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distribution of available water level data that will be used i11 each of the model calibration <br />periods. An important aspect of selecting the transient calibration period is that it contains a <br />large munber of the wells that include at least four measurements i11 a given year i11 order to <br />adequately evaluate seasonal changes in water levels. Figure 3-4 shows the number of wells <br />that have at least four measurements per year. Many of these are from SPDSS-installed wells <br />that are equipped with data loggers. As shown uz the Figure 3-4, periods with a relatively large <br />number of wells t11at fit this criterion (defined here as at least 20 wells) include 1967-68, 1974-78, <br />1989-94, and 2000-2006. The last two of these periods generally include over 50 wells per year. <br />Table 3-1 lists the number of wells and measurements available for eac11 period. <br />Table 3-1 Summary of Groundwater Measurements for Model Calibration <br /> Steady-state Transient <br /> Calibration Period Calibration Period Validation Period <br /> 1991-1994 1999 - 2005 1950 - 2006 <br />Number of wells measured 391 605 4,811 <br />Number of measurements 3,205 23,826 75,049 <br />Number of wells with at least 82 170 431 <br />4 measurements in a ear <br />3.1.2 Streamflow <br />Arulual streamflow at three gages located throughout the model area were used to evaluate <br />hydrologic conditions: <br />• Sout11 Platte River near Kersey (6754000) gage, <br />• Soutll Platte River at Henderson (6720500) gage, <br />• Cache la Poudre River near Greeley (6752500) gage. <br />The data from these gages were compared to eac11 other over the period of record; relatively wet <br />and dry cycles at the three gages were fowld to be in general agreement, as shown on Figure 3- <br />5. The average flow at each gage for various periods of record including each calibration period <br />are shown in Table 3-2. The difference in average flows for eac11 period of record is compared <br />to the full period of record for each gage and was used i11 the evaluation. <br />The results in Table 3-2 show that average Streamflow was higher during the study period <br />(1950-2005) than the period of record average flow for eac11 gage by 8 to 18%. T11e steady-state <br />period 11as lower Streamflow t11an the period of record average, by about 15%. T11e Streamflow <br />during this period is lower t11an bot11 the full period of record and t11e study period but other <br />criteria (suc11 as the number of observation wells) support t11e use of t11e selected calibration <br />periods. The transient period has Streamflow ranging from -8 to 5% of the average flow for <br />each gage. This period shows significant variability in streamflow (Figure 3-5), wit111999 <br />showing very high flows at the Kersey and Henderson gages, 2002 showing some of the lowest <br />flows on record, and 2005 having average flows. <br />P~ SPDSS T~8 2 Final TM 10-08-0t~.doc 7 <br />