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Monthly municipal ground water pumping data were collated <br />from two sources: data for 1971 -76 were estimated using power con- <br />sumption data (Boyle Engineering Corp. 1990); data for 1977 -94 <br />were metered by the city of La Junta. Monthly irrigation ground <br />water withdrawals were estimated with power consumption data. <br />Irrigation pumping for 1971 -85 was estimated by Boyle Engineering <br />Corp. (1990); pumping for 1986 -94 was estimated with power <br />consumption data for individual wells using the same methodology <br />as Boyle Engineering Corp. (1990). Ground water pumped for <br />irrigation purposes was equally applied to the cell in which the pump <br />was located and the two adjacent cells in the north and south direc- <br />tion. This application practice was chosen for each well's applied <br />water because it closely matched application practices documented <br />in the field and it was convenient for finite difference approxima- <br />tions used in the model. The amount of ground water pumped every <br />year varied inversely with surface water applications (Figure 4). <br />During 1972 -82, the average annual irrigation application rate <br />from ground water sources was about 0.6 m/yr, which represented <br />about 50% of the total applied water. Application from ground <br />water sources in 1983 -94 decreased to 0.4 m/yr, or 33% of the total <br />applied water. <br />P oientiale-vapot rwmpkation and consumptive use by plants <br />were estimated using a modified Blaney - Criddle equation. This <br />equation incorporates mean tmonttiij% tetitperature, monthly percent <br />of daylight hours, and an empirical consumptive use crop coefficient. <br />Phreatophyte evapotranspiration was assumed to take place at all <br />stream cells at a rate equal to potential evapotranspiration. <br />Ground water underflow takes place across the alluvium at the <br />upstream and downstream ends of the study area. Using Darcy's law, <br />underflow through each of the boundary cells was calculated on the <br />basis of the hydraulic gradients that existed at the beginning of the <br />study period (February 1971). Underflow into and out of the study <br />area was estimated to equal 0.05 m /s and 2.5 X 10 m /s, respec- <br />tively. These rates were assumed to remain constant throughout the <br />study period. Ground water flow between the alluvium and the val- <br />ley walls and the underlying bedrock surface was assumed to be <br />insignificant, due to the relative impermeability of the bedrock. <br />Water quality data used in the solute transport calculations <br />include dissolved solids concentration of the river, Fort Lyon <br />Canal, and recharge from applied irrigation water. Mean monthly <br />salinity in the river at La Junta and in the Fort Lyon Canal was esti- <br />mated using linear regression with streamflow as the independent <br />variable and dissolved solids concentration as the dependent vari- <br />able. Salinity of the applied surface water was assumed to equal the <br />salinity of the canal. Salinity of ground water recharge from applied <br />irrigation water was based on the total salt mass of the applied water <br />minus the salt mass lost to tailwater. The increase in total dis- <br />solved solids concentration of the recharge was related to the <br />decrease in recharge volume resulting from evapotranspiration <br />losses of the applied water. <br />Model Calibration <br />The selection of the initial aquifer and boundary conditions was <br />based on the previous work by Person and Konikow (1986). In the <br />calibration process, several aquifer parameters were adjusted within <br />a limited range to obtain a best fit between measured and simulated <br />ground water levels and ground water dissolved solids concentra- <br />tion. The parameters adjusted included effective porosity, stream bed <br />and canal bed transmissivity, and the ground water recharge frac- <br />tion. Through trial and error, a best fit was obtained with the orig- <br />inal parameter values for effective porosity and transmissivity of the <br />stream bed and canal bed; effective porosity was 0.2, transmissiv- <br />ity of the stream bed and canal bed varied spatially and ranged from <br />4.7 m /s to 0.02 m /s and from 1.9 X 10 -3 m /s to 0.01 m /s, <br />respectively. It was necessary to change the recharge fraction <br />from the values used in the initial study. The recharge fraction is the <br />fraction of the total applied water (including precipitation) that is <br />recharged to the aquifer. It was calculated based on the following <br />equations, depending on the value of the normalized applied water: <br />Rf w(L +1 -1I +1 if W2.1 <br />R = + 1 ifws1 <br />where <br />w is the normalized applied water (A/E) <br />A is the total applied water <br />E is the potential evapotranspiration <br />R is the recharge fraction (R/A) <br />R is the total recharge <br />L is the recharge parameter (Konikow and Bredehoeft 1974a). <br />The recharge parameter is a dimensionless fitting parameter that <br />cLC:COUrlts for file l:Gtltbined effects Gf JGVeral ptiysiCai, ClititauiC and <br />model characteristics. Model calculated ground water salinity was <br />sensitive to changes in the recharge parameter. Konikow and <br />Bredehoeft (1974a) used an average recharge fraction of about <br />32% over the one year study period. For the current study, a best fit <br />between the calculated and observed ground water quality and <br />quantity over the 24 year study period was obtained by using an <br />average recharge fraction of about 36 %. <br />The model was calibrated for the 24 year period to average <br />measured aquifer water levels and dissolved solids concentration <br />data. Water level data were collected at 12 wells throughout the 24 <br />year study period; water level measurements were typically made <br />in March. For calibration purposes, a monthly average water level <br />was calculated from all water level measurements made during a <br />particular month. A comparison of the measured and simulated <br />average water levels is shown in Figure 5. The average error <br />between simulated and measured average water levels (simulated <br />to measured) was — 0.12 m. Although the general temporal trend <br />of the measured water levels was reasonably well simulated <br />(Figure 5), the model tended to overestimate water levels prior to <br />1981 and underestimate water levels after 1981. The relatively small <br />number of wells (12) used to compute the average water levels <br />probably resulted in some of the error between the measured and <br />JI 1 14 A <br />Yenn <br />Figure 5. Comparison of model - simulated average monthly ground <br />water levels and average measured water levels. <br />79 <br />two Im Im UW /M6 <br />