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Stimulated Effects of Irrigation on Salinity in the Arkansas River Valley in CO
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Stimulated Effects of Irrigation on Salinity in the Arkansas River Valley in CO
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7/20/2010 2:54:25 PM
Creation date
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Metadata
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Water Supply Protection
Description
ARCA
State
CO
KS
Basin
Arkansas
Water Division
2
Date
1/1/1998
Author
Ground Water Vol. 36(1), Karin Goff, Michael E. Lewis, Mark A. Person, Leonard F. Konikow
Title
Stimulated Effects of Irrigation on Salinity in the Arkansas River Valley in CO
Water Supply Pro - Doc Type
Report/Study
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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 />
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