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The decree approach requires estimation of reservoir depth in the capacity to surface area <br />conversion calculations. In order to accomplish this, assumptions must be made <br />concerning average reservoir depth, how well the volume to surface area ratio represents <br />this average depth, and how accurately the calculations represent the actual reservoir <br />shape. Our area-capacity analyses based on key reservoirs indicated that an average <br />depth of 17 feet was appropriate for reservoirs in the South Platte drainage basin. <br />However, an average depth of around 12 feet would have provided better agreement <br />between the GIS and Decree Methods. <br />The decree approach requires a careful filtering of the HydroBase query results. Many of <br />the storage reservoirs in Water Division 1 are located off-channel with diversions to <br />storage occurring in a different Water District. Consequently, a simple search for the <br />storage facility WDID cannot be used to separate non-key reservoirs from key reservoirs. <br />A more advance filtering procedure is required together with a time consuming, visual <br />inspection of the query results. <br />Surface areas determined from the GIS method represents actual reservoir operations and <br />water use, whereas the decree method measures surface area based on the total water <br />storage allowed by law. Decrees are sometimes larger than the capacity of the dam <br />structure plus second fill storage rights are not coded differently from first fill storage <br />rights in HydroBase. A visual inspection, plus knowledge of actual reservoir size, is <br />required to remove second fill rights from the analyses. In addition, water use is typically <br />not equal to the total decreed storage right of a particular reservoir and varies from <br />reservoir to reservoir. For reservoirs that store transbasin water, such as Carter Lake and <br />Horsetooth Reservoir, net storage rights for native water that are far less than the actual <br />reservoir capacity. This discrepancy between actual storage and decreed storage maybe <br />an issue for aggregate reservoirs as well. <br />Evaporation rates have been estimated by Water District based on average annual <br />evaporation rates estimated by NOAA, then distributed to monthly rates using the SEO <br />recommended methodology. Monthly distributions vary above and below elevation 6500 <br />feet, and where required, Water District rates are divided into "upper" and "lower" rates <br />(see SPDSS Task Memorandum 53.3). Therefore, reservoir must be aggregated for areas <br />above and below 6500 feet. Reservoir decrees have location information believed to be <br />accurate, however elevation is generally not stored in HydroBase. The GIS approach <br />allows reservoirs to easily be grouped within a Water District above and below 6500 feet. <br />Stock Pond Area -Approach and Results <br />The following process was used to estimate stock pond areas. <br />A list of non jurisdictional tanks and erosion control structures and associated volumes was <br />queried from HydroBase. The Colorado Division of Water Resources has established the <br />following criteria for non jurisdictional dams, livestock tanks, and erosion control structures <br />in the Rules and Regulations for Dam Safety and Dam Construction (2 CCR 402-1, 1988): <br />• Non-jurisdictional Dam: creates a reservoir that is less than 100 acre-ft in capacity, <br />or is less than 20 acres of surface area, and is less than 10 feet high. <br />Page 8 of 14 <br />