Building water resilience in the face of cascading wildfire risks

Abstract

Severe wildfire is altering the natural and the built environment and posing risks to environmental and societal health and well-being, including cascading impacts to water systems and built water infrastructure. Research on wildfire-resilient water systems is growing but not keeping pace with the scale and severity of wildfire impacts, despite their intensifying threat. In this study, we evaluate the state of knowledge regarding wildfire-related hazards to water systems. We propose a holistic framework to assess interactions and feedback loops between water quality, quantity, and infrastructure hazards as determinants of post-fire water availability and access. Efforts to address the evolving threat of wildfires to water systems will require more interdisciplinary research on the complex relationships shaping wildfire’s threat to water availability and access. To support this, we need reliable long-term data availability, consistent metrics, greater research in shared contexts, more extensive research beyond the burn area, and multistakeholder collaboration on wildfire risks to water systems.

Navigating the growing complexity of wildfire hazards to water supply systems requires adopting a holistic approach.

INTRODUCTION

Wildfires are becoming increasingly frequent and destructive across the globe because of a confluence of factors, including climate change, fire suppression regimes, land management policies, and human encroachment into wildlands (1–6). The rising occurrence of drought due to climate change amplifies these effects, which also increasingly stresses natural hydrologic systems and water supplies. In the United States, forests make up 36% of the total land area but contribute 50% of the total surface water yield, and federally owned forests supply the majority of water for populations in the West (7, 8). Within these forested watersheds that supply drinking water, an average of 29% of the forested areas are at high or very high risk of fire (9). The repeated occurrences of wildfires in recent years have revealed water system vulnerabilities and subsequent impacts of hazardous wildfires [e.g., (10, 11)]. Our water supply networks, both natural hydrologic systems and built infrastructure, are at risk of degradation, threatening environmental and societal health and well-being. Current research in understanding, developing, and implementing practices that promote fire-resilient water systems is not keeping pace with the scale and severity of the threat.

Severe wildfires can lead to adverse effects on the local environment during and after the blaze. However, the movement of air and water extends these impacts beyond the immediate burn area, with consequences for rural and downstream communities. Sediment and contaminants released from vegetation, soils, and human structures/built environment during fire events are eventually flushed from the burn area or deposited atmospherically into surface waters [e.g., (12–16)]. In addition, wildfires may disrupt the normal ecosystem processes that maintain the water balance, resulting in elevated runoff and flood risk [e.g., (17–19)]. The infrastructure necessary to treat and distribute water to various communities may also suffer direct or indirect damage as a result of wildfires (11, 20).

The need for more advanced knowledge of these threats and effective mitigation strategies was made apparent during the 2018 and 2020 fire seasons in the western United States which caused multiple water system challenges. Fires and subsequent storms, floods, and debris flows resulted in water quality declines, water and hydroelectric service interruptions, and damage to critical infrastructure. Water managers in affected areas were forced to periodically suspend water intake from local rivers following fire events when the levels of sediment and contamination were deemed too extreme for treatment facilities (21). As a result of these and previous fire seasons, in some fire-prone regions, a few water agencies have started to invest heavily in actions that mitigate wildfire-related threats to the watersheds that supply their drinking water (22).

Unfortunately, resource allocation and spending for water supply systems and infrastructure protection have not grown in parallel to the risk and are highly variable between communities (23, 24). Some communities have developed detailed source water protection strategies [e.g., (25, 26)], yet many have not adequately assessed and planned for the risks posed by wildfires to their water systems (24). Further, not all source water protection strategies provide adequate consideration of the multiple threats wildfires present to water systems [e.g., (27)]. Fragmented knowledge surrounding the threats wildfires pose to water systems creates added challenges for water and fire managers seeking to appropriately mitigate and respond to these threats.

The expansion of the wildland-urban interface (WUI) in recent decades has further challenged forest and water managers. The encroachment of urban communities into wildland areas has contributed to substantially elevated wildfire risk and increased human exposure to post-fire water hazards, such as flooding and water distribution network contamination (4). Resource constraints have made it difficult for forest management agencies to proactively mitigate increasing fire risk in the rapidly expanding WUI (28). However, spending on wildfire response is at an all-time high. Federal wildfire suppression costs have gone from an annual average of over $426 million to $1.7 billion from 2000 to 2021 (29).

These resource constraints, coupled with continually evolving wildfire-related risks, have led to many challenges in protecting threatened water systems. The secondary post-fire hazards that threaten water systems continue to grow, and as these hazards increasingly co-occur, reoccur, and propagate across spatial and temporal scales, the complexity of the management landscape grows as well. To navigate this mounting complexity, a holistic approach can enable effective assessment, mitigation, and responses to wildfire-related risks to water systems. Here, we aim to unify the literature spanning the water system impacts of wildfire to inform assessments of water system vulnerability to the hazard of wildfire.

In this study, we conduct a systematic review to evaluate the current state of knowledge regarding wildfire-related hazards to water quality, water quantity, and water infrastructure. We provide an overview of the literature, including both peer-reviewed and gray literature, and identify the current state of knowledge on the interconnected wildfire impacts to the water system. Next, we assess how changes to the biogeophysical characteristics of a watershed can lead and contribute to these hazards to shed light on the land use planning and management efforts that can mitigate these risks. We present a holistic framework that identifies the multiple linkages and feedback loops between wildfire and water availability and propose several strategies to support wildfire and water resilience at the WUI and beyond.

RESULTS

The state of the literature

We reviewed 212 publications (177 peer-reviewed publications and 35 gray literature) to assess the state of the knowledge for wildfire hazards and their linkages to water quantity, quality, and infrastructure. Of the 177 peer-reviewed publications, 120 featured named, distinct wildfire events. Eighteen named wildfires appeared in three or more of the peer-reviewed studies and are summarized in table S1. The United States was overrepresented in the reviewed content representing 111 of the 120 named wildfires. There was more geographical diversity in the reviewed gray literature spanning across the United States, Canada, and Australia. All publications were written between 1999 and 2022, although the publication date was not used in the inclusion criteria (Fig. 1). We classified water supply systems as source water (i.e., natural infrastructure), which included water quality and quantity parameters, and water infrastructure (i.e., built infrastructure). Nearly one-third of the peer-reviewed publications investigated impacts to water quantity and water quality, respectively, while fewer than 3% examined the effects of wildfire on built water infrastructure alone. The gray literature publications, in contrast, frequently addressed all three hazards to some degree, indicating a discrepancy between what practitioners recognize as threats and the research that is being conducted. In sum, the literature is unbalanced in its treatment of water system impacts of wildfire. Water quality and water quantity impacts are well represented in the peer-reviewed literature, and research in these areas continues to grow. However, the impact on built water infrastructure and the interlinks and cascading effects of wildfire are far less frequently studied but more likely to be included in practitioner-oriented gray literature.

Fig. 1. The state of the peer-reviewed and gray literature assessing wildfire impacts to water systems over time and amongst water categories.

The 212 peer-reviewed and gray literature items reviewed by publication year (A). The percentage of peer-reviewed publications represented by category (B). Publications in 2022 are not shown on the bar graph as papers were collected midway through the year.

Eight key categories of wildfire-related hazards to water systems were identified through our systematic review (Fig. 2). These hazards are not evenly distributed over time and space and can arise sequentially or concurrently depending on the biogeophysical context and human response. While wildfire is a natural process necessary for resilient forested ecosystems in many landscapes, the expansive, high-severity, and high-intensity wildfires common in the 21st century often result in the loss of normal ecosystem functions, which are critical to maintaining and stabilizing water systems. These disruptions can precipitate a range of hazards that threaten water quality, quantity, and infrastructure including floods, landslides, and debris flows, decreased snowpack retention and early snowmelt, and surface and groundwater contamination. The volume of research exploring these post-fire impacts on water quality and quantity has grown substantially in recent years.

Fig. 2. Wildfires threaten water quality, quantity, and infrastructure.

The outer ring of the figure depicts the eight key wildfire-induced hazards to water systems identified in this review.

Wildfire impacts to water quality, quantity, and infrastructure

The growing body of research on water quality and water quantity impacts to wildfires has well characterized many of the main trends associated with wildfire burns. Wildfires can fundamentally change water quality by accelerating erosion and liberating constituents from organic materials, soils, and the built environment that are then readily delivered to surface waters via wet and dry depositional processes (12–16). Elevated concentrations of heavy metals, trace elements, nitrogen, phosphorus, and organic carbon are commonly detected in fire-affected watersheds [e.g., (15, 30)]. In many contexts, the most severe declines in water quality correspond with the first post-fire flushes and high-intensity rainfall events [e.g. (16, 31, 32)]. Ash, sediment, and other contaminants accumulate on the soil surface during fires, and high levels of runoff produced during the first post-fire storms and snowmelt mobilize the constituents in substantial quantities, flushing them into surface waters (33, 34).

Water quality recovery typically occurs within 2 to 3 years after fire, with the most severe impacts observed within the first year (35, 36). However, even transient declines in water quality can present substantial water treatment challenges. Notably, delayed water quality recovery has been observed, sometimes attributed to post-fire drought (37–40). Changes in water quality are also leading to challenges in the treatment of drinking water (41). Many treatment facilities are not prepared to meet these challenges, which can (but not always) require additional processes, lead to increased costs, and may even result in supply disruption (15, 41).

Wildfires typically also result in water quantity changes with temporarily increased runoff, streamflow, and flood risk. The loss of vegetation as a result of low- to high-severity wildfires leads to decreased evapotranspiration and interception [e.g., (17, 42, 43)]. Combined with fire-related soil water repellency, these factors have been found to result in considerable increases in overland flow and water yield, particularly during high-intensity rainfall events [e.g., (17, 42, 44–47)]. Wildfires may also lead to earlier and increased snowmelt as fire-related debris decreases albedo and the loss of forest canopy increases solar radiation reaching the snowpack (48–52). Typically, streamflow peaks in the first year following the fire but can remain elevated for many years [e.g., (44, 53)].

Built water infrastructure ranging from treatment facilities and reservoirs to aboveground pipes and water meters may be directly damaged when exposed to flames and extreme temperatures associated with wildfire. Direct damage to water distribution systems, which can interrupt service and contaminate drinking water, threatens water utilities and their customers as wildfires increasingly occur within the WUI (54). Several recent studies have identified water contamination in and around burnt structures as a result of heat damage to plastic plumbing components as well as backflow of contaminated water and air into the water distribution system (11, 55). Following California’s Tubbs Fire of 2017 and Camp Fire of 2018, community-wide water supply disruptions occurred because of contamination from volatile organic compounds (11, 20). The risk of these events will likely rise with the growing incidence of wildfire at the WUI (11, 20), with cascading impacts to downstream communities. Built infrastructure may also suffer indirect damage as a result of secondary post-fire hazards such as flooding and debris flows. Reservoir sedimentation is a costly indirect consequence of wildfire that can result in interruptions to water supply (10, 56, 57).

Infrastructure provides the terminal connection in the path linking source water to community water users. Therefore, infrastructural vulnerabilities to the direct and indirect impact of wildfire may create a bottleneck constraining water availability and access. As mentioned in the previous section, this area has received less attention in the literature, and, thus, our understanding of this bottleneck is limited. The cumulative effects of multiple water quality and quantity hazards challenge the built capacity of many water systems.

Controls on wildfire water system impacts

The biogeophysical context of a region influences both its vulnerability to wildfire and its susceptibility to generating post-fire hazards to water. The interaction of climate patterns, topography, soil characteristics, vegetation regime, and hydrologic traits with fire severity, intensity, and extent determines impacts to water systems. For instance, regional climate patterns influence fire risk, the timing and duration of fire season, post-fire precipitation, the pace of vegetative recovery, and water availability. Topography affects post-fire erosion, landslide, and debris flow susceptibility as well as soil moisture and snowmelt dynamics. Soil properties influence post-fire infiltration and runoff processes, erodibility, and availability of constituents that may affect water chemistry. The vegetation regime also plays a considerable role in fire risk, severity, and recovery. Forest density, vegetation aridity, and fuel availability increase fire risk and severity (3, 26, 58). The accumulation of fuel loads and increased forest density and homogeneity due to legacy fire suppression regimes coupled with climate change have resulted in heightened fire risk across the western United States and elsewhere (59–62). Last, hydrologic traits such as peak flow, seasonality, and baseflow influence post-fire changes to streamflow, flood risk, and the magnitude of water quantity impacts. Wildfires damage soil and vegetation, deteriorating the hydrologic condition of watersheds, leading to elevated water yield and flood risk [e.g., (44, 63, 64)].

The coincidence of biogeophysical characteristics determines wildfire conditions, but, in turn, the direct and indirect impacts of wildfire may transform the biogeophysical characteristics of an affected area. post-fire erosion, landslides, and debris flows can alter topography [e.g., (10, 65–67)]. Burning and high temperatures can destroy soil organic material, harm the soil microbiology, and lead to the formation of a soil water-repellent layer on the soil surface [e.g., (13, 68)]. Fires may thin vegetation, promote the growth of fire-tolerant species, or otherwise permanently or temporarily alter the vegetation regime [e.g., (17, 69)]. These effects, in turn, can alter the normal water balance leading to an increased and more variable water yield (17, 42, 44–47).

Wildfire severity, intensity, and extent control the degree of impact on local biogeophysical characteristics as well as the severity of post-fire water quality, quantity, and infrastructure hazards. Changes to water quantity and quality typically increase with burn severity and extent [for example, see (65, 66)]. Low-severity fires, such as prescribed burns, typically do not produce harmful water quality, quantity, or infrastructure impacts (64, 70), making a compelling case for active fire management. These fires can increase water yield but, unlike catastrophic wildfires, do so in a way that mimics natural processes (64, 71). In contrast, areas burned at high severity typically experience more acute water hazards including debris flows and flooding [for example, see (18, 72, 73)]. Similarly, watersheds burned to a greater extent typically experience more severe water quality impairment, while the impacts are typically diluted in watersheds with limited burn area (74).

post-fire hazards to water quality, quantity, and built infrastructure may occur days to years following the initial fire with consequences for water availability and access. As the threat of wildfire grows and evolves with climate change, so too will wildfire impacts to water availability and access [e.g., (75, 76)]. However, much of our present understanding of these hazards remains fragmented as the current research on the wildfire water nexus often does not address the interactions between the three hazards as shown in Fig. 1.

Consequences of and opportunities for land use planning and management

Land use planning and management decisions both before and after a severe wildfire have multispatial and temporal impacts on water systems and wildfire risk, exposure, and recovery. Land use planning and forest management practices centered around fire suppression have contributed to the elevated fire risk in the western United States and across the globe [e.g., (61)]. Human encroachment into the WUI has further contributed to the rise in wildfire occurrence (4, 77, 78), which has also increased wildfire exposure for communities and infrastructure in the expanding WUI (79). The impacts of these land use planning and management trends on wildfire risk are further compounded by the effects of climate change (4). Given the current state of the literature, it is unclear if there has already been an increase in destructive impacts to water infrastructure systems, but the increase in large fire frequency in and near the WUI suggests that this is and will continue to occur. The key pre-fire mitigation and post-fire adaptation strategies to enhance water system resilience to fire hazards have been summarized in Table 1.

Table 1. Overview of approaches proposed in the reviewed literature to attenuate wildfire risks to water systems.

Actions to decrease vulnerability	References	Actions to increase resilience	References
Fuels reduction and thinning	(23, 26, 97)	Improved prediction and warning systems	(63)
Improved wildfire containment	(98)	Collaborative water management	(92)
Update building codes	(11, 95)	Erosion control	(80, 95)
Coupled wildfire and water risk assessments	(99)	Water treatability assessments	(27)
Afforestation with native species	(46)	Water infrastructure redundancies	(90)
Managed fire	(100)	Flexible water sourcing/diversion	(23)
Identify policy priorities	(11)	Safe disposal of damaged infrastructure and waste	(101)
		Forest restoration	(69)

pre-fire management approaches to mitigating impacts to water systems have become increasingly important in recent years as fuel conditions have grown more hazardous. Fuel reductions (e.g., prescribed and cultural burns, managed wildfire, and mechanical thinning) are frequently the first line of defense against wildfire. Positioning infrastructure and creating defensible spaces to reduce exposure to future wildfires in addition to ensuring water infrastructure redundancies, flexible water sourcing and diversion, building code updates, and enhanced remote operation capabilities for water facilities are all management options to better insulate the water supply from post-fire disruptions.

Postwildfire management and adaptation actions to effectively respond to risks to both natural (e.g., watersheds) and built (e.g., pipes and reservoirs) water infrastructure can help ensure future water availability as wildfires give rise to increasing water supply disruptions. Unfortunately, few strategies currently exist to effectively mitigate post-fire water system hazards across large areas. Erosion control methods (e.g., hydromulching, reseeding, and debris dams) have been frequently deployed to mitigate water quality declines [e.g., (80, 81)], but the implementation of these measures across extensive burn areas is often infeasible.

Last, it is important to acknowledge the human and institutional infrastructure required to support resilient water systems. Wildfire impacts to water systems intersect with multiple types of governing agencies, including those with land-use, fire, and water related authorities that span from local to national scales. In addition, community-based organizations and nongovernmental organizations can be very active in this space (82, 83). Risk reduction strategies, such as prescribed burns, face multiple barriers to implementation from a sociopolitical perspective, including availability of financial and technical resources and stringent regulations for the practice (84). The ability of these entities to coordinate across scales for both risk reduction and post-fire response is an important factor in a region’s ability to support wildfire-resilient water systems in a way that is just and equitable (85).

A holistic approach

Climate change is exacerbating the threat of severe wildfires and the ability of communities to access safe, affordable, and acceptable water for both potable and nonpotable uses—what we refer to here as water availability and access. Additional preparation is necessary to ensure that our critical water infrastructure can withstand the escalating hazards of wildfire events. This underpins the need for a holistic approach to better assessing aggregate risk and anticipate emerging post-fire impacts to water systems at multiple scales. Such an approach can enable a deeper understanding of the linkages between source water, wildfire, and infrastructure and inform effective and resilient management of water systems in the evolving context of wildfire and climate change.

While current research questions may blend water quality, quantity, and infrastructure concerns, the consideration of all three components concurrently is needed to develop a holistic approach to assessing post-fire risk to water systems. In pursuit of filling this gap, wildfire research should seek to enable a multidimensional understanding of wildfire risks to water systems. Interdisciplinary and transdisciplinary methods are well positioned to understand the effects of post-fire water quality, quantity, and infrastructure interactions. Research questions themselves need not be holistic to be useful but rather can work within a holistic systems perspective to fill identified gaps. This research can also further inform pre-fire mitigation strategies through feedback loops.

Here, we propose a holistic framework to assess interactions and feedback loops between water quality, quantity, and infrastructure as determinants of post-fire water availability and access (Fig. 3). These relationships are drawn from across the existing literature. This paper fills existing research gaps by offering a comprehensive and holistic view, summarizing our assessment of the linkages between wildfire and water availability.

Fig. 3. A holistic framework representing water availability as a function of fire-affected water quality, quantity, and infrastructure interactions.

Building on the literature reviewed above, in this framework, the biogeophysical characteristics of a watershed, specifically climate patterns, topography, soil characteristics, vegetation regime, and hydrologic traits, determine source water quality and quantity, as well as wildfire severity, intensity, and extent. In turn, the severity, extent, and intensity of wildfires generate hazards that directly and indirectly affect source water and infrastructure and, ultimately, water access and availability. Climate change acts on the biogeophysical and water supply systems, increasing risk and vulnerability to post-fire water hazards. However, land use planning (e.g., residential development, water treatment, distribution infrastructure, and roads) and management actions (e.g., forest management including thinning and prescribed fire and erosion control) to decrease vulnerability and increase resilience can alter the biogeophysical characteristics of watersheds and water supply systems to mitigate and adapt to these post-fire water hazards. However, some present management practices themselves, such as fire suppression, increase vulnerability and challenge resilience. Appropriate selection and implementation of management plans are essential for mitigating and adapting to the harmful effects of climate change. For example, selective tree thinning and prescribed burning in overgrown forests, as part of comprehensive forest management, can not only increase landscape resilience to drought but also increase streamflow in some geographies (69).

DISCUSSION

Supporting water system resilience requires connecting science to policy and decision-making through investment in data and information technology infrastructure. The extant literature is primarily concerned with identifying post-fire outcomes for fire-affected water systems. In contrast, less effort has been made to identify the mechanisms controlling the nature, timing, duration, severity, and spatial extent of these hazards nor the adverse outcomes they typically produce and possible risk reduction associated with various management strategies. Therefore, while we can glean general approaches to increasing system resilience, we struggle to link this research with effective management applications based on their unique connections to hydrologic processes.

For instance, it is well known that vegetative recovery is often an important precondition for post-fire hydrologic recovery, but the precise mechanisms by which revegetation prompts hydrologic recovery is not fully known. The role of root density, community structure, forest age, and other characteristics in the return to pre-fire hydrology is not well understood but could provide valuable insight into the pace of the post-fire recovery process. Knowledge of this kind as it relates to post-fire water hazards could play a role in informing the design of effective, holistic risk assessment and hazard mitigation strategies and policies. To clarify the linkages between wildfire, water quality, water quantity, and water infrastructure impacts, reliable long-term data availability, consistent metrics, greater research in shared contexts, more extensive research beyond the burn area, and multistakeholder collaboration are needed.

Baseline data availability is a perennial challenge of research, as well as decision-making, at the wildfire-water nexus

Many fire-prone regions lack long-term, continuous monitoring of water quality and quantity due to the cost and human resources required to install and operate in situ monitoring devices. Luckily, the advent of remote sensing has enabled better, cheaper, and more frequent measurements that can complement these on-the-ground readings, but many challenges remain. Inadequate availability of pre-fire baseline data for fire-affected watersheds presents a challenge for researchers, typically requiring them to identify similar, unaffected watersheds to act as controls and perform additional monitoring at those sites. Data-rich regions where managers and researchers have invested in monitoring infrastructure are overrepresented in the literature as the availability of long-term water quality and quantity data is a prerequisite for the before-after analysis of post-wildfire impacts. In addition, post-fire water monitoring is typically limited to 3 to 5 years. Limited long-term pre-fire and post-fire water quality and quantity monitoring limits our understanding of the post-fire recovery timeline across diverse climatic, ecological, and burn contexts.

Inconsistent use of vegetative, hydrologic, and soil recovery metrics across publications challenges our understanding of postfire water systems recovery

The lack of uniformity in recovery indicators makes it difficult to identify the mechanisms guiding water system recovery and to compare findings across publications. Vegetative recovery, often measured through remote sensing methods, is frequently assessed with metrics including the normalized difference vegetation index and related geospatial indicators. However, changes in community structure and measures of diversity, richness, and/or evenness assessed from the ground are also used. Hydrologic recovery indicators used include the return to pre-fire streamflow, evapotranspiration, and water chemistry. Last, soil recovery has been identified by changes in soil hydraulic conductivity, structure, and other conditions, including the deterioration of the hydrophobic layer. Integrating, linking, and standardizing these multiple metrics could enhance our overall understanding and connect learnings across studies and regions.

Inadequate definitions and/or measurement approaches to classifying burn severity in the WUI

Remotely sensed Monitoring Trends in Burn Severity and Burn Area Reflectance Classification data dominate in the scientific literature, but these and other conventional burn severity definitions and datasets fail in WUI and urban geographies, which are increasingly consequential in research at the wildfire-water nexus. Burn severity data are most often derived from Monitoring Trends in Burn Severity for fire research in forested watersheds but are occasionally assessed on the basis of soil burn severity, loss of soil organic matter, or observations of visual indicators including canopy damage, root damage, debris consumed, and ash characteristics (86–88). However, research in the WUI demands unique measures of burn severity. For example, geospatial measures of burn severity that rely on changes in vegetation may be inaccurate for fires that affect built environments, such as those occurring at the WUI (21). Schulze and Fischer (20) use the density of damaged structures to measure burn severity following WUI fire, although opportunity remains to further develop approaches for WUI and urban contexts.

While WUI fires are increasing in occurrence, the majority of research is focused on undeveloped watersheds

Wildfire research must target all areas of risk, including WUI contexts, to support policy and decision-making. The watersheds that have received attention in the literature are often undeveloped headwaters that have typically burned at moderate to high severity across a large extent (>50%). While adverse impacts have been documented across low- to high-severity fires and small to large watersheds, some effects may be dampened at large spatial scales (89, 90). Since watershed size likely influences the timing, duration, and severity of post-fire impacts on water systems, watersheds of varying scales may respond differently to management interventions. Thus, research at multiple spatial scales, burn severities, and levels of development is needed.

In addition, much of the reviewed literature evaluates the effects of wildfire on water quality and quantity within burned watersheds. However, the effects both upstream, because of aerial deposition, and downstream of the outlet in burned watersheds have received limited attention. A greater understanding of these system impacts that cascade into the WUI, particularly over time, is needed to identify the spatial and temporal risks for management planning purposes. Research on the WUI has grown substantially in recent years, but many challenges exist including data constraints due to privacy concerns, lack of baseline data given the continually changing rapid development, and challenge of methodological standardization in defining and studying the context (4, 91).

Supporting resilience in the WUI requires greater collaboration between stakeholders and management communities

Often, watershed boundaries fall across multiple jurisdictions and property types, which presents a challenge for both pre-fire hazard mitigation and post-fire adaptation efforts (69, 92). Across the literature, there is a recurring emphasis on the need for greater institutional coordination and clarification of responsibilities in hazard prevention and response. A shared understanding of the spatial and temporal distribution of post-fire water hazards can aid in these collaborative efforts by allowing communities to identify intertwined vulnerabilities and hazard mitigation opportunities. Improved collaboration allows for a quicker response to post-fire water hazards and contributes to greater trust in management agencies, which ultimately helps attenuate community vulnerability to post-fire water hazards (24, 26, 92, 93).

In addition, water infrastructure vulnerability is closely intertwined with energy, transportation, and other critical infrastructure as wildfires may block or damage roads, inhibiting access to water treatment facilities or other infrastructure, and failure of the electrical grid may disrupt operations (54, 94, 95). Multistakeholder and utility coordination is critical to anticipating and responding to these hazards and funding pre-fire mitigation strategies. Understanding the local context and characteristics of WUI communities can help inform both their individual response to wildfires and how they can best collaborate with others (96). Further, more efficient modes of coordination between fire and water managers are needed to enhance mitigation and adaptation capacity.

Toward wildfire resilient water systems

The risk of wildfires to water availability and access across the world is growing rapidly, especially in arid and semiarid regions. In the face of these growing risks, the framework we present here for understanding the linkages between the biogeophysical context, wildfire, source water, and infrastructure, which shape water availability and access, can help resource managers and decision-makers build and maintain resilient water systems and communities. By adopting a holistic perspective on the interrelated effects of wildfires on water systems, decision-makers may better consider long- and short-term outcomes of management decisions, identify key intervention opportunities with the potential to generate multiple co-benefits across the system, and evaluate the tradeoffs between mitigation and adaptation management actions under different conditions.

To support efforts to plan for, mitigate, and respond to the evolving threat of wildfires to water systems, we need an interdisciplinary approach to research that explores the complex relationships shaping wildfire’s threat to water availability and access. This is not to undermine the need for targeted research on specific impacts but rather emphasize that a holistic approach can build upon targeted research to identify and fill gaps in our knowledge and management approaches. Greater collaboration is needed to reimagine wildfire and water research and better enable integrated knowledge and decision-making. Addressing the gaps in baseline data, uniform metrics, and WUI-specific studies is vital for supporting this interdisciplinary and convergence research. Of particular need is research into the social and distributional impact of post-fire water hazards. This knowledge will directly inform risk assessments for fire-vulnerable communities, which are largely disadvantaged and rural populations. Further, there is an opportunity for policy improvements to promote wildfire resilience of natural and built water infrastructure, but additional investigation is needed to identify the most salient needs.

MATERIALS AND METHODS

Experimental design

We reviewed 212 items from the English-language peer-reviewed (177 publications) and gray literature (35 publications) (Fig. 4). For the full list of publications reviewed, see data S1. Initially, we identified publications from paper citations and keyword searches in Google Scholar that included at least one wildfire-related and water-related term in each search. Wildfire related terms included “wildfire,” “wildland-urban interface,” and “post-fire” and water-related terms included “water,” “water quality,” “water quantity,” “infrastructure,” “hydrology,” and “water yield.” Several searches were also conducted including the names of severe wildfires in the western United States (i.e., Tubbs, Camp, Cerro Grande, and Carr Fires) with known water impacts. We did not define specific geographical inclusion criteria and accepted global publications; however, with the inclusion of specific fire search terms and focus on English-language literature, our review is skewed toward studies based in the western United States.

Fig. 4. Summary of the review process.

We included studies that either (i) tested empirically the relationship between wildfires and some dimension of water impacts (e.g., water quality, water quantity, or water infrastructure), (ii) reviewed literature that empirically tested the relationship between wildfires and one or more dimensions of water impacts, (iii) described theoretical models for understanding wildfire impacts to water systems, or (iv) described, analyzed, or prescribed practitioner approaches to mitigating wildfire impacts to water systems or increase the resilience of water systems to wildfire. We excluded papers that did not methodically examine water impacts for wildfires, such as those that identified water impacts as a post-fire concern but did not address them as a primary focus of the study. Empirical studies, meta-analyses, reviews, case studies, and commentaries were all accepted on the basis of our inclusion criteria. Ultimately, this process resulted in the identification of 184 papers. To supplement this narrative review, a series of systematic database searches were conducted in Web of Science, SCOPUS, and Engineering Village. These searches included two or more of the following terms, always with a combination of wildfire and water-related terms: “wildfire,” “water,” “*wildfire,” “water*,” {wildland urban interface}, {reservoir sedimentation}, and {water systems}. These searches resulted in the addition of 29 relevant publications to the review.

Supplementary Materials

This PDF file includes:

Tables S1 and S2

Legend for data S1

References

Other Supplementary Material for this manuscript includes the following:

Data S1