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0002';2 ~ .; " .'.... >.' ~~:"- . ~v'-'" .~. ",', }.-, .' .." ..' ....; Transit losses were calculated for transmountain return flows that ranged from 1 to 100 ft3js and for native streamflows that. ranged from 0 to 1,000 ft3js. Sources .of transit loss for which transit loss calculations were made are: bank-storage, evaporation, and channel storage. Although there may be increases in transmountain return flow (transit gains) on some days that result from return of water previously lost to bank storage and channel storage, the long-term effect is a transit loss. Therefore, the term "transit gains" is used only in reference to the gains from bank storage and channel storage. Approach Transit loss in a complex stream-aquifer system, such as the Fountain Creek valley (see "Description of Study Area" section in this report), only can be determined readily by use of computer models. These models provide the capability to simulate streamflow in a given stream and to simulate the interaction of the streamflow with an alluvial aquifer with reasonable accuracy. The U.S. Geological Survey's J349 computer program (Land, 1977) was selected for use in this study. This model, however, has no provision for computing evaporation. Therefore, evaporation was computed using the evapora- tion-loss component from another model that has been used to determine transit losses for reservoir releases on the lower Arkansas River (Livingston, 1978). The evaporation-loss component was modified and adapted to this study.. Determination and application of transit losses as described in this report consisted of five basic steps: (]) Identification of all potential .transit losses and evaluation of applicability to the present study; (2) calibration and verification of the streamflow-routing model; (3) determination of bank-storage and channel-storage losses with the calibrated and verified model; (4) determination of evaporation losses; and (5) development of a technique by which the transit-loss determinations could be applied in daily calculation and administration of transmountain return-flow exchanges. The application technique is illustrated with an example that uses actual transmountain return-flow, streamflow, and streamflow-diversion data. Interim Exchange Agreement Transmountain return flows discharged into Fountain Creek at the Colorado Springs Wastewater Treatment Facility currently (1986) are being exchanged at the Arkansas River in accordance with an interim exchange agreement established between the Division Engineer, Colorado Division of Water Resources, and the city of Colorado Springs. As designated by this agreement, if Fountain. Creek is a continuous stream, either transit losses are assessed at 0.07 percent per mile if the flow at streamflow-gaging station 07106300 Fountain Creek near Pinon (fig. 1) is 500 ft3/s or more, or transit losses are assessed at ]0 percent for the study reach if the flow at station 07106300 is less than 500 ft3js. No exchange of transmountain return flow can be made if streamflow is not continuous in Fountain Creek. The interim exchange rates just described apply during the irrigation season from April 1 to October 31. During the nonirrigation season (November 1 to March 31), transit losses always are assessed at 0.07 percent per mile (Gary M. Bostrom, City of Colorado Springs, Department of Public Utilities, written commun., ]985). i I 4