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<br />'. <br /> <br />, <br /> <br />0252 <br /> <br />if irrigation canals were lined in one area of the <br />system, one would like to answer (at least) ~ <br />questlon9: Will the water savings from the reduction <br />of canal seepage losses lead to a si~niflcant in- <br />creaSe in degree of Biltisfaction of lr["l~ation loIater <br />requirements? Will canal lining result in a sl~nlfi- <br />~ decreage In aquiCer return flows to the streae? <br />Other questions ~hould be raised if farmers in- <br />vest 1n measures which ....111 increase farm irrigation <br />efficiency. Will the result be a 5JKnific~nt amount <br />of "ne...." ""ater avail.able for other beneUcial uses~ <br />What effect would be Been in aquifer return flows to <br />the river and in the amount of water flowing out of <br />the state1 <br /> <br />If pumping from the aquifer were allowed to in- <br />crease, lolould the result be chaos in the priority wa- <br />ter rights system? Would les9 water be tost down- <br />stream out of the state? <br /> <br />A COMPUTER MODEL OF THE WATER SYSTEM <br /> <br />The answers to the many a~d varied questions <br />which naturally come to mind reg~rding the system's <br />behavior ~nder changed physical or managerial cir- <br />cumstances are very difficult to secure for a complex <br />water use system such as the South Platte basin, par- <br />ticularly if the answers are to be quantitative at the <br />operational level. The only feasible approach is to <br />simulate the ...,ater system with a model in which vari- <br />ous elements can be manipulated. <br />A computer program was written to represent <br />(simulate) in great detail the physical and o~era- <br />tional characteristics of the lower South Platte <br />basin. The resulting developed program (model) 8imu- <br />lates lJ reach of the SLlutlJ Platte from a point 91tght- <br />ly upstream of the Balzac U.S.G.S. stream gauging <br />station to a point slightly downstream from the JuleB- <br />burg gauge at the Colorado-Nebraska state lin~. The <br />length of the simulated river is approximately 90 <br />miles. <br /> <br />, <br />Ill....... <br /> <br />The model is .able to predict behavior of water <br />the aquifer (flo~ and water table elevation) at 1057 <br />points on a grid 93 cells long by 14 cells wide sure <br />imposed on the 5tudy area. The river is thus divide, <br />into 9) separate r~ache8. each of which is mudel~d t <br />predict ground water inflow (retutn (low), diversion. <br />and instream flow. Pumping from the existing wells <br />in each grid ceLL i5 included 1n this modei. <br />Hydrologic conditions of the river-aquifer sys- <br />tem are computed at weekly intervals for a period of <br />10 years. Historical data forlll the basis for tbe <br />roodel. These include weekly diversions at )3 points <br />on the river, weekly streamflows at Balzac and Jules <br />burg, weekly precipitation and crop consumptive wate <br />requi~ementa. etc. <br />Tne main computer program performs the 8ame s~- <br />quence of calculations for every week. Schematicall" <br />the steps in the calculations are as foltows: <br /> <br />" <br />L Given the river inflow into the study ateal~ <br />the le~al water availability is determined at each' <br />diversion point. This legal water availability is <br />calculated as the upstream river i~fl~ pius the aqu. <br />fer return flows upstream of the diversiOn point <br />minus the sum of all diversions of higher seniority. <br />regardless. of locati.:Hl on the river. The calcula- <br />tions arc performed starting with the diversion of' <br />the most senior priority date down to the one ~ith <br />most junior priority. Note that the phYSical water <br />availabil~ty at a diversion point exceeds the legal <br />~ater availabtlity by the amount of right9 more <br />G~nior located farther downstream. <br /> <br />2. Given the just calculated legal an.d physi- <br />cal ~ater availabilities, a decision is made as <br />to the actual amount of water to be diverted for <br />the week from each diversion point. For e~ample, ~ <br />purel~ historical ~ater allocation strategy consists <br />of the precise diversion that was made historically <br />9n that date. (The model was calibrated using this <br />data). A E-urelv legal strategy consists of divert- <br />ing exactly the full ~ater right (no more, no lessJ <br /> <br />2 <br />