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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

January 1, 1994

Methods were developed by which transit losses could be determined for transmountain return flows in 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 interacts with an alluvial aquifer. The study approach 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 calculated; 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. Net loss to bank storage can vary from about 50% for a 0-day recovery period to about 2% for a 180-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% 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% of a given daily transmountain return-flow release, depending on month of year. Evaporation losses are permanently lost from the system. (USGS)