Identifying Streamflow Shifts Induced By Wildfires in Mountain Basins Under Summer Precipitation

High severity wildfires drastically alter the hydrologic response in headwater catchments, as a consequence of reductions in vegetation cover and modifications of soil hydraulic properties. These changes lead to an increased probability of flash-floods in steep-slope mountain watersheds. This study investigates the changes in hydrologic response for post-fire conditions at two burned basins in Colorado as observed from time series of streamflow, precipitation and remotely sensed vegetation density. We examine the event and seasonal hydrologic shifts as a function of vegetation cover which is measured by the Enhanced Vegetation Index (EVI). First, we compare flow duration curves of 15-min streamflows pre and post fire. Subsequently, we study the event scale changes induced by wildfire as measured by the  runoff coefficient (RC), response time (RT) and peak flow (Q_pk). At the seasonal scale we explore the yearly evolution of runoff coefficient and peak flow and their relationship with a normalized EVI (NEVI) to identify a recovery hysteresis pathway. Our findings support the idea that for similar burned areas relative to total basin surface, forested watersheds evidence the largest streamflow changes. Flow duration curves depict significant post-fire increases in the high-range streamflows (low probability of exceedence) on the order of 1900% in forested and 500% in shrubland dominated basins with respect to pre-fire conditions. For a similar-precipitation and antecedent soil moisture, burned watersheds significantly showed a decrease in response time and increase in runoff coefficient relative to pre-fire for two isolated hydrologic events. At the seasonal scale, the expected increase in NEVI translates into increases in RC and Q_pk with a hysteresis effect driven by vegetation recovery, precipitation volumes and antecedent soil moisture. This study provides new insights to understand the physical processes triggered by fire that influence watershed responses and increase flash-flooding risks.