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atmospheric carbon dioxide (IPCC, 1990).4 Given the greater uncertainty about both the magnitude and the direction of regional precipitation changes, both increases and. decreases in precipitation are frequently modeled. Much of the effort to understand climate has focused on the development of computer models that simulate many of the intricate and Intertwined phenomena that make up the climate. The most complex of these models, GCMs, are detailed, time-dependent, three-dimensional, numerical simulations that include atmospheric motions, heat exchanges, and important land-ocean-ice interactions (IPCC. 1990). Climate models. however. are still simple when compared with the complexities of the real climate system. For Instance, current GCMs handle cloud formation and ocean currents quite primitively, although these are important climatic processes (Ramanathan, 1981). Oceans are generally modeled as simple slabs. and only some of the GCMs take heat transport by currents and circulation into account. In addition. the models use a smoothed topographic profile that precludes an accurate representation of regional orographic effects. Despite these limitations. general circulation models currently provide the best Information available on the response of the atmosphere to Increasing concentrations of greenhouse gases, as well as valuable Insights Into the potential Impacts across broad regions (IPCC, 1990). In theory, GCM estimates of changes In hydrologic variables, such as runoff. could be used directly to estimate changes In water resources (see, for example. USEPA, 1984). In practice. however, GCM-generated hydrologic data suffer from two major limitations. First. the spatial resolution of GCMs Is too coarse to provide hydrologic Information on a scale typically of Interest to hydrologlsts.s Present 4 Regional temperature changes, however, may be higher or lower. 5GCM resolution Is unlikely to dramatically Improve for many years because of the extreme cost of hlgh- speed computer tlme--a factor of two Increase In resolution requires approximately a factor of eight Increase In computer time [Somerville, 1987]. With a typical model resolution of 4.5 degrees latitude by 7.5 degrees longitude and nine vertical layers In the atmosphere, computing one year of weather at 3O-mlnute Intervals takes 10 hours of computer time on a Cray XMP computer--one of the fastest In the world. 6