Colorado Water Resources Circular No. 20

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S 0'63 Transpiration losses An increase in transpiration losses along the Arkansas River is associated with bank storage during a reservoir release. This process, however, is extremely complex. Because evaporation losses, which are probably of the same magnitude as transpiration losses, are not a sig- nigicant source of transit loss, transpiration losses were assumed to be negligible. RECESSION FOLLOWING RESERVOIR RELEASES The previous discussions of the four potential sources of transit loss have shown that evaporation and inadvertent diversions remove release water throughout the release period. Channel and bank storage, on the other hand, have temporarily stored release water at a rate which, although initially high, rapidly decreases with time. When the stage declines, channel and bank storage will release the stored water to the stream in a similar fashion. Obviously, canal operation during this recession period will determine how much of the stored water is recovered. The recessions at the Avondale and Pueblo gaging stations for many reservoir releases to Colorado Canal were studied to determine how much of the water in channel and bank storage is divertible. Average reces- sions for various river flows at Colorado Canal are shown in figure 11. The recessions have been modified slightly to represent only release water which the Colorado Canal can economically and practically divert. For example, figure 11 indicates that 400 ft /s reservoir release water will require about 26 hours to recede following the initial decrease in stage beginning the recession. After 26 hours, the release water still in the river is not divertible by the canal. The discharge shown in figure 11 includes water released from both channel storage and bank storage. Comparison with figure 4 indicates that all water in channel storage arrives at the Colorado Canal head - gate soon enough to be diverted. Therefore, there is no transit loss due to channel storage assuming water will enter and leave channel storage at about the same rate. This is probably a valid assumption. Figure 6 can represent water leaving bank storage if the assump- tion is made that water leaves bank storage at the same rate as it enters. This assumption is theoretically invalid because (1) the short duration of increase in river stage causes nonequilibrium effects, (2) river stage decreases are more gradual than stage increases, and (3) saturated thicknesses may change aquifer transmissivity. Because suf- ficient data were not available, figure 6 was not adjusted for this phenomena. 29