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Last modified
9/26/2011 8:31:55 AM
Creation date
7/10/2008 2:15:40 PM
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Decision Support Systems
Title
CRDSS Task 1.5 - Variable Efficiency Evaluation - Compare StateMod Variable Efficiency and Soil Moisture Accounting Historic Model Results to Previous CRDSS Model Results and Historic Measurements
Description
The purpose of this task was to run the monthly Historic Yampa River Basin model with the irrigation requirement file (*.ddc) created in Task 1.2 and the baseflows developed in Task 1.3.
Decision Support - Doc Type
Task Memorandum
Date
11/5/2001
DSS Category
Surface Water
DSS
Colorado River
Basin
Colorado Mainstem
Contract/PO #
C153933, C154062
Grant Type
Non-Reimbursable
Bill Number
SB92-87, HB93-1273, SB94-029, HB95-1155, SB96-153, HB97-008
Prepared By
Leonard Rice Engineering
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• Both the Previous CRDSS analysis and the Enhanced analysis predict streamflows and <br />diversions during the representative wet-year (1984) that are essentially the same as <br />gaged. <br />• Reservoir calibration does not appear to be better or worse during varying hydrologic <br />conditions for either the Previous CRDSS or Enhanced current analysis. This is likely the <br />result of the operating rule used in the Historic analyses that release stored water to meet <br />targets. <br />• The Enhanced analysis predicts consumptive use estimates for explicit structures that are <br />closer to the consumptive use estimated by the Yampa River Basin StateCU analysis in <br />dry, average, and wet hydrologic periods. <br />• The Enhanced StateMod simulation using the variable efficiency and soil moisture <br />accounting options better represents the interaction between diversions, irrigation water <br />requirements, and return flows. The Enhanced results more accurately represent the crop <br />consumptive use results from the StateCU modeling efforts in the Yampa River Basin <br />and are consistent with the methodology used in Colorado by engineers to determine <br />historic consumptive use for water right transfers and augmentation planning. <br />About 20 percent of the structures in the Yampa River Basin have late irrigation season <br />calculated efficiencies equal to the 60 percent maximum efficiency set for the basin. A <br />more detailed study of basin-wide conveyance and application efficiencies may result in a <br />lower maximum efficiency. A preliminary analysis showed that a lower maximum <br />efficiency would likely predict streamflows closer to gaged during dry periods. <br />The Enhanced analysis predicts the largest diversion shortages in Water District 54. The <br />calculated efficiencies for most of the structures in Water District 54 are at the 60 percent <br />maximum. If a lower maximum efficiency was determined to be more representative of <br />conditions in the basin, more of the diverted water would be returned to the river and <br />made available for downstream diversions. A preliminary analysis showed that this <br />would likely reduce the shortages in this water district, and increase the consumptive use. <br />The methodology used to develop "estimated" diversions for aggregate structures uses <br />monthly irrigation water requirements divided by monthly basin-wide average <br />efficiencies. These average efficiencies tend to determine inflated "estimated" diversions <br />during dry and average years because they assume a full water supply was obtained. This <br />method also tends to determine smaller "estimated" diversions during wet years. Ideally, <br />aggregate "estimated" diversions would be determined using available historic data, <br />appropriate data filling techniques and, potentially, efficiencies that vary by month and <br />year. As shown in Water District 54, some sub-basins generally operate at a higher <br />efficiency than others do, therefore at a minimum it may be appropriate to determine <br />aggregate "estimated" diversions using average monthly efficiencies determined by water <br />district rather than the total basin. <br />Recommendations <br />The variable efficiency and soil moisture accounting algorithms are believed to provide a better <br />representation of the interaction between diversions, irrigation water requirements, and return <br />flows. The fact that calibration did not significantly improve with this approach may be an <br />VariableEff Taskl-5 16 of 17 November 5, 2001 <br />
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