Regional patterns of postwildfire streamflow response in the Western United States: The importance of scale‐specific connectivity

Abstract

Wildfires can impact streamflow by modifying net precipitation, infiltration, evapotranspiration, snowmelt, and hillslope run‐off pathways. Regional differences in fire trends and postwildfire streamflow responses across the conterminous United States have spurred concerns about the impact on streamflow in forests that serve as water resource areas. This is notably the case for the Western United States, where fire activity and burn severity have increased in conjunction with climate change and increased forest density due to human fire suppression. In this review, we discuss the effects of wildfire on hydrological processes with a special focus on regional differences in postwildfire streamflow responses in forests. Postwildfire peak flows and annual water yields are generally higher in regions with a Mediterranean or semi‐arid climate (Southern California and the Southwest) compared to the highlands (Rocky Mountains and the Pacific Northwest), where fire‐induced changes in hydraulic connectivity along the hillslope results in the delivery of more water, more rapidly to streams. No clear streamflow response patterns have been identified in the humid subtropical Southeastern United States, where most fires are prescribed fires with a low burn severity, and more research is needed in that region. Improved assessment of postwildfire streamflow relies on quantitative spatial knowledge of landscape variables such as prestorm soil moisture, burn severity and correlations with soil surface sealing, water repellency, and ash deposition. The latest studies furthermore emphasize that understanding the effects of hydrological processes on postwildfire dynamic hydraulic connectivity, notably at the hillslope and watershed scales, and the relationship between overlapping disturbances including those other than wildfire is necessary for the development of risk assessment tools.

Forest restoration as a strategy to mitigate climate impacts on wildfire, vegetation, and water in semiarid forests

“…A number of gauged catchments in Arizona have experienced large wildfires and the records from these events suggest short‐term increases in runoff (Hallema et al. ). Moderate to severe wildfires are also associated with substantial reductions in water quality due to increased sediment mobilization (Campbell et al.…”

“…Effects of fire on runoff.-High-intensity wildfire has a substantial effect on the hydrologic cycle. A number of gauged catchments in Arizona have experienced large wildfires and the records from these events suggest short-term increases in runoff (Hallema et al 2017). Moderate to severe wildfires are also associated with substantial reductions in water quality due to increased sediment mobilization (Campbell et al 1977, Malmon et al 2007), so it is unlikely that runoff increases from wildfire would be beneficial to habitat or suitable for human use.…”

Ecohydrological processes and ecosystem services in the Anthropocene: a review

“…The reason was that clearcutting reduced snow accumulation compared to non-disturbed forests. Disturbance events such as wildfire not only affect evapotranspiration but also reduce the rate of infiltration into the soil, leading to enhanced streamflow in regions with a Mediterranean or semi-arid climate (Hallema et al 2017). The streamflow regulation functions of forests depend on many watershed factors including background climate, forest structure (i.e., species composition, ages), soil properties such as hydraulic conductivity (Wilcox et al 2003b), geology, and watershed size ).…”

Reframing the Challenge of Global Wildfire Threats to Water Supplies

“…The majority of research on fire effects on surface water resources is within the ecohydrological domain and has occurred at the small plot to watershed scale (Hallema, Sun, Bladon, et al, 2017). However, even within this water domain, interactions are complex and not fully understood.…”