Simulating Surface and Subsurface Water Balance Changes Due to Burn Severity

Abstract

Core Ideas Burn severity data informed a hydrologic model to assess water balance changes. Loss of vegetation and evapotranspiration exceeded impact of increased runoff. High burn severity may cause drier site conditions due to increased runoff. Change in evapotranspiration acts at long timescales while runoff is event based. Forest fires have a significant impact on hydrology, such as reduced infiltration rates leading to increased flooding. However, post‐fire water balance changes and the competing hydrologic response of increased runoff and evapotranspiration as a function of burn severity are not well understood. Comparing pre‐ and post‐fire water balance changes is challenging because measurements of fire‐disturbed landscapes with the previously undisturbed character are impractical due to non‐repetitive observational conditions. We used a physically based modeling experiment to incorporate burn severity data from the Las Conchas fire to approximate model parameterization to evaluate continuous water balance progression for pre‐ and post‐fire simulations using the same forcing conditions. Fire disturbance decreased evapotranspiration and increased overland flow response to precipitation events. The reduction of evapotranspiration often dominated the new water balance compared with the increase in overland flow, resulting in higher soil moisture. However, this modeling experiment also identified a tipping point where increased overland flow from high burn severity sites eclipses the effect of reduced evapotranspiration on the water balance, causing comparatively drier post‐fire soils. In particular, high burn severity sites approach a threshold that results in larger changes to overland flow than changes in evapotranspiration, potentially moving the site to an overland flow dominated regime. The shifts in water balance components have implications for how site conditions will change under a range of burn severity scenarios.