Thesis and Dissertation Abstracts
 

Juan de Dios Benavides-Solorio 

Post-Fire Runoff and Erosion at the Plot and Hillslope Scale, Colorado Front Range

            Forest ecosystems in the Colorado Front Range are at very high risk for large increases in runoff and erosion after wildfires.  This research was proposed because there is very limited data on post-fire runoff and erosion rates and the factors that control these rates.  This research focussed on two different scales: 1) runoff and erosion rates from small plots subjected to high-intensity artificial rainfall, and 2) sediment production rates at the hillslope scale from prescribed and wild fires of different ages.  The results will help predict the effect of future fires and design more effective rehabilitation treatments.

On the small plots 70-85 mm of mean rainfall was applied in 60 minutes, and runoff/rainfall ratios generally exceeded 45%.  The high rainfall rate meant that runoff/rainfall ratios were only slightly higher from plots burned at high severity than from low severity/unburned plots.  Mean runoff/rainfall ratios in recently-burned, high-severity plots decreased by 15-30% from the first to second years after burning, but were still high relative to runoff rates from simulations on the 1994 Hourglass wildfire.  Post-fire soil water repellency was the main control on runoff/rainfall ratios. 

Mean sediment yields from rainfall simulations on high severity sites in the Bobcat wildfire were 1,280 g m^-2 in 2000 and 1,230 g m^-2 in 2001.  Sediment yields from high severity sites in the Lower Flowers prescribed fire decreased from 850 g m^-2 in 2000 to 350 g m^-2 in 2001.  High severity plots yielded 16-33 times more sediment than low severity and unburned plots.  Simulations on high severity plots in the 1994 Hourglass wildfire yielded only slightly more sediment than unburned plots, indicating that recovery was nearly complete after 6 years.  Univariate and multivariate analysis showed that percent bare soil was the dominant control on sediment yields, although percent silt and the runoff/rainfall ratio were significant factors for high severity sites. 

At the hillslope scale sediment production rates exceeded 10 Mg ha^-1 yr^-1 from sites burned at high severity in a recent wildfire, and only 0.1-4 Mg ha^-1 yr^-1 from high severity sites in recent prescribed fires.  High severity sites in the Bobcat wildfire produced 75 times more sediment than moderate severity sites.   Summer rainstorms generated at least 73% of the sediment at all sites.  Sediment production rates from swales or small drainages were 2-3 times higher than planar hillslopes.  Four to six years are required for sediment production rates to approach the values from sites burned at low severity.  Multivariate modeling showed that sediment production rates were a function of fire severity, percent bare soil, rainfall erosivity, soil water repellency, and soil particle size.  The best model had a R² of 0.77, and this declined to 0.62 for a two-parameter model using percent bare soil and rainfall erosivity.  Model validation was satisfactory, but more data are needed for complete testing.

The runoff and erosion rates measured in this study are high relative to the results from most other studies in burned areas.  Areas burned at high severity are at particularly high risk for at least the first 2-3 years after burning.  To be effective post-fire rehabilitation treatments must immediately provide ground cover and maintain this for at least two years.  Rehabilitation efforts also should focus on reducing flow velocities in swales and small drainages to reduce channel erosion.

Joe Wagenbrenner

Effectiveness of Burned Area Emergency Rehabilitation Treatments, Colorado Front Range

Wildfires can increase the amount of runoff and erosion by several orders of magnitude compared to unburned areas.  Burned area emergency rehabilitation (BAER) treatments are installed to minimize the increase in erosion from burned hillslopes, but there have been few quantitative studies on the effectiveness of these treatments.

This study assessed the effectiveness of seeding, straw mulching, and contour felling in areas burned by the Bobcat Fire in June 2000 near Loveland, Colorado.  Sediment yields were measured using thirty-four sediment fences from August 2000 to October 2001.  Percent ground cover was measured in fall 2000, spring 2001, and fall 2001.  The installation quality and sediment storage capacity of the contour felling treatment were assessed at seven sites in three wildfires along the Colorado Front Range.

In fall 2000, the untreated control plots averaged 67% bare soil.  The mulched plots only had 26% bare soil.  Percent vegetative cover increased for all treatments over the study period, but only the mulched plots had significantly less percent bare soil than the controls and other treatments throughout the study (p £  0.016 in all cases).

In summer 2000, the mean sediment yield for the controls was at least 6,200 kg ha^-1, and none of the treatments significantly reduced sediment yields relative to the controls.  The sediment yields from the control plots averaged 9,500 kg ha^-1 in summer

2001, while the mean was 520 kg ha^-1 for the plots that were mulched in 2000, 23 kg ha^-1 for the plots mulched in 2001, and 2,800 kg ha^-1 for the newly installed contour-felled plots.  In 2001 the sediment yields from these three treatments were significantly less than the sediment yields from the controls (p ≤  0.0001 in each case).

The installation quality of the contour felled logs was highly variable between sites.  Failure rates ranged from 10 to 70%, with placement or movement off contour the dominant cause of failure.  The mean storage capacity for the seven sites was 16 m³ ha^-1, or approximately 18,000 kg ha^-1, which is greater than the mean sediment yield for the untreated plots in 2001.

Seeding was not effective at increasing cover or reducing sediment yields compared to the controls, even for storms with return periods of less than one year.  Mulching was the most effective treatment at reducing sediment yields.  This effectiveness extended to slopes of 19 to 69%, and persisted through summer 2001.  Contour felling had the capacity to reduce sediment yields from smaller storms.  However, the effectiveness of contour felling may not persist, depending on the timing and magnitude of storms as well as proper installation of the logs.

These results should help land managers determine the benefits associated with a given BAER treatment in the Colorado Front Range.  The importance of percent cover at reducing sediment yields should be considered in the design of more effective treatments.

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