, many studies attribute the post-fire erosion to soil hydrophobicity, whereas the real cause of erosion is likely vegetation loss as seen in the result that ground cover correlates with erosion. There appears to be a need to perform a controlled study on soil hydrophobicity to study the effects of hydrophobicity in undurned and burned sites. Since Martin and Moody (2001) identified hydrophobicity in both unburned and burned sites, this is a possible study and would fill a gap in understanding if hydrophobicity contributes to erosion in addition to vegetation loss. To reiterate, many studies attribute the post-fire erosion to soil hydrophobicity, whereas the real cause of erosion is likely vegetation loss as seen in the result that ground cover correlates with erosion. There appears to be a need to perform a controlled study on soil hydrophobicity to study the effects of hydrophobicity in undurned and burned sites. Since Martin and Moody (2001) identified hydrophobicity in both unburned and burned sites, this is a possible study and would fill a gap in understanding if hydrophobicity contributes to erosion in addition to vegetation loss.

15. 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 The determininats research has identified with some amount of confidence (but a level of variation) for the formation of hydrophobic layers in Colorado are fire severity, soil texture and soil moisture. The studies remind us there is variation and un certainty with these results potentially due to measuring methods and regional variations and complexities. Unlike much of the research exclaims, there is no evidence hydrophobicity is the main cause of post-fire erosion. The loss of vegetation associated with fire is the factor that increases sediment yield, although hydrophobic soils may contribute (more studies need to be performed to know this). Post-fire erosion mitigation will likely be successful if ground cover is encouraged. Further research should focus on region-specific studies to understand the more complex interactions of determinants of soil hydrophobicity formation. Also, a controlled study featuring burned, unburned regions displaying hydrophobicity and lack of hydrophobicity would be a good opportunity to understand the contribution, if any, hydrophobicity has on erosion. Top Picture: http://www.donpearman.com/fire/pix/hillsidehouses3burned.lg.jpeg Bottom Picture: http://www.greenvillerancheria.com/epa-pics_files/image020.jpg Center Picture: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpgThe determininats research has identified with some amount of confidence (but a level of variation) for the formation of hydrophobic layers in Colorado are fire severity, soil texture and soil moisture. The studies remind us there is variation and un certainty with these results potentially due to measuring methods and regional variations and complexities. Unlike much of the research exclaims, there is no evidence hydrophobicity is the main cause of post-fire erosion. The loss of vegetation associated with fire is the factor that increases sediment yield, although hydrophobic soils may contribute (more studies need to be performed to know this). Post-fire erosion mitigation will likely be successful if ground cover is encouraged. Further research should focus on region-specific studies to understand the more complex interactions of determinants of soil hydrophobicity formation. Also, a controlled study featuring burned, unburned regions displaying hydrophobicity and lack of hydrophobicity would be a good opportunity to understand the contribution, if any, hydrophobicity has on erosion. Top Picture: http://www.donpearman.com/fire/pix/hillsidehouses3burned.lg.jpeg Bottom Picture: http://www.greenvillerancheria.com/epa-pics_files/image020.jpg Center Picture: http://www.fs.fed.us/r2/psicc/hayres/baer/scarification_1.jpg

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.