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Title: SELECTION OF PARAMETER VALUES TO MODEL POST-FIRE RUNOFF AND SEDIMENT TRANSPORT AT THE WATERSHED SCALE IN SOUTHWESTERN FORESTS 1734

Author
item Canfield, Howard
item Goodrich, David - Dave
item BURNS, I. - UNIVERSITY OF ARIZONA

Submitted to: American Society Of Civil Engineers Watershed Management Conference
Publication Type: Proceedings
Publication Acceptance Date: 3/15/2005
Publication Date: 7/19/2005
Citation: Canfield, H.E., Goodrich, D.C., Burns, I.S. 2005. Selection of parameter values to model post-fire runoff and sediment transport at the watershed scale in southwestern forests. Proc. ASCE Watershed Manage. Conf., July 19-22, Williamsburg, VA. 2005 CDROM.

Interpretive Summary: Erosion and flooding increase following wildfire. Land managers and specialists who develop plans to reduce the erosion and flooding on people and facilities below the burned watersheds (Burned Area Emergency Rehabilitation (BAER) teams) must be able to estimate these post-fire changes. This paper describes how values in computer models must be changed to account for the impacts of fire, so that they can be useful to people needing to design rehabilitation plans. Since wildfire primarily impacts soils and vegetation cover on hillslopes, it is appropriate to assume that changes in hillslope conditions will result in changes in flood peak, flood volume and eroded sediment. The AGWA (Automated Geospatial Watershed Assessment) hydrologic modeling tool employs both a model based mostly on statistical relationships (SWAT) and a model that attempts to describe the processes of erosion and flooding mathematically (KINEROS2). The models were applied on two burned watersheds. Analysis of data from the Marshall Gulch watershed near Tucson, Arizona, showed that flood peak changes following fire are greater than flood volume changes. Changing model input values in the KINEROS2 model to make it better describe the flooding pattern Starmer Canyon near Los Alamos, New Mexico shows a pattern of change over time that is consistent with watershed recovery. The input values that best fit the flooding pattern show that the hillslopes must have more vegetation and litter which slows the water, reducing flooding and erosion. Therefore, the most important change the models must incorporate is a change from very smooth, bare conditions following the fire.

Technical Abstract: Erosion and runoff have been observed to increase following fire. Land managers and Burned Area Emergency Rehabilitation (BAER) teams must be able to estimate these post-fire changes. Studies of post-fire erosion on burned watersheds show that the concentrations of sediment eroded from burned rangeland and forested hillslopes in the southwestern United States can be extremely high. Since wildfire primarily impacts soils and vegetation cover on hillslopes, it is appropriate to assume that changes in hillslope conditions will result in changes in runoff peak, volume and sediment yield. The AGWA (Automated Geospatial Watershed Assessment www.tucson.ars.ag.gov/agwa) hydrologic modeling tool employs both an empirical model (SWAT) and a more process-based model (KINEROS2). In order to study how these models should be modified to provide land managers with a means to assess the impact of fire, the models were applied on two burned watersheds. Analysis of data from the Marshall Gulch watershed near Tucson, Arizona, indicates that changes in runoff volume are small compared to changes in peak runoff. The application of the KINEROS2 model to burned conditions at the Starmer Canyon near Los Alamos, New Mexico shows a pattern of change over time that is consistent with watershed recovery. Calibrated hillslope roughness values are consistent with independent estimates for roughness under bare conditions following the fire to roughness consistent with forested conditions three years later. The modeling also indicated that increasing hillslope roughness over time accounts for much of the change in runoff response.