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<br />. <br /> <br />TRANSIT LOSSES DURING RESERVOIR RELEASES <br /> <br />III <br /> <br />i,1 <br />Ii <br /> <br />To interpret results from the computer model, an administrative decision <br />was made concerning the definition of transit loss. The volume of water <br />released is rather easy to determine. The volume of water arriving <br />downstream, however, is a function of time because release water leaving <br />channel and bank storage continues to arrive at the diversion point long <br />after the end of the release. Following discussions with representatives of <br />the Office of the Colorado State Engineer and the Southeastern Colorado Water <br />Conservancy District, a uniform method by which this time would be determined <br />was established. Based on the release hydrograph predicted by the model, the <br />specific time allowed is that required for the release discharge to diminish <br />to less than 5 percent of its maximum rate at the diversion point. For <br />example, if for a release rate of 120 ft3/s the maximum discharge arriving at <br />a downstream ditch is 100 ft3/s (as predicted by the model), then the volume <br />of release water that would arrive at the ditch after the release discharge <br />had diminished to less than 5 ft3/s (again, as predicted by the model) would <br />be the transit Zoss as defined for this report. This method assumes that the <br />entire release volume arriving at the downstream diversion canal (as pre- <br />dicted by the model) is diverted into the ditch up to this point in time. <br /> <br />Through analysis of the model results for numerous hypothetical releases <br />from Pueblo Reservoir, relationships were developed for each sub reach between <br />transit loss and the antecedent river conditions, the distance downstream <br />from Pueblo Reservoir, the rate and duration of reservoir release, and the <br />time of year. The May-through-October transit losses in each subreach for an <br />arbitrarily chosen base reZease of 100 ft3/s for 10 days during various ante- <br />cedent river conditions In that subreach are shown in table 4. These losses <br />range from 0.043 percent per mile in subreach 3 when the antecedent subreach <br />flow Is 4,000 ft3/s, to 0.481 percent per mile in subreach 5 when the antece- <br />dent subreach flow is 5 ft3/s. Adjustment factors applied to the base-release <br />transit loss to correct for the actual rate and duration of the ,release are <br />shown in table 5. Both tables also give an example of how they are used to <br />determine the transit loss of a hypothetical reservoir release. Because of <br />differences In evaporation rates, transit losses for releases made during <br />November through April are about 7 percent less than the summer losses <br />determined from tables 4 and 5. <br /> <br /> <br />Model results also indicate that about 80 percent of the total loss can <br />be attributed to bank storage and about 10 percent to channel storage. The <br />remaining 10 percent of the transit loss is evaporated water. This <br />evaporated water and perhaps a small part of the water in bank storage that <br />Is withdrawn by wells or is evapotranspired are the only true water losses <br />from the stream-aquifer system. Thus, transit loss to a downstream on- <br />channel reservoir, which has the capability of collecting virtually all water <br />in bank and channel storage in the recession of a release from an upstream <br />reservoir, is only about 10 percent of the transit losses as determined from <br />tables 4 and 5. <br /> <br />17 <br /> <br />/'-'- <br /> <br />, <br />