A variety of techniques are available for studying the hydrologic impacts of climatic change, including stochastic methods that rely primarily on statistical techniques and deterministic or conceptual models that use physically based, mathematical descriptions of hydrologic phenomena. Because climatic changes are expected to alter underlying statistical relationships among variables, stochastic models are probably inappropriate for climate-impact studies. In contrast, deterministic techniques should be more robust so long as the modeled processes (such as percolation, soil-moisture storage, and snow melt) are not expected to change significantly under a carbon dioxide-altered climate. To date, climate-impact studies on the Colorado River basin have been limited to stochastic methods (Revelle and Waggoner, 1983; Stockton and Boggess, 1979). In contrast, this project used a conceptual hydrologic model to study the sensitivity of the basin to greenhouse warming.

The large size of the Colorado basin complicates the development of a hydrologic model. This study used a conceptual model developed and operated by the National Weather Service River Forecasting Service (NWSRFS) in Salt Lake City, Utah (Burnash et al., 1974; Anderson, 1973). This model has advantages and limitations, described in detail in Nash and Gleick (1991), but its success as a forecasting tool provides reason for believing that the model has the capability to simulate the effects of changes in temperature and precipitation. The NWSRFS models the upper Colorado basin as a series of approximately 50 small subbasins that are linked together. In addition, an aggregated model has been developed that divides the entire upper Colorado basin into two elevation zones and uses a limited number of meteorological stations to predict inflow into Lake Powell. In addition to this two-elevation model, we selected three subbasins that were known to make a substantial contribution to basin flow: the White River at Meeker, the East River at Almont, and the Animas River at Durango.

To assess the potential impacts of climatic change on runoff in the Colorado River basin, scenarios of changes in temperature and precipitation were used as inputs into the NWSRFS model. For this study, we relied on purely hypothetical scenarios as well as scenarios derived from the outputs of general circulation models (GCMs). These scenarios are listed in Table 9.1. The baseline data for the model consist of six-hourly data for the years 1949 through 1983, inclusive.

Using the results of the NWSRFS model, we chose a range of plausible runoff scenarios with which to assess the sensitivity of