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.

Can Landsat-Derived Variables Related to Energy Balance Improve Understanding of Burn Severity From Current Operational Techniques?

“…Our results agree with previous studies showing that ET decreases after a fire event (e.g. [9,89] in semi-arid; [15,90] in boreal forests; and [13,35,91] in temperate conifer forests). In Mediterranean ecosystem, Sánchez et al [17] studied the temporal evolution of changes insurface energy balance over an eleven-year period following fire in a maritime pine-shrub mixed forest (Spain).…”

“…However, forested areas affected by high burn severity show a decrease in soil moisture. In this case, the increase in surface runoff exceeds the reduction in ET and leads to drier post-fire soils [9,12]. Post-fire changes in vegetation, soil and water balance corroborate wildfires impact on energy balance [13][14][15].…”

“…Fire reduces evapotranspiration (ET) and increases surface runoff following rainfall [9]. Usually, the increase in surface runoff is lower than the decrease in ET, causing an increase in soil moisture documented by different studies (e.g., [10,11]).…”

Progress in simplifying hydrologic model parameterization for broad applications to post‐wildfire flooding and debris‐flow hazards

“…Numerical modeling experiments also showed that roughness parameter values varied with spatial scale, which has implications for the transferability of calibrated roughness parameters to other sites. Since the hydrologic processes in burned basins have been altered and there may be limited time to gather information prior to the onset of rainfall, it is not uncommon for post-fire modeling efforts to estimate hydraulic roughness based on values previously calibrated at other burned sites (Istanbulluoglu and Bras, 2005;Cydzik and Hogue, 2009;Wagenbrenner et al, 2010;Atchley et al, 2018;McGuire et al, 2018). Numerical experiment 1 showed that the best-fit effective Manning n eff values for the entire watershed decreased as a power-law function of drainage area (a proxy for flow discharge).…”

Climate change and climate-driven disturbances in the San Juan River sub-basin of the Colorado River

“…Selection of the appropriate timescale is an important aspect to consider in vegetation disturbance studies. For example, research focused on forest disturbance over a single summer season, such as post-fire hydrology in New Mexico shows that surface runoff and recharge both rise following fire (Atchley et al, 2018). However, these studies do not measure the year-long change in water balance, which is critical for snow-driven systems.…”