Post-fire Soil Water Repellency : Persistence and Soil Moisture Thresholds

and Thomas, 2000). A study of soil water repellency after five different fires in the Colorado Front Range found Fire-induced soil water repellency is a key control on post-fire that burn severity, percentage of sand, and soil moisture runoff and erosion rates, but there are few data on the persistence of soil water repellency and the soil moisture threshold at which water were the primary controlling variables (Huffman et al., repellent soils become hydrophilic. This study used repeated sampling 2001). Time since burning was not a significant control to quantify changes in soil water repellency over time and identify on soil water repellency because of the large variability soil moisture thresholds for the loss of soil water repellency. The in soil water repellency values between fires. The strongstudy area was a wildfire in the northern Colorado Front Range that est soil water repellency was observed in prescribed burned 43 km2 of ponderosa and lodgepole pine forests in June 2000. fires 4 and 7 mo after burning, and significant water Soil water repellency and soil moisture were measured periodically repellency was still present 22 mo after burning in the from June 2000 through June 2001 at 36 sites stratified by burn severity and nine unburned sites. Water repellency was assessed in the field case of the Crosier Mountain fire (Huffman et al., 2001). at depths of 0 to 18 cm using the critical surface tension (CST) test. Soil Given the variability between fires, repeated measurewater repellency was strongest in sites burned at high and moderate ments from one fire are needed to more accurately severity, decreased with increasing depth, and was spatially highly determine the persistence of fire-induced soil water revariable. The fire-induced soil water repellency progressively weakpellency. ened and became statistically nondetectable by 1 yr after burning. The The generalized linear model developed in Huffman effect of time since burning on soil water repellency was increasingly et al. (2001) showed that soil water repellency signifisignificant with increasing burn severity and progressively less important with increasing soil depth. The soil moisture thresholds at which cantly decreased with increasing surface soil moisture. water repellent soils become hydrophilic apparently increase with Other studies have suggested a soil moisture threshold increasing burn severity. The data suggest soil moisture thresholds of or transition zone at which soils cease to be water repelapproximately 10% for unburned sites, 13% for sites burned at low lent, as soils eventually wet up due to the strong hydrauseverity, and no less than 26% for sites burned at moderate and lic gradient and movement of water vapor (DeBano, high severity. 1981). Once wet, soils are no longer water repellent until they again dry out (Doerr and Thomas, 2000). For unburned soils, reported soil moisture thresholds range F soil water repellency is often cited as from 2 to 5% for a dune sand (Dekker et al., 2001) to a key factor controlling post-fire runoff and erosion 5 to 12% for naturally hydrophobic soils in Denmark rates (e.g., Morris and Moses, 1987; Imeson et al., 1992; (de Jonge et al., 1999), and 34 to 38% for clayey peat Shakesby et al., 2000; Letey, 2001). The strength and soils (Dekker and Ritsema, 1996). persistence of fire-induced water repellency is of particFor burned areas there are almost no data on the ular concern in the Colorado Front Range, as recent soil moisture threshold for the elimination of soil water forest fires have been followed by severe flooding, erorepellency. Doerr and Thomas (2000) noted an absence sion, and sedimentation (e.g., Moody and Martin, 2001; of soil water repellency in a coarse-textured forest soil Graham, 2003). Millions of dollars are being spent on in Portugal once the soil moisture content exceeded 28%. emergency treatments in burned areas to reduce postfire increases in runoff and erosion and the associated A visual examination of the data from Huffman et al. threats to property, aquatic resources, and domestic wa(2001) suggests that the soil moisture threshold in the ter supplies (Robichaud et al., 2000). The assessment Colorado Front Range may range from about 12% in of post-fire erosion risks and design of rehabilitation unburned areas to a minimum of 25% in severely burned treatments require an understanding of how soil water areas, but a more rigorous analysis was hampered by repellency changes over time and under different envithe limited amount of soil water repellency data at high ronmental conditions. soil moisture contents. Studies in other areas have shown that several factors The objectives of this study were to determine: (i) the affect the strength and persistence of post-fire soil water persistence of post-fire soil water repellency by repeated repellency, including burn severity, vegetation type, soil measurements within a single fire; (ii) if there is a soil texture, soil moisture, and time since burning (DeBano, moisture threshold for the loss of post-fire soil water 1981; de Jonge et al., 1999; DeBano, 2000a, 2000b; Doerr repellency; and (iii) if the soil moisture threshold varies with fire severity. These results are needed to design L.H. MacDonald, Dep. of Forest, Rangeland, and Watershed Stewardpost-fire rehabilitation treatments, identify the necesship, Colorado State Univ., Fort Collins, CO 80523-1472; and E.L. Huffman, Allegheny National Forest, 222 Liberty St., Warren, PA, sary design life for those treatments, and indicate when 16365. Received 22 Sept. 2003. *Corresponding author (leemac@cnr. post-fire flooding and erosion risks may be substancolostate.edu). tially reduced. Published in Soil Sci. Soc. Am. J. 68:1729–1734 (2004).  Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CST, critical surface tension.