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:~-::.....', ..": ., .,:.. ,'.~ ., .'.. ....;.. (.~:. ..' ..... ...,. METHODS TO DETERMINE TRANSIT LOSSES FOR RETURN FLOWS OF TRANSMOUNTAIN WATER IN FOUNTAIN CREEK BETWEEN COLORADO SPRINGS AND THE ARKANSAS RIVER, COLORADO 000239 By Gerhard Kuhn ABSTRACT .-.."' Methods were developed by which traasit losses could be determined for transmountain return flows for a reach of Fountain Creek between.Colorado Springs, Colorado, and its confluence with the Arkansas River. The study reach is a complex hydrologic system, wherein a substantially variable streamflow, both in timing and quantity of flow, interacts with an alluvial aquifer. Tributary streamflow, streamflow diversion, return flow, and ground-water withdrawal are factors that affect the system. The approach for determining transit losses included: (1) Calibration and verification of a streamflow-routing model that contained a bank-storage- discharge component; (2) use of the model to develop the methods by which transit losses could be determined; and (3) design of an application method for calculating daily transit loss using the model results. Sources of transit losses that were studied are bank storage, channel storage, and evaporation. ~... : .. Magnitude of bank-storage loss primarily depends on duration of a recovery period during which water lost to bank storage is returned to the stream. ,Bank-storage loss also depends on the transmountain return flow and native streamflow conditions in Fountain Creek. Net loss to bank storage can vary from about 50 percent for a O-day recovery period to about 2 percent for a lBO-day recovery period. Virtually all water lost to bank storage could be returned to the stream with longer recovery periods. Channel storage loss was determined to be about 10 percent of a release quantity. Because the loss on any given day is totally recovered in the form of gains from channel storage on the subsequent day, channel storage is a temporary transit loss. Evaporation loss generally is less than 5 percent of a given daily trans- mountain return-flow release, depending on month of year. Evaporation losses are permanently lost from the system. The study results are applicable for transmountain return flows that range from 1 to 100 cubic feet per second and for native streamflows that range from 0 to 1,000 cubic feet per second. .-'. INTRODUCTION The city of Colorado Springs, like most large cities along the Front Range of Colorado, augments its water supplies with water imported from the western slope of the Continental Divide (transmountain water). Such water is foreign to the basin in which it is used, and Colorado water laws that govern use of foreign water are different from laws that govern use of water that originates in the basin (native water). ". ..-.,... .<'. 1 ... . .' .~., ~':