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Daily discharge data from the identified dischargers was not 1listorically recorded for the <br />desired period of record and therefore could not be directly obtained, with fl1e exception of <br />intermittent daily records obtained from the Denver Metro Wastewater District. In order to <br />include daily discharge data for all of the major industrial and municipal dischargers in the <br />mass balance calculations, daily values were estimated. <br />Industrial and municipal daily data was estimated using two separate procedures. hdustrial <br />discharge was estimated using 1listorical monthly discharge flow from approximately 2002 <br />tllroug112007 obtained from t11e EPA. To calculate daily values, average monthly values were <br />calculated and then divided by t11e number of days in the month. This method was deternuned <br />to be adequate for the purposes of this evaluation. <br />Municipal discharge was based on population data and an Indoor Return Rates calculated for <br />each municipality by county included in the SPDSS study area. The population data set was <br />developed using available population data from HydroBase and a linear interpolation method <br />to fill missing data consistent with Task 66.2 (LRE, 200'7b). hldoor Return Rates for each key <br />municipality within a county were based on engineering estimates provided by the SPDSS <br />Consumptive Use contractor. The reported rates were utilized to estimate gallons per capita per <br />day (gpcd) of wastewater flow directly discharged i11to fl1e South Platte River, Cherry Creek or <br />the Cache la Poudre River. The yearly population value was multiplied by the gpcd to obtain an <br />average discharge value for eac11 municipality. T11e resultant yearly discharge values were <br />varied on a monthly basis to account for seasonal variations in water use. The basis for t11e <br />monthly variations was developed from wastewater discharge obtained from fl1e Metro <br />Wastewater Reclamation District. <br />2.0 Methodology <br />A spreadsheet-based approach called t11e Pilot Point method was used to estimate monthly <br />baseflow values for each reach evaluated for the study period, which was from 1950 to 2005. <br />The Pilot Point method determines a daily water balance by summing $Ze measured daily <br />surface water inflows and outflows for each reach. This is defined for the purposes of this TM <br />as the mass balance. T11e Pilot Po>11t method then applies constraints to limit extreme flow <br />values, and then applies a lonb term average to produce a smoothed result of net gauz or loss <br />which is defined as the estimated baseflow. <br />Prior to implementing the Pilot Po>11t method to estimate baseflow, the use of a traditional mass <br />balance estimation method was evaluated. The mass balance method is better suited to <br />estimating baseflow in watersheds that are not influenced by human activities. Flows >11 the <br />South Platte River are highly influenced by agricultural diversions and other human activities <br />so the mass balance method could not accurately estimate baseflow. A comparison of the mass <br />balance approach and the Pilot Point method is discussed on Appendix C. <br />The Pilot Point method was initially implemented to estimate stream gains and losses for <br />groundwater modeling in t11e Republican River Basin and was later adapted to estimating <br />baseflow in Rio Grande River Basin in Colorado. T11e original method used on the Republican <br />was aspreadsheet-graphical method that utilized control or "pilot" points with an <br />SPDSS Phase 4 Task 46 Technical Memorandum -Final <br />0~4i 10i0~ <br />