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<br />00-2~5~ <br /> <br />Chapter II <br />DEVELOPMENT OF A PHYSICALLY BASED <br />DISTRIBUTED PARAMETER RAINFALL RUNOFF MODEL <br /> <br />In 1991. Reclamation's Upper Colorado Regional Office began work to develop a physically based <br />rainfall runoff model to simulate the natural surface water hydrologic syslem in the Upper <br />Gunnison River Basin. For the purposes of this document, this model shall be referred to as the <br />Gunnison Model. The Gunnison Model was created by Reclamation with the assistance and <br />technical support of the USGS. The original focus in developing Ihe Gunnison Model was to <br />develop a tool which could be used to assess the effects of changing climate on the magnitude and <br />timing of runoff in the Gunnison River Basin. Subsequently, the Gunnison Model was modified <br />for use as a "real time" model for Ihe operation of Blue Mesa, Morrow Point, and Crystal <br />Reservoirs. <br /> <br />The Gunnison Model simulates the nalural hydrologic system for the entire Upper Gunnison River <br />Basin. (See figure 2.) The Upper Gunnison River Basin covers 10,270 square kilometers <br />(3,965 square miles) and comprises all areas which ultimately flow 10 the Gunnison River at <br />Crystal Dam; 57 percent of the natural flow in the Gunnison River Basin originates in the Upper <br />Gunnison River Basin.' The Gunnison Model was created using the Precipitation Runoff <br />Modeling System (PRMS) (Leavesley et aI., 1983) running inside the MMS graphical framework. <br /> <br />Prior to discussing the development of the Gunnison Model, an overview of PRMS and MMS <br />shall be rendered. <br /> <br />OVERVIEW OF PRMS <br /> <br />Originally developed by the USGS, PRMS is a distributed parameter, physical-process watershed <br />modeL As opposed to a "lumped" parameter model, a distributed parameter model accounts for <br />the spatial variability of parameters within a watershed. PRMS is a physical model, in that the <br />algorithms embodied in the model closely mimic physical hydrologic processes. Because of this, <br /> <br />I Source" Colorado Riyer Simulation System natural Ilow database. <br /> <br />5 <br />