Elsevier

Journal of Hydrology

Volumes 231–232, 29 May 2000, Pages 220-229
Journal of Hydrology

Fire effects on infiltration rates after prescribed fire in Northern Rocky Mountain forests, USA

https://doi.org/10.1016/S0022-1694(00)00196-7Get rights and content

Abstract

Infiltration rates in undisturbed forest environments are generally high. These high infiltration rates may be reduced when forest management activities such as timber harvesting and/or prescribed fires are used. Post-harvest residue burning is a common site preparation treatment used in the Northern Rocky Mountains, USA, to reduce forest fuels and to prepare sites for natural and artificial tree regeneration. Prescribed burn operations attempt to leave sites with the surface condition of a low-severity burn. However, some of the areas often experience surface conditions associated with a high-severity burn which may result in hydrophobic or water repellent conditions. In this study, infiltration rates were measured after logging slash was broadcast burned from two prescribed burns. The two sites were in Northern Rocky coniferous forests of Douglas-fir/lodgepole pine and ponderosa pine/Douglas-fir. Simulated rainfall was applied to one-square meter plots in three, 30-min applications at 94 mm h−1 within the three surface conditions found after the burn: unburned-undisturbed areas, low-severity burn areas and high-severity burn areas.

Runoff hydrographs from the rainfall simulations were relatively constant from the plots that were in unburned-undisturbed areas and in areas subjected to a low-severity burn. These constant runoff rates indicate constant hydraulic conductivity values for these surface conditions even though there was variation between plots. Hydrographs from the rainfall simulation plots located within areas of high-severity burn indicate greater runoff rates than the plots in low-severity burn areas especially during the initial stages of the first rainfall event. These runoff rates decreased to a constant rate for the last 10 min of the event. These results indicate hydrophobic or water repellent soil conditions, which temporarily cause a 10–40% reduction in hydraulic conductivity values when compared to a normal infiltrating soil condition. Since variability was high for these forest conditions, cumulative distribution algorithms of hydraulic conductivity provide a means to account for the inherent variability associated with these hillslopes and different surface conditions cause by fire.

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

References (32)

  • R.D Bond

    The influence of the microflora on the physical properties of soils. II. Field studies on water repellent sands

    Aust. J. Soil Res.

    (1964)
  • I.A Campbell

    Stream discharge, suspended sediment and erosion rates in the Red Deer River basin, Alberta, Canada

    Int. Assoc. Hydrol. Sci.

    (1977)
  • DeBano, L.F., 1981. Water repellent soils: a state of the art, Gen. Tech. Rpt. PSW-46. USDA For. Serv., Pacific...
  • L.F DeBano et al.

    Water repellent soils: their implications in forestry

    J. Forestry

    (1973)
  • DeBano, L.F., Osborn, J.F., Krammes, J.S., Letey, J., Jr., 1967. Soil wettability and wetting agents…our current...
  • L.F DeBano et al.

    The transfer of heat and hydrophobic substances during burning

    Soil Sci. Soc. Am. J.

    (1976)
  • L.W Dekker et al.

    Fingerlike wetting patterns in two water-repellent loam soils

    J. Environ. Qual.

    (1995)
  • L.W Dekker et al.

    Preferential flow paths in a water repellent clay soil with grass cover

    Water Resour. Res.

    (1996)
  • Dyrness, C.T., 1976. Effect of wildfire on soil wettability in the High Cascades of Oregon. Res. Paper PNW-202. USDA...
  • R.H Hawkins et al.

    Steady-state analysis of infiltration and overland flow for spatially-varied hillslopes

    Water Resour. Bull.

    (1987)
  • D Hillel

    Introduction to Soil Physics

    (1982)
  • Holzhey, C.S., 1969. Water-repellent soil in Southern California. In: DeBano, L.F., Letey, J. (Eds.), Water-Repellent...
  • C.H Luce et al.

    Parameter identification for a runoff model for forest roads

    Water Resour. Res.

    (1994)
  • R.H McCuen et al.

    Hydrologic Modeling: Statistical Methods and Applications

    (1986)
  • D.H McNabb et al.

    Infiltration, water repellency, and soil moisture content after broadcast burning a forest site in southwest Oregon

    J. Soil Water Conserv.

    (1989)
  • Meeuwig, R.O., 1971. Infiltration and water repellency in granitic soils. Res. Paper INT-111. USDA For. Serv.,...
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