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<br />002473 <br /> <br />Quantification of Augmentation Needs <br /> <br />A practical approach was needed to determine when river flows should be augmented. Such an <br />approach was developed by the Service in consultation with the subcommittee, using upper and <br />lower set points like a thermostat to turn augmentation on when streamflows fall below a specified <br />lower set point or threshold and turn augmentation off once streamflows reach a specified upper <br />threshold (Figure 9). These thresholds bracket the summer/winter flow targets previously described <br />and are applied in accordance with the following protocol: <br /> <br />. When unaugmented stream flows fall below the seasonally appropriate lower threshold, <br />water would be delivered at a fixed rate until such time as augmented stream flows <br />exceed the seasonally appropriate upper threshold. <br /> <br />. At that point, augmentation would cease until unaugmented flows again fall below the <br />lower threshold. <br /> <br />. Streamflow augmentation would continue throughout the augmentation period (July- <br />March), in accordance with this protocol, or until the available augmentation water <br />supply has been exhausted. <br /> <br />The objective of this protocol is to emulate, as closely as possible, historical base flows such that <br />stream flows do not fall below the 93-cfs and 124-cfs flow targets with any greater frequency, <br />magnitude or duration than occurred historically. A variety of thresholds and augmentation rates <br />were evaluated using simulated daily flows based on the 90-year CROSS data to determine which <br />of these augmentation scenarios best simulates historic stream flows under a variety of hydrologic <br />conditions. In addition, the volume of water needed to satisfy each scenario was estimated. <br /> <br />:! <br /> <br />Thresholds were defined according to three criteria: <br /> <br />I. Flow targets (previously described) <br /> <br />2. Differential - Five values (80, 60, 50,40 and 30 cfs) were selected, representing the <br />numerical difference between upper and lower thresholds. Increasing the differential <br />lowers the lower threshold and raises the upper threshold. Decreasing the differential <br />raises the lower threshold and lowers the upper threshold. Too small a differential would <br />require continual, intermittent augmentation. <br /> <br />3. Skew - Five values of skew were selected (+25%, +10%, 0%, -10% and -25% of the <br />differential) the net effect of which is to increase or decrease both thresholds by the same <br />amount relative to flow targets. At 0% skew, flow targets are centered between upper <br />and lower thresholds. <br /> <br />For example, with a flow target of93 cfs, a differential of60 cfs, and 0% skew, lower and upper <br />thresholds would be 63 cfs (93 - 30) and 123 cfs (93 + 30), respectively. Whereas, with +25% skew <br />(25% x 60 = 15 cfs), the thresholds would be 78 cfs (63 + 15) and 138 cfs (123 + 15) and with -25% <br />skew the thresholds would be 48 cfs (63 - 15) and 108 cfs (123 - 15). Because positive skew raises <br />and negative skew lowers both thresholds by the same amount, positive skew would call for more <br />water, and negative skew would call for less water relative to 0% skew. <br /> <br />Management Plan for Endangered Fishes in the Yampa River Basin <br /> <br />36 <br />