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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damage annually. Miles estimated that 81,000 hectares within the
valley were irrigated with class C4 water, which is the U. S. Salinity
Laboratory's highest rating of salinity hazard for crops.
The objecti ve of this study was to estimate how changes in his-
torical irrigation practices could affect the quantity and quality of
ground Water and surface water in the Arkansas River valley.
Mathematical modeling represents a powerful 1001 to help evalu-
ate the potential impact of changes in irrigation management prac-
tices on the quality and quantity of surface and ground water. In the
Murray Basin in Australia, for example, ground water flow mod-
eling has been used to detennine the effective-ness of alternative
ground water management schemes to prevent stream and land salin.
isation associated with irrigation (Simpson and Herczeg 1991;
Ghassemi et al. 1989). Within the Roswell and Carlsbad Basins in
New Mex.ico, analytical and three-dimensional ground water flow
modeling are currently being used to assess the impact of reduced
ground water pumping on changes in base flow to the Pecos River
in order to satisfy an interstate water compact between New Mexico
and Texas (BarrollI996). A two-dimensional flow and solute trans-
port model(Konikow and Bredehoeft 1978) ~as applied to a ]7.7
km reach of the Arkansas River to examine the relations between
hydrologic stresses (such as irrigation, pumping, and streamflow);
ground-water levelS-in ti1eaIluvlal aqUIfer; a:i1dSaliiiity of ground
water and surface water. Rather than using this model in a strictly
predictive sense, the hydrologic and chemical consequences of
various modeled irrigation management scenarios were compared
to historic conditions.
This model has been applied to this site previously (Konikow
and Bredehoeft I 974a, 1974b; Person and Konikow ]986) and
was originally developed for this area in the Konikow and
Bredehoeft (1974a) study. Konikow and Bredehoeft (1974a) mod-
eled the period from 1971-72 to evaluate how ground water and river
salinity changes in response to varying hydrologic stresses. Konikow
and Bredehoeft (1974b) extended the simulation period to ]975 by
recycling the first year of input data. They assumed that the hydro-
logic stresses measured in the 1971-72 study period were repre-
sentative of long~term averages. The simulation was extended to
1982 by Person and Konikow (1986); however, the model was mod-
ified by incorporating an unsaturated zone solute transport function
and by allowing the quantity of the applied surface water to vary
based on actual streamflow measurements during 1972-82. These
studies emphasized the difficulties in using models to make pre-
dictions of future hydrologic and water quality conditions sub-
stantially beyond the model calibration period. The present study
uses a 24 year study period as the basis for model calibration, a sub-
stantially longer period than previous studies. Natural and anthro-
pogenic effects throughout the 24 year study period produced a high
degree of variability in many hydrologic conditions, including

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Figure 1. Location of study area.

river and irrigation canal flow, ground water withdrawals, and sur-
face water quality. The present st~dy tncorporated this variability
by using monthly data of these conditions to estimate the potential
effects of changes in inigation practices.

Description of Study Area
The study area is located in the alluvial valley of the Arkansas
River, near La Junta in southeastern Colorado (Figure I), and is
about 17.7 km long and 2.4 km wide. This area was chosen because
its hydrologic framework and water use patterns are representative
of much of the Arkansas River valley downstream from Pueblo.
because of the availability of detailed hydrologic data (hydrology.
geology, water use, anJ water quality). and because of the availability
of a previously calibrated model (Konikow and Bredehoef[
]974a).The climate is semi-arid; mean annual precipitation is about
27.9 cmlyr; and mean annual temperature is l2T. The primary crops
in the study area are alfalfa. corn, sorghum, and wheat. About
1993 hectares (about 27%) of land in the study area are flood irri-
gated with surface water and ground water. For the purposes of mod~
eling, the irrigated acreage in the study area was subdivided into
three sub-areas. This subdivision is based on land use patterns, irri-
gation application rates, and inigation canal administration proce-
dures. Sub-areas], 2, and 3 compose about 20, 33, and 47% of the
total irrigated area in the study area. re.spectively (igure 2).
Surface water is diverted for irrigation from the Arkansas
River by the Fort Lyon Canal at its headgate, which is located
about 1.6 km west of the upstream end of the study area. The
unlined canal follows the northern boundary of the study area
(Figure 2) :Uld generally diverts a substantial portion of the stream-
flow from the river (Figure 3). Direct-tlow water rights for the canal

Figure 2. Map of study area, sub-areas of irrigated acreage, and aquifer boundaries.

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