The susceptibility of society to the harmful effects of hydrologic variability and the implications of climate variability and change necessitate a reassessment of the water planning and management principles developed in the 20th century. Significant changes in many key hydrologic design variables (including the quantity and quality of water) and hydrologic extremes are being experienced around the Nation. Paleoclimate analyses and climate projections suggest persistent droughts and wet periods over the continental United States that are longer, cover more area, and are more intense than what was experienced in the 20th century. An evolving future, which can only be partially anticipated, adds to this risk. Furthermore, while hydroclimatic extremes are projected to increase in frequency, accurate predictions of changes in extremes at a particular location are not yet possible. Instead, climate projections provide a glimpse of possible future conditions and help to scope the plausible range of changes.
A central challenge to water planning and management is learning to plan for plausible future climate conditions that are wider in range than those experienced in the past (see Figure 3.3) (see also Ch. 28: Adaptation, KM 5). Doing so requires approaches that evaluate plans over many possible futures instead of just one, incorporate real-time monitoring and forecast products to better manage extremes when they occur, and update policies and engineering principles with the best available geoscience-based understanding of global change. The challenge is both scientific, in terms of developing and evaluating these approaches, and institutional–political, in terms of updating the regulatory–legal and institutional structures that constrain innovation in water management, planning, and infrastructure design.
One approach is to focus on better managing variability, which is likely the dominant source of operational uncertainty for many water systems.115 An example of this approach is incorporating monitoring of current conditions and forecasts of near-term future conditions (days to weeks to seasons) in lieu of stationary operating rules based on historical expectations. Forecasts of near-term hydrologic conditions can provide the basis for adaptive reservoir operations, but they require flexible operating rules. New York City, for example, altered existing operational guidelines to implement adaptive reservoir operations based on current hydrologic conditions to better meet new concerns for ecological flow requirements in addition to water supply goals.116 In another example, the International Joint Commission adopted a new operating plan for Upper Great Lakes water levels; the plan is based on the ability to provide acceptable performance, as defined by stakeholders, over thousands of possible future climates.117 The plan includes forecast-based operations and a funded adaptive management process linking observatories and information systems to water-release decisions to address unanticipated change.118 In addition, updating operations and optimizing for changing conditions as they occur provide additional operating flexibility for water supply, flood risk reduction, and hydropower reservoirs.119,120,121 Finally, financial instruments and water trading provide avenues for managing the effects of variability on water competition, especially between urban water supply and agricultural water use.122,123,124
Better management of variability does not eliminate the need for long-term planning that responds to plausible climate changes (see Figure 3.3). Major water utilities provide examples of planning that focus on identifying and managing vulnerabilities to a wide range of uncertain future conditions, rather than evaluating performance for a single future.125 For example, Tampa Bay Water employed 1,000 realizations of future demand and future supply to evaluate their preparedness for future conditions.126 Alternatively, Denver Water used a small set of carefully selected future climate and socioeconomic development scenarios to explore possible future vulnerabilities.125 The World Bank published a set of specific guidelines for implementing such robustness-based approaches in water investment evaluation.127 As described in Key Message 2, the nature of hydrologic extremes and their rarity complicate the detection of meaningful trends in flood risk,128 while traditional trend detection methods may lead to missed trends and underpreparation.129 In response to these challenges, the U.S. Army Corps of Engineers is exploring robustness to a wide range of trends and expected regret as metrics for evaluating flood management strategies,130,131 including the increased incorporation of natural infrastructure.132
Actions taken by communities and the managers of water systems of all sizes can help prepare the Nation for the water-related risks of climate variability and change. The risks associated with a changing climate are compounded by inadequate attention to the state of water infrastructure and insufficient maintenance. Developing new water management and planning approaches may require updating the regulatory, legal, and institutional structures that constrain innovation in water management, community planning, and infrastructure design.133,134 Furthermore, adequate maintenance and sufficient funding to monitor, maintain, and adapt water policy and infrastructure would help overcome many of these challenges. Continued collaboration on transboundary watershed coordination and agreements on both surface water and groundwater with Canada and Mexico are among the actions that could facilitate more sustainable binational water management practices.
Developing and implementing new approaches pose special challenges for smaller, rural, and other communities with limited financial and technical resources. The development and adoption of new approaches can be facilitated by assessments that compare the effectiveness of new management and planning approaches across regions; greater exchange of emerging expertise among water managers; and better conveyance of the underlying climate and water science to communities, managers, and other decision-makers.135,136