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001307 . . Draft Task 9 Technical Memorandum 205 square miles. To accurately estimate hydrologic conditions at the dam, a correction factor was applied to translate the data from gages 9245500 and 9245000 downstream. This correction factor was developed by considering elevation influences an<! additional contributing drainage area. Total annual yield at the North and Main Forks confluence was multiplied by a factor of 1.265 to produce an average annual yield at the Elkhead Dam site of 64,550 af. This factor is considerably less than the factor which would be used if only contributing drainage area was considered and reflects influence of decreased elevation on precipitation. The annual total was then disaggregated into monthly values by applying an average annual distribution observed over the periods of record for the sum of gages 9245000 and 9245500. By comparison, a 1987 report by Morris Knudson Consulting Engineers estimated the average annual yield of Elkhead Creek at the site of the dam to be approximately 71,200 af. 4. The method of generating inflow hydrology for Elkhead Reservoir was checked by continuing to translate the data from gages 9245500 and 9245000 all the way down to the confluence with the Yampa River. At this point, the synthetic data was compared with data recorded at USGS gage 9246500 for which data are available for the years 1910 through 1918. This comparison suggested that the method of downstream translation of gage data may over estimate the actual yield of Elkhead Creek at the confluence with the Yampa River by approximately 10 percent. To some degree this is a result of difficulty correcting for elevation influences in the contributing drainage area and the error is likely more pronounced the further downstream the data is moved. Furthermore, this comparison may be somewhat inappropriate in that an average gage value between 1910 and 1918 is compared to an average estimated value between 1930 and 1982. Finally, the roughly 10% error between predicted and recorded values at the confluence with the Yampa River is only slightly greater than the expected gage error. Significance of Elkhead Creek Water Rights Several direct flow water rights are located on Elkhead Creek upstream and downstream of Elkhead Dam. The majority of the rights are associated with very small ditches or are appropriations on small springs or very small tributaries. However a few of these rights could affect the storable yield of Elkhead Creek. The effect of water rights diverting above gages 9245000 and 9245500 is implicit in the estimated yield of Elkhead Creek because the estimate is based on the depleted gage records. However, when translating the gage records at gages 9245000 and,92455oo down to the reservoir site, the effects of ditches which divert in the intervening reaches must be considered. 3 ;,li;..'_J;;