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<br />CWCB North Fork <br />7/5/2005 <br />Page 2 <br /> <br />Future water use will be estimated for each subbasin using landuse breakdowns, irrigated <br />acres, and population estimates derived from the GIS coverages. Land uses for the entire <br />study area are shown in Figure 2. For each subbasin, land use acreage, irrigated acres for each <br />land use type, parcel breakdowns, and population estimates, were calculated using ArcGIS <br />9.0, and are shown in Appendix A. The following discussion describes the methodology used <br />to obtain the desired quantities. <br /> <br />The land use parcel coverages were intersected with the subbasin coverage, resulting in <br />land use parcels with the subbasin attributes and matching subbasin boundaries. The <br />intersect tool in ArcGIS splits a feature (in this case a land use parcel polygon) with another <br />specified feature (subbasins), where they overlap; therefore, if a landuse parcel polygon <br />overlaps two subbasins, the parcel will be split into two polygons when the intersect tool is <br />used. If a land use parcel polygon extends beyond the subbasin boundary, the intersect tool <br />will only retain the portion of the landuse parcel polygon within the subbasin boundary. <br />After the intersect was performed, the area of each land use parcel was recalculated, and the <br />attribute tables were exported to Excel. <br /> <br />Irrigated acres for various land use classifications were determined by intersecting the <br />irrigated acre coverage with the subbasin coverage, resulting in a new irrigated acres <br />coverage with the attributes of the subbasin coverage, and matching boundaries. This <br />intersected irrigated acre coverage was then unioned with the land use coverage to obtain <br />irrigated acres for each subbasin with various land use classifications, and matching land use <br />boundaries. The union tool, like the intersect tool, will split features at overlapping <br />boundaries and add the attributes of each coverage, however, the union tool will retain all <br />portions of the two polygons, not just the overlapping portions. This tool is helpful when <br />features outside the extent of the "clipped" feature must be retained. The irrigated acre area <br />was recalculated after the union, and the attribute table was exported to Excel. <br /> <br />Population and population density of each subbasin were calculated from 2000 census TIGER <br />data. Census tracts intersecting the study area are shown in Figure 3. Seven census tracts <br />overlap the subbasin boundaries (census tracts (963600, 963800, 963900, 964600, 964900, <br />965000, and 966200). These seven census tracts contain a portion of ten municipalities <br />(Crawford, Crested Butte, Delta, Gunnison, Hotchkiss, Marble, Mount Crested Butte, Paonia, <br />Pitkin, and Olathe). To obtain an accurate population distribution, population densities were <br />calculated for incorporated and unincorporated portions of each census tract separately. It <br />was assumed that no people live on public land. Landuse was divided into public or private <br />land, and population density was calculated based on the total area of private parcels within <br />that census tract. <br /> <br />The incorporated area population densities were calculated by dividing the population of the <br />municipality by the area of private land (obtained from the landuse parcel coverage) within <br />that municipality. Unincorporated population density for each census tract was calculated by <br /> <br />S :\GI S\working\Jinal_ memo\CW CB _ N _Fork0701 05.doc <br />