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<br />UUU/ilt <br /> <br />2.3 ASSUMPTIONS AND ANALYSIS PROCEDURES <br /> <br />2.3.1 Model <br /> <br />Evaluation and analysis of the various alternatives was performed using StateMod, the State of <br />Colorado's Stream Simulation Model. StateMod was originally developed for the Colorado River <br />Decision Support System (CRDSS) and is a monthly water allocation and accounting model with <br />flexibility to become a daily model. It can be used to make comparative analyses for assessing various <br />water management policies and practices utilized in the administration of a river basin. The model's <br />operation is governed by basin hydrology, water rights associated with diversion and storage structures, <br />and the operating rules used for the diversion and storage structures. StateMod uses four types of <br />water rights: direct flow rights, instream flow rights, reservoir storage rights, and operational rights. <br />Water rights are sorted by priority and river administration is simulated using the Prior Appropriation <br />Doctrine. <br /> <br />StateMod was selected for this analysis for several reasons. It can be operated on a monthly time step <br />that allows for fast execution and a reasonable volume of output to be analyzed. The simulations on a <br />monthly time step will generally be adequate for assessing the impacts of potential changes to water <br />management policies within the basin. In those cases where the impacts need to be examined at <br />shorter time intervals, StateMod can be set up to run on a daily time step. Input to the daily version of <br />the model can be daily data for stream flow, diversion demands, and reservoir targets, or a set of values <br />for distributing the monthly data. <br /> <br />Routing of reservoir releases Qag times, etc.) is not included in either the monthly or daily model <br />because: (1) StateMod is primarily a planning model; (2) the additional detail required to properly- <br />implement reservoir releases with a travel time component is not justified because the system would <br />have to include some kind of forecasting to know a reservoir release is required before a reservoir <br />demand occurs; and (3) the volume of water associated with a potential under-release that occurs by <br />ignoring travel time before a reservoir demand is known is offset by the potential over-release that <br />occurs after the demand is satisfied. <br /> <br />I <br /> <br />2.3.2 Baseline Hydrology: C1 Data Set <br /> <br />2.3.2.1 C1 Data Set. The PBO employs the hydrology from the C1 Data Set developed for the PBO <br />for investigative purposes. In the Cl Data Set, irrigation demands are calculated from average <br />irrigation efficiencies for the study period, 1975-1991. Irrigation efficiencies were calculated for <br />CRDSS based on historic data for the Colorado River Basin in Colorado (Moore, 1999). The C1 Data <br />Set also includes "backcasting" of 1995 water year demands throughout the entire study period for <br />selected major structures. Average annual depletions for these existing demands are used for every <br />year. Depletions from demands that were in place for only a portion of the 1975-1991 period were <br />included in the C 1 Data Set for the entire study period. Depletions from new demands such as Ruedi <br />Reservoir Round 1 and 2 water sales are also included. [See Appendix F of the Final PBO (U.s. Fish <br />and Wildlife Service, December 20, 1999) for a listing of all projects included in the backcast 1995 <br />demand levels.] <br /> <br />P:\Data \GEN\CWCB\19665\Repon Phase 2\FinalRepon12.02\Finat Draft _ Repon(1~3),doc <br /> <br />17 <br />