Biophysical drivers of short-term change in evapotranspiration after fire as estimated through the SSEBop Landsat-based model

Abstract

Wildfires in Mediterranean countries are increasingly frequent, extensive, and ecologically damaging, impacting not only vegetation and soil but also the water cycle, specifically altering evapotranspiration (ET). Following a wildfire, ET values experience a sharp decline, which persists until vegetation returns to its pre-fire state. This study examines the factors influencing this reduction, focusing on fire severity, topography, ecosystem type (broadleaf, conifer, mixed forests, and shrublands), and pre-fire fuel conditions, including fractional vegetation cover (FCOVER) from PROSAIL-D RTM inversion of Landsat 8 OLI images and structural complexity from Sentinel-1 SAR, on ET 1-year after fire. Given the heterogeneous nature of Mediterranean landscapes, where vegetation and water availability vary widely, fine spatial resolution ET models are essential. This study utilized the Operational Simplified Surface Energy Balance (SSEBop) model to estimate ET from Landsat imagery, focusing on four major wildfires that occurred in Spain and Portugal in 2022. Random Forest regression identified fire severity and pre-fire FCOVER as the most influential factors in ET reduction. Results showed that fire severity’s impact on ET reduction followed a consistent pattern across ecosystems, with the greatest relative reductions observed in shrublands, followed by conifer and broadleaf forests. The most pronounced reductions occurred in areas of higher fire severity. In conclusion, fire severity emerges as a key driver of short-term changes in ET in Mediterranean environments. This study underscores the value of Landsat-based ET models as reliable tools for assessing the ecological consequences of fire severity in these regions.