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Past studies of the hydrologic impacts of climatic change can be divided into two categories: (1) stochastic methods that rely primarily on statistical techniques for evaluating the hydrologic characteristics of a region or for extending the existing hydrologic record (such as Schwarz [1977], Revelle and Waggoner [1983], and Stockton and Boggess [1979]); and (2) deterministic or conceptual methods that use physically based, mathematical descriptions of hydrologic phenomena (Nemec and Schaake, 1982; Gleick, 1986, 1987a,b; Mather and Feddema, 1986; Cohen, 1986; Flaschka,.m...&, 1987; Bultot~, 1988; Lettenmaier and Gan, 1990). To date, climate-impact studies of the Colorado River Basin have been limited to stochastic methods (Revelle and Waggoner, 1983; Stockton and Boggess, 1979). These studies necessarily assume, however, that the relationships among temperature, precipitation, and streamflow will remain unchanged under future climatic conditions. In contrast, this study used a conceptual hydrologic model to study the sensitivity of the basin to greenhouse warming. A recent attempt to use a deterministic model to study climatic impacts on a small sub-basin of the Colorado River is presented in Schaake (1990). In this project we expand upon that work by incorporating additional climate scenarios and modeling additional sub-basins. By modeling actual hydrologic processes (e.g. percolation, soil-moisture storage, snowmelt, etc.), deterministic techniques incorporate an additional level of complexity. So long as these hydrologic processes do not change significantly under a C~ -altered climate, deterministic models should be more robust than derived statistical relationships between meteorologic variables and streamflow. In fact, however, all attempts to study the impacts of climatic change using hydrologic models are limited by their dependency on historic data, which may not be applicable to future conditions. Descriotion of the Model The large size of the Colorado River Basin complicates the development of a physically based hydrologic model; indeed, no completely satisfactory basin model exists. As a result, we modeled several sub-basins In the Upper Colorado River Basin, using a conceptual hydrologic model developed and operated by the National Weather Service River Forecasting Service (NWSRFS) In Salt Lake City, Utah. These models 11