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<br />Chapter II <br /> <br />Development of a Physically Based Distributed <br />Parameter Rainfall Runoff Model <br /> <br />Another important faClOr in delineating basin HRU's is the spatial resolution for these HRU's. <br />The modeler must determine the methodology by which 10 break a basin into homogenous units. <br />The more physical characteristics that are considered in this delineation, the more HRU's are <br />ultimately produced, resulting in a higher spatial resolution. Spatial resolution in PRMS models is <br />generally quite high. In examining PRMS models created by other researchers, it was found that <br />PRMS is usually applied on small to medium-sized watersheds (watersheds with total areas of less <br />than 1,000 square kilometers) and that up to 50 HRU's were used in spatial delineation. <br /> <br />In the Gunnison Climate Study, however, the goal was to develop a single PRMS model for the <br />entire Upper Gunnison River Basin (over 10,000 square kilometers). Because of this, it was <br />decided to pursue a somewhat lower spatial resolution approach than is generally applied with <br />PRMS, in the attempt to successfully model the entire Upper Gunnison River Basin. <br /> <br />In order to use GIS to develop HRU's for the Gunnison Model, a comprehensive GIS database <br />was first assimilated for the Gunnison River Basin. Spatial data layers (coverages) included in the <br />Gunnison GIS database are described in table 2. <br /> <br />It was hypothesized that an HRU delineation that is based on these categories would result in a <br />somewhat coarser spatial resolution than is generally applied with PRMS. Still, it was felt that <br />such a characterization would account for enough spatial variability to successfully model the <br />entire Upper Gunnison River Basin. <br /> <br />To develop model HRU's, the Upper Gunnison River Basin was partitioned using three <br />categories: (I) subbasin, (2) aspect, and (3) land cover. These categories are depicted in <br />figures 4, 5, and 6, respectively.' <br /> <br />GIS processing was performed using ARCIINFO software produced by Environmental Systems <br />Research Institute. To automate the development of HRU's, a spatial processing program was <br />developed using ARC Macro Language (AML). <br /> <br />2 For figures 5 and 6, which depict aspect and land cover, only the Taylor Park Basin is shown. Due to the high <br />degree of detail in these two data sets, they have nol been displayed for the entire Upper Gunnison River Basin. <br /> <br />10 <br />