RILL EROSION: EVIDENCE FOR ADJUSTMENTS IN HILL-SLOPE HYDROLOGY AFTER WILDFIRE

The form of a watershed is a function of the dynamic relationship between the climate and the physical properties of the landscape. Erosion as a result of this dynamic relationship is largely controlled by the mechanisms by which water travels through the hill-slope system. After a disturbance that alters the hydrologic mechanisms, one can expect an alteration in the erosional processes and a readjustment of form. Wildfire has the ability to alter hill-slope hydrologic processes which in turn can alter the erosional processes acting on the landscape. This may have consequences if incisive erosion begins operating in regions of the watershed that are normally dominated by diffusive processes. Erosion processes can be quantified as functions of slope and drainage area. The dominant erosion processes operating in watersheds can therefore be evaluated using slope-drainage area relationships for the watershed. The purpose of this study was to determine if rills, small channel-like features commonly found on hill-slopes after wildfire, exist in regions of the watershed that are not typically subject to incisive erosion. This is accomplished by mapping the topographic position of new rills in two recently burned watersheds in the inter-mountain west and determining if the rill-heads plot in diffusive regions of a site specific landscape dissection plot. Because rills are formed by incisive processes, where the rill-heads plot in slope-area space indicate the erosional regime dominating in that region of the watershed. If the rill-heads plot in regions predicted to be dominated by incisive processes it indicates there is a correlation between the predicted erosional process and the erosional feature. However, if the rill-heads plot in modeled diffusive regions it indicates an inbalance between the erosional process and erosional feature. The area-slope relationships of seven channel heads and forty-two rills in a burned watershed near Los Alamos New Mexico, and of nine channel heads and twenty-one rills in a burned watershed near Bailey Colorado were plotted. The rill-heads at both sites lie in regions predicted to be dominated by diffusive erosion. These results imply that there has been a change in the long term hydrologic processes and short term erosional processes are now at work on the landscape.