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

Robichaud, P. R. 2000. Fire effects on infiltration rates after prescribed fire in northern Rocky Mountain forests, USA. Journal of Hydrology. 231-232(2000): 220-229.

Keywords: Water repellent, rainfall simulation, forest fire, hydraulic conductivity, water repellency, infiltration

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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 mountain coniferous forests of Douglas-fir/lodgepole pine and ponderosa pine/Douglas-fir. Simulated rainfall was applied to one-square meter plots in three, 30-minute 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 minutes of the event. These results indicate hydrophobic or water repellent soil conditions, which temporarily cause a 10 to 40 percent reduction in hydraulic conductivity values when compared to a normal infiltrating soil condition. Because 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 caused by fire.

Moscow FSL publication no. 2000g