Yampa River Basin Water Resources Planning Model Final Report

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4.1.4 Historical Streamflow File - yampa.rih The yampa.rih file contains historical streamflow records or estimates that are used for baseflow generation and model calibration. Table 4.1.3a summarizes the gages used and their average annual flow. Appendix C, Section 7 gives the technique used to fill data gaps, along with the data filling results. The locations of streamflow gages modeled are shown in Figure 4.1.1a. For a more detailed discussion on the historical streamflow file, see the StateMod Users' Manual. Approximately 26 USGS streamflow gages were initially identified for inclusion in the Yampa Model. Not all gaging stations were in operation during the entire study period, and of the 26 gaging _ stations identified, 18 were used. Table 4.1.3a presents the historical flows for the water years 1975- 1991 and the average estimated natural flows at each of the 18 USGS gages used in the baseflow development and model calibration. The Phase IIIa enhancements did not impact the historical streamflow file. 4.1.5 Baseflow File - yampa. rib The baseflow file contains the information that allows baseflows estimated at gaged locations to be distributed to ungaged locations throughout the basin. Proration factors were developed at selected points throughout the river basin using the following guidelines: • The most upstream diversion structure in each tributary • Critical points of interest, such as river confluence points • All reservoir locations The yampa.rib input file contains the proration factors used by StateMod to distribute gaged flows throughout the river basin. At a given baseflow location, proration factors were developed using one of two techniques, the gain approach or the nearby gage approach. For the gain approach, proration factors were defined as the fraction of the reach gain that was estimated to be tributary to a baseflow point. These factors were determined by normalizing the local volumetric runoff (local area times local average precipitation) by the corresponding incremental runoff. The incremental runoff was determined by taking the difference of the total volumetric runoff between successive USGS gage locations. The gain approach was used as the default technique in the Yampa Model. However, at several locations, the prorated hydrograph developed using the gain approach showed an attenuated shape that was not representative of a "natural' hydrograph. This occurred predominantly in the headwater areas where the hydrograph is dominated by runoff from spring snow melt. In these situations, the ' nearby gage approach was used. For the nearby gage approach, the proration factor was determined as a function of a stream gage in a neighboring tributary, by comparing the local volumetric runoff. This procedure was applied to the following tributaries: r River System 4 -5