WATER REPELLENCY AND FIRE IN SAGEBRUSH ECOSYSTEMS OF THE NORTHERN
GREAT BASIN, USA.
Frederick B. Pierson, USDA, Agricultural Research Service, Northwest Watershed Research
Center, Boise, Idaho, USA. E-mail: fpierson@nwrc.ars.usda.gov
Peter R. Robichaud, USDA, Forest Service, Rocky Mountain Research Station, Moscow, Idaho,
USA.
Severe wildfires have occurred across the western United States over the past decade. Past
management practices and fire suppression policies have left wild lands with high fuel loads
resulting in larger wildfires with high burn severities. This combined with more people living in
suburbs and commuting to work in urban areas has created a greater wildfire threat to human life
and community developments. Threats not only include the fire itself, but in some areas the
threat of flooding can also occur. Many communities are built along rivers at the base of large
steep watersheds. An estimated $40 million is spent every year in the United States to mitigate
the effects of wildfires and reduce the risk of flash floods and debris flows. Many factors are
considered when determining where to apply erosion control treatments. Past research in
forested and chaparral-dominated communities has indicated that wildfires cause a significant
increase in soil water repellency resulting in more runoff and erosion potential. Little data exists
for other shrub lands and grasslands, therefore findings from forests and chaparral are
extrapolated to the post-fire management of many other plant communities. A series of studies
were conducted to better understand the impacts of wildfire and subsequent recovery on hillslope
runoff and erosion processes within steep watersheds with coarse-textured soils and sagebrushbunchgrass plant communities. In general, year to year changes in infiltration capacity have been
larger than the impact of fire on infiltration due to natural variations in soil water repellency.
Under dry conditions, soil water repellency can be greater and more persistent on unburned
areas compared to burned areas. Recovery in years following fire can result in less water
repellency, improved infiltration capacities and reduced runoff amounts compared to unburned
conditions. However wildfires do consume organic ground cover that protects the soil surface,
thus runoff can easily move down slope with greater velocity and erosion potential. These results
suggest that postfire treatments within these plant communities should focus on erosion control
and not on improving infiltration capacity by “breaking-up” soil water repellent layers within the
soil.