The 2002 Hayman Fire was the largest fire in recent Colorado history (558 km2). The extent of high severity combustion and possible effects on Denver’s water supply focussed public attention on the effects of wildfire on water quality. We monitored stream chemistry, temperature and sediment before the fire and at monthly intervals for 5 years after the fire. The proportional extent of a basin that was burned or that burned at high severity was closely related to post-fire streamwater nitrate and turbidity. Basins that burned at high severity on >45% of their area had twice the streamwater nitrate and four times the turbidity as basins burned to a lower extent; these analytes remained elevated through 5 years post-fire. In those basins, the highest post-fire streamwater nitrate concentrations (23% of USA drinking water standards) were measured during spring, the peak discharge period. Summer streamwater was 4.0°C higher in burned streams on average compared with unburned streams; these persistent post-fire stream temperature increases are probably sufficient to alter aquatic habitat suitability. Owing to the slow pace of tree colonisation and forest regrowth, recovery of the watersheds burned by the Hayman Fire will continue for decades.

This short paper provides the framework and introduction to this special issue of International Journal of Wildland Fire. Its eight papers were selected from those presented at two consecutive conferences held in 2018 in Europe and the USA that focussed on the impacts of wildfire on factors that regulate streamflow, water quality, sediment transport, and aquatic habitats. Despite decades of watershed research, our understanding of the effects of wildfires on the processes that regulate clean water supply remains limited. Here, we summarise the key challenges and research needs in this interdisciplinary field and evaluate the contributions the eight special issue papers make to improved understanding of wildfire impacts on watershed processes. We also outline research priorities aimed at improving our ability to predict and, where necessary, mitigate wildfire impacts on watersheds. Achieving these advances is all the more pressing given the increasing extent and severity of wildfires in many areas that are the source of clean water for major population centres.

Wildfires commonly increase nutrient, carbon, sediment and metal inputs to streams, yet the factors responsible for the type, magnitude and duration of water quality effects are poorly understood. Prior work by the current authors found increased nitrogen, phosphorus and cation exports were common the first 5 post-fire years from a synthesis of 159 wildfires across the western United States. In the current study, an analysis is undertaken to determine factors that best explain post-fire streamwater responses observed in those watersheds. Increased post-fire total nitrogen and phosphorus loading were proportional to the catchment extent of moderate and high burn severity. While post-fire dissolved metal concentrations were correlated with pre-fire soil organic matter. Total metal concentration increased where post-fire Normalised Difference Vegetation Index, a remote sensing indicator of live green vegetation, was low. When pre-fire soil field capacity exceeded 17%, there was a 750% median increase in total metals export to streams. Overall, the current analysis identified burn severity, post-fire vegetation cover and several soil properties as the key variables explaining extended post-fire water quality response across a broad range of conditions found in the western US.