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  • The Influence of Climate Change and Climatic Variability on the Hydrologie Regime and Water Resources (Proceedings of the Vancouver Symposium, August 1987). IAHSPubl. no. 168, 1987.

    Global climatic changes and regional hydrology: impacts and responses

    Peter H. Gleick Energy and Resources Group University of California Berkeley , California, USA

    ABSTRACT As the atmospheric concentration of carbon dioxide and other trace gases increases, changes in global and regional climatic conditions will lead to a wide range of hydrologie impacts, including changes in the timing and magnitude of runoff and soil moisture. These hydrologie changes, in turn, will result in diverse economic, social, and political consequences.

    The nature of the regional hydrologie effects depends on changes in the climatic conditions and the water- resource characteristics of the region. The research conducted to date has identified a wide range of potential problems—as well as some possible advantages—that might result from plausible changes in climate estimated by state-of-the art general circulation models.

    These hydrologie changes fall into a series of distinct categories, including: changes in the timing of water availability; changes in the magnitude of water availabi- lity; changes in the hydrologie variability; and effects on water quality. Similarly, diverse societal responses to the hydrologie changes are available, including adaptation, mitigation, and prevention. Each of these responses depends on the quality of the information available on future impacts and on the perceived importance of the effects. This paper discusses the extent and character of hydrologie changes that could result from global climatic changes, together with the options available for hydrolo- gists and water planners.

    Introduction

    Growing attention is being paid to climatic changes that may result from increasing atmospheric concentration of carbon dioxide and other trace gases. While the direct effects of changes in climatic conditions can be severe—as can be seen by the effects of existing climatic variability—we must also pay attention to the wide range of indirect effects, such as changes in agricultural productivity, changes in sea-level, and changes in water resources. This latter category is one of the most important and yet least well-understood consequence of future changes in climate. Hydrologie impacts may include major alterations in the timing and magnitude of surface runoff and soil-moisture availability, and changes in the quality of freshwater resources. Associated with these effects will be a wide

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  • 390 P.H. Gleick

    range of economic, environmental, and societal impacts» This paper discusses the likely extent and character of important hydrologie changes that could result from global climatic changes, together with the options available to hydrologists and planners for dealing with the most severe impacts»

    The limited research conducted to date has identified a wide range of potential problems—as well as possible advantages—that might result from plausible changes in climate» These hydrologie changes fall into distinct categories, including: changes in the timing and magnitude of water availability; changes in the frequency and severity of severe events, and effects on water quality» Similarly, diverse societal responses to the hydrologie changes are possible, including adaptation, mitigation, and prevention» Each of these responses depends on the quality of the information available about future impacts and on the perceived importance of the effects»

    Future climatic changes

    Despite the fact that hydrologists need accurate information on climatic means and variability in order to develop appropriate water- resource designs and rules of operation, details of future climatic conditions cannot yet be predicted with any high degree of confidence» The principal reasons for this inability to clearly identify future climatic changes are the complexities of the ocean-atmosphere-land interactions, the difficulties of developing satisfactory computer models to reproduce these interactions, and uncertainties about our actions that affect climatic conditions»

    The problem is that, at present, while there are many ways in which climate may be affected by human actions, we are unable to see clearly either the direction of future climatic changes or nature of their societal impacts» Because we are unable to "do the experiment" directly, we must attempt to model climate and climatic changes—an imprecise alternative because of the complexity of the global climate system» Much of the effort of trying to understand the atmospheric system has focused on the development of large-scale computer models of the many intricate and intertwined phenomena that make up the climate» The most complex of these models - general circulation models (GCMs) - are detailed, time-dependent, three-dimensional numerical simulations that include atmospheric motions, heat exchanges and important land-ocean-ice interactions (see, Manabe 1969a, 1969b; Schlesinger and Gates 1980; Manabe and Stouffer 1980; Wetherald and Manabe 1981; Ramanathan 1981; Manabe et al» 1981; Hansen et al» 1983, 1984; Washington and Meehl 1983, 1984)»

    GCMs permit us to begin to evaluate some of the implications for global climatic patterns of increasing concentrations of radiatively- active atmospheric gases» While many uncertainties remain, a consensus is now beginning to form about the direction and magnitude of certain major impacts, such as increases in global-average temperatures and changes in the intensity and distribution of the global hydrologie cycle»

    Unfortunately, state-of-the-art general circulation models are large and expensive to operate» Furthermore, while GCMs are invaluab- le for identifying some climatic sensitivities and changes in global

  • Effects of climatic change on regional hydrology 391

    climatic characteristics, they have two limitations that reduce their value to researchers interested in more detailed assessments of water resources: (1) they are unable to provide much detail on regional or local impacts, and (2) they are unable to provide much detail on small-scale surface hydrology. Until our ability to model climate improves, we must use other methods to either enhance the information available from GCMs or provide insights now unavailable from them.

    Plausible future hydrologie changes

    The attention focused on large-scale GCMs in recent years results in large part from their relative sophistication compared to other models. Yet this attention has also highlighted the need for new methods of hydrologie assessment. Recently there have been some serious efforts to evaluate the regional hydrologie implications of climatic changes (Schwarz 1977; Stockton and Boggess 1979, Nemec and Schaake 1982; Revelle and Waggoner 1983; Flaschka 1984; U.S. Environ- mental Protection Agency 1984, Cohen 1986, Gleick 1985, 1986a,b, 1987c). These works have provided the first evidence that relatively small changes in regional precipitation and évapotranspiration patterns might result in significant changes in regional water availability.

    Methods for evaluating the hydrologie impacts of climatic changes include using historical data to evaluate the effects of past fluctuations in precipitation and temperature on runoff and soil moisture; determining the sensitivity of runoff and soil moisture to hypothetical changes in the magnitude and timing of precipitation and temperature; and incorporating regionally disaggregated changes in temperature and precipitation predicted by GCMs into more accurate regional hydrologie models. While none of these methods - individual- ly - can provide much reliable information on future changes, each can provide insights into specific hydrologie vulnerabilities to climatic change.

    Future hydrologie changes: what can we expect?

    Changes in climate may cause changes in a variety of hydrologie variables, including the timing, location, duration, and extent of precipitation, runoff, soil moisture, and extreme events. These impacts can be categorized in a variety of different ways. One useful method, shown in Table 1, is to separate the impacts by the spatial and temporal scales involved, with additional separation for the different statistical moments of interest and the distinction between political and geophysical boundaries. In the following sections, the most plausible and worrisome changes in water availability are described. These changes are not the only hydrologie effects that will occur, and not all of these will occur at any one place or at any one time. Nevertheless, we should pay particular attention to these impacts because they are more likely to occur, they are harder to mitigate, and they may be more disruptive than other climatic effects.

  • 392 P.H. Gleick

    Table 1 Hydrologie effects of climatic change

    Hydrologie Variable of Interest

    Useful Precipitation Surface Runoff Available Soil Moisture Groundwater Temperature Monsoonality (Onset, Ending, Intensity, Location) Storm Events

    Temporal Scale of Interest

    Long-Term (greater than annual) Annual Seasonal (two to six months) Monthly Daily

    Spatial Scale of Interest (Political)

    Global 10® km2

    Continental 10^ km2

    Country/Region 10° km2

    Local 10J - 10s km2

    Spatial Scale of Interest (Hydrologie)

    Global 10® km2

    Continental 10^ km2

    Regional 10^ - 10° km2

    Watershe

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