SIMULATED EFFECTS OF IRRIGATION ON SALINITY IN THE ARKANSAS RIVER VALLEY IN COLORADO

Agricultural irrigation has a substantial impact on water quantity and quality in the lower Arkansas River valley of southeastern Colorado, A two-dimensional now and solute transport model was used to evaluate the potential effects of changes in irrigation on the quantity and qua lity of water in the alluvial aquifer and in the Arkansas River along an 17.7 km reach of the river, The model was calibrated to aquifer wat er level and dissolved solids concentration data collected throughout the 24 year study period (1971-95). Two categories of irrigation manag ement were simulated with the calibrated model: (1) a decrease in grou nd water withdrawals for irrigation; and (2) cessation of all irrigati on from ground water and surface water sources, In the modeled categor y of decreased irrigation from ground water pumping, there was a resul ting 6.9% decrease in the average monthly ground water salinity, a 0.6 % decrease in average monthly river salinity, and an 11.1% increase in ground water return flows to the river, In the modeled category of th e cessation of all irrigation, average monthly ground water salinity d ecreased by 25%; average monthly river salinity decreased by 4.4%; and ground water return nons to the river decreased by an average of 64%. In all scenarios, simulated ground water salinity decreased relative to historical conditions for about 12 years before reaching a new dyna mic equilibrium condition, Aquifer water levels were not sensitive to any. of the modeled scenarios, These potential changes in salinity cou ld result in improved mater quality for irrigation purposes downstream from the affected area.