Wildfire and the Future of Water Supply Kevin D. Bladon,* ,†,‡ Monica B. Emelko, § Uldis Silins, † and Micheal Stone ∥ † Department of Renewable Resources, University of Alberta, Edmonton, Alberta T6G 2H1, Canada ‡ Department of Forest Engineering, Resources, and Management, Oregon State University, Corvallis, Oregon 97331, United States § Department of Civil and Environmental Engineering, University of Waterloo, Waterloo, ON N2L 3G1, Canada ∥ Department of Geography and Environmental Management, University of Waterloo, Waterloo, ON N2L 3G1, Canada In many parts of the world, forests provide high quality water for domestic, agricultural, industrial, and ecological needs, with water supplies in those regions inextricably linked to forest health. Wildfires have the potential to have devastating effects on aquatic ecosystems and community drinking water supply through impacts on water quantity and quality. In recent decades, a combination of fuel load accumulation, climate change, extensive droughts, and increased human presence in forests have resulted in increases in area burned and wildfire severitya trend predicted to continue. Thus, the implications of wildfire for many downstream water uses are increasingly concerning, particularly the provision of safe drinking water, which may require additional treatment infrastructure and increased operations and maintenance costs in communities downstream of impacted landscapes. A better understanding of the effects of wildfire on water is needed to develop effective adaptation and mitigation strategies to protect globally critical water supplies originating in forested environments. I n many parts of the world, forests provide high quality water for domestic, agricultural, industrial, and ecological needs  water supplies in those regions are inextricably linked to forest health. As population and related pressures on already stressed watersheds increase, awareness regarding the effects of large- scale landscape change on the water cycle grows increasingly important for the protection of aquatic ecosystem health, and the sustainability of downstream water supplies. The challenge of balancing competing demands is amplified by significant changes in water quality, quantity, timing, and availability that may result from human activities, as well as climate change and related land disturbances (e.g., wildfire, insect pests) in forested regions. 1 As a result, there has been a paradigm shift in forest management in many regions of the world toward a greater emphasis on preserving clean and abundant water flows from forests. 2 The high quality and substantial quantity of water flowing from most forests makes these resources particularly vulnerable to impacts of natural and man-made land disturbances. Many of the effects of urbanization, agriculture, and forestry on water quality and quantity, and the associated linkages to both human and aquatic ecosystem health, are well documented. The impacts of land disturbances, such as wildfires, are less understood. Although wildfires can be important in maintaining complex and productive aquatic ecosystems, 3 severe wildfires have the potential to be devastating to aquatic ecosystems because they may release significant amounts of sediment, 4 nutrients, 5 heavy metals, 6 and other contaminants. 7 These impacts may recover within a few years or last for numerous decades and extend far beyond the forested headwaters. 8,9 In recent decades, many regions of the world have experienced changes in seasonal weather that have resulted in earlier and longer fire seasons. 10,11 These trends are largely attributable to climate change and are likely to continue. 12 Thus, the implications of wildfire for many downstream water uses are increasingly concerning, particularly the provision of safe drinking water, which may require additional treatment infrastructure and increased operations and maintenance costs in communities downstream of impacted landscapes. 13 A better understanding of the effects of wildfire on water is needed to develop effective adaptation and mitigation strategies to protect globally critical water supplies originating in forested environ- ments. ■ FORESTS: CRITICAL SOURCES OF WATER SUPPLY Forests cover approximately 31% (4 billion hectares) of the total global land mass, providing a broad range of economic and ecological goods and services that include natural storage, filtration, and provision of drinking water supplies. Most healthy forests produce high quality water because they grow in regions with high annual precipitation and produce large quantities of runoff with relatively low contaminant concen- trations. High quality water from forested source watersheds can have substantial economic benefits by limiting costs for extensive drinking water treatment and associated infra- structure. It has been estimated that the value of the natural storage and filtration of water provided by global forests is approximately $4.1-trillion US (2013) per year. 14 Not surprisingly, many population centers rely heavily on the water provided by forests. For example, almost two-thirds of the municipalities in the United States and about one-third of the world’s largest cities, including Tokyo, Melbourne, Los Feature pubs.acs.org/est © XXXX American Chemical Society A dx.doi.org/10.1021/es500130g | Environ. Sci. Technol. XXXX, XXX, XXX−XXX