Wildfires can temporarily increase runoff and erosion rates in forested watersheds by several orders of magnitude, and these accelerated runoff and erosion rates have considerable geomorphic, ecological and societal significance. Surface sealing -- the formation of a thin surface layer with a very low hydraulic conductivity -- may contribute to increases in runoff and erosion after fire either through rainsplash induced compaction and disaggregation, the clogging of soil pores by ash, or the formation of low conductivity ash crusts. However, there has been very little work on the extent to which surface sealing contributes to increased runoff and erosion after fire, the sealing mechanisms involved, or the factors that contribute to an increased potential for sealing, and surface sealing effects have not been incorporated into predictive models of post fire runoff and erosion. This project will conduct an intensive, field and lab based research study to (1) determine the extent to which surface sealing contributes to increased runoff and erosion after wildfires in the western U.S.; (2) determine the various mechanisms of surface sealing and the factors that lead to an increased probability of seal formation; and, (3) develop a predictive model of the probability of surface sealing after fires, based upon the soil and ash properties in burned areas. The research will be based on six field and lab experiments. The work will result in quantitative, process-based understanding of post-fire surface sealing and new understanding of wildfire related runoff and erosion processes.
According to the National Interagency Fire Center, the 2012 wildfire season within the American West was 30 % above average and consumed over 9 million acres, an area greater than the state of Maryland. This increase in wildfire activity has been associated with shifts in climate over the last forty years, the legacy of human wildfire suppression and the increase in bark beetle activity throughout the West (Climate Central, 2012). Increases in spring and summer temperatures over the years, as well as decreases in winter snowpacks, have increased the typical burning season by two and a half months; making the wildfire season 75 days longer than 40 years ago (Westerling et al., 2006). Within the Rocky Mountain region the average number of wildfires over 400 hectares have quadrupled and the frequency of large wildfires, greater than 4,000 hectares, have increased seven times since the 1970’s (Climate Central, 2012). The effect of this increase wildfire activity on resource management and ecosystem function is of increased interest to agencies and land managers within the Northern Rocky Mountain region of the United States. The research conducted during this grant contributed to an ever-growing knowledge base of post-fire systems, with emphasis on how variations in wildfire ash characteristics influence post-fire hydrological response and surface sealing within these systems (Figure 1). Overall this research finds that ash should not be considered a generic term, as not all ash is created equal, in regards to its effects on immediate post-fire systems. Variability in the effect of ash on runoff following wildfires, and hence the often conflicting findings reported in the literature, can be partially explained by variations observed in the hydrologic properties of ash. Wildfire ash is an important element of post-fire landscapes and should be categorized and taken into consideration when assessing post-fire ecosystems and hazards. The main conclusions, from laboratory and field based experiments carried out during this research, are as follows: I) The initial physical, chemical, and hydrological properties of vegetative ash within the Northern Rocky Mountain region mainly vary with combustion temperature / fire severity. The saturated hydraulic conductivity of ash varies substantially, covering three orders of magnitude. Furthermore, the hydraulic conductivity of some ash can decrease an order of magnitude following initial hydration, with such decreases associated with the formation of a surface seal in the form of an ash crust. The hydration of ash containing oxides results in the formation of carbonate and therefore it is suggested that ash color is not an acceptable metric to differentiate between variations in the hydrologic response of ash, as both oxide and carbonate ash exhibit high chroma values. Instead carbonate content in ash is suggested as a more reliable variable if measured prior to post-fire rainfall (Balfour and Woods, 2013). II) There is potential for the variability in the hydrological properties of ash to affect initial post-fire infiltration and runoff response in ash-covered soils. Variability in the effect of ash on runoff responses following wildfires could be partially explained by variations observed in the hydrologic properties of ash. The hydrologic response of low and high combustion ash, associated with physical (grain size) and chemical (carbonate formation) properties, could prompt the formation of surface seals in post-fire systems by either creating a low conductive ash layer or an ash crust, while mid-combustion, carbonate dominated, ash may explain reported buffering effects of ash layers due to its high water retention (Bodi et al., 2014). III) The use of air permeametery and a sorptivity probe are viable methodologies for obtaining initial ash saturated hydraulic conductivity and sorptivity values respectively in the laboratory. Furthermore laboratory based measurements; conducted on disturbed ash samples, can accurately reflected field based measurements. Air permeametry and sorptivity probes require relatively low volumes of ash, allowing essential information regarding ash characteristics to be obtained in the laboratory and easily incorporated into modeling systems aimed at predicting post-fire infiltration response (Balfour, 2014). IV) The hydrological properties of ash layers were shown to change over time. While numerous authors have previously commented on the presence of an ash crust within post-fire ecosystem, this work is the first to document the formation of an in-situ ash crust recently after wildfire activity (Figure 2). The formation of an ash crust was documented to decreases ash hydraulic conductivity by an order of magnitude, as well as significantly decreasing ash layer bulk density and porosity. While raindrop impact increases the robustness of an ash crust, raindrop impact alone is not sufficient to form an ash crust, instead mineralogical transformations must occur to produce a hydrologically relevant ash crust. Therefore initial ash composition, the presence of oxides and a hydrating rainfall event are all necessary precursors for crust formation. Ash crust formation, however, does not occur following all severe wildfire events (Balfour et al., 2014).
