Hydrologic evaluation of flood flows from a burned watershed

Abstract

Forest fires often alter the balance between rainfall and resulting runoff from natural watersheds.This may result in flooding 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 fire burned approximately 42% of the watershed.Subsequent storms during the summer caused flows in Mud Canyon that led to flooding downstream at the town of Mescalero.The flooding was likely the result of inadequate remediation, neither human nor natural, during the months between the fire and the storms.While the summer storms that followed the spring fire had an updated magnitude to be expected every ten years, the resulting updated flooding was more on the order of a five hundred-year event.A purpose of this study was to determine the amount of rainfall that produced the estimated flood flows.This result could then be used to determine the degree to which the burned portion of the watershed caused the flooding.The Burned Area Emergency Rehabilitation (BAER) Plan concluded that there was not a serious threat of flooding, based on the fact that the soils in the watershed were well-drained and not hydrophobic.This updated study focused on the new methodologies used to evaluate the hydrologic runoff potential of a burned watershed when few variables were actually measured.The paper concludes that the loss of ground cover, particularly for relatively steep watersheds, should be seriously considered when evaluating the potential for flooding 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 flood flows.