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Comparison of the GIS and Decree Methods
<br />As noted above, the Decree Method was used to estimate non-key reservoir volume, surface
<br />area, and resulting evaporation in previous CDSS efforts where GIS water body coverages had
<br />not been developed. Table 3 provides a side-by-side comparison of the results of the GIS method
<br />and the Decree Method (including grave pits). Inmost Water Districts, there are significant
<br />differences between the two methods. With the exception of Districts 23, 48, and 64, surface
<br />areas estimates from the Decreed Method were less than those from the GIS method.
<br />Table 3
<br />Comparison Aggregate Reservoir Surface Area by Water District
<br />GIS Method vs. Decree Method
<br />
<br />
<br />District Surface Area by
<br />GIS Method
<br />Acres Surface Area by
<br />Decree Method
<br />Acres Difference
<br />(Acres)
<br />GIS-Decree
<br />Difference
<br />Diff/GIS
<br />South Platte
<br />1 5,279 3,076 2,203 42%
<br />2 5,667 1,289 4,378 77%
<br />3 6,693 4,900 1,793 27%
<br />4 4,956 3,105 1,851 37%
<br />5 4,161 2,063 2,098 50%
<br />6 3,717 3,376 341 9%
<br />7 2,583 2,467 116 4%
<br />8 1,896 1,438 458 24%
<br />9 848 664 184 22%
<br />23 994 2,558 -1,564 -157%
<br />48 442 707 -265 -60%
<br />64 1,056 1,070 -14 -1%
<br />80 397 391 6 2%
<br />Total 38,691 27,105 11,586 30%
<br />North Platte
<br />47 4,994 4,421 573 11
<br />In most cases, reservoir owners were able to provide actual capacities for the reservoirs detailed
<br />in the Task 5 Operating Memoranda. Table 4 shows 28 key reservoirs' user-supplied capacity
<br />and the corresponding net decreed storage extracted from HydroBase using the methodology
<br />described above. Based on information gathered for Task 5, we know that several of the
<br />reservoirs have second and even third fill rights -those rights are included in the net decreed
<br />storage (Empire Reservoir, Prewitt Reservoir, Eleven Mile Reservoir, Tarryall Reservoir). In
<br />many cases, the second fill rights are not for the same storage amount as the first fill rights, so
<br />they are not easily identified.
<br />In addition, again based on information gathered for Task 5, we know that several of the
<br />reservoirs have capacities much greater than decreed storage, because transbasin water is the
<br />primary source and doesn't require a South Platte decree (Horsetooth Reservoir, Carter Lake
<br />Reservoir, Spinney Mountain Reservoir).
<br />Page 6 of 14
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