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In these scenarios, flow was maintained in the Fort Lyon Canal to <br />satisfy downstream irrigation demands outside of the study area. In <br />the four scenarios, average ground water salinity decreases ranged <br />from 4.6 to 25% (Table 3). The decrease in ground water salinity that <br />resulted in scenarios 1 to 3 was related to the decreased irrigated <br />acreage and the historically applied irrigation water in those subar- <br />eas. The largest decrease in ground water salinity was observed when <br />irrigation ceased in all three sub -areas (scenario 4). In scenario 4, <br />average monthly ground water salinity decreased from 2180 to <br />1630 mg/L (Table 3). For all scenarios, ground water salinity <br />decreased steadily for about 12 years following the beginning of the <br />simulation period and then approached steady -state conditions <br />thereafter (Figure 12). This steady -state condition is coincident <br />with the simulated aquifer residence time of 10 to 15 years as <br />reported by Konikow and Person (1985). The decreases in river salin- <br />ity were related to the magnitude of the decreases in irrigation in sce- <br />narios 1 to 4 (Table 3), and reached an average maximum decrease <br />of 4.4 %. Salinity decreased in the river because of a decrease in the <br />quantity and salinity of return flows from the aquifer. <br />Water levels were relatively insensitive to changes in irri- <br />gated acreage. Although the river experienced a net gain in flow <br />between the upstream and downstream ends of the study area, the <br />decreases In net gains In strearnflow ranged from 7.9 to 64% in the <br />four scenarios (Table 3). Scenario 4 had the largest effect on <br />streamflow gains, decreasing the gains from 0.18 to 0.06 m /s. <br />In the preceding four scenarios, the effect of the hydraulic head <br />in the canal was large, and leakage from the canal was a major com- <br />ponent of aquifer recharge. Based on model calculations, average <br />daily recharge to the aquifer from the canal was about 0.17 m /s. <br />The canal is 193 km long, has a sandy bottom, and delivers water <br />to about 37,500 hectares of irrigated land (Gronning Engineering <br />Co. 1993). In the model simulations, 5.5% of the water diverted into <br />the canal was applied to 1993 hectares in the study area. Therefore, <br />most of the water diverted from the river into the canal flows <br />through the study area for delivery to downstream irrigators. <br />Consequently, regardless of the modeled scenarios, the large vol- <br />ume of water remaining in the canal provides a substantial amount <br />(about 22 %) of recharge to the aquifer underlying the study area. <br />A simulation was performed to determine the consequence on <br />the water quality and quantity of removing all the water from the <br />canal. Simulation results were quite different when the scenario of <br />100% decrease in irrigated acreage was run with no water in the <br />canal, thus eliminating recharge to the aquifer from the canal. This <br />scenario duplicates conditions that could be associated with the trans- <br />fer of all water from an entire irrigation system, which could take <br />6 _ - <br />place if all water rights in a canal system were purchased and that d <br />water was transferred to other uses (e.g., municipal or industrial). <br />In that case, the transferred water typically would be diverted from <br />the river upstream from the study area. Therefore, the transferred <br />water would not represent a recharge source for the aquifer. In the <br />scenarios of 100% decrease in irrigated acreage with and without <br />flow in the canal, average monthly streamflow gains decreased 64 <br />and 86 %, respectively (Table 4). Average monthly water levels in <br />the aquifer declined about 0.09 and 0.18 m, respectively (Table 4). <br />This degree of change in the average water level may be the result <br />of the model assumption that transmissivity doesn't change over <br />time. In future modeling efforts, it may be advantageous to define <br />transmissivity as a function of the total saturated thickness. Salinity <br />in the aquifer and river did not vary substantially between the sim- <br />ulations with or without flow in the canal (Table 4). In the model <br />simulation with flow in the canal, recharge was supplied from <br />both the canal and the river; whereas, in the model simulation <br />with no flow in the canal, most recharge was supplied from the river. <br />Both sources of recharge have nearly identical salinity. <br />Discussion <br />This study demonstrates the nature and magnitude of changes <br />in flow and salinity that could occur in response to the evaluated <br />changes in water use practices in the study area. It would be of great <br />value if model results from this study can be extended to other por- <br />tions of the lower Arkansas River valley in order to assess changes <br />in water quality and quantity at the basin scale. Because the data <br />requirements (e.g., salinity levels, recharge, ground water pumpage, <br />aquifer characteristics, etc.) for the model are not available for the <br />entire valley, it would be difficult to construct a similar model over <br />the entire valley. While the states of Colorado and Kansas have each <br />developed basin -wide models that evaluate the effects of post -com- <br />pact wells and the winter water reservoir storage program, many <br />simplifying assumptions were made to develop these models and <br />both models had their shortcomings (Simpson 1996). Furthermore, <br />neither of the models addresses the issue of water quality. <br />It is possible to use a simpler model that would require less data <br />to estimate the effects that changes in irrigation practices could have <br />on the quantity and quality of water over the entire valley. For exam- <br />ple, McLin (198 1) used a generalized lumped parameter model on <br />the same study area as the Konikow and Bredehoeft (1974b) model. <br />During the March 1971 to February 1972 study period, McLin's <br />results compared favorably with the results obtained by Konikow <br />and Bredehoeft (1974b). While limitations exist in representing <br />84 <br />Table 4 <br />Model Results for Scenarios of Complete Decrease in Irrigated Acreage With and Without Flow in the Canal <br />Alluvial Aquifer <br />Arkansas River <br />Average Monthly <br />Average Monthly Water <br />Average Monthly Average Monthly <br />Model Run Salinity (mg/L) <br />Level (m above mean sea level) <br />Salinity (mg/L) Streamflow Gains (m /s) <br />Base Condition 2180 <br />1224.42 <br />1810 0.176 <br />Complete Decrease in Irrigated Acreage <br />With Canal Flow 1630 (-25 <br />1224.33 <br />1730 (-4.4 0.064 (-64 <br />Without Canal Flow 1650(-24 <br />1224.24 <br />1740 (-3.9 0.025 (-86 <br />'Numbers in parentheses indicate the percent difference between the modeled base condition and the indicated scenario. <br />84 <br />