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.. j_: ~'l: Simulated Effects of Irrigation on Salinity in the Arkansas River Valley in Colorado by Karin Got'!", Michael E. Lewisb, Mark A. Person<, and Leonard F. KOlUkowd Abstract Agricultural irrigation has a substantial impact on water quantity and quality in the lower Arkansas River valley of southeastern Colorado. A two.dimensional flow and solute transport model was used to evaluate the potential efTeets of changes in irrigation on the quantity and quality of water in the alluvial aquifer and in the Arkansas River along an 17.7 km reach of the river. The model wassalibrated to aquifer water level and dissolved solids concentration data collected throughout the 24 year study period (1971. 95).1\'0 categories ofirrigation management were simulated with the calibrated model: (1) a decrease in ground water withdrawals for irrigation; and (2) cessation of all irrigation from ground water and surface water sources. In the modeled category of - - ---decreascdirrigation-from.ground-water.pumping,therewas.a -resulting6.9%-deerease-in-the.average.monthly.ground-watcr_salin._ ily, a 0.6% decrease in average monthly river salinity, and an 11.1 % increase in ground water return flows to the river. In the mod- eled category of the cessation of all irrigation, average monthiy ground water salinity decreased by 25 %; 3,,'erage monthly ii,'er salinity decreased hy 4.4%; and ground water return Oows to the river decreased by an average of64%. In all scenarios, simulated ground water salinity decreased relative to hi,torical conditions for about 12 years before reaching a new dynamic equilibrium con- dition. Aquifer water levels were not sensitive to any of the modeled scenarios. These potential changes in salinity could result in improved water quality for irrigation purposes downstream from the alTected area. Introduction Degradation of the quality of surface and ground water in arid to semi-arid regions in general, and in the lower Arkansa5 Rh/er valley of southeastern Colorado in particular. are closely related to extensive- agricultural diversions and usage, primarily because the consumption of irrigation water by evapotranspiration increases pro- portionately the salinity of return flows. These effects are mediated during high streamflow periods within such a region when salinity tends to be relatively low because of dilution. Beginning in 1995 in the Arkansas River Basin. ground water withdrawals have been more closely monitored and Coiorado ground water pumping reg- ulations have been more strictly enfmced by \he state of Colorado than in the past. Therefore. the volwne of water pumped in the future may substantially decrease relative to historic withdrawals. Adkins (1996) stated that, with respect to the new pumping regulations, "some farmers will have to curtail their acreages and some will be forced out of business." In addition, about 19.440 hectares of his- torically irrigated land in the lower Arkansas River valley, as of 199G. have ceased to be irrigated following the purchase of agricultural water rights for municipal supplies (Howe et at 1990). These 'Integrity Solutions, 2900 Lone Oak Pwky., Eagan, Minnesota 55121. bU.S. Geological Survey, 201 W. 8th Street, Pueblo, Colorado 81003. CDepartmenl of Geology and Geophysics, University of Minnesota. Minneapolis. Minnesota 55455. dU.S. Geological Survey. 431 National Center, Reston. Virginia 22092. Received May 1996, accepted March 1997. 76 changes in water use in the lower Arkansas River valley could affect the interaction between surface and ground water as wel1 as water quality. Al\hough ?mblems associated with salini\)' and irrigated agriculture are common throughout the western United States, the impact of proposed changes in water management practices on water quality along the Arkansas River valley have not been systemati- cally studied. \ Batie and Healy (1983) describe excess salinity as the "most pervasi ve problem associated with irrigated agriculture." Lefkoff and Gorelick (1990) estimated that about 25 to 35% of the irrigated land in the western United States is serioosly affected by elevated salinity. The Ark.ansas River valley in southeastern Colorado is no exception: irrigated land in this area receives irrigation water from the Arkansas River, which is one of the most saline rivers in the United States (Miles 1977). Elevated salinity in ground water and surface water in the lower Arkansas River valley is primarily related to the down-valley reuse of water for irrigation. Although much of the applied irrigation water is lost to evapotranspiration, some of the applied water recharges the aquifer and provides return flow to the river. The concentrating effect of evapotranspiration. and lhe subsequent reuse of the irrigation return nows, serves to increase salinity down valley. In this paper, "salinity" is used to mean total dissolved solids concentration. Average salinity in the lower Arkansas Ri ver ranges from about 500 mgIL at Pueblo to 3500 mgIL at the Colorado-Kansas state line about 255 kIn downstream (Cain 1987). Average salinity in the alluvial aquifer ranges from about 1500 mgIL near Pueblo to 4100 mgIL at the state line (Cain 1987). Miles (1977) reponed that elevated salinity (> 2000 mglL) in the Arkansas River valley was causing several million dollars of crop Vol. 36, No. I-GROUND WATER-january-February 1998 II \ I I