7 Sep 2009, 9:23pm
Ecology Management
by admin

Causes of Post-Fire Runoff and Erosion: Water Repellency, Cover, or Soil Sealing?

Isaac J. Larsen, Lee H. MacDonald, Ethan Brown, Daniella Rough, Matthew J. Welsh, Joseph H. Pietraszek, Zamir Libohova, Juan de Dios Benavides-Solorio, Keelin Schaffrath. 2009. Causes of Post-Fire Runoff and Erosion: Water Repellency, Cover, or Soil Sealing? Soil Sci. Soc. Am. J. 73:1393-1407

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Few studies have attempted to isolate the various factors that may cause the observed increases in peak flows and erosion after high-severity wildfires. This study evaluated the effects of burning by: (i) comparing soil water repellency, surface cover, and sediment yields from severely burned hillslopes, unburned hillslopes, and hillslopes where the surface cover was removed by raking; and (ii) conducting rainfall simulations to compare runoff , erosion, and surface sealing from two soils with varying ash cover. The fire-enhanced soil water repellency was only stronger on the burned hillslopes than the unburned hillslopes in the first summer after burning. For the first 5 yr after burning, the mean sediment yield from the burned hillslopes was 32 Mg ha-1, whereas the unburned hillslopes generated almost no sediment. Sediment yields from the raked and burned hillslopes were indistinguishable when they had comparable surface cover, rainfall erosivity, and soil water repellency values. The rainfall simulations on ash-covered plots generated only 21 to 49% as much runoff and 42 to 67% as much sediment as the plots with no ash cover. Soil thin sections showed that the bare plots rapidly developed a structural soil seal. Successive simulations quickly eroded the ash cover and increased runoff and sediment yields to the levels observed from the bare plots. The results indicate that: (i) post-fire sediment yields were primarily due to the loss of surface cover rather than fire-enhanced soil water repellency; (ii) surface cover is important because it inhibits soil sealing; and (iii) ash temporarily prevents soil sealing and reduces post-fire runoff and sediment yields.


Wildfires increase hillslope- and watershed-scale runoff and sediment yields by several orders of magnitude (e.g., Prosser and Williams, 1998; Robichaud and Brown, 1999; Moody and Martin, 2001; Benavides-Solorio and MacDonald, 2005; Malmon et al., 2007). Land use and climate change have increased, or are projected to increase, the size and frequency of fires in many wildland environments (e.g., Mouillot et al., 2002; Hennessy et al., 2005; Westerling et al., 2006). The increase in fire risk is generating considerable concern about the potential adverse effects on water quality, aquatic habitat, and water supply systems (Rinne, 1996; Robichaud et al., 2000; Moody and Martin, 2001; Burton, 2005).

The large increases in runoff and sediment yields after high-severity fires have been attributed to several factors, including: (i) soil water repellency (DeBano, 2000; Doerr et al., 2000); (ii) loss of surface cover ( Johansen et al., 2001; Pannkuk and Robichaud, 2003); (iii) soil sealing by sediment particles (Lowdermilk, 1930; Neary et al., 1999); and (iv) soil sealing by ash particles (Mallik et al., 1984; Etiégni and Campbell, 1991). The problem is that the relative contribution of each factor to the observed increases in post-fire runoff and sediment yields is largely unknown (Shakesby et al., 2000; Letey, 2001). This lack of understanding hampers our ability to predict post-fire sediment yields and design effective post-fire rehabilitation treatments.

Burning has been shown to induce or enhance soil water repellency (hydrophobicity) in a variety of shrub and forest ecosystems (Doerr et al., 2009), and this increase in soil water repellency has been commonly cited as a primary cause of the observed post-fire increases in peak flows and sediment yields (e.g., Krammes and Osborn, 1969; DeBano, 1981, 2000; Robichaud, 2000; Shakesby et al., 2000). High-severity fires also alter the vegetative cover and characteristics of the mineral soil, making it very difficult to separate the effects of fire-induced soil water repellency from other changes in soil characteristics and surface cover. Doerr and Moody (2004) explicitly stated that the linkage between soil water repellency and post-fire runoff and erosion rates has rarely been demonstrated.

Our previous work in the Colorado Front Range has shown that post-fire soil water repellency breaks down within 1 to 2 yr after burning (MacDonald and Huff man, 2004). For the same fires, 3 to 5 yr may be required before hillslope-scale sediment yields return to background levels (Benavides-Solorio and MacDonald, 2005; Pietraszek, 2006; Wagenbrenner et al., 2006). This discrepancy in time implies that fire-induced soil water repellency is not the primary cause of the observed increases in runoff and erosion after high-severity wildfires. …

In severely burned areas, the loss of surface cover is greatly confounded by the increase in soil water repellency noted above, a decrease in soil organic matter and an associated reduction in aggregate stability (Giovannini and Lucchesi, 1983; Soto et al., 1991; Badí and Martí, 2003), and the presence of ash, which has been reported to induce soil sealing and inhibit the infiltration of runoff (e.g., Mallik et al., 1984; Durgin, 1985; Gabet and Sternberg, 2008).

Soil sealing refers to the development of a thin (0.1–1.0 mm), dense soil layer at the mineral soil surface. The hydraulic conductivity of this seal or crust can be several orders of magnitude lower than that of the underlying soil (McIntyre, 1958; Terry and Shakesby, 1993; Shainberg and Levy, 1996; Assouline and Mualem, 2000; Assouline, 2004). Soil seals are categorized as either structural seals, which form due to raindrop impact and rapid wetting, or depositional seals, which form due to the settling of fine particles carried by runoff (Assouline, 2004). The processes that contribute to the formation of structural soil seals include: (i) the destruction of soil aggregates by raindrop impact and slaking; (ii) soil compaction and realignment of surface particles by raindrops; and (iii) pore clogging by the physical movement of fine particles or the chemical dispersion of clays (Assouline, 2004).

Structural soil seals are an important control on runoff rates in agricultural areas (e.g., Radcliff e et al., 1991; Bajracharya and Lal, 1998; Assouline, 2004), and it has been suggested that highseverity fires can increase the likelihood of soil sealing by several processes. …

The overall goal of this study is to better understand the relative effects of soil water repellency, loss of surface cover, and soil sealing on post-fire sediment yields by a combination of field studies and rainfall simulation experiments. …


The rapid decay of soil water repellency after burning suggests that soil water repellency cannot be the primary cause of the observed increases in post-fire runoff and surface erosion. The severely burned areas at the Hayman and Schoonover fires generally had stronger soil water repellency than the unburned areas only through the first summer after burning (Fig. 4). An even more rapid decline in post-f re soil water repellency was observed after the June 2000 Bobcat fire in the northern Colorado Front Range (MacDonald and Huffman, 2004), yet sediment yields were high in each of the first 2 yr after burning (Benavides-Solorio and MacDonald, 2005; Wagenbrenner et al., 2006). Rapid declines in post-fire soil water repellency also have been documented for other conifer and chaparral ecosystems in western North America (Henderson and Golding, 1983; McNabb et al., 1989; Hubbert et al., 2006). The longer duration of high post-fire sediment yields relative to the fire-induced soil water repellency, as shown in Fig. 4, is strong evidence that fire-enhanced soil water repellency is not the dominant control on post-fire sediment yields. …

The burning and raking experiments demonstrated that surface cover was an important control on post-fire sediment yields, which is consistent with the results from other studies (e.g., Morris and Moses, 1987; Cerdà, 1998a; Prosser and Williams, 1998; Robichaud and Brown, 1999; Benavides-Solorio and MacDonald, 2001, 2005; Johansen et al., 2001). The problem is that the process(es) that drives the observed empirical relationship between bare soil percentage and post-fire sediment yields has heretofore been unclear, as high-severity fires reduce both soil organic matter and aggregate stability. These changes increase soil erodibility and are conducive to soil sealing. The Hayman fire did not significantly reduce soil organic matter (Libohova, 2004), and hence the loss of organic matter cannot explain the observed increase in sediment yields. …

More process-based studies provide strong evidence that a fire-induced increase in soil erodibility was not the primary cause of the increased sediment production after the Hayman wildfire. Our field data showed no evidence of overland flow or surface erosion on convergent hillslopes, while the first post-fire storm caused extensive rilling in the swale axes (Libohova, 2004). During successive storms, these rills incised up to several decimeters into the underlying soil, and this soil was not altered by the 2002 fires. Detailed measurements indicate that these rills and small gullies contributed 60 to 80% of the hillslope sediment yields measured after the Hayman and Schoonover fires (Pietraszek, 2006). Since this unaltered soil was the primary source of sediment, we infer that the fire-induced changes in soil erodibility and aggregate stability were not the primary causes of the observed increase in sediment yields. The same argument can be made for the raked hillslopes where similar rilling was observed. …

The key question is what caused the observed increase in runoff, and both the single and successive rainfall simulation experiments demonstrated that the development of a structural soil seal on the bare soil treatment led to the measured increases in runoff. The simulation experiments also demonstrated that an ash cover initially prevented soil sealing, but runoff rates sharply increased as the ash cover was removed by successive rain events.

Numerous other studies also have shown that a wide variety of surface cover types can protect against soil sealing; these include straw mulch, crop residues, leaves, grasses, cryptogams, and stones (Morin and Benyamini, 1977; Poesen, 1986; Kinnell et al., 1990; Moss and Watson, 1991; Ruan et al., 2001). Hence, the relationship between sediment yields and surface cover shown in Fig. 1 can be best explained by the surface cover percentage controlling the extent of soil sealing, which in turn controls runoff production and surface erosion by overland flow. …

The critical role of soil sealing for post-fire runoff and sediment yields has important implications for the design and application of post-fire emergency rehabilitation treatments. Several studies have shown that application of straw mulch reduces post-fire sediment yields by at least 90% in the first 1 to 3 yr after burning (Bautista et al., 1996; Wagenbrenner et al., 2006; Rough, 2007), and this further confirms the importance of surface cover in controlling post-fire sediment yields. Our present study provides the mechanistic explanation as to why post-fire mulching is effective, as it protects the mineral soil from raindrop impact and soil sealing. Experimental and plot-scale rainfall simulations conducted under unburned conditions also have shown that mulch reduces soil sealing (Morin and Benyamini, 1977; Morin et al., 1989; Moss and Watson, 1991).

In contrast to mulching, seeding generally does not reduce post-fire sediment yields (Robichaud et al., 2000). The lack of effectiveness has to be attributed to the fact that seeding generally does not significantly increase revegetation rates and the amount of surface cover (Robichaud et al., 2000; Wagenbrenner et al., 2006; Rough, 2007). These empirical observations also can be mechanistically explained as a lack of protection against soil sealing. Taken together, these results show that the most effective treatments for reducing post-fire sediment yields are those that immediately increase the amount of surface cover and thereby inhibit soil sealing. …

The predominant role of soil sealing means that the most effective post-fire rehabilitation treatments will be those that immediately increase the amount of surface cover.

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