GEOMORPHIC AND HYDROLOGIC IMPACTS OF SUCCESSIVE WILDFIRES: INSIGHTS FROM THE 2021 TELEGRAPH FIRE IN CENTRAL ARIZONA

The geomorphic and hydrologic impacts of wildfire are often considered to be independent of prior wildfires, with the magnitude of the impact often being a function of soil burn severity in the most recent fire. In many conceptual models for post-fire recovery, geomorphic and hydrologic systems return to a state that is similar to nearby areas that have not recently burned. As fire and climate regimes change, however, landscapes may begin experiencing fire on more frequent intervals that allow for minimal recovery between fires. For example, the 2021 Telegraph Fire burned a large portion of the Pinal Mountains in central Arizona, including much of an area that had recently burned in the 2017 Pinal Fire. In this study, we examine soil hydraulic properties and debris flow activity in the first several months following the 2021 Telegraph Fire and compare them with observations and measurements made after the 2017 Pinal Fire. Following the 2017 Pinal Fire, we established five monitored watersheds (< 1 km²) to assess changes in soil hydraulic properties and debris flow activity as the landscape recovered. Several of these watersheds burned again in the 2021 Telegraph Fire, providing an opportunity to explore how successive fires influence hydrologic and geomorphic response. Field measurements of soil hydraulic properties made with mini disk tension infiltrometers indicate reductions in sorptivity and wetting front potential following the Telegrpah Fire that are roughly equivalent to those observed immediately after the Pinal Fire, despite the fact that the area experienced moderate-high soil burn severity during the Pinal Fire and low soil burn severity during the Telegraph Fire. Preliminary observations of increased debris-flow activity within areas burned during both the Pinal and Telegraph Fires, relative to observations following the Pinal Fire, suggest that areas burned by both fires may be more susceptible to extreme geomorphic and hydrologic responses.