Integrating Fire Effects into an Ecohydrologic Model for Simulating Fire Regimes

Ecohydrologic models are used to dynamically simulate vegetation growth/ mortality and their interaction with water and nutrient fluxes. Although disturbances such as wildfire are a natural part of the landscape in environments such as the Western US, wildfire is generally included only as an exogenous variable in ecohydrologic models. An alternative approach is to integrate wildfire directly into ecohydrologic models so that wildfire ignition, spread and effects are driven by simulated landscape conditions within the model. This approach allows for the simulation of natural fire regimes and may provide more robust estimates of long-term ecological variables such as forest health, carbon sequestration and water use. For this study, we detail a fire-effects model that has been developed to couple a fire-spread model, WMFire, with an ecohydrologic model, RHESSys. The fire-effects model is designed for use with a simple two-stratum representation of canopy structure and computes losses following fire spread to a given landscape patch. Losses to a modeled litter layer, coarse woody debris layer and understory vegetation layer are determined based on a patch-level integrated measure of fuel loads, moisture levels, wind speed, and topography. Losses to an overstory vegetation layer are based on understory biomass consumed by the wildfire. The fire effects model was found to replicate the expected impacts of wildfire on vegetation and litter. Further, the fully coupled RHESSys-WMFire model was tested in four Western US watersheds with different vegetation/climate/fire characteristics and preliminary results indicated that the model was able to reproduce the disparate fire regimes. We highlight remaining challenges with simulating fire effects in ecohydrologic models using simplified representations of canopy structures and litter fuels. This study demonstrates the potential for integrating fire into ecohydrologic models for simulating future fire regimes.