River catchments supply resources such as water, food and energy, while being economically tied to their urban areas through trade. Urban areas are the driver of regional, national and global economies. The complex interrelationship between these urban areas and their supporting catchments is a vital aspect of a city’s economic success. Cities offer opportunities for employment, culture and social interaction; this has resulted in the growth of the global urban population from 34% in 1960 to 55.7% in 2019 [1
], with WHO projections suggesting growth to 68% by 2050 [2
]. The speed at which cities are increasing their exposure needs to be matched by measures to reduce vulnerability. This rapid urban expansion is taking place against a background of climate change, the impact of which is uncertain. It is, however, clear that cities within the UK, as well as internationally, are already impacted by hydrological extremes (floods and droughts: hydrohazards), which cause economic damages year on year, affecting homes, businesses, food security and energy supplies, and increasing population vulnerability. These hazards are set to intensify (in magnitude, frequency and duration) in the future due to the influence of climate change. One way in which the UK can manage the potential future impacts from increasing exposure to natural climate-related hazards is by improving river catchment resilience. This paper will explore the question:
How can we use catchment resilience as a unifying concept in catchment management and regulation—particularly in light of climate risks, population growth and other pressures?
This paper summarises current literature following the findings of our previously published papers [3
]—these are structured critical reviews focusing on climate change adaptation to hydrohazards, and flood management resilience. We use these reviews as a basis to develop and broaden our arguments; and relate these to the concept of catchment resilience in order to identify promising research and management gaps. Specifically, we will focus on catchment resilience for hydrohazard management, where we define hydrohazards as floods and droughts, thus we address both hydrological extremes.
The first paper [3
] systematically reviews literature for climate change adaptation to hydrohazards, and we explore available methods for their ability to address complexity. The paper [3
] identifies that research into climate change adaptation to hydrohazards suffers from a substantial lack of complexity-smart approaches. It highlights that complex climate change challenges are quickly outgrowing our ‘classic’ approaches, and thus there is a need to develop new methods. The second paper [4
] systematically reviews the academic literature on flood resilience to explore how resilience is assessed, operationalised and implemented. The paper concludes [4
] that resilience requires fluidity in concepts and recognition of context in order to recognise different ways to be resilient. The paper highlights gaps in current understanding of the importance of temporal and spatial scales when considering resilience and recommends a complex adaptive systems approach which accounts for the interdependencies between spatiotemporal scales, and couples human and physical systems to allow for new dynamics to emerge.
These two structured reviews are used as a basis to develop our perspective on catchment resilience. The first only focusses on existing methods for both floods and droughts, whilst the second is specific to floods. Within this paper, we expand on these findings in order to address catchment resilience, which necessarily requires consideration of both extremes of the hydrological cycle (floods and droughts), within the context of the water management unit (i.e., a catchment). Furthermore catchment resilience requires a forward-looking perspective in which critical external influences are considered (e.g., climate change and urbanisation). Thus, we use the findings of these papers as building blocks to expand our perspectives from floods to hydrohazards, within the context of a catchment, in order to answer the question posed above.
This paper will introduce the concept of a catchment as a complex adaptive system at the nexus of the natural, social, and technical realms (Section 2
). This paper will move on to discuss resilience and its concepts, characteristics and methods by which to explore it in a catchment context (Section 3
), alongside a review of current methods capable of considering catchment resilience in the context of complex adaptive systems (Section 4
). Finally, we finish with identifying some promising research gaps and explore a future research agenda (Section 5
2. The Catchment as a Complex Natural-Social-Technical (NST) System
Catchments are complex systems. Within one river catchment or basin, there will be a large diversity of land use, each of which presents a different pressure, exposure, driver or buffer in the system and fulfils a particular role. For example, urban areas are economic and infrastructure hubs [5
], and simultaneously resource users (water, energy and food), runoff and pollution sources (e.g., from impermeable land), and central points of vulnerability (due to their high population density).
Flood hazards result from excess water from one or multiple sources (e.g., coastal, fluvial, or surface water), while drought hazard arises from a deficit of flow (hydrological), soil moisture (agricultural) or precipitation (meteorological) over a period of time. A hazard acts as what we might perceive as an ‘active’ trigger for impacts within a catchment. However, impacts are a consequence not just of this active trigger, but also the latent conditions within the catchment—its exposure, vulnerability, and level of resilience. In this paper, we consider exposure to ‘include people, infrastructure, housing, production capacities and other tangible human assets located in a hazard-prone area’. For example, for flood exposure, this might be the assets located within an active floodplain area; and for droughts, assets, goods, etc., which are directly at risk from short- or medium-term droughts. Vulnerability is defined as the ‘conditions determined by physical, social, economic and environmental factors or processes which increase the susceptibility of an individual, a community, assets or systems to the impacts of hazards’ (following the definitions by UNISDR [6
]). Vulnerability to floods [7
] can then be expanded to the extent to which a system [interacting human, social and technical components] is susceptible to flood exposure in combination with its ability to adapt. This can be expanded in a similar manner for droughts [8
]. Resilience is defined as “the ability of a system, community or society exposed to hazards to resist, absorb, accommodate to and recover from the effects of a hazard in a timely and efficient manner, for example the preservation and restoration of its essential basic structures and functions” [6
] p. 24. The water management policy context within Europe recognises the need to consider the catchment unit as the basis for assessment and management. For example, both the Water Framework Directive [Directive 2000/60/EC] and the Floods Directive [Directive 2007/60/EC] recognise the spatial unit of the catchment, and its interacting parts as a basis for understanding processes. There is, however, still an emphasis on the interaction of the natural catchment system more than the social and technical components.
The combined exposure, vulnerability, and resilience of a catchment can result in impacts affecting agriculture (e.g., [9
]), infrastructure (e.g., [10
]), human health (e.g., [11
]), economic activity (e.g., [12
], government and institutional practices (e.g., [13
]), cultural heritage and community (e.g., [14
]), and of course the environment (e.g., [15
]). Despite the policy context mentioned earlier, in the last five years (2014–2019), the UK has experienced several significant natural hazard-related disasters [16
], affecting over 100,000 people and resulting in over £3.5Bn of direct economic damages. These disasters have been the result of different natural hazards including fluvial floods, convective or frontal storms and heatwaves. Each hazard has different characteristics, causing different impacts spatially and temporally across the country. Hydrohazards are known to have devastating economic and social consequences for different sectors (e.g., transport, energy generation and supply, communications networks), and communities (e.g., rural, urban). In particular, understanding the vulnerability of exposed sectors or populations can shed light on disproportionately affected members of society, thereby informing more effective adaptation strategy development to improve resilience within a river catchment.
Natural hazards are also projected to increase in frequency and magnitude [17
], and may occur concurrently or in close succession. This is why it is critical to begin characterising catchments as complex systems, rather than a set of neatly isolated parts. In so doing, we can more effectively unpack the dynamics in each catchment that might lead to different types and degrees of impact. Recently established research threads in social-ecological systems and sociohydrology [20
] recognise the dynamic link between natural processes and social systems, which is particularly pertinent in the catchment context. In this vein, Tempels and Hartmann [21
] p. 873 propose resilience as a “fluid frontier” to conceptualise the interdependencies of ecological and human systems.
We take this further to argue that the catchment system can be loosely characterised as consisting of three dynamically-linked subsystems: natural (including physical processes, e.g., hydrology, hydrogeology, geomorphology, sediment transport, nutrient cycles, and ecosystem functions), social (processes driven by intangible human values and priorities, e.g., community cohesion, health, and economic standing), and technical (physical infrastructure that is in some way human made or human influenced, e.g., transport, energy provision, and communications). We specifically include the technical within this framework as the interactions between infrastructure and the natural environment and society are critical to their function. For example, building a flood wall may improve flood exposure for some, but have feedbacks on the natural environment whilst also acting to erode the perception of risk of those living directly behind them [22
]. Figure 1
shows these three subsystems as mutually coupled in an inextricable way, whereby a change in one subsystem may trigger a feedback in another. We argue that to consider the true resilience of a catchment, the complexity of the system must be acknowledged, with feedbacks and interactions explicitly considered.
For example, flooded communities may experience long-term health impacts arising from psychological impacts from the fear of repeat flooding [23
]. Likewise, a person or community with poor health (e.g., retirement village) may be less able to invest time and resources in future flood adaptation measures, and as a result experience the impacts of the next flood to a greater degree [23
]. Of course these are examples which are fairly straightforward in nature. In reality, there is an overwhelming number of feedbacks and interactions, and which of these will be key to a catchment’s resilience is often elusive. Figure 1
suggests a handful of typical issues occurring within a catchment boundary. These exemplify current focus points for resilience research, and how these are situated within the three subsystems and their overlaps. Traditional unidisciplinary research typically sits at the edges of the nexus [18
], studying specific phenomena or known feedback loops, in order to explain and adjust the wider system from that perspective. Research exists which addresses the intersection of two domains (e.g socio-technical studies [26
]. Ideally, initial research on a catchment’s resilience should be situated at the nexus [7
], to acknowledge the importance of interactions between subsystems. Currently, our research suggests that only approximately 20% of hydrohazards research addresses any type of interaction, despite the wider complexity literature pointing to these as an underlying source of emergence [3
In conclusion, catchments are complex systems with interrelated natural, social, and technical aspects. The exposure, vulnerability, and resilience of these aspects (separately and in combination) are the latent conditions which when triggered by a specific hazard, result in catchment impacts. Figure 2
illustrates our conceptual framework for catchment resilience as a unifying concept for hydrohazard management. The framework presents our view of ‘what’ catchment resilience is, situated at the nexus of the natural-social-technical system. Within this nexus, we can explore the ‘how’ of catchment resilience by considering the feedbacks between exposure, vulnerability and resilience. The different aspects of resilience outlined in Section 3
help to consider the interactions within a catchment. Section 4
considers the methods capable of accounting for this complexity. These interactions within the system are key to understanding its overall behaviour, but these are often not captured. By considering the different aspects of resilience, we can start to think of the ‘why’ of catchment resilience, in particular the interaction between short-term exposure and longer-term objectives [30
], which are explored in Section 5
3. How to Be Resilient: Bounce Back, Absorb and Transform
There is an overwhelming body of literature which seeks to define and measure resilience. Originating in field of ecology [31
], the resilience concept has developed over the intervening half century and pervades the discourse in many disciplines [4
]. Fundamentally, resilience relates to a system’s ability to resume functionality in the wake of a perturbation. However, the recent popularity of the term resilience has led to ambiguity surrounding definitive application of the concept [4
Our recent structured review [4
] on resilience literature pertaining to the flood risk management field observed that there are differences in definition across the discipline. For example, Restemeyer et al. [32
] state that resilience centres on robustness, adaptability and transformability; Nguyen and James [33
] point to speed of recovery, magnitude of disturbance relative to a threshold, and ability to learn/adapt/transform; Hegger et al. [34
] define the capacity to resist, capacity to absorb/recover, and capacity to transform. The striking commonalities between these studies is the construction of resilience as a tripartite concept and the specific inclusion of transformation as a component of resilience [4
]. These observations point towards resilience going beyond the mitigation of impacts and reducing probability of exposure, and exploring the opportunities which arise from a hazard (in this case floods) [4
Martin-Breen and Anderies [35
] reviewed 50 years of resilience research to produce a resilience spectrum of increasing complexity, which consists of three interdisciplinary frameworks—engineering resilience, systems resilience, and complex adaptive systems resilience—reflecting the different aspects of resilience. This resilience spectrum was used in [4
] to inform the review and to match the flood resilience definitions to the resilience aspects of each framework in order to identify where flood resilience studies are currently situated along this spectrum. Martin-Breen and Anderies [35
] provide case studies for each framework in the broader resilience literature, and Philip et al. [36
] have used the engineering and complex adaptive systems framework to inform their research on assessing long-term impacts of flooding.
3.1. Engineering Resilience
Engineering resilience is to “withstand a large disturbance without, in the end, changing, disintegrating, or becoming permanently damaged; to return to normal quickly; and to distort less in the face of such stresses” [35
]. It should be noted that engineering resilience is not constrained to this engineering discipline, rather it is a widely used conceptual framework. Therefore it is not exclusive to physical ‘hard-engineered’ infrastructure (e.g., road networks), rather it indicates the ability to bounce back, and is associated with the emergency recovery stage of a shock event [4
]. According to Martin-Breen and Anderies [35
], resilience from this perspective is about decreasing a hazard-specific risk and restoring conditions to a precrisis state. A strength of framing resilience in this way, they argue, is that it makes the concept straightforward to understand, model, measure and manage. However, its simplicity is also a major limitation when we focus on engineering resilience alone. By focusing on aspects such as ‘withstand’ and ‘bounce back’ ‘return to normal quickly’, it maintains the status quo, which has been argued to be detrimental to future resilience [35
]. In other words, is returning to ‘normal’ conditions always advisable? Acknowledging additional aspects of resilience expands the space to consider whether future change is needed.
3.2. Systems Resilience
Systems resilience is “maintaining system function in the event of a disturbance” [35
], and this framework increases the complexity of the engineering resilience, where the aim is to keep things functional as opposed to identical. When we consider that the “world is in flux”, we acknowledge that there are slower variables of resilience as a result of interacting parts within a system, which have an impact during a shock event [35
]. As such, it is necessary to couple engineering resilience and its focus on a relatively short and specific hazard event, with a systems resilience understanding of longer-term and wider-scoped system dynamics. The goal of systems resilience is to ensure that system components can still function during a crisis [35
]. System resilience includes aspects such as ‘absorb’, ‘maintain’, ‘cope’ and ‘function’ [4
]. However, ensuring that the catchment system can continue to operate as normal may not be enough. Similar to the limitations of seeking only engineering resilience, is maintaining the normal operating rules of the system always advisable? Does the ability of a catchment to absorb a shock today mean that we are adequately prepared for the future? In the face of climate change, such an assumption becomes increasingly dubious.
3.3. Complex Adaptive Systems Resilience
Complex adaptive systems resilience is the “ability to withstand, recover from, and reorganise in response to crisis” [35
]. According to Martin-Breen and Anderies [35
], this framework acknowledges not only adaptation in response to a shock event, but the ability of systems to generate new ways of operating to achieve longer-term resilience. Transformability is a key element of the complex adaptive framework, which is the ability of a system component to assume a new function [35
]. Key aspects from this framework include ‘transform’, ‘adapt’, and ‘learn’ [4
]. As one would expect, acknowledging complexity makes operationalising more complex, which requires innovative methods to capture such dynamics and a truly interdisciplinary approach.
Whilst Martin-Breen and Anderies [35
] give definitions for each framework, these are complementary and not mutually exclusive. We would argue against limiting the resilience concept to one specific framework [4
]. In general, the literature which addresses all three resilience frameworks refutes the false dichotomy of infrastructure vs. nature, or control vs. chaos. In reality, there are shades of grey between these black and white concepts. For example, [21
] both discuss the nuances of resistance vs. resilience, even arguing that resistance measures are an inherent part of resilience. Tempels and Hartmann [21
] further discussed robustness vs. flexibility, and the need to take a balanced approach to these rather than prioritising one or the other, as they are not on opposite ends of a spectrum but instead overlap in many ways. Indeed, defining resilience is the source of much contention in the literature [37
], perhaps because resilience is often linked to real-world complex adaptive systems, which are also notoriously context-dependent and difficult to define.
Instead, we would argue in favour of acknowledging the different aspects of resilience that align with each framework for a more holistic and complete understanding of catchments. All three frameworks (in isolation or combination) can be matched to different catchment issues. For example, on the one hand, the Netherlands can be perceived to be resilient to flooding because they are highly advanced in their ability to control flooding, leading to less frequent flooding and lower flood damages compared to England [29
]. On the other hand, England could be perceived to be more resilient to flooding due to its high capacity to absorb and adapt to flooding, allowing England to perform well in terms of response and recovery [34
]. We argue [4
] that one framework perspective is not ‘more resilient’ than another, but that these differences emphasise the fluidity of the concept, where certain aspects of resilience are prioritised depending on their relative importance. In other words, we consider that the concept of the “fluid frontier” [21
] is not only applicable to natural, social, and technical interactions but also to our operationalisation of true resilience. Whilst resilience is truly present in all three frameworks, which aspects are most applicable will depend on the context of how natural, social, and technical aspects are interlinked in a given catchment.
3.4. Resilience Frameworks
When all three resilience frameworks are accounted for, we are forced to consider the interactions, not only between natural, social, and technical aspects but also between spatial and temporal scales the system [38
]. One framework of resilience cannot be considered in isolation without having a feedback to other aspects of resilience. However, the current state of play lacks this integrated conceptualisation. From our structured literature review [4
], we found that only 15% of flood risk management papers accounted for all three frameworks in their definitions of resilience. The majority of papers consider engineering resilience alone; systems resilience alone; or engineering and systems aspects of resilience. This indicates that the majority of existing work in this area does not perceive resilience to be an iterative, adaptive process with the ability to transform. In other words, catchments are not yet widely understood as complex adaptive systems, limiting the instances in which the three resilience frameworks can be precisely applied.