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IV. Reservoir Yield Analysis <br />The annual yield of Grand Valley Lake that could be delivered every year was estimated using a <br />reservoir operations model developed in Excel for a study period of 1975 through 2005. This period <br />contains at least one short intensely dry year (1977) as well as a more sustained drought (2001 through <br />2004). In general, reservoir yield is a result of the interplay among the water supply (inflow) time series, <br />the demand or release (outflow) time series, and the capacity of the reservoir. Two reservoirs of different <br />capacity were analyzed: a 154,500-af reservoir (Alignment 1), and a 39,800-af reservoir (Alignment 2). <br />The inflow and outflow time series for the analyses are described below. <br />A. Water Rights/Supply <br />Water available to an off-stream reservoir is constrained by diversion capacity of the feeder canal, and <br />legally available supply at the diversion site. This section describes how the legally available supply, <br />which is independent of reservoir size, was estimated. <br />AECOM relied on the CDSS Gunnison River Water Resources Planning Model (CWCB and CDWR, <br />' 2007), and specifically, the Baseline scenario model, for the model period time series of available flow. <br />This version of model input is described in CDSS Water Resources Planning Model documenta.tion as <br />follows: <br />' <br /> ( � <br /> I ' <br /> I' <br /> I ' <br />� <br />The Baseline data set is intended as a generic representation of recent conditions on the Gunnison <br />River, to be used for "what �' analyses. It represents one interpretation of current use, operating, <br />and administrative conditions, as though they prevailed throughout the modeling period. All existing <br />water resources systems are on line and operational in the model ..., as are junior rights and modern <br />levels of demand. The data set is a starting point, which the user may choose to add to or adapt for a <br />given application or interpretation of probable demands and near-term conditions. <br />The amount of water that a brand new right can legally divert, once all downstream senior diverters' <br />needs have been met was calculated for each time step (month) and at each node. Because the model <br />tracks return flows and treats them as part of the water supply available to downstream users, the CDSS <br />model estimates legally available supply more accurately than a simpler approach such as adjusting <br />historical gage flow by the combined downstream water rights or historical diversions. The CDSS <br />Gunnison model has a node representing the USGS gage Gunnison River near Lazear, Colorado, which <br />was located during its operating life (1961 through 1985) very close to the Grand Valley Lake gravity- <br />flow diversion location, shown on Figure 1 on page 12. Model output at this node was used to represent <br />river conditions at the diversion site. <br />According to the Baseline model, annual (1975 — 2005) physical flow at the Lazear gage averages 1.14 <br />' maf, of which 1.02 maf is available to a junior right. Water is least available from January through <br />March, and was unavailable in late summer of dry years (1977, 1990, and 2002 through 2004). May and <br /> I ' <br />� <br />� AECOM <br />37 <br />2/22/10 <br />