Wildfire impacts on water: improving impact assessment though model adaptation

Wildfires are a threat to water security worldwide, due to the negative effect of the post-fire mobilization of sediments and associated nutrients and contaminants on the waterbodies located downstream of burned areas. Such impacts have been assessed in field studies and, more recently, also through modelling approaches. Models are valuable tools for anticipating the potential negative impacts of wildfires, allowing to test different environmental scenarios. The state of the art in post-fire model adaptation has shown that most studies simulate the hydrological and erosion response in the first post-fire year in situ, without considering the cascading effects on downstream waterbodies. In addition, few studies have evaluated the long-term impacts of wildfires, likely due to the limited available data. Among the existing gaps in post-fire modelling, ash transport has recently been identified as a priority. The lack of ash modelling studies has been ascribed to the limited understanding of ash behavior and the difficulties of incorporating ash-related processes into the structure of existing models.

As a way to fulfill these research gaps and advance the state of the art in post-fire hydrological modeling, the authors provided several contributions in recent years.

For instance, a watershed model has been coupled with a reservoir model to simulate the effects of fires on drinking water supplies, using the outputs of the main streams as inputs to the reservoir branches. As most simulations commonly end at the watershed outlet, a simple methodology was proposed to assess how the impacts on watercourses propagate to the drinking water supply inlet. The results showed that integrated modeling frameworks are critical for anticipating the off-site impacts of fires.

Post-fire management can also influence the impacts of fires beyond the first post-fire rainfall events, when the soil is exposed and ash and sediment transport is greatest. Another modelling exercise evaluates the long-term impacts of different post-fire management options, more specifically terracing, mulching and natural recovery, on water availability and quality.

As post-fire ash and sediment mobilization is typically limited to the duration of the rainfall events, which typically lasts for a few hours, hydrological models that run at a daily time-step can underestimate the environmental impacts of fires. To improve the knowledge of post-fire hydrological processes at event-based scale, two hydrological models (LISEM and MOHID) were calibrated, accounting for burn severity and initial soil moisture conditions before each specific rainfall event.

The work done in the past years is expected to be of added value for the post-fire modeling community, providing future directions on post-fire hydrological modelling studies.