Co-Conveners:

Patrick Mulholland, Oak Ridge National Laboratory

Katherine Jacobs, Arizona Department of Water Resources

Rapporteur:

Kenneth Strzepek, University of Colorado

Scott Chaplin, Rocky Mountain Institute

Archivists:

Michael Sale, Oak Ridge National Laboratory

Eugene Stakhiv, U.S. Army Corps of Engineers

Water resources are fundamental to almost all environmental and socioeconomic activities. The response of water resources to climate variability and change has important ramifications for all other sectors. Likewise, environmental and human responses to climate variability and change will have important effects on water resources. Thus, water resources can be viewed as the key integrating component in comprehensive assessments of effects of climate variability and change.

Water resource implications associated with climate change are complex, and vary substantially between regions, with the watershed or river basin being the primary response unit for surface waters and the groundwater basin the primary response unit for deep aquifers. Consequently, assessments of effects of climate on water resources require detailed information and analysis at local and regional scales. The issues that face each region of the U.S. are unique; it is difficult to assess climate stresses at the national level. For example, in some coastal areas, the primary water resource issue is salt water intrusion. Groundwater contamination, loss of riparian habitat and aquifer subsidence are of great concern in interior areas where groundwater is the primary supply for human activities. Managers of surface water systems must prepare for the extremes of drought and flooding. It is important that the geographic differences in water resource issues, and the complexity of the overlapping institutional jurisdictions be recognized in the context of the national assessment.

High Priority Issues and Questions:

High priority issues and questions with regard to water resources investigations include: (1) large uncertainty in future climate scenarios at the regional and subregional scales, and need for predictive tools; (2) large uncertainties in evapotranspiration and human demand responses to changes in climate; (3) uncertainties concerning the ability of water resources management institutions to respond to climate changes; (4) aging water resources management infrastructure; and (5) water quality stresses that may accompany increases in climate extremes and lack of sufficient monitoring to identify episodic changes in water quality.

Water resources modeling capabilities at the river basin scale are generally good, but these models require accurate climate data as forcing functions (spatial and temporal patterns in temperature and precipitation). Assessments of effects of climate variability and change are limited more by uncertainty in climate forcing functions than by inadequate hydrologic or management models. In the case of precipitation scenarios, the uncertainty in future climate extends both to changes in mean conditions and changes in the size and frequency of extremes, such as large rainfall events and prolonged periods with little or no rainfall. These climate uncertainties make detailed assessments of future climate effects on water resources very difficult at this time.

There have been increases in annual precipitation and in the severity of extreme events (both floods and droughts) in many regions of the U.S. in recent decades. These changes in climate characteristics provide an important opportunity for retrospective analyses of water resources response to climate variability and change. This opportunity should be exploited more fully.

Large uncertainties in relationships between ecosystem/human water use and climate variability and change also limit our ability to perform assessments of water resources under future climates. In particular, the effects of clouds on evapotranspiration rates, and changes in cloudiness, vegetation type, and plant water use under a warmer climate with higher CO₂ concentrations are uncertain. These uncertainties are important limitations in our understanding of hydrologic responses to future climate scenarios. Important limitations of water resources management models involve uncertainties in the interactive effects of changes in climate and socioeconomic factors on future municipal, agricultural, and industrial demands for water. For example, the interactions between population growth, development and implementation of water conserving or demanding technologies, and government regulations under a changing climate will have important but uncertain effects on future water demands. There is a need to link climate, hydrologic, economic and social models to achieve a more reliable and multidisciplinary understanding of the range of possible effects of climate change.

The lack of connection between land use planning and water resource considerations increases the vulnerability of human populations and ecosystems to climatic extremes. Decisions made by individuals and institutions often increase the likelihood of weather-related disasters. Environmental values such as protection of endangered species and fragile ecosystems are in competition with recreational pressure and urban development. Water rights, especially in the west, are experiencing new stresses as Native American water rights are quantified and habitat protection requires larger quantities of water. These conditions may be exacerbated by climate change.

Wise management of water resources given future climate uncertainties requires institutional flexibility and coordination. An important issue in water resources management is whether sufficient flexibility and coordination exist at the river basin or groundwater basin scales. This is particularly problematic in regions such as the east where many small agencies have jurisdiction over limited areas and specific purposes or where water resources are largely unmanaged. It is rare for hydrologic boundaries to be coterminous with political boundaries, and transboundary water issues occur at the local, regional, state and international level. We lack a good understanding of how the collective water management system will respond to the interactive stresses of changes in climate and socioeconomic factors at local and regional levels. We also lack a good understanding of how climate variability and change will alter the demands for instream uses of water associated with habitat protection for species of particular concern. Higher summer temperatures and/or more severe summer droughts may increase instream water demands for species protection in some regions, but warmer winter temperatures or higher flows may reduce instream demands in others.

Aging water resources infrastructure will stress the ability of water managers to meet future demands for water supply and flood management if climate changes rapidly or dramatically. An important issue is the level of vulnerability in particular regions or river basins for a given a range of climate scenarios. Water resource managers need to have an understanding of the degree of risk associated with identifiable climate conditions. A related issue is whether there is adequate communication of updated hydrological response analyses and climate change scenarios allowing for sufficient lead time to upgrade or construct new infrastructure. There may be a need to reassess the underlying assumptions that our infrastructure and hydrologic models are based on, to ensure that we adapt to changes in our environment incrementally, rather than waiting for a crisis to ask the difficult questions that lead to innovation and better management.

Water quality problems are important stresses on water resources in many areas. Although improvements in wastewater treatment spurred by government regulations have resulted in reductions in point source loadings in recent years, runoff from urban and agricultural lands continues to be a serious water quality problem. Increases in hydrologic extremes may exacerbate water quality problems via greater erosion and runoff of contaminants from urban and agricultural areas under more intensive rainfall events and reduced dilution of point source effluents during more severe droughts, particularly in summer. Further, current water quality monitoring programs are generally inadequate to determine relationships between climate variability and water quality response for many land use types and to identify changes in water quality resulting from future changes in climate. Monitoring is an extremely important element in improving our understanding of the interactive effects of climate and human activities on water quality, interactions that are likely to become more important if climate variability increases.

Coping Strategies:

In terms of water availability, the water management system is central to strategies for coping with climate variation and change. While technical advances that improve the efficiency of water use and the effectiveness of water management infrastructure will play an important role in coping with climate change, the most beneficial coping responses will likely come from "soft" strategies involving institutional adjustments. Additional institutional flexibility to respond to climate change can be developed through: (1) facilitating water banking and water transfers; (2) creative storage options including conjunctive management of groundwater and surface water; (3) conservation efforts that are targeted to respond to specific water resource conditions; (4) improved integration of water quality and water quantity management across federal, state and local jurisdictions; (5) resolution of Indian water rights claims through negotiated settlements; and (6) taking a longer-term view in water resource planning. Creative regulatory reform to integrate the objectives of the Clean Water Act, the Safe Drinking Water Act, the Endangered Species Act and others could significantly increase the flexibility of the water management system. Water conservation and land use management plans (particularly for floodplains) that account for and make use of technology advances and socioeconomic trends may be among the most important elements of water management strategies to cope with changes in climate. Among the "hard" strategies, greater connectivity of infrastructure and revisions to infrastructure sizing requirements based on near past and projected future hydrologic responses are also important coping responses.

The ability to maintain or improve water quality with climate change will require additional storm water management efforts in urban areas and new chemical application and runoff management strategies in agricultural areas. A better understanding of the relationship between water quality, ecosystem effects and human health is needed. More comprehensive land use management strategies, particularly involving riparian areas, will help in coping with increases in hydrologic extremes. Continued reductions in industrial and municipal effluent discharges using improved technology and waste stream controls will help to mitigate adverse effects on water quality of lower river flows or longer periods of low flow, particularly in summer. Finally, maintaining and even expanding water quality monitoring activities will help to identify deteriorating trends related to climate that then can be addressed with targeted management strategies.

This is a truly critical time to be developing the baseline data necessary to respond to climatic change. Yet, basic monitoring of precipitation, snowpack, stream flow, groundwater levels, and water quality is currently being cut back in many areas due to loss of funding. Given the importance of the climate change issue, additional monitoring and research funding is needed. It is also important to integrate data bases so that data that do exist are accessible and can be analyzed.

Ideas about the Water Resources Assessment:

An assessment approach that uses a small number (3 or 4) of climate scenarios that span the range of plausible future climates for individual regions is recommended. Regional down scaling of coupled atmosphere-ocean GCMs can now provide such a set of plausible, alternative climate scenarios for use with river basin hydrologic models to produce a range of potential responses. The analysis of water resources response should then focus on the resiliency of environmental and management systems to the alternative scenarios, identifying the most problematic or uncertain scenario-response couplings for more detailed analysis and future scientific study to reduce uncertainties.

The assessment should also attempt to make better use of retrospective analyses of responses of water resources (including physical characteristics and management actions) to climate variability and change in the recent past. Although the retrospective approach is limited in terms of defining responses to the full range of future climate characteristics, it nonetheless provides an indication of system sensitivity and response direction and should be used where sufficient time series data on climate and water resources response exist.

Finally, education is paramount in the assessment effort. Consideration of strategies to improve communication of the vulnerabilities and uncertainties between scientists and water resource managers, industry representatives, public officials, and the public at large will greatly aid development of a consensus on many issues involving adaptation and mitigation.