Droughts aggravate impacts on post-fire evapotranspiration recovery in the Western U.S.

Intensifying droughts and recent increases in the fire occurrences in Western United States (WUS) have turned into a pervasive ecological and hydrological disturbance that often causes considerable impacts to underlying vegetation, soil, and the ecosystem. Fires modulate ecosystem evapotranspiration (ET) immediately after burning by directly affecting the aboveground vegetation, which increases bare ground cover and reduces transpirational leaf surface area. The reduced evaporative capacity over the burned area contributes to wetter soils and increases the amount of water available for surface runoff, leading to greater potential for flash flooding and debris flows, compared to neighboring unburned areas. Droughts exacerbate disturbances from fire and increase vegetation mortality both in the forested and non-forested landscapes. Thus, understanding the post-fire hydrologic response to compounding fire and drought disturbances is paramount for effective risk management and mitigation of hydrologic hazards.

In this study, we quantify the impact of fires on ET and its post-fire recovery as a function of drought stresses, burn severity, and landcover for fires that occurred from 2014-2020 in the WUS. We use high-resolution evapotranspiration estimates from the OpenET dataset, six different landcover products, and fire properties from Monitoring Trends in Burn Severity (MTBS) gridded dataset to analyze trends in ET over fire-impacted and unimpacted regions. We show that ecoregions dominated by non-forests (grasslands and shrublands) are more susceptible to drought stresses, which amplify fire-induced ET decline and, subsequently shift water flux partitioning. In contrast, severely burned forests respond with slow or incomplete recovery, but with a lower sensitivity to dry extremes. We quantify how drought exerts additional stress on vegetation burned with high severity. This stress sustains the fire-induced shifts in the water balance and further inhibits recovery to pre-fire conditions. The vulnerability of different vegetation types to drought plays a dominant role ­in regulating hydrologic recovery dynamics, which offers an improved understanding of interactions between droughts and wildland fires for risk modeling and hazard management.