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Ooa9~5 CRDSS Memorandum To: From: Date: RE: File, Ray Alvarado Ross Bethe], Doug Stenzel January 20, 1997 Regeneration of Baseflow Hydrology for Yampa Phase III Modeling As work progressed on the Phase III enhancement modeling of the Yampa River Basin by Boyle Engineering, it was noted with concern that large losses existed at several baseflow node. Table 1, is a portion of the xtb summary of input data for the Phase II Yampa River Basin model and indicates that the average annual loss at the mouth of the Williams Fork River is about 56,000 acre-feet and the average annual loss at the mouth of the Litt]e Snake River is about 116,000 acre-feet. Boyle Engineering reviewed the Little Snake Basin baseflow gains and made adjustments to remove the large negative losses. Review of base flows in selected other parts of the Yampa River Basin was performed with the resulting opinion that some portions of the Phase II Yampa River baseflow generation were undocumented, inconsistent or incorrect. Reviewing the hydrology at individual base flow nodes was a time consuming process and it was decided that it would be more efficient to undertake an effort to develop documented and consistent baseflow hydrology above the Maybell Gage. The revision of the baseflows upstream of the gaging station at Maybell were primarily conducted by Doug Stenzel with assistance/direction from Ross Bethel. The primary steps in the revision of the baseflows are described in Tab]e 2. Following the baseflow derivation described in Table 2, the summary of input data (.xtb) file was created and the baseflow gains reviewed (see Table 3). In several situations where the baseflow hydrology would serve multiple diversion structures downstream, the contributing drainage basin assigned to the baseflow node was redefmed to include the contributing area available to the downstream structures. Problems were also noted in the area between the Yampa River at Steamboat, Elk River at Clark and Yampa River at Hayden (including Trout Creek) which area used the adjacent basin strategy. The five baseflow nodes in this area were changed to be estimated by the proration strategy rather than the adjacent basin strategy. Table 4 presents the revised area and weighted precipitation assigned to each baseflow node for baseflows. A color coded map of the basetlow points and associated drainage basins is being prepared and will be attached to this memo. Simulations for the historic scenario were performed with the revised hydrology and compared to the Phase II simulation results. The revised hydrology was generally found to offer small improvement to the calibration (streamflow and diversions) upstream of Hayden and small deterioration of the Phase II calibration in the lower part of the Yampa River Basin. Table 5 provides differences between historic and simulated streamflow for both a Phase II simulation and a simulation with revised hydrology. The bottom line is a difference between the simulation of about 700 af at the Maybell gage. This difference represents less than .1 of one percent of the gage flow.