Post-fire mulching for runoff and erosion mitigation. Part II: Effectiveness in reducing runoff and sediment yields from small catchments

Robichaud, Peter R.; Wagenbrenner, Joseph W.; Lewis, Sarah A.; Ashmun, Louise E.; Brown, Robert E.; Wohlgemuth, Peter M. 2013. Post-fire mulching for runoff and erosion mitigation. Part II: Effectiveness in reducing runoff and sediment yields from small catchments. Catena 105: 93–111.

Keywords: Straw mulch, Hydromulch, post-fire rehabilitation, Treatment effectiveness, California, Colorado

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Abstract: Agricultural straw, hydromulch, and wood shred or wood strand mulches increasingly are being used as post-fire hillslope treatments, but the differences in effectiveness among these mulch treatments are not fully understood. Following the 2002 Hayman fire in central Colorado and the 2003 Cedar fire in southern California, matched catchments were monitored for five to seven post-fire years to determine the effectiveness of wheat straw mulch (Hayman fire only) and hydromulch in reducing post-fire runoff, peak flow rates, and sediment yields from natural rainfall. Measured runoff and sediment yields were caused by short duration high intensity summer storms at the Hayman fire and long duration winter rains at the Cedar fire. The wheat straw mulch treatment significantly reduced peak flow rates and sediment yields at the Hayman fire. The annual peak flow rates in the first two post-fire years in the straw mulch catchment were 4.5 and 3.9 m³ s^-1 km^-2 (respectively) as compared to 4.3 and 7.1 m³ s^-1 km^-2 (respectively) in the control. In post-fire years one and two, the maximum event sediment yields in the straw mulch catchment were 7.2 and 10 Mg ha^-1, respectively, which were less than half of the maximum event sediment yields in the control catchment (19 and 24 Mg ha^-1, respectively). The straw mulch catchment had no detectable runoff or sediment yield after the second post-fire year, but the control catchment continued to have measurable runoff and sediment yields through the seventh post-fire year. The straw mulch treatment effect in runoff reduction was not significant in the statistical model. Total ground cover was 80% immediately after the application of straw mulch, and decreased to 10% by the end of first post-fire year, yet total ground cover values remained high as litter and vegetation, including invasive cheatgrass, increased.

The hydromulch cover at both fires declined rapidly and provided less than 10% of the ground cover within 2.5 months after application at which point the catchment was presumed to be untreated. Due to differences in precipitation, the three catchments at the Cedar fire had significantly different hydrologic responses during the presumed untreated portion of the study, which precluded evaluation of treatment effectiveness during the short treated period. The peak flow responses from the hydromulch and control catchments at the Hayman fire were also different during the presumed untreated period and were not tested. Although the runoff and sediment yields did not differ during the presumed untreated period and were tested for treatment effects, the Hayman hydromulch treatment did not significantly affect either response during the first post-fire year the presumed treated period.

Unit-area sediment yields from the catchments were similar to those measured on hillslope plots at both the Hayman and Cedar fires in the first post-fire years, but in later years the sediment yields from the catchments were at least double the sediment yields measured on hillslope plots. The longer periods of greater erosion rates in the catchments likely reflect the addition of channel erosion processes and a difference in hydrologic connectivity at the catchment scale.

Moscow FSL publication no. 2013b