Laserfiche WebLink
<br />Graph A in Figure 9 depicts hypothetical historic and future hydrographs for the same modeling <br />period. The historic hydrograph (blue) is further differentiated into flows greater than the 93-cfs <br />flows target (light blue) and flows less than 93 cfs (dark blue). In Graph A, the future hydrograph <br />(tan) is unaugmented. Graph B duplicates the two hydrographs in Graph A, but overlays the effects <br />of augmentation (pink) on the future hydrograph. Ideally, flow augmentation should precisely <br />compensate for the difference between future flows and historic flows less than 93 cfs, filling the <br />dark blue area completely. Although the application of flow thresholds cannot precisely match the <br />historic hydrograph, such an augmentation protocol can reasonably approximate it. Granted this is <br />an idealized example. For example, augmentation in Graph B assumes an immediate and direct <br />effect on stream flows. That is, augmented flows (pink) are the exact sum of future flows (tan) plus <br />the volume of augmentation delivered. It does not consider potential lag time or attenuation of <br />delivered flows as a function of distance from their augmentation source(s). But it assumes that <br />sufficient water is provided to offset any transit losses from the source(s) to the point of delivery. <br />Augmentation volumes were estimated using these same assumptions. However, because the effects <br />oflag time and flow attenuation diminish as the duration of augmentation increases, these effects <br />are considered insignificant for modeling purposes. <br /> <br />Three different augmentation rates were selected for each threshold differential, in proportion to <br />differential. Augmentation rates were never more than 90% nor less than 50% of differential. <br />Therefore, the highest augmentation rates were associated with the largest differentials and the <br />lowest rates with the smallest differentials. Rates greater than or equal to 100% of each differential <br />were not considered due to their potential to produce a "yo-yo" effect. Rates less than 50% of <br />differential were not evaluated because at values of skew less than or equal to zero, they failed to <br />achieve flow targets. If water supplies were unlimited, more aggressive augmentation scenarios <br />(i.e., positive skew, larger differentials, higher augmentation rates) would require more water than <br />less aggressive strategies. However, if supplies were limited, more aggressive strategies would <br />exhaust supplies earlier and potentially fail to meet augmentation needs later in the season. The <br />following evaluation assumes supplies are limited to 6,000 AF delivered to the Maybell gage. <br /> <br />Seventy-five augmentation scenarios (5 differential values x 5 skew values x 3 augmentation rates) <br />were evaluated and ranked according to their relative performance in achieving augmentation <br />objectives. No one scenario performed better than all the others under all demand and hydrologic <br />conditions. And none ofthe scenarios was superior under 2045 demand conditions during the driest <br />hydrologic conditions. Howsver, the analysis revealed that less aggressive scenarios (negative skew <br />and lower augmentation rates) failed to achieve the flow targets with greater frequency than did <br />more aggressive scenarios. Four of the best performers, which all used the maximum value of skew <br />(+25%), satisfied an average 63-71 % of gross deficits during dry conditions (Table 17). Moreover, <br />they met 79-90% of net deficits under moderately dry hydrologic conditions and 41-65% under <br />average hydrologic conditions. All scenarios performed better under moderately dry conditions than <br />they did under average hydrologic conditions. The most and least aggressive strategies did not <br />perform as well as an intermediate strategy. This can be attributed to more aggressive strategies <br />over-augmenting initially and using all of the available water earlier than less aggressive strategies, <br />which continued to provide a lower level of augmentation, as needed, for the duration of the base- <br />flow period. Although the least aggressive strategies allow for augmentation to continue longer <br />during the base-flow period, they may not provide sufficient volume in dry years to satisfy larger <br />deficits, which may be better satisfied by somewhat more aggressive strategies. <br /> <br />Management Plan for Endangered Fishes in the Yampa River Basin <br /> <br />38 <br />