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<br />Chapter II <br /> <br />Development of a Physically Based Distributed <br />Parameter Rainfall Runoff Model <br /> <br />The algorithm used in this program shall be described in four steps. Many of the specific <br /> <br /> <br />details have been omitted in this description in the hope that those readers not proficient in <br /> <br /> <br />GIS processing will be able to assimilate the basic methodology used. <br /> <br />I. The aspect data set was filtered. All the polygons (triangular facets) in the <br />aspect data set were placed into one of three categories: (I) north, (2) south, or <br />(3) east-west. Facets with aspect azimuths between 0 and 60 degrees or 300 and <br />360 degrees were attributed as north. Facets with azimuths between 120 and <br />240 degrees were attributed as south. All other facets were attributed as east-west] <br /> <br />2. The land cover spatial data set was filtered. Each area (polygon) in this data <br />set was attributed as belonging to one of three classes: (I) range, (2) forested, or <br />(3) tundra. The aspect data set was filtered to keep the total number of HRU's <br />developed at a manageable level and because 96 percent of the lands in the Upper <br />Gunnison River Basin fell into one of these three classes (range, forested, or <br />tundra). <br /> <br />3. The filtered and attributed aspect data set, the filtered land cover data set, and <br />the subbasin data set were then "unioned" together. A union is a spatial operation <br />that overlays one polygon data set with another to create a new polygon data set. <br />The spatial locations of polygons (including their attributes) are joined to derive a <br />new data relationship in the output data set. The result of this union processing <br />was the creation of a new spatial data layer, the HRU data layer. <br /> <br />4. The resultant HRU data layer was then filtered. Defined HRU's with areas less <br />than I square kilometer were placed in adjacent HRU categories. <br /> <br />The resultant HRU spatial data layer contains over 40,000 polygons. Each of these polygons <br />contains an identifier with respect to its subbasin, land cover, and aspect. However, even though <br />the HRU data set has a large number of polygons, there are only 132 unique identifiers with <br /> <br />.' Facets thal were nm (slopes less than 5 degrees) were allribUled as east.wesl. <br /> <br />12 <br />