Yampa River Basin Research Final Synthesis Report

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� I MEMORANDUM TO: Dave Frick — Ayres Associates FROM: Chuck Brendecke, Ed Armbruster - Hydrosphere SUBJECT: Summary of Hydrosphere Model Runs to Evaluate Instream Flows ' DATE: July 26, 1999 This memorandum summarizes assumptions and results of selected Yampa River Basin model runs made by Hydrosphere during studies conducted between 1991 and 1996. The summary focuses on the hydrologic implications of simulated water management strategies to meet different instream flows at Juniper Canyon and Maybell. The principal assumptions associated with each of these model scenarios, along with tables of resulting instream flows and reservoir releases, are described below. Definition of Scenarios Numerous model scenarios were run using Hydrosphere's Yampa River Basin Model in connection with Phase I and Phase II planning studies completed in 1993 and 1995, respectively. Subsequent to the completion of the Phase II study, additional model scenarios were developed to further examine a variety of operation and management options ( "O &M Studies'). Of the more than fifty model scenarios run over the course of these efforts, six are described below. All these model scenarios simulated storage releases from an enlarged Elkhead Reservoir. The six scenarios include: • The Phase I Study scenario, which simulates an instream flow water right based on the contemplated draft of the Juniper Project and having a priority date of 1954. Reservoir storage releases are made to satisfy existing and future in -basin demands, some of which are called out by the 1954 instream flow right. • The Phase II Study scenario, which simulates flow targets based on Preliminary Flow Recommendations developed in 1994. In -basin demands are assumed to divert free from any call for instream flows. Reservoir storage releases are then made year around to bring Maybell flows up to the level of the targets, if they are not met otherwise. • The Flow Rec. - Junior scenario, which simulates flow targets based on Revised Flow Recommendations made by the USFWS. Reservoir releases are made year around to bring Maybell flows up to the target level. 0 The CWCB - Junior scenario, which simulates flow targets based on the now - withdrawn CWCB instream flow filing. Reservoir releases are made year around to bring Maybell flows up to the target level. Appendix A Hydrosphere Resource Consultants, 1002 Walnut Suite 200, Boulder, CO 80302