A spatial evaluation of global wildfire-water risks to human and natural systems
Graphical abstract
Introduction
Ensuring water security, which is defined as the assurance of sufficient and safe freshwater resources for human development and ecosystem functioning, has long been a challenge in developing countries (United Nations, 2005), and is a growing issue in more developed countries as population pressures increase consumption and pollution (Norman et al., 2012). Despite measurable improvements within the past decades, water insecurity still threatens or affects many countries. For instance, > 2-billion people do not have access to an improved source of water (United Nations, 2016, Gain et al., 2016). Many of the critical issues are due to water pollution, diversion, or depletion (Hoekstra and Mekonnen, 2012, Meybeck, 2003, Schwarzenbach et al., 2010). Complex relationships among social stability, ecosystem health, and freshwater availability have been recognised, all of which condition water security (Dodds et al., 2013, Padowski et al., 2015, Rockström, 2009). These relationships may be modified or enhanced by the occurrence of extreme natural disturbances (Grigg, 2003, Huppert and Sparks, 2006), thereby increasing the challenge of maintaining or achieving water security (Hall and Borgomeo, 2013, Srinivasan et al., 2012).
Recent catastrophic wildfires, characterized by extreme fire behaviour leading to life and infrastructure losses (Cruz et al., 2012), in the USA (e.g. California and Colorado), Canada, and Chile have drawn attention to the nexus among fire, water, and societies (Martin, 2016). These natural disasters have increased interest in the wide range of consequences a severe and large wildfire can have on the reliability of surface freshwater resources (Emelko et al., 2011, Kinoshita et al., 2016). The hydrogeomorphic effects of wildfires can be numerous, spatially extensive, and long-lasting. These effects include increased annual water yields and peak flows, shifts in the timing of runoff due to earlier snowmelt, and decreased water quality due to high sediment and nutrient loads (Shakesby and Doerr, 2006). Bladon et al. (2008) and Emelko et al. (2015) noted significantly higher concentration of trace elements, phosphorus and organic carbon in the water downstream of severely burned sites, persisting after several years. In the USA, Hallema et al. (2016) attributed to wildfire a + 219% increase in annual water yield in a watershed in Arizona, while Moody and Martin (2001b) documented a 200-fold increase in erosion rates in two watersheds in Colorado. Conedera et al. (2003) recorded a 200-year flood in a mountain catchment in Switzerland induced by a 10-year precipitation event, which are otherwise observed for a 40-year precipitation event in an unburned basin and with much higher flow velocity. Post-fire hydrogeomorphic hazards may consequently expose water resources to drastic quality and quantity changes that can impair downstream water supply of human and natural communities.
These post-fire impacts on the downstream water supply can result in substantial economic costs (Emelko et al., 2011, Emelko and Sham, 2014), and adversely affect human and environmental health (Finlay et al., 2012, Writer and Murphy, 2012). Greater erosion rates in burned watershed have increased sedimentation in reservoirs regulating drinking-water provision (Moody and Martin, 2004, Smith et al., 2011), thereby reducing their storage capacity and their lifespan. The increased concentration of dissolved organic carbon, often documented, pose serious issues for water treatability as it favours the formation of carcinogenic disinfection by-products (Writer et al., 2014). Other hazardous chemicals, such as lead or arsenic, can accumulate downstream in quantities far greater than what is prescribed for drinking-water quality by the World Health Organization (Tecle and Neary, 2015). The trophic chain of riverine and lacustrine ecosystems can be highly disturbed by changes in turbidity and chemical element concentration (Tobergte and Curtis, 2016) leading to decrease in ecosystem health with consequences on fisheries and recreational use of water (Tecle and Neary, 2015). The water security of downstream human and natural communities may, therefore, be threatened, making them vulnerable to risk from wildfire (hereafter ‘wildfire-water risk’ [WWR]) (Bladon et al., 2014, Robinne et al., 2016, Thompson et al., 2013). Seven years after the 2002 Hayman Fire in Colorado, Denver Water had to invest $30 million to dredge the city's reservoirs, which was filled with sediments transported from burned areas (Denver Water, 2010). In 2014, the Sydney Catchment Authority, in Australia, had to shut down a water treatment plant after heavy water contamination by ashes (Santín et al., 2015). As an emerging risk to coupled human-water systems, the WWR has been gaining in interest for the past decade. However, the threat it represents to global water security remains to be understood in a context of planetary change (Bogardi et al., 2012), in which extreme weather events, such as droughts and floods (Mann et al., 2017) are predicted to become increasingly common.
Global composite indices are commonly used in water security assessment (Garrick and Hall, 2014, Vörösmarty et al., 2010), risk analysis (De Bono and Mora, 2014, Dilley et al., 2005, Peduzzi et al., 2009), and other diverse environmental questions (Freudenberger et al., 2012, Halpern et al., 2009). Composite indices are efficient tools to explore complex environmental processes and to convey high-value information to policy-makers in an easily understandable manner (Gregory et al., 2013). They also help detect temporal and spatial trends in the evolution of a process, making them valuable to monitor policies effectiveness (OECD, 2008). However, a robust composite index requires a well-structured analytical framework. The Driving forces-Pressure-State-Impact-Responses (DPSIR) framework (EEA, 1999) simplifies complex causal relations between human and natural systems at several spatial scales (Bitterman et al., 2016, Freudenberger et al., 2010). It has been successfully applied to questions related to risk evaluation, water resources management, biodiversity protection, and economics (Freudenberger et al., 2012, Halpern et al., 2009). Meybeck (2003) contends the DPSIR framework as an appropriate tool for the analysis of global issues impacting freshwater quality and availability. The novelty of the WWR and its inherent complexity make it a good candidate for a DPSIR analysis. This framework is considered a problem structuring method that can help organising the numerous natural and social processes involved in the characterization of the risk and thus provide a tool to develop targeted policies (Gregory et al., 2013).
The present study adapts the DPSIR framework to produce the first global-scale assessment of the wildfire risks to water security. Our objective is threefold: 1) develop a reference WWR spatial analysis framework at a global scale, 2) understand the current geography of the WWR according to the different criteria involved, and 3) raise awareness of WWR issues to global water security challenges. To do so, we demonstrate the benefit of the DPSIR risk-based framework to creating a spatially explicit index. This index is then used to produce a global map showing the geography of the risk. We finally discuss the importance of our approach to the understanding of wildfire risks to water security and the questions posed by future global changes.
Section snippets
Data
For clarity, we present hereafter the 33 global datasets we used according to the five Drivers-Pressure-State-State-Impact categories, and we briefly explain their use as indicators (Table 1). Although our application of the DPSIR framework deviates from that from the original by EEA (1999), our adaptation of this approach remains similar to numerous other studies (Maxim et al., 2009). As no specific data depository representing the diversity of post-fire issues is currently available, we
Results
Values of the global composite index of the wildfire-water risk range from 0.25 to 77.27, with a mean of 18.11 and a standard deviation of 12. A closer look indicates that ~ 3.5% (Score ≥ 40) of the global area is at a substantially greater risk from wildfire impacts on water than other regions of the world. However, approximately 45.5% of the terrestrial area of the earth is at a moderate risk (Score = 20–40), while ~ 51% is at a relatively low risk (Score < 20). Greater risk scores are mostly found
Discussion
The creation of a spatial index showing the geography of wildfire-water risks to water security was motivated by three objectives: to create a robust framework, to study the WWR's geography, and to raise awareness about the WWR. We believe that the DPSIR framework for the analysis of the WWR is robust, in line with other studies presenting the DPSIR as a useful tool for the development of environmental indicators, from a global to a regional scale (Freudenberger et al., 2010). It also provides
Conclusion
The work presented here gives a global overview of the wildfire-water risk to water security. As any global index, its primary aim resides in giving an overall perspective of an issue that could affect most parts of the globe. In line with the Sendai Framework for Disaster Risk Reduction, we used the DPSIR analysis framework to provide new insights and raise awareness about this emerging risk. Although actual risk management actions take place at finer scales, a global view of this growing
Acknowledgements
We want to thank the numerous researchers who provided data for this analysis: Ben Collen from the Zoological Society of London; Ben Poulter from Montana State University; Fay Johnston from University of Tasmania; Leho Tedersoo from University of Tartu; Huan Wu from University of Maryland; Yukiko Hirabayashi from University of Tokyo; Victoria Naipal from the Max Planck Institute for Meteorology.
This research received funding from the Western Partnership for Wildland Fire Science, University of
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