Significant gaps remain in the knowledge base that informs both projections of climate change impacts on water resources and governance strategies that can curb demand and build adaptive capacity of water systems. Critical research needs include the following.
Improved projections of changes in the water cycle at regional and seasonal time scales. Because water most directly affects society at the watershed or regional level, improved regional-scale projections of changes in precipitation, soil moisture, runoff, and groundwater availability on seasonal to multidecadal time scales are needed to inform water management and planning decisions, especially decisions related to long-term infrastructure investments. Likewise, projections of changes in the frequency and intensity of severe storms, floods, and droughts are critical both for water management planning and for adapting the natural and human systems that depend on water resources. This will require new multiscale modeling approaches, such as nesting cloud-resolving climate models into regional weather models and then coupling these models to land surface models that are capable of simulating the hydrologic cycle, vegetation, multiple soil layers, ground water, and stream flow. These models will also need to reliably project changes in storm paths and modes of regional climate variability.
Long-term observations for measuring and predicting hydrologic changes and planning management responses. Improved physical observations are needed to monitor the impacts of climate change on water systems and to support model development and adaptation planning. Improved observations would also improve short-term hydrological forecasts. New technologies are needed to allow continuous high-precision measurements of inventories and fluxes of water, including precipitation, groundwater, soil moisture, snow, evapotranspiration, and stream flow. Time-series data related to human demographics, economic trends, vulnerabilities to changes in water quantity and quality, and human exposures and sensitivities to water contamination are also important, and should be made available in an integrated framework with physical observations to support integrated analysis and decision making.
Improved tools and approaches for decision making under uncertainty and complexity. Water resource managers are faced with making many important and complex decisions under uncertainty. To support more robust and effective decisions and strategies, further advances are needed in ensemble and integrated approaches to modeling, scenario building and comparison, and identification of no-regrets options. To improve the use and usability of climate knowledge in decision making, research is also needed on effective decision-support tools, such as forecasts, climate services, and methods for making complex trade-offs under uncertainty (see Chapter 4 for additional details).
Impacts of climate change on diverse water uses. Climate change will affect many water-related activities and sectors, including navigation, recreation, tourism, human health, drinking water, agriculture, hydroelectric power generation, and the ecological integrity of aquatic and terrestrial ecosystems. Continued and expanded research in all of these areas, and on the economics of water supply, demand, and costs of adaptation, is needed across and between different water-dependent sectors. The potential for local, state, and international disputes over water resources is also an area where further study is warranted (see Chapter 16). Another need is for better understanding of how institutions and behavior shape vulnerability and offer opportunities to adapt to changing water regimes.
Develop vulnerability assessments and integrative management approaches to respond effectively to changes in water resources. Changes in water resources are anticipated to affect coupled human-environment systems in a variety of ways and in interaction with many other environmental stresses. Assessing which water supplies and human-environment systems are most vulnerable to climate change will require analysis of place-based environmental conditions as well as social conditions and management needs. Frameworks need to be developed and tested for such assessments, and new integrative water resource management and adaptation approaches are needed for managing water in the context of climate change. Finally, the effects of actions taken to limit the magnitude of climate change (or adapt to other impacts)on water resources need to be more systematically assessed and accounted for in climate-related decision making.
Increase understanding of water institutions and governance, and design effective systems for the future. Water institutions of the future will have to deal with the complexity of multiple and interacting stresses as well as equity and economic issues related to water use. Reconciling water entitlements across different water systems, making water systems more flexible in the face of change, and shaping an institutional environment that encourages water conservation and reuse are only some of the challenges facing water resource institutions as climate change progresses. To improve our ability to design and deploy water institutions, more research is needed on governance mechanisms such as water markets, public-private partnerships, and community-based management. Evaluation of legacy effects of past infrastructure and management decisions will assist in understanding path-dependent effects, but only to the extent that such lessons are relevant to constantly evolving conditions.
Improve water engineering and technologies. Many water management systems are currently constrained by existing water infrastructures, many of which are old and need replacement. Thus, attention needs to be given to the development and implementation of more efficient water delivery systems. New technologies for water storage, supply, treatment, and recycling will also be needed, as will more efficient residential, commercial, and agricultural end-use technologies.
Evaluate effects of water resource use on climate. Changes in land and water use affect local and regional climate through effects on land-atmosphere interaction, particularly changes in evapotranspiration. The role of ecosystems in recycling precipitation, influencing stream flow, and mitigating droughts is particularly important. Improving our understanding of the effects of water and land use on regional climate will be an important component of developing local and regional integrated climate change responses.
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