Factors that Affect Post-fire Watershed Response and Treatment Effectiveness

Factors Unrelated to Fire

• Topography–The erosion rate generally increases as slope and hillslope length (flow path) increase. In addition, the drainage pattern (as determined by geologic terrain) can concentrate or dissipate erosive energy (Moody and Martin 2009a; Scott and others 2009). Longer flow paths and convergent hillslopes (swales) allow overland flow to concentrate into rill flow, which has higher erosive power and causes the majority of surface erosion (Libohova 2004).
• Rainfall characteristics, especially rainfall intensity–Intense short duration storms, characterized by high rainfall intensity and low rainfall amounts, have been associated with high stream peak flows and significant erosion events after fires (DeBano and others 1998; Moody and Martin 2001; Neary and others 2005b; Robichaud 2005, 2008a). The potential rainfall amounts, intensities, and seasonal patterns directly impact post-fire hillslope treatment effectiveness, it is essential to consider the potential rainfall regime of the burned area when selecting treatments.

Fire-dependent Factors

• Burn severity–A qualitative measure of the effects of fire on ecosystem properties and is usually evaluated by the degree of soil heating and/or mortality of vegetation (Agee 2007). Several factors that impact post-fire flooding and erosion response are included in the assessment of burn severity, and higher burn severity is associated with larger and more rapid watershed responses to rainfall (DeBano and others 1998; Moody and others 2008). Forest ecologists define burn severity by the degree of overstory plant mortality where overstory mortality below approximately 30 percent is considered low severity, 30 to 70 percent is considered moderate severity, and greater than 70 percent is considered high severity (Agee 2007).
• Soil burn severity–The fire effects of soil heating and the consumption of organic material on the soil surface and near-surface lead to changes in soil properties that generally reduce soil infiltration and increase soil erodibility (Benavides-Solorio and MacDonald 2001; Doerr and others 2006). The degree of soil burn severity is dependent on the peak temperatures and the duration of those temperatures within the soil. Observable post-fire ground parameters (for example, amount and condition of ground cover, ash color and depth, soil structure, presence of fine roots, and soil water repellency) are often used to classify soil burn severity.
  • Soil water repellency–Fire-induced soil water repellency has been directly linked to soil burn severity (DeBano 2000; Doerr and others 2006; Robichaud and Hungerford 2000) and to reduced infiltration (Cerdà and Robichaud 2009; Robichaud 2000). Although the presence of fire-induced soil water repellency is generally confined to the top few inches of the soil, the presence and degree vary widely across the burned landscape. In addition, the effects of soil water repellency can vary over time depending on soil moisture, with water repellency being most pronounced during dry conditions and reduced or absent following prolonged wet conditions (Doerr and others 2009).
  • Soil erodibility–Soil erodibility is an estimate of the ability of soils to resist erosion, based on the many factors, but predominantly on soil texture, soil structure, and organic matter content (Hillel 1998). When the organic material in the upper soil is consumed by fire, the soil can become disaggregated and, as a result, more erodible. In addition, the collapse in soil structure decreases both total porosity and pore size which reduces infiltration rates (DeBano and others 2005).
• Time since the fire–Natural recovery of native vegetation reduces erosion over time. The greatest erosion usually is measured during the first post-fire year, and the second post-fire year and subsequent years can be an order of magnitude lower (Robichaud and Brown 1999; Pierson and others 2001; Robichaud and others 2008a). However, recovery rates vary by climate and vegetation type.