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represented by the "fullest' outline during the 2001 season. According to Division 1 personnel, <br />most reservoirs filled to capacity in 2001. Readily available information was used to assign <br />appropriate Water District identifier attributes to reservoirs. Water bodies with less than three <br />acres of surface area are estimated to be ponds used for stock purposes only. SPDSS <br />memorandum 89.10 documents the process used by RTi. <br />The following process was used to aggregate reservoirs using the GIS approach. Note that <br />standard ArcGIS analyses were used in all steps. <br />1. Key reservoirs and reservoirs within key reservoir systems were excluded from the <br />analysis by linking the key reservoir list from Task 5 with the GIS shapefile. <br />2. Surface area of natural lakes was estimated and excluded from the analysis. Initially, this <br />was done using a USGS high resolution National Hydrography Dataset (NHD) shapefile <br />of water features, which includes lakes, ponds, and reservoirs. The NHD shapefile <br />offered accurate outlines of a majority of the surface water features that appear on USGS <br />24,000-scale topographic quadrangles. However, the criteria used to distinguish man- <br />made reservoirs from naturally occurring water bodies were not sufficient for this <br />investigation. In general, reservoirs were coded, along with natural lakes, as a lake or <br />pond because the criteria to determine a reservoir were based solely on the reservoir <br />having an apparent rectangular geometry, or other "not natural" shape. For example, in <br />Water Districts 4, 5, and 6, the NPID shapefile codes only 120 water features as <br />"reservoirs", mostly along the Front Range. These features are relatively small and none <br />of the larger (or key) reservoirs are identified. <br />Therefore, natural lakes were identified by comparing small, high-altitude lakes in the <br />Waterbodies.shp file to 24,000-scale USGS topographic maps directly for Water Districts <br />4 and 5. Natural lakes that appear in the shapefile generally occur high in the watershed <br />and were identified based on their size, shape, name, and diversion structure association. <br />A natural lake was estimated to be small, have no apparent dam structures or geometry <br />that would suggest the presence of a dam, not be identified as a reservoir, and have no <br />man-made diversion structures associated with it. When this process was applied to <br />Water Districts 4 and 5, natural lakes were estimated to account for approximately 5 <br />percent of the total surface area above 6500 feet. Based on this analysis, total water body <br />surface area above 6500 feet was decreased by 5 percent. <br />The remaining water bodies were aggregated. Average monthly evaporation estimates <br />were determined in SPDSS Task 53.3. During this task, Water Districts that span both <br />plains and mountainous areas were split to reflect the different evaporation characteristics <br />above and below 6500 feet. The 5 percent natural lake surface area value estimated in <br />Step 2 was removed from surface areas in "upper" portions of Water Districts (above <br />6500 feet) for all Water Districts except 1, 2, 8, and 64. The entire water body surface <br />area in the remaining four Water Districts was used, since there are few natural lakes <br />along the Front Range and in the eastern plains. <br />Page2of14 <br />