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I I I I I I I I I I I I I I I I I I I 4.3 EFFECTS OF TAYLOR PARK RESERVOIR REGULATION Taylor Park Reservoir is located on the Taylor River approximately 16 miles upstream of the confluence with the Gunnison River. Construction of the reservoir began in 1935 with the first releases made in 1937. The Bureau of Reclamation owns and operates the dam in conjunction with the Blue Mesa Reservoir (downstream on the Gunnison River) primarily to augment irrigation supply in the later portions of the growing season. All releases are made directly downstream; there are no diverted releases from this reservoir. The reservoir does not serve as a flood control facility; however, a primary operations objective is to avoid discharge flowing over Taylor Park Dam's uncontrolled spillway. In addition, attempts have reportedly been made to reduce peak flows if a request is made for reduction in outflows (Stanton, 2002). Current operation practices tend to reduce peak flows as some of the peak runoff is retained for release throughout the irrigation season extending into October. However, if the reservoir is full, higher releases are possible. The Taylor Park Reservoir downstream regulated flow data (discharge records for Taylor River below Reservoir) are used in conjunction with changes in reservoir storage to generate the downstream unregulated or natural flow data. The unregulated flow data is then compared to regulated data to determine the effects of the reservoir. A detailed explanation of the procedure used to develop the unregulated flow data follows. The daily change in storage in the reservoir, reported in acre-feet-per-day, is converted to cubic-feet-per- second. Positive change in daily storage represents water that would have been in the channel just downstream of the reservoir; it is therefore added to the regulated flow data. A negative change in daily storage indicates that the discharge, as reported at the Taylor River below Reservoir gage, should be decreased to accurately represent what would have been flowing in the channel. Storage records for Taylor Park Reservoir are available from the Bureau of Reclamation beginning on October 1, 1958. Inflow records are also available; however, inflow data is calculated by Reclamation using outflow records (Taylor River below Reservoir) and daily storage information. Therefore, this data does not provide any additional information for this analysis. The reported storage values are somewhat sporadic due in part to the measuring techniques and to weather conditions such as wind and rain. The daily values of storage are therefore averaged over a three-day period to help account for the inherent variability in the reported values.2 An unregulated flow daily data set is created for the Taylor Park below Reservoir gage beginning on October 1, 1958 and extending to September 30, 2000. This flow data series provides 42 years of daily flows and annual maximum daily flow values. The annual maximum daily flow value must be adjusted to estimate the instantaneous peak value used to perform flow frequency analyses. This is done by estimating a ratio of instantaneous peak flow values to the daily flow value reported on the same day for a similar gage with both data sets available. The Taylor River at Taylor Park Gage is located upstream of the reservoir. Because this gage is not affected by the reservoir, the instantaneous peaks and corresponding daily value sets from this gage are used to determine the ratio. The resulting ratio of 1.2481 is applied to the unregulated annual maximum daily flows at the Taylor River below Reservoir gage, located just downstream of the reservoir outlet. The regulated instantaneous peaks, as reported at the Taylor River below Reservoir stream gage, are compared to the simulated unregulated instantaneous peaks for the same location. The results, depicted in Figure 4.2.1, indicate, as expected, that the peaks regulated by the Taylor Reservoir are less than what would have occurred in an unregulated scenario. 2 This method of using a three-day average is recommended by Coil Stanton of the Bureau of Reclamation, Grand Junction office. (Coli Stanton, Hydraulic Engineer, Water Resources Group, personal communication, 2003) 12