WSP05265

• File Number: 8170
• Description: Arkansas Basin Water Quality Issues
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 2/1/1998
• Author: Goff Lewis Person Ko
• Title: Simulated Effects of Irrigation on Salinity in the Arkansas River Valley in Colorado
• Water Supply Pro - Doc Type: Report/Study

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Range of Error in Water Levels (meters)

Figure 6. FrequenC}' distribution of errors in model-simulated water
levels (simulated minus measured) for individual well locations in
1912,1982, and 1995.

Table 1
Average Observed and Simulated Salinity
ill the Alluvial Aquifer
O~s.eN~ 1J.IlM'I""....dS'ito\;,n\\) Si,m\ll.:ilt~d Salinity
Salinil}' Spatially Spatially
Arithmeticl WeightedJ,l Weightedl,l Percent
Date Average (mgIL) Merage (mgIL) Average (mgIL) Error
February 1972 2262 (25) 2177 (25) 2140(5541 -1.7
F"ebru31')' 198.2 2269 (24) 2146 (24) 2370(554) 10.4
February 1995 1779 (25) 1983 (25) 1770 (554) -10.7
lSpatial1y weigh(~d average computed by the Thel~sell ml:thod .....llh all available data
forcachevem.
2Number In pareJUheses inwc;lles the number of sanlpleslt\ni\e-dlfference cells used In
th~ avo::raging procedure.

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L_~Ch 1993

1238,..

Figure'. Measured and simulated water levels in the aUuvial aquifer,
February 1972, March 1985, and March 1993. Dark surface is shllU-
lated water table. Spheres are measured water le\'els for individual
weUs. 'Where the spheres intersect the dark surface, the calculated 3f1d
measured water levels are the same.

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simulated water I~vels. The frequency "d~~tribtition of errors in
the model-calculated water levels for the 12 individual wells is
shown in Figure 6. For 11 of tile 12 wells. the errors were relatively
small (0 m 10 1.2 m) and reasonably unbiased. Model simulations
for one of the 12 wells were consistently poor and under pre-
dicted, thus making the overall frequency distribution appear neg-
atively skewed (Figure 6).
The simulated water table at individual locations was in good
agreement with measured water levels at individual observation
wells through lime. Figure 7 shows three snapshots of simulaled ver-
sus measured water levels for the monlhs of Febmary 1972 (Figure
7a), March 1985 (Figure 7b), and March 1993 (Figure 7c). These
snapshots are representative of most months throughout the 24
year simulation period. For most wells, there is no consistent bias
in the flow calibrations; simulated water levels fluctuate randomly
about measured elevations in Fi.gure 7. Small errors in simulated ver.
sus measured water levels at individual wells should not have a sig-
nificant effect on the calculated regional hydraulic gradients and
velocities, which are factors that strongly control solute transport.
Dissolved solids concentration data v.."ere measured in water
samples collected from 25 to 26 wells in Febmary of 197 I. 1972,
-12.82 :mcl199S An ~ttemRl~J1S_lDade to sample the ~arne wdLnet=-_
work that was used throughout the study period. In several instances,
wells were destroyed or in a condition mai. prevented sampling. In
these instances, new wells that were located near the old wells were
sampled. The Febmary 1971 data were tile basis for defining ini-
tial conditions for the simulation. The simulated salinity (represented
as a spatially weighted average computed by the Theissen method)
was within 11 % of measured values for 1972, 1982. and 1995
(Table I). A comparison of tile measured and simulated spatial dis-
tributions of salinity indicates that there is reasonably good agree-
ment between the general salinity patlerns with some exceplions
(Figure 8), In 1972 and 1982, a ridge of high salinity (>3000
mgIL) existed in the middle of the area (Figure 8a and b). Although
this general pattern was well simulated, the model overestimated
salinity in some areas for 1982 (Figure 8b). The salinity was found
to vary substantially over a short distance in wells sampled during
the study period. This l,5 due \0 differences in pumping rates across
tbe study area, and temporal differences in the salinity of the water
leaking into the aquifer from the irrigation canaL The variabilily in
salinity over a shon distance and the relatively small number of wells
sampled (25 to 26) might aceOUn! for some of the discrepancy
between simulated and measured salinity in areas where the model
tended to overestimate. In Febmary 1995, measured salinity had
decreased substantially across the study area (Figure 8c). probably
in response to decreased ground water pumping and increased irri-
gation from surface waler sources. The February 1995 salinity
paltern was reasonably well simulated by the model (Figure 8e),
although the model tended to underestimate salinity at the individual
weU locations. The frequency distribution of the error in model-cal-
culated salinity for individual wells sampled in 1972. 1982, and 1995
is shown in Figure 9. Although some of the errors are ratjJer large.
the errors are reasonably distributed about zero, which indicates rel-
atively unbiased estimates. Ba'ied on the results of the model cal-
ibration, the model satisfactorily reproduces hisloric spatial patterns
of ground water levels and salinity, although prediction errors for
some individual well locations are relatively large. Therefore, the
obvious strength of the model is to simulate general water level and
salinity trends or patterns, which is well suited to its intended use
in the srudy.