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ABSTRACT A physically based distributed parameter rainfall runoff model was developed for the upper Gunnison River Basin in western Colorado. The model was developed using the Precipitation Runoff Modeling System (PRMS), running inside the Modular Modeling System (MMS) graphical framework. Characterization of the watershed and development of model distributed parameters was accomplished, in large part, through the use of Geographic Information Systems technology. The model created was unique in that its areal extent (the upper Gunnison River Basin) was orders of magnitude larger than any previous applications of PRMS. In combination with this large areal extent, a lower spatial resolution was used for model distributed parameters than is generally applied with PRMS. Statistical and visual analyses indicate a close match between simulated and observed streamflow. While using a lower spatial resolution causes some generalization of the simulated hydrograph, the general shape of the hydrograph and aggregated volumes of water can be effectively simulated using such an approach. It is concluded that PRMS can be applied successfully on large river basins using a lower spatial resolution approach when the output of interest is weekly, monthly, or yearly volumes of water. The model was used to perform global climate change analysis in the Gunnison River Basin. Based on a "best guess" climate scenario developed by Dennis (1991) for the Gunnison River Basin (under the assumption of doubled atmospheric carbon dioxide), the rainfall runoff model predicts a decrease in streamflows of 5 percent, with significant changes in the distribution of streamflow throughout the year. Sensitivity model runs were also performed on an array of altered historical climate data. These model simulations show that streamflows change significantly as precipitation and temperature change. However, due to the uncertainty of future climate, limited information pertinent to water resource management planning activities is gained by such analysis. The model was also used as a "real time" operations model to predict inflows to major reservoirs in the upper Gunnison River Basin. Climate data obtained through satellite telemetry was used as input data in these model runs. Inflow forecasts developed with the model were used in the operation of Blue Mesa, Morrow Point, and Crystal Reservoirs in water years 1993 and 1994. The model helped water managers improve the operation of the reservoirs during the 2 test years. It is concluded that PRMS within the MMS graphical framework can be successfully applied to operational modeling. Ancillary to the development of the rainfall runoff model, several computerized systems were developed to assist in general water resource management activities. These systems include: (I) the Upper Colorado Hydrologic Database. (2) the Upper Colorado Hydrologic Information System, and (3) MMS.