HYDRO-GEOMORPHIC IMPACTS OF SUCCESSIVE WILDFIRES IN AREAS WITH DIFFERENT FIRE EXPOSURES: LESSONS LEARNED FROM THE SANTA CATALINA MOUNTAINS, TUCSON, AZ

The Santa Catalina Mountains (SCM), located just north of Tucson, Arizona, have a rich history of wildfires. Prior to 1995, the largest historical wildfires documented in the SCM were less than 7,000 ac. Since then, however, the size and characteristics of wildfires have dramatically changed as drought conditions have intensified. For example, in 2002 the Bullock Fire burned ~ 32,000 ac. east of the Catalina Highway, which divides the mountain, and in 2003 the Aspen Fire burned ~ 85,000 ac. west of the highway. In 2017, the Burro Fire burned ~ 27,000 ac. on the far east side of the SCM, reburning a portion of the Bullock burn scar. Most recently, in June 2020, the Bighorn Fire burned ~ 120,000 ac. across most of the SCM, reburning through the Aspen, Bullock and Burro burn scars. In this study, we assess the hydro-geomorphic impacts of areas with different fire exposures by stratifying sites based on number of fires in the last 25 years (0, 1, 2+) and soil burn severity of each fire (unburned, low, moderate-high), a key driver of post-fire hydro-geomorphic impacts. We used minidisk infiltrometers to evaluate soil hydrologic properties (saturated hydraulic conductivity, sorptivity, wetting front potential), as well as rainfall data and field observations of flows (debris flow, flood, no response) to document geomorphic responses. Immediately after the Bighorn Fire, we established 6 study sites to assess soil hydrologic properties, and monitored 20+ basins, 6 of which were instrumented with pressure sensors or nodal seismometers and rain gauges. The 2020 monsoon was abnormally dry, with only 8 storm days recorded at the highest elevations and 3 days at lower elevations. Debris flows in high elevation, forested basins were triggered by a rainstorm with a peak 15-minute rainfall intensity (I15) of 55 mm/h, the highest I15 recorded at that gauge since the Bighorn Fire. At other rain gauges, however, higher I15 intensities were recorded on other days, including an I15 of 93 mm/h in a low elevation, desert basin, yet debris flows were not generated. During spring 2021, we added 20 additional study sites to assess soil hydrologic properties from areas with different fire exposures, and we continue to monitor geomorphic responses in study basins.