全球变化知识资源中心

Managing freshwater resources sustainably under future climatic and hydrological uncertainty poses novel challenges. Rehabilitation of ageing infrastructure and construction of new dams are widely viewed as solutions to diminish climate risk, but attaining the broad goal of freshwater sustainability will require expansion of the prevailing water resources management paradigm beyond narrow economic criteria to include socially valued ecosystem functions and services. We introduce a new decision framework, eco-engineering decision scaling (EEDS), that explicitly and quantitatively explores trade-offs in stakeholder-defined engineering and ecological performance metrics across a range of possible management actions under unknown future hydrological and climate states. We illustrate its potential application through a hypothetical case study of the Iowa River, USA. EEDS holds promise as a powerful framework for operationalizing freshwater sustainability under future hydrological uncertainty by fostering collaboration across historically conflicting perspectives of water resource engineering and river conservation ecology to design and operate water infrastructure for social and environmental benefits.

Securing the supply and equitable allocation of fresh water to support human well-being while sustaining healthy, functioning ecosystems is one of the grand environmental challenges of the twenty-first century, particularly in light of accelerating stressors from climate change, population growth and economic development. Rehabilitation of ageing infrastructure and construction of new infrastructure are now widely viewed as engineering solutions to mitigate future climatic uncertainty in the hydrologic cycle¹. Indeed, the construction of tens of thousands of dams in the twentieth century helped secure water supplies and fuel economic development in industrialized countries, and developing economies are now pursuing massive new infrastructure projects with thousands of new dams proposed for hydropower production and water supply security².

Despite the economic stimulus provided by many dams historically, the global experience with dam building warns that traditional approaches to water infrastructure development in a rapidly changing world carry severe risks of economic and environmental failure. First, large water projects are very capital-intensive and long-lived, costing billions of dollars to plan, build and maintain. Yet, they are vulnerable to biased economic analyses³, cost overruns and construction delays, and changing environmental, economic and social conditions that can diminish projected benefits^{4, 5}. Under a variable and changing climate, large water infrastructure may even risk becoming stranded assets⁶. Second, the principles of economic efficiency inherent in cost-benefit analysis dominate project design and performance assessment, making integration of social and environmental benefits and costs into a comprehensive economic evaluation a significant challenge^{7, 8}. These costs can be substantial, as evidenced by human displacement^{5, 9}, local species extinctions¹⁰ and the loss of ecosystem services such as floodplain fisheries and other amenities^{11, 12}.

As unanticipated economic, social and environmental costs accumulate with ageing water infrastructure, society is investing in restoration projects to partially undo longstanding environmental degradation, including modifying flow releases from dams^{13, 14} and, in some cases, dam removal¹⁵. As global-scale impairment of aquatic ecosystem function becomes increasingly documented and articulated^{16, 17}, there is urgent need for a broader conception of sustainable water resource management that formulates environmental health as a necessary ingredient for water security and the social well-being it supports^{18, 19, 20}. Notably, new national directives are emerging to develop and manage river ecosystems in more environmentally sustainable ways that retain social benefits, including in the USA²¹, Europe^{22, 23} and Australia²⁴.

Here, we ask if a more sustainable water management philosophy can be forged to guide investment in, and design of, water infrastructure while avoiding adverse (and sometimes irreversible) social and environmental consequences. We consider 'sustainable water management systems' to be those that meet the needs of society over the lifetime of the infrastructure while also maintaining key ecological functions that support the long-term provision of ecosystem goods, services and values, including biodiversity maintenance. These systems would embrace the principle of resilience, that is, the capacity to persist with functional integrity under changing social and environmental conditions²⁵. Indications of this emerging perspective are reflected in calls for greater focus on demand-side management, rather than supply-side solutions²⁶, as well as 'green' infrastructure approaches, such as 'soft-path' solutions²⁷ and managed natural systems²⁸. Deep uncertainty about future climate raises significant concerns about how to achieve long-term economic benefits and performance reliability of major water projects^{29, 30}. This unprecedented uncertainty renders traditional approaches to the design of long-lived infrastructure inadequate, requiring new decision-making approaches³¹. In the context of a changing (non-stationary) hydrologic cycle, incorporation of alternative design and management principles can be viewed as reducing risk in infrastructure investment by enhancing 'robustness' (satisfactory performance under a wide range of uncertain futures) and 'adaptive capacity' (the ability to be modified rapidly and economically in response to changing, unforeseen conditions)^{32, 33}.

Planning for resilient, robust and adaptive water infrastructure to achieve social, economic and environmental objectives under a highly uncertain future presents novel challenges. First, contrasting paradigms in water resource engineering and in conservation ecology have dominated the broader societal debate about infrastructure design and operation over the past several decades^{34, 35}, and these perspectives have typically been antagonistic. However, the fields of water resource engineering and conservation ecology are now independently re-examining long-held, foundational assumptions, in no small part because of concerns about climate change and other forms of non-stationarity (Box 1). These philosophical shifts are subsequently creating the possibility of revisiting ingrained presumptions about barriers to collaboratively attaining more sustainable water resource management. Second, methods for integrating ecological principles into water infrastructure design and operation to satisfy multiple objectives have been proposed⁸ but are not well established in practice^{36, 37}. The key question emerges of how to operationalize sustainable water management to couple engineering design principles with ecosystem requirements in the context of non-stationary stressors (for example, changes in climate, water use, population growth and land-use change).

Box 1: Shifting paradigms in water resource engineering and conservation ecology.

Rapid climate change, population growth and economic trends are generating unprecedented uncertainty about how to achieve sustainability targets for water management and ecosystem conservation, as well as simultaneous opportunities to find common ground. First, traditional water resource engineering is struggling with climate non-stationarity (unknowable uncertainty about future hydrologic conditions) and seeking new approaches to guide infrastructure planning and avenues for secure economic investment under a wide range of climate scenarios⁷⁷. Second, climate variability and change plus the pervasive effects of human activities on ecosystems are broadly challenging conservation and restoration ecology, which have traditionally defined ecosystem management targets by reference to historical ('natural') conditions and focused on habitat reserve strategies⁷⁸. Emerging perspectives in aquatic ecology now place biological conservation in the context of highly altered and non-stationary hydrosystems that require active management within human-dominated landscapes to sustain critical ecosystem functions^{79, 80, 81}. These perspectives align with a broader conservation approach of “managing for resilience”⁸², which focuses on maintaining key processes and relationships in social-ecological systems so that they are robust against a wide range and variety of perturbation from climate or other stressors. This paradigm represents a departure from traditional conservation biology in that it emphasizes the endurance of system-wide properties (rather than a sole focus on individual species) and promotes reconciliation of conservation objectives with the alteration of natural systems by human influences^{81, 83}. Together, emerging paradigms in ecology and engineering are giving rise to potential new levels of cooperation and communication across these (traditionally conflicting) disciplines. For example, ecologists are now developing socially contextualized conservation tools to inform water infrastructure management ('environmental flows'^{50, 84, 85}) and water resource engineers are actively exploring how to incorporate these into infrastructure operations^{86, 87} with implications for multiple objective evaluation approaches^{8, 88}.

Expansion of the existing decision scaling framework to consider both engineering and ecological performance affords a powerful new analytical approach to operationalize sustainable water resource management in the face of future hydrologic uncertainty. We refer to the integrated analysis of these complementary domains as eco-engineering decision scaling (EEDS), and it builds from the multiple-objective decision scaling approach used for policy evaluation in the International Upper Great Lakes Study^{53, 54}. The conceptual significance of EEDS is that it allows explicit evaluation of trade-offs between engineering design features and socially valued ecological performance associated with water resource development. More specifically, this trade-off analysis occurs in the initial stages of project development, so that economic, engineering and ecological vulnerabilities can be simultaneously compared. Such early evaluation of ecosystem vulnerability is necessary to reveal a range of potential design and management options in complex social-ecological systems⁵⁵. This new approach is closely aligned with planning principles that engineers often follow, such as the Principles and Guidelines used by the US Army Corps of Engineers (USACE)⁵⁶, and similar guidance documents in Europe⁵⁷.

The EEDS framework significantly contrasts with approaches typically used to assess the environmental impacts of water infrastructure projects, and it can be summarized as a five-step process shown visually in Fig. 1 and described in detail in Box 2. Traditionally, initial project conception and design are driven by economic assessment of expected direct costs (for example, financing, construction and maintenance) and benefits (for example, revenue from hydropower production and water supply or avoided damages). Typically, several competing economically viable alternatives are developed in engineering designs before environmental impacts are considered. In the EEDS approach, however, both engineering and environmental performances are quantified and simultaneously compared across management alternatives under the range of future uncertainty. Ecological performance indicators must be clearly defined and quantitative, but significantly, they need not be monetized (which is often challenging or infeasible⁵⁸) to allow comparison with traditional economic indicators. Furthermore, the EEDS approach can accommodate multiple performance metrics representing a diverse suite of economic, social and ecological objectives (for example, ref. 8). For the sake of simplicity, we present only two metrics in our conceptual framework (Fig. 1) and three metrics in our case study below. Ultimately, stakeholders assess viable decision pathways based on the aggregate performance of all metrics and implement management options according to values and preferences.

See main text and Box 2 for a detailed description of each step.

We illustrate the EEDS framework through a hypothetical example of a water resource decision problem for an existing flood management project. Coralville Dam was constructed in 1958 by the USACE on the Iowa River to protect Iowa City and downstream farmlands from flooding (Fig. 2). Iowa City also has a series of floodplain levees in place to reduce flood risk. Since 1990, several severe runoff events have resulted in unscheduled water releases from the dam spillway, raising concerns that extreme floods are becoming more frequent and that current management operations are inadequate for controlling flood risk. We apply EEDS to evaluate the potential economic costs associated with altered climate regimes that increase flood risk and explore how alternative flood-control management strategies could affect both engineering and ecological performance indicators. Extensive data on dam operations, system hydrology and flood inundation risk (Supplementary Information) make the system amenable to a hypothetical exploration of EEDS in a plausible management scenario.

This map shows Coralville Dam with flooding spillways and the extent of the 2008 flood that breached some levees in Iowa City (urban footprint shown in grey) and extensively inundated downstream floodplain farmland and riparian habitats (dark blue).

Deep uncertainty about future hydrology undermines traditional approaches for the design and operation of water infrastructure to achieve 'reliable' performance^{29, 30} and poses an unprecedented challenge for sustaining healthy, resilient freshwater ecosystems. On a global scale, current infrastructure (dams and irrigation works) is extensive and a significant driver of freshwater ecosystem degradation^{16, 60, 69, 70, 71}. Historical evidence clearly indicates that human decisions on the design, location and operation (or reoperation) of water infrastructure such as dams will have both immediate and long-term effects on the health and resilience of freshwater ecosystem function and biodiversity^{72, 73}. Given the inevitability of much new and redesigned water infrastructure, a new spirit of cooperation and collaboration among water resource engineers and conservation ecologists is needed to improve design and operate water infrastructure efficiently to meet both human and ecosystem needs in a socially acceptable and sustainable way.

EEDS is a framework that can provide a transparent process for operationalizing sustainable water management through integration of socio-environmental objectives in a decision-oriented vulnerability assessment framework. This approach has several strengths. First, it is designed to manage risk of uncertainty and provide guidance to managers and decision makers by focusing on the vulnerability of engineering and ecological indicators to a range of hydrologic futures. It does not rely solely on downscaled GCM projections to assess climate risks but can include a wide range of sources of hydrologic non-stationarity, including historical and palaeoclimate records and modelled land-use change information and changing water allocations.

Second, EEDS represents only a relatively small adjustment to the existing water management decision-making processes. The key change is in assessing ecosystem vulnerabilities equally and early in the design process, so that trade-offs can be identified and addressed accurately in the beginning of the planning process and thus help inform social choices⁵⁵. While engineering objectives of a project may sometimes be perceived as irreconcilable with ecological performance targets, it is possible that strategies for satisfying even modest ecological objectives may improve economic performance of water infrastructure systems, as has been shown with the restoration of coastal wetlands for wave-surge protection^URL: