Methods to evaluate floods on a burned watershed

Abstract

This updated study focused on the methodologies, tools, and applications used to evaluate the hydrologic runoff potential of a burned watershed when few variables were actually measured. Forest fi res often alter the balance between rainfall and the resulting runoff from natural watersheds. This may result in fl ooding from the burned watershed at locations downstream. Such was the case for the Mud Canyon watershed on New Mexico’s Mescalero Apache Indian Reservation. In the spring, a fi re burned approximately 42% of the watershed. Subsequent storms during the summer caused fl ows in Mud Canyon that led to fldownstream at the town of Mescalero. The fl ooding was likely the result of inadequate remediation, neither human nor natural, during the months between the fi re and the storms. While the summer storms that followed the spring fi re had an updated magnitude to be expected every 10 years, the resulting updated fl ooding was more on the order of a 500-year event. A purpose of this study was to determine the amount of rainfall that produced the estimated fl ood fl ows. This result could then be used to determine the degree to which the burned portion of the watershed caused the fl ooding. The Burned Area Emergency Rehabilitation (BAER) Plan concluded that there was not a serious threat of flbased on the fact that the soils in the watershed were well-drained and not hydrophobic. The paper concludes that the loss of ground cover, particularly for relatively steep watersheds, should be seriously considered when evaluating the potential for fl ooding on a burned watershed. The methods used for this updated hydrologic analysis of Mud Canyon, as outlined in this paper, are applicable for future analyses of burned watersheds to determine the extent to which loss of ground cover contributes to increased fl ood fl ows.