A CONUS-scale study of wildfire and evapotranspiration: Spatial and temporal response and controlling factors

Evapotranspiration (ET) accounts for a substantial portion of regional water budgets in much of the southeast and fire-prone western United States (US). Even small changes in ET rates can translate to meaningful shifts in runoff patterns and makes forecasting the direction and magnitude of wildfire-induced ET alteration of critical importance. We use 1 km ET estimates from the operational Simplified Surface Energy Balance (SSEBop) product for the conterminous US (CONUS) to evaluate post-fire ET and evaporation ratio (ET/P) shifts in the first five post-fire years in approximately 5500 unique fires. Pixels with similar ET/P responses to fire are grouped through k-means clustering and the resultant cluster distribution is explored over space and time. The largest changes in post-fire ET/P are observed in the southwestern CONUS where first-year ratios are reduced by 50 to 90% and pre fire ratios are rarely recovered by post-fire year five. Regional and intra-fire ET/P response variability is also highest in the western CONUS where climatic, topographic, and ecologic gradients are steep. Post-fire ET/P modifications are small to negligible in the east-southeast CONUS, and 18% of all pixels analyzed exhibit small to moderate increases in post-fire year one ET/P. A comparison of burned and unburned pixel pairs confirms the role of fire in the shifts but also indicates a high degree of background variability in the ET and precipitation data. Although the biggest percent ET/P reductions occur in shrub/scrub landscapes in much of the west, the biggest magnitude ET changes often occur in evergreen forests. Higher burn severities are consistently correlated with greater post-fire ET/P reductions, while relationships between post-fire ET/P shifts and numerous other landscape attributes (e.g., pre-fire vegetation type) vary in both direction and magnitude in different parts of the CONUS. Further work can be conducted to refine controlling relationships within more homogeneous subregions.

Automated summary

Evapotranspiration (ET) plays a significant role in regional water budgets in the southeast and fire-prone western United States. Small changes in ET rates can lead to meaningful shifts in runoff patterns, making it crucial to forecast the direction and magnitude of wildfire-induced ET alterations. Post-fire ET and evaporation ratio (ET/P) shifts were evaluated in approximately 5500 unique fires across the conterminous US using 1 km ET estimates from the Simplified Surface Energy Balance (SSEBop) product. Results show that the largest changes in post-fire ET/P occur in the southwestern US, while the eastern southeast US experiences small to negligible modifications. Regional and intra-fire variability in ET/P responses were also observed, influenced by factors such as climatic, topographic, and ecologic gradients.