HR: 1340h
AN: H43F-1694    [Abstracts]
TI: An Experimental Study of the Effects of Litter and Duff Consumption and Ash Formation on Post-Fire Runoff.
AU: * Woods, S W
EM: scott.woods@cfc.umt.edu
AF: University of Montana, 32 Campus Drive, Missoula, MT 59802, United States
AU: Balfour, V
EM: victoria.balfour@grizmail.umt.edu
AF: University of Montana, 32 Campus Drive, Missoula, MT 59802, United States
AB: Consumption of the litter and duff layers in forest wildfires can lead to substantial increases in the frequency and magnitude of overland flow. These increases result from the loss of storage in the organic surface layer, reduced surface roughness, and from sealing of the exposed mineral soil surface. The presence of an ash layer may accentuate surface sealing by providing an additional source of fine material, or it may reduce runoff by storing rainfall and by protecting the soil surface from raindrop impacts. We used simulated rainfall experiments to assess the effects of litter and duff consumption and the presence of ash layers of varying thickness on post fire runoff at two forested sites in western Montana, one with sandy loam soils formed out of granodiorite and the other with gravelly silt loam soils formed out of argillite. At each site we measured the runoff from simulated rainfall in replicated 0.5 m² plots before and after application of the following treatments: 1) burning with a fuel load of 90 Mg ha^-1, 2) manual removal of the litter and duff layers, 3) addition of 0.5, 2.5 and 5 cm of ash to plots from which the litter and duff had previously been removed, and 4) addition of the same depths of ash to burned plots at the sandy loam site. In the burned plots the surface litter and duff layers were completely consumed and a <1cm layer of black and gray ash and char was formed, indicating a moderate severity burn. The mean soil temperature in the upper 1 cm of the mineral soil was 70° C, and there was no detectable increase in water repellency. The mean final infiltration capacity of the burned sandy loam plots was 35 mm hr^-1 compared to a pre-fire mean of 87 mm hr^-1, while in the gravelly silt loam plots the pre- and post burn infiltration capacities (27 and 31 mm hr^{- 1}) were not significantly different. Manual removal of the litter and duff layers reduced the mean final infiltration capacity in the sandy loam plots from 64 mm hr^-1 to 40 mm hr^-1 and in the gravelly silt loam plots from 23 mm hr^-1 to 16 mm hr^-1. We attribute decreases in infiltration due to the burning and duff removal treatments primarily to surface sealing. In the sandy loam plots, burning may have had a greater effect on infiltration than duff removal because the thin ash layer in the burned plots provided an additional source of fine material. In the gravelly silt loam plots, macropores located around rock fragments helped to minimize sealing effects. The addition of 0.5 cm of ash to the burned granitic plots resulted in a 20 mm hr^-1 decrease in the final infiltration rate, and this was also probably due to surface sealing. However, the overall effect of ash addition was to increase the cumulative infiltration in proportion to the ash thickness and to maintain a higher average infiltration rate, indicating that while thin (<1 cm) ash layers may promote sealing, thicker ash layers help to reduce the runoff rate by providing additional storage for rainfall and by protecting the soil surface from raindrop impacts.
DE: 1817 Extreme events
DE: 1826 Geomorphology: hillslope (1625)
DE: 1838 Infiltration
DE: 1865 Soils (0486)
SC: Hydrology [H]
MN: 2007 Fall Meeting