Soil water repellency and infiltration in coarse-textured soils of burned and unburned sagebrush ecosystems
Introduction
Soil water repellency has been well documented in burned and unburned soils on semi-arid rangeland (Salih et al., 1973, Pierson et al., 2001, Pierson et al., 2008), chaparral (Hubbert et al., 2006), and forested environments (Robichaud, 2000, Huffman et al., 2001, MacDonald and Huffman, 2004, Doerr et al., 2006, Lewis et al., 2006). Wildfires have often been associated with the formation of water repellent soil conditions thought to decrease infiltration and increase runoff and soil erosion (DeBano et al., 1998, Robichaud, 2000). During combustion hydrophobic organic compounds in litter and topsoil are volatized and released upwards to the atmosphere and downwards into the soil profile along a temperature gradient. Downward translocated hydrophobic compounds condense on cooler soil particles at or below the soil surface forming water repellent conditions (DeBano et al., 1976). Natural water repellent soil conditions also occur in unburned plant communities due to coating of soil particles with hydrophobic compounds leached from organic matter accumulations, by-products of microbial activity, and or fungal growth under thick layers of litter and duff material (Savage et al., 1972, DeBano, 2000, Doerr et al., 2000). Under unburned conditions litter and vegetation cover promote water storage and mitigate water repellency impacts on infiltration and erosion (Rauzi et al., 1968, Blackburn et al., 1986). Fire removes this protective covering, exposing the soil to raindrop impact and removing barriers to overland flow (Moffet et al., 2007, Pierson et al., 2008).
Recent studies have identified seasonal variability in the presence and strength of soil water repellency under burned and unburned conditions (Doerr and Thomas, 2000, Dekker et al., 2001, Huffman et al., 2001). Doerr and Thomas (2000) observed seasonal patterns in soil water repellency correlated to rainfall patterns, biological productivity, and spatial variations in water repellency during soil wetting. Dekker et al. (2001) demonstrated that soil water repellency is a function of soil water content, that critical soil water thresholds demarcate wettable and water repellent soil conditions, and that the relationship between moisture content and soil water repellency is affected by drying regime. In a multiple fire study, Huffman et al. (2001) found that time since burning was not a significant predictor of soil water repellency in pine forests of the Colorado Front Range and noted the water repellent soils became wettable when soil moisture levels exceeded 12 to 25%. These studies indicate that seasonal variability in site characteristics that influence soil water repellency can confound assessment of long-term soil water repellency persistence (Doerr et al., in press).
The temporal dynamics of soil water repellency and respective impacts on rangeland hydrology and erosion have received little attention in the literature. Pierson et al., 2001, Pierson et al., 2002 used rainfall simulations to investigate infiltration, runoff, and erosion processes on burned and unburned steeply-sloped sagebrush sites with coarse-textured soils. Burned shrub coppice microsites (areas underneath shrub canopy) had significantly lower infiltration compared to unburned shrub coppices in both studies. The lowest infiltration rates were observed on unburned interspaces (areas between shrub canopies) that were densely covered in very dry litter and senescent grasses. Pierson et al. (2001) developed a Water Repellency Index (WRI) to quantify the relative impact of soil water repellency on infiltration. The index differs from a previous water repellency index presented by Tillman et al. (1989) that is a ratio of the intrinsic sorptivity of ethanol to that for water. The Pierson et al. (2001) index scales the difference in final and minimum infiltration rates by the final infiltration rate. Using this WRI, Pierson et al. (2001) found burned coppice microsites had an average reduction of 28% in infiltration immediately post-fire with relatively little variability between plots. Pierson et al. (2002) attributed results to naturally strong soil water repellency, but did not explicitly measure soil water repellency in their experiment.
The goal of this paper is to document the effects of fire on the spatial and temporal variability in soil water repellency and potential impacts on infiltration and runoff on sagebrush-dominated landscapes. The objective of this paper is to test if relationships exist between directly measured soil water repellency, and the spatial and temporal variation in infiltration capacity and runoff generation on burned and unburned microsites within coarse-textured sagebrush-dominated ecosystems. The results of Pierson et al., 2001, Pierson et al., 2002 are extended by adding data, analysis, and interpretation. Data from an additional independent study is also included to broaden the inference space of conclusion.
Section snippets
Eighth Street Wildfire (1996)
The Eighth Street Wildfire study area (43°40′00″ latitude 116°06′48″ longitude) is located 5 km north of Boise, Idaho, USA, in the Boise Foothills. The fire burned 6070 ha at low to high intensities in late summer 1996 and was the focus of the hydrologic experiments presented in Pierson et al. (2002). The research area mean elevation is 1400 m. Annual precipitation ranges from 350 mm at lower elevations to 750 mm near ridgelines, most of which falls between November and May. Mean annual air
Experimental design
The Eighth Street site was sampled with 10 rainfall simulation runoff plots (0.5 m2) for each treatment 1 year after the fire (Pierson et al., 2002). Treatments included slope, aspect (north and south) and fire severity (unburned, moderate, and high). Additional plots were sampled 2 years after the fire only on the high fire severity treatments. The plots were arranged with half the plots randomly placed on coppice microsites and half on interspace microsites. Burned sites were closely matched
Soils and vegetation
All sites had similar coarse texture soils and soil water contents. Average sand content ranged from 69 to 84% (Table 1). Antecedent surface soil water contents were low for all hillslopes during all years of each study (Table 1). Bulk densities were similar across all sites but were 30% higher on burned than unburned hillslopes at the Eighth Street and Denio sites (Table 1). Bulk density at the Breaks site was nearly equal under burned and unburned conditions.
All study sites had similar
Patterns in water repellency
Experiments on two steeply-sloped sagebrush sites with coarse-textured soils found strongly water repellent soils under unburned conditions (Table 2). Background water repellency at the Denio and Breaks study sites was strong at the mineral soil surface immediately below the duff layer on coppices and interspaces (Fig. 1, Fig. 2). Water repellency on unburned semi-arid landscapes is commonly highest at the surface of the soil profile (Huffman et al., 2001). Surface WDPT at the Denio and Breaks
Conclusions
Runoff from two wildfires and one prescribed fire on coarse-textured and steeply-sloped sagebrush sites in this study was more influenced by the combined removal of vegetation and pre-fire strongly water repellent soils than fire influenced soil water repellency. Fire removal of vegetation on coppice microsites greatly reduced infiltration even though the water repellency strength generally decreased. Increased runoff on coppice microsites following burning then resulted from concurrent strong
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