FIRE-RELATED DEBRIS FLOWS IN THE SONORAN DESERTSCRUB PLANT COMMUNITY

Fire can make landscapes more susceptible to debris flows by reducing soil infiltration capacity and decreasing vegetation cover. As fire becomes more impactful within different plant communities and geographic regions, there is a need to expand efforts to document fire-driven changes to runoff and debris flow activity. For example, fire has historically been infrequent in the Sonoran Desertscrub plant community due to the amount and continuity of fuel. The presence of invasive grasses, among other factors, helped to sustain several large wildfires in Arizona in 2019 and 2020 that impacted portions of the Sonoran Desertscrub plant community. We monitored debris-flow activity within small (< 1 km²) watersheds burned in two of these fires, the 2019 Woodbury Fire and 2020 Bighorn Fire, over 2+ years. We deployed rain gauges to quantify the intensity and duration of rainfall required to initiate debris flows following fire. In addition, we used mini disk tension infiltrometers to estimate the magnitude and temporal persistence of fire-driven changes to soil hydraulic properties. We found that soil infiltration capacity decreased following both fires and that debris flow activity increased relative to nearby unburned areas. Our findings indicate that rainfall intensities needed to trigger debris flows in recently burned watersheds within the Sonoran Desertscrub plant community are substantially greater than previously reported rainfall thresholds in Arizona that were derived using data from higher elevation forests. Results provide insight into postfire debris flow hazards in a region where the size and severity of fires is increasing. Results also help expand existing postfire debris flow inventories by providing observations in a plant community where there is a paucity of data.