Fire effects on infiltration rates after prescribed fire in Northern Rocky Mountain forests, USA
Introduction
Water infiltration is defined as the flow of water from the soil surface into the soil profile. The rate at which water is transmitted through soil is highly dependent upon the surface conditions. In forest environments, various surface conditions can exist and it is important to characterize these conditions and their effect on infiltration.
Runoff from harvested and burned hillslopes varies from extensive to minor. The major determining factor is the amount of disturbance to the surface material which is usually organic debris (commonly referred to as duff or forest floor) that protects the underlying mineral soil. Disturbance may be from tree harvesting operations, road building, or fire. All of these activities may impact the protective duff layer. Adverse effects on the duff layer by burning depend upon the severity of fire (Robichaud et al., 1993, Robichaud and Waldrop, 1994, Robichaud, 1996). Post-fire condition of the surface horizons are important because they determine the amount of mineral soil exposed to raindrop splash, overland flow and the development of water repellent soil conditions (DeBano, 1981). Observations from previous studies (Robichaud et al., 1993) suggest there are four different surface/hydrologic conditions to monitor which affect infiltration. These conditions are: (1) areas subjected to high-severity burns (possibly hydrophobic); (2) areas subjected to low-severity burns; (3) areas with bare soil due to log dragging, log landings, skid trials, or roads; and (4) unburned-undisturbed areas.
Numerous observations of water repellent soil conditions have been reported throughout the western USA and the world. Water repellency caused by wildfires has received the most attention in southern California chaparral (DeBano et al., 1967, DeBano and Rice, 1973), although it has been reported after forest wildfires (Megahan and Molitor, 1975, Dyrness, 1976, Campbell, 1977) and on rangelands (Richardson and Hole, 1978, Soto et al., 1994).
In burned soils, severity of water repellency not only depends on soil texture, but is also related to fire intensity, antecedent soil-water content and fuel conditions (DeBano et al., 1976, Robichaud and Hungerford, 2000, Robichaud, 1996). Under field conditions, the water-repellent layer is usually not continuous, so irregular wetting patterns are common (Bond, 1964, Meeuwig, 1971, DeBano, 1981, Dekker and Ritsema, 1995, Dekker and Ritsema, 1996). Water repellency induced by a low-to-moderate severity prescribed burn is usually of short duration. For example, in southwestern Oregon, soil wettability resulting from a late spring wildfire burn returned to near normal levels after the fall rains began (McNabb et al., 1989). After a late summer wildfire in the Oregon Cascade Mountains, Dyrness (1976) found that soil wettability in stands of lodgepole pine (Pinus contorta) experiencing burns of low-severity recovered more rapidly than soils experiencing burns of high-severity. By the sixth year after the fire, wettability of the soils that experienced both low- and high-severity burns approached that of unburned soil.
The most apparent hydrologic effect of hydrophobic soil conditions is the reduction of infiltration which can induce erosion by overland flow (DeBano et al., 1967). Infiltration curves reflect increasing wettability over time once the soil is placed in contact with water. Infiltration increases with time because the hydrophobic substances responsible for water repellency are slightly water soluble and slowly dissolve, thereby increasing wettability (DeBano, 1981). Researchers have documented persistence of hydrophobic conditions from weeks to years (DeBano et al., 1967, Holzhey, 1969). In general, hydrophobicity is broken up, or is sufficiently washed away, within one to two years after a fire.
The objective of this study was to determine infiltration characteristics of forest soils burned at different severities. These calculated hydraulic conductivity values provide important input parameters for use in current erosion prediction models that describe hydrologic responses for various surface conditions typically encountered in forest environments.
Section snippets
Field sites
The first site, Slate Point (7 ha), was located on the West Fork Ranger District of the Bitterroot National Forest in western Montana, USA. This location has a Douglas-fir (Pseudotsuga menziesii)/lodgepole pine (Pinus contorta) forest. The habitat type is Douglas-fir/twinflower (Linnaea borealis) (Pfister et al., 1977). Slopes within the study area range from 30 to 70% with a northern aspect. Elevation range from 1620 to 1780 m. The soils (83% sand, 12% silt, 5% clay with 33% gravel component)
Fire descriptions
Ignition techniques, fuel moisture and weather during the Slate Point burn produced an intense fire concentrated in the center of the unit, whereas the Hermada burn produced a low intensity fire (Table 1). Maximum temperatures within the duff were 69–612°C lasting 3–8 min at the Slate Point site, whereas at the Hermada site maximum temperatures were only 119–187°C in the duff. Spatially varied surface conditions occurred after both prescribed burns. Duff depths averaged 47 mm prior to the fire
Conclusions
Variable surface conditions are common in forest environments especially after prescribed fires. Small-scale rainfall simulation techniques provide a reliable method to determine hydraulic conductivity for these various surface conditions. Two prescribed burns were conducted and both produced variable infiltration rates related to burn severity. When hydrophobic conditions were present, marked changes in the runoff hydrographs over time allowed for the determination a hydrophobic hydraulic
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