Rill Erosion: Evidence for Adjustments in Hill-Slope Hydrology After Wildfire

The purpose of this study is to determine if rills, which are commonly found after wildfire, form in regions of a watershed that are not typically subjected to incisive erosion. This was accomplished by mapping the topographic position of new rills formed in two recently burned watersheds and performing a slope-area analysis that is specific to that hillside. The site specific slope-area analysis was used to delineate regions of the hillside that undergo diffusive or incisive erosional processes. To meet these objectives, the distance from the ridge line down to the rill head was used to estimate the drainage area for that rill and the slope of the hill, near to where the rill formed, was assumed to be the slope at the rill head. The same information was measured for channel heads in the area. The drainage areas for the channel heads were measured using a topographic map and the slopes of the hillside at the channel heads were measured. The slope-area relations for each rill head and each channel head were then plotted on the site specific, process delineated, slope-area graph and the region in which the data lie were noted. This procedure was performed for 21 rill heads and 9 channel heads burned by the Hi Meadow fire near Bailey, Colorado, and for 42 rill heads and 7 channel heads burned by the Cerro Grande fire near Los Alamos, New Mexico. Results for the pre-bum slope-area analysis show that the channel heads for both Hi Meadow and Cerro Grande plot in regions predicted to be dominated by saturation overland flow and landsliding. This result verifies the validity of the slope-area analysis because these two processes are considered by many to be fundamental in channel formation. The rill heads for both Hi Meadow and Cerro Grande plot in regions predicted to be saturated and dominated by diffusive erosion. The post-bum slope-area analysis for both field sites show that the critical up-slope drainage areas necessary for erosion are reduced. Graphically, the erosion thresholds move towards the x-axis and essentially drop out from under the plotted rill heads. The rill heads now exist in regions that undergo the incisional processes of saturation overland flow or infiltration excess overland flow. The net effect of wildfire on the landscape is to reduce resistance to erosion thereby reducing the amount of up-slope drainage area necessary for erosion. This allows for an expansion of the channel network onto hillslopes that do not normally experience incisional erosive processes. Using rills as an indicator of the erosional process acting on the landscape, and using the erosional process as an indicator of the hillslope hydrologic pathways, this study concludes there is a short-term deviation in hydrologic flow paths from sub-surface to surface that only occurs after wildfire has altered the physical properties of the soil and watershed as a whole. These new hydrologic pathways will remain until the hillslope returns to equilibrium conditions by overcoming the effects of the wildfire