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Stimulated Effects of Irrigation on Salinity in the Arkansas River Valley in CO
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Stimulated Effects of Irrigation on Salinity in the Arkansas River Valley in CO
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7/20/2010 2:54:25 PM
Creation date
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Water Supply Protection
Description
ARCA
State
CO
KS
Basin
Arkansas
Water Division
2
Date
1/1/1998
Author
Ground Water Vol. 36(1), Karin Goff, Michael E. Lewis, Mark A. Person, Leonard F. Konikow
Title
Stimulated Effects of Irrigation on Salinity in the Arkansas River Valley in CO
Water Supply Pro - Doc Type
Report/Study
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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 />
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