Temporal persistence of postfire flood hazards under present and future climate conditions in southern Arizona, USA

Abstract. Changes to soil hydraulic properties that reduce infiltration capacity following fire can increase flash flood risks. These risks are exacerbated by rainfall intensification associated with a warming climate. However, the potential effects of climate-change-driven rainfall intensification on postfire floods remain largely unexplored. Using rainfall and runoff observations from a 49.4 km² watershed in southern Arizona, USA, and a hydrological model (KINEROS2), we examined the temporal evolution following a historic fire of three crucial hydrologic parameters: soil saturated hydraulic conductivity (K_sp), net capillary drive (G_p), and hydraulic roughness (n_c). We explored how the effect of fire on these parameters may influence peak flow under future climate scenarios derived from CMIP6, specifically the medium emissions scenario (SSP245) and high emissions scenario (SSP585). Results demonstrate an increase in K_sp from 11 mm/hr in the first postfire year to 60 mm/hr in postfire year three. G_p similarly increased from 19 mm in the first postfire year to 30 mm in the third, while n_c was relatively constant. The highest simulated Q_p occurred in the first postfire year. Under the SSP245 scenario, the likelihood of a 100-yr flood is projected to be twice as large by the middle century relative to its historical magnitude. Simulations further indicate that the maximum expected discharge associated with a postfire flood, as derived from historical data, could be triggered by a 10-yr rainstorm under the SSP585 scenario by the late century. Simulations also demonstrate that rainfall intensification will lead to greater persistence of elevated flood hazards following fire by late century under both the SSP245 and SSP585 scenarios.