Both the natural variability of the hydrologic cycle and potential disruptions of that cycle resulting from possible climate change can affect water supply and thus water management in the western United States. Uncertainty about both types of change poses a challenge for water resource managers. At the request of the Bureau of Reclamation, a committee of the Water Science and Technology Board convened a colloquium on November 14–16, 1990, to draw together material on climate change and climate variability and to explore possible water management responses. This proceedings contains an overview of that colloquium, ''Managing Water Resources in the West Under Conditions of Climate Uncertainty,'' and the individual papers presented there.
As discussed in a related National Research Council report, Policy Implications of Greenhouse Warming, increases in atmospheric greenhouse gas concentrations probably will be followed by increases in average temperature. However, we cannot predict how rapidly these changes will occur, how intense they will be, or what regional changes in temperature, precipitation, wind speed, and frost occurrence can be expected. So far, no large or rapid increases in the global average temperature are evident. But if the projections being developed by general circulation models (GCMs) prove to be accurate, the stresses on the planet and its inhabitants would be serious, and substantial responses would be needed (National Research Council, 1991).
Changes in climate—whether brought about by natural variability or by global climate change—would of course have great effects on western water management. Four broad climatic classifications—humid, subhumid, semiarid, and arid—are relevant to western water. Different climatic zones exhibit differences in
differences in streamflow, vegetation, evapotranspiration, variability of precipitation, and other factors important to water management. Some of the most noticeable changes in catchment hydrologic fluxes (rainfall amount, evaporation rate and amount, and streamflow rates) and states (soil moisture depth and distribution in time and space) are found in threshold climates—that is, climates where relatively small changes in vegetative state or meteorological inputs have amplified effects.
One important topic, the hydroclimatology and ecosystems of urban areas, many of which are located in hydrologic threshold regions, has not been included explicitly in the colloquium. The relevant issues for urban areas are those associated with massive importation of water and subsequent changes in the natural hydrologic balance. It is not uncommon for imported water to be the equivalent of about twice the annual rainfall volume. Transfers of this magnitude influence the hydrologic cycle locally at the mesoscale (horizontal distances of 20 to 30 km) and influence the hydroclimatological balance at the export locations.
In an editorial written to mark the beginning of the second quarter century of the journal Water Resources Research, Charles Howe emphasized the role of technology, institutions, and politics in water resource management. He concluded:
. . . [I]t remains true, as it was [25 years ago], that socially responsible decisions require broad public participation, channeled through appropriate institutions. Institutions must change in response to changing public values, and institutional change is costly, but vital. Democracy, unfortunately for some, is messy and costly, but we will be better off pursuing the right goals somewhat inefficiently than pursuing the wrong goal efficiently (Howe, 1990).
It was in the spirit of these observations that the colloquium proceeded. All present—participants from academia, industry, and government—were eager to discuss the implications of possible climate change and to find ways to increase the resilience of our water systems in the face of increased uncertainty.
SHARING WATER RESOURCES
Edith Brown Weiss began the colloquium with a keynote address entitled "Sharing Water Resources with Future Gener-
ations." She presented a theory of intergenerational equity that relates the present generation to past and future generations and that relates the human species to the natural system of which we are a part. Both relationships can be viewed as trusts, in which the present generation is simultaneously a beneficiary and a trustee.
One significant concern is that traditional economic measures may not be sufficient to ensure that the state of the resources left for future generations reflects present diversity and quality. Her concept of intergenerational equity can be applied to many aspects of society but is particularly relevant to water. "If we are not careful today, we can leave our children a huge bill for cleaning up rivers and lakes we have polluted. We can leave our great-grandchildren a nearly irreversible legacy of eroded watersheds, polluted ground water, and contaminated river bottoms. Water is vital. Simple fairness demands that we conserve it for future generations and that we find ways to consider the interest of future generations in the decisions we make today."
The presentations that followed Dr. Brown Weiss's introduction touched a wide spectrum of issues related to climate and climate change, including measurements of relevant variables and the status of modeling these phenomena at the global scale. The colloquium also explored management responses to climate variability and the affected publics, both present and future.
THE SCIENCE OF CLIMATE CHANGE AND CLIMATE VARIABILITY
The late Roger Revelle opened the first session and commented broadly about factors that affect climate. He made a clear distinction between what is considered "weather" and what is considered "climate." Weather, he noted, is the instantaneous condition of the atmosphere at any particular time and place; climate is the average of weather over some time period. Dr. Revelle expressed his belief that there is good reason to expect that the increase of greenhouse gases in the atmosphere will cause climate warming, but that the degree of warming is very difficult to estimate. Whatever the degree, he predicted profound effects on some aspects of water resources—changes in demand for water, changes in supply, and changes in the general circulation of the atmosphere. Beyond these crude predictions, what we are really faced with is a lack of certainty—a lack of understanding—of what may happen.
Robert Dickinson gave the first formal paper, a primer on climate change. He emphasized the extreme importance of the earth's atmosphere to all life and noted that the portion of the atmosphere that principally generates our climate and weather is relatively thin, extending only about 50,000 feet (16,667 meters) above the earth's surface. (To put this in perspective, if the earth is imagined to be a sphere three feet in diameter, this layer is just four hundredths of an inch thick.) Human activity is changing the infrared radiant transmission properties of this life-sustaining layer. Dr. Dickinson gave a clear indication of the complexity (and relative crudeness) of existing and likely future general circulation models (GCMs). During this presentation and the discussion that followed, he noted the need for better hierarchical links with mesoscale precipitation models. Hillslope flow production, which feeds drainage networks, depends on the spatial and temporal distribution of precipitation and evapotranspiration. We are not yet at the state of development where the output generated by GCMs (where generally the smallest grid is on the order of 200 km by 200 km) can be used as input for hydrologic models for predicting activity (such as streamflow, moisture states, and evapotranspiration) at the catchment scale (where the area included is only ten to a few thousand square km). These links are important areas for research and development.
Kevin Trenberth discussed the recent climatological record and emphasized that many of the data series collected for one set of purposes were measured without uniform techniques. Consequently, few, if any, of the data sets are suitable for detecting global warming or cooling signals. Atmospheric patterns, sea surface temperature anomalies, and land-based hydrology are connected, but the linkages are not readily discernable from existing records or helpful for forecasting. Given observations concerning the heterogeneity of records, renewed examination of existing spacetime series of data using some of the tools of modern spatial statistics might show some correlative links.
Malcolm Hughes, David Meko, and Charles Stockton focused on paleo climate information, which scientists use to infer climate states prior to relatively recent direct measurement of climatic variables. The various paleo records indicate that the assumption of weak statistical stationarity (e.g., the average and variance do not change with time) may not be tenable except for relatively short periods. Information may be contained in the various paleo records, but those data may need to be analyzed differently using newer and evolving spatial-temporal statistical tools to reveal any
possible significant signals. This is an area where additional research is needed.
Implications of Climate Variability for Management
Stephen Burges and Bruce Kimball chaired the second session, which covered the specific topics of plant-water-atmospheric trace gas relationships, water sources, water resource management, economics, and water law.
Leon Hartwell Allen examined the implications of global warming on plant growth, looking in particular at trace gas enriched and changed thermal environments. The work has implications for future water use in crop production, catchment hydrology and redistribution of water within a catchment, and with linkages with the mesoscale and GCM-scale weather and climate predictions. The growth of plant biomass has been shown to be linearly related to cumulative evapotranspiration. In the case of soybeans, for instance, a doubling of carbon dioxide concentration with no climate change is likely to increase photosynthesis by about 50 percent, growth by about 40 percent, and seed yields by 30 percent. This is consistent with data on other species. Evapotranspiration rates are likely to increase by 5 percent for each 1°C temperature rise. The work that shows changed biomass and water use has been done at laboratory and relatively small "controlled field" scales. Much work remains to be done to evaluate or estimate such effects at the catchment scale.
Marshall Moss addressed the vexing problem of linkages between catchment-scale hydrology and the larger GCM scale. He emphasized the need for careful use of catchment information to assist in calibrating and validating GCMs. He encouraged everyone to think carefully about the "uncertainty of hydrologic uncertainty." There is a crucial area of research in determining what temporal scales are used to describe hydrologic variability. Decadal average "surrogate streamflow" obtained from paleo records shows that some streamflow patterns derived through GCM modeling are well beyond any historical experience; outputs from hydrologic models that are not in accord with longer term observed patterns in the paleo records are extremely unlikely to happen.
Linda Nash examined alternative possible streamflow scenarios and showed how they could influence storage reservoir states and power production in the Colorado River System. Although there is no way to validate such "what-if" scenarios, they illustrate the
range of what might be experienced. This approach is one of few options open for evaluating the influence of climatic variability on water resource system infrastructure. Analysts using this approach should also consider alternative water uses and demands and alternative flow volume scenarios. Many who are concerned about climate change focus on water shortage. In discussing this paper, Robert Dickinson reiterated the importance of examining both increased and decreased streamflow volume scenarios; if climate change brings a warmer atmosphere, increased runoff might occur in the mountainous West.
John Dracup used the Colorado River system to illustrate issues in managing large-scale water resource systems under a changing climate scenario. He emphasized that the historical record is rich in extremes of high and low flow and much can be learned by studying these extremes. During discussion, it was suggested that much knowledge might be gained about system operations for high flow scenarios by using the extremely large 1983 flood volume as a baseline to which alternative operations might be compared. The potential utility of using historical records to explore complex system operations needs further investigation. The Colorado River illustration shows how improved flexibility in the operation of systems offers opportunities to cope with potential long-term change in annual streamflow volume and temporal distribution patterns.
Kenneth Frederick examined the possible economic implications of climate variability on western water supplies. He indicated that the Bureau of Reclamation will need to play a larger role in facilitating water transfers. Demand management and water marketing are both potentially important tools for building system resiliency for managing water resources during droughts under present climatic variability as well as for accommodating possible long-term reductions in "natural" supply. Another significant issue that ties in with the work presented by Leon Hartwell Allen is the biological consequences of water use and climate variability. Economic assessments of biological resources are primitive at best; work is needed to develop appropriate measures for improved economic analyses.
Dan Tarlock discussed western water law in a climate change context. He believes there are now obvious limits to population growth in the West and these may be affected by climate change, especially if the changes reduce river flow volumes and aquifer recharge. In the spirit of Dr. Brown Weiss's keynote address, Mr. Tarlock also addressed the issue of the rights of future gener-
ations. He noted that there are no legal obligations to future generations—that aquifers can be mined of their water or soil damaged irreparably without consideration for future needs. Consequently, there is no legal incentive to develop conservation strategies or to make efforts to delay use of water into the future. Mr. Tarlock also said that water transfers from farm use to other uses may prove to be one option to mitigate some of the impacts of a changing climate.
John Schaake discussed the role of streamflow forecasting in managing water resources. Improvements in forecasting are being made using new observation schemes and increased computing power. One approach is the National Weather Service's "Extended Streamflow Prediction" (ESP) activity. Most forecasting assumes statistical stationarity in the scenarios used, but Dr. Schaake noted that a fundamental question of science needs to be addressed: when will it be necessary to abandon the use of statistically stationary precipitation sequences in the ESP technique? Improved forecasting will take advantage of newly developed models and use of denser spatial sampling of precipitation, snow cover, water content, and atmospheric temperature. A crucial concern is for deployment of sufficient measurement stations to take advantage of data handling and modeling capabilities. He also noted that it is important to maintain the existing network of long-term hydrologic measurement stations because, while imperfect, they provide the best available data base for comparison purposes.
MANAGEMENT RESPONSES TO CLIMATE VARIABILITY
Gilbert White chaired the third session, which focused on the types of management responses possible to cope with climate variability. He noted that there are many serious resource management concerns, and that in the long run the maintenance of soil and vegetation worldwide could actually be more important than any increase in atmospheric trace gases. He reminded participants that loss of soil and vegetation is an ongoing and extremely serious problem whether or not there is any climate change. When considering climatic issues and management strategies, soil and vegetation preservation should be a paramount concern.
John Keane discussed water management in Arizona's Salt and Verde rivers. The region has a long history of susceptibility to climate variability and thus has lessons to offer related to climate change. This arid region—once dominated by an Indian culture
that disappeared, possibly because of unreliability of water sources—relies on winter generated river flow that is susceptible to warmer atmospheric conditions. The existing water management system of six dams and ground water storage was designed to overcome the region's natural variability.
The Salt-Verde system has been managed to take advantage of the presence of large aquifers by recharging (and pumping) for prolonged periods. This opens up the possibility of joint operation with recharge on the order of years during wet periods and pumping for years during prolonged dry periods. This concept is now known as "cyclic storage" (see, for example, Lettenmaier and Burges, 1982) and may be of considerable importance for making the best use of surface and ground water supplies in arid and semiarid environments where the present climate is naturally variable. Water stored in the ground is a relatively well-known quantity. Water that is to be delivered by precipitation is much less certain. Legal restrictions in the region create difficulties for implementing extensive artificial recharge.
Dale Bucks described present and possible improvements in agricultural water management. He reiterated the fact that agriculture is influenced by both water supply and trace gases. Furthermore, he estimated that agricultural practice contributes approximately 26 percent of the trace gases annually to the atmosphere. He emphasized the need for increased understanding of process hydrology to determine the effects of climatic variability or changes in variability. Increased atmospheric carbon dioxide concentrations can increase plant production provided that there is adequate water. The combined uncertainties presented by the natural variability of western water regimes and the possible consequences of climate change are cause for improved water conservation and management of irrigated agriculture at all levels, from principal supplier to end user.
Daniel Sheer emphasized the need for a renewed examination of how water resource systems are operated. He noted that present natural variability is large, while the rate of long-term climate change is small relative to natural variability. Thus he believes that many factors affect the short-term needs of system operations more than climate change. In considering how water systems are operated, the goal is to have the best possible mix of benefits; unfortunately, there is no clear agreement on what array of benefits to consider. One often forgotten objective concerning urban water supplies is the paramount need for water for fire protection, the basis for many of the water systems initially installed in the United States.
Computer simulations of system behavior under different constraints provides a way to examine how changed operations may be beneficial. Dynamic computer-generated illustrations of water movement throughout a system are an effective way to communicate these possibilities. Such simulations will show in what locations it may be effective to modify existing law to build in resiliency against the effects of uncertain supplies as demand levels increase.
Wayne Marchant and Arnett Dennis discussed weather modification as a possible means of adapting to climate variability. The U.S. Bureau of Reclamation and other organizations have been involved in cloud seeding research for almost 30 years. The greatest potential benefits occur in threshold hydrologic environments where an increase in precipitation goes largely to streamflow production rather than to soil moisture retention. Drs. Marchant and Dennis reported on planned activities for precipitation augmentation in the 13,000 square miles of the upper Colorado basin (largely about 9000 feet elevation), from which most of the river flow is produced. Although doubt remains concerning the benefits of cloud seeding, there is sufficient potential that the opportunity cannot be ignored.
PUBLIC INVOLVEMENT IN WATER RESOURCE DECISIONMAKING
The colloquium concluded with a panel discussion moderated by Helen Ingram. The participants were Jim Carrier, Jim Dyer, and Roger Kasperson. Participants focused on the public's understanding of climate variability and change. Several themes emerged during the panel discussion. One was that the print media have a mission to report news and that they do not have an obligation to build public consensus. Balanced coverage of scientific issues is difficult to achieve because particular individuals may be more quotable and persuasive than others. Other experts with valid information may not convey that information effectively to reporters. Examination of the media's coverage of climate change indicates that most environmental organizations have given increased attention to global climate change since 1986. Although greenhouse warming is of concern to environmental organizations, it was still relatively low on a 1990 list of public concerns. Another issue raised is how climate change might influence life-style. Perhaps the most obvious action will be the need for personal use
of water-saving technologies. A personal and community sense of supply-cost relationships will be needed before people will be willing to take actions that reflect avoidable costs.
MANAGING WESTERN WATER UNDER PRESENT VARIABILITY AND FOR AN UNCERTAIN FUTURE
The purpose of the colloquium was to provide a compilation of knowledge for those involved with water management in the West to use to build research and development activities and foster institutional capabilities to handle the multiple facets of climate variability. Many issues merit consideration.
Natural variability in hydrologic processes is all-pervasive. Consequently, resource management allocation decisions are made in an inherently uncertain, and thus risky, setting. Present methods address variability from a statistically stationary perspective. Possible changes in variability and in the level of hydrologic states and fluxes that might accompany climate change must be addressed by nonstationary statistical methods. Unfortunately, there is no basis for doing this in other than a "what-if" manner. The issue is truly "trans-scientific;" that is, there are crucial questions of science involved but the present tools and methods of science are not adequate for answering them (Weinberg, 1972).
Hydrologic variability and its influence on water management was a significant component of many of the papers. In one discussion, Stephen Burges demonstrated examples of different forms of variability from a time series of tree ring data and an annual streamflow volume record from a hydrologic regime sensitive to climatic variations. Based on measures of persistence from the flow record, he demonstrated what that means in terms of supply reliability for a reservoir of fixed capacity, or alternatively how large a reservoir is needed to meet various contracted supply amounts. High reliabilities for large demands can only be attained with impracticably large reservoir capacities. There is little doubt that institutional arrangements for alternatives of supply and demand management must be explored in light of such information.
Research, Development, and Institutional Issues
Despite the difficulties posed by attempting to describe an unknown future, much could be achieved by incorporating what
is known about present day variability into water resource management and allocation. Specific observations made by colloquium participants indicate that research, development, and institutional attention should be given to the following issues:
General circulation models (GCMs) should be linked with process level hydrologic representations at the catchment scale. GCMs need to be coupled to models capable of representing hydrologic interactions at scales on the order of a few, to tens, to hundreds and thousands of square kilometers. Modeling present and recent past climate in this context would ensure that the GCM representations are plausible.
Establishment and augmentation of benchmark monitoring networks is an important consideration. Environmental monitoring is conducted throughout the United States and the West but these efforts are not designed to be a sampling network to detect and monitor global climate change. The existing network of climatological, stream gaging, and water quality stations are largely unsuitable for making other than general estimates of what may have happened over a relatively recent period. The limited benchmark stream gaging network operated by the U.S. Geological Survey provides a good model for one component of a broader monitoring program.
Measurements of plant production in carbon dioxide enriched environments should be translated upwards from the laboratory and small field scales. Much work needs to be done for a range of catchment scales to determine the linkages between precipitation, temperature, evapotranspiration, and water yield before alternative water demand and supply scenarios can be considered reliable. Such work needs to be explored in threshold climatic regions where small perturbations could make substantial changes in the local vegetation and temporal water distribution pattern.
Historical streamflow and surrogate records should be examined for their possible use in validating the scenarios created with increasingly refined GCMs. Historical streamflow and surrogate records should be examined for their possible use in validating the scenarios being generated by increasingly refined GCMs. Hydrologic measures have been and continue to be made at the catchment scale, while GCM outputs (estimates of precipitation, evaporation, and runoff) are on a vastly larger scale. Much work needs to be done to determine how to disaggregate GCM outputs to the catchment scale. Also, since hydrologic measurements are of short duration relative to climate change influences, paleo records can
act as surrogate hydrological information for longer time periods. There is information in recent hydrologic and climatologic records as well as in paleo records that could be used to calibrate GCMs to make them more compatible with the variable hydrologies of the many catchments typically contained within a GCM cell.
Opportunities exist to improve water resource system operation. To develop improved economic measures, work is needed to include biologic resources in economic benefit evaluations. When considering alternative operation policies for multiple water resource systems, flexibility is a crucial consideration. For example, in the Colorado River system it is unclear if there will be increased or decreased flow and how the flow patterns would be distributed differently in space and time throughout the system, even if the vegetation remained relatively constant. This suggests that many institutional questions regarding mechanisms for using varying amounts of water differently (depending on shortage or excess supply situations) should be examined.
Improved streamflow volume forecasts for water resource system operations are needed. If within-year streamflow variability changes and there is no change in the annual average flow volumes at major locations within a river basin, water managers will need improved methods to forecast near-to intermediate-term streamflow volumes. This need argues strongly for redesigning present river forecast systems to take advantage of present hydrologic and environmental monitoring systems. Such systems must include greatly improved measurements of precipitation (spatial density of measurements) and averaged direct measurements of evapotranspiration to permit closer estimates of the water budget for a given catchment.
Many promising technologies exist that may help reduce uncertainty in water resource supplies. Technologies to increase the resilience of water systems to variability should be evaluated for their utility. Precipitation enhancement, for instance, might prove useful in locations where there would be benefits from maintaining streamflow patterns similar to those that have been experienced during the period of record.
The concept of safe yield engenders a false sense of security. This concept is particularly frail in reduced water availability scenarios; any mechanism that builds in the understanding that shortages will occur is desirable. This may be a key consideration for Bureau of Reclamation staff and members of the multiple publics who will need to confront water allocation policy issues.
Opportunities for the Bureau of Reclamation
Many people consider the Bureau of Reclamation to be the West's primary water resource management agency. As such, the bureau could play a key role in facilitating the region's adaptation to future changes in water supply and demand. Any impediments to reallocating supplies need to be reduced to permit flexible response to changing conditions. The prospect of climate change adds to present uncertainties associated with supply and demand but it does not alter the need to develop flexible and efficient water management. Climate change impacts, if they occur, are uncertain, they are likely to occur slowly and to be relatively small in comparison to natural variability. Thus management flexibility will be critical regardless of the causes of changes in climate. Three institutional issues arose repeatedly during the colloquium that might help provide needed management flexibility:
Voluntary transfers of federally supplied water might provide temporary response for shorter-term phenomena such as severe drought or permanent responses to supply and demand.
Water users, principally farmers, can be active partners in efforts to improve water management if provided with incentives to adopt alternative, efficient water use practices.
Innovative uses of federal facilities, such as making federal storage and distribution facilities available for water transfers even if federal water is not involved, should be evaluated carefully.
To be better prepared to deal with the uncertainties offered by possible changes in climate, water managers should be working now to increase the resilience of existing water systems to normal climatic variability. This flexibility would pay off in the short term when periods of especially wet or dry weather affect water availability. It would also pay dividends over the long term should climate change occur. There appear to be many opportunities to modify how water is managed in the western United States to provide for robust institutional settings that can accommodate hydro-climatic variability and, in the longer term, possible climatic change. Conservation, increased water use efficiency, marginal cost pricing of water, water transfers, alternative uses of existing infrastructure, and explicit consideration of present day variability in determining system supply reliabilities are but a few of the topics discussed in the papers in this proceedings. No single actor
can implement a completely effective water management system. Close cooperation among federal, state, and private suppliers and users is essential.
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