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PROBLEM <br />The Lower Arkansas River Valley in southeast <br />Colorado is primarily devoted to agricultural <br />production and is dependent on the quantity <br />and quality of irrigation water. Recent issues <br />in the administration of water rights in Colorado <br />have the potential to change ground -water <br />surface -water relations and water quality. <br />OBJECTIVE <br />To estimate changes in salinity in the alluvial <br />aquifer and the Arkansas River as a result of <br />the following changes in irrigation practices: <br />(1) reductions in irrigation from ground -water <br />pumpage: and (2) reductions in irrigated acre- <br />age following the transfer of water rights from <br />agricultural to municipal use. <br />SCOPE <br />Several scenarios of potential changes in hist- <br />orical irrigation patterns were modeled with <br />the U.S. Geological Survey Method of Character- <br />istics solute transport model. The study area is <br />an 11 -mile reach of the Arkansas River near <br />La Junta, Colorado. The study period was from <br />February 1971 through February 1995. <br />i <br />-- - ----- - ---------- <br />Measured Salinity Concentrations -- February 1995 <br />Simulated Salinity Concentrations -- February 1995 <br />In each of the sampling events (February 1972, 1982, and 1995) 25 to 31 wells were <br />sampled. The model was calibrated to the measured aquifer salinity concentrations. <br />Cor.caADO L "- <br />4 - -- <br />r.e,l <br />Location of model boundaries <br />Crowley study Area <br />Aux ss I I _ <br />Bubb r T <br />I Pueblo ' <br />Prower <br />106• � `\ i( I'! oltxo 1j <br />104' 108' <br />o to 20 30 40 60 MLES <br />Location of study area <br />Appmilmayy5fea of rrder pperrate There era l,B20 xresof r,ieyaa b,dinO,e eAldy aee,Thep,Ynary sap re e9alla, corn, <br />bn9b appleC Vb9ood 1,9ga1oniran so,9hm,aM W,eat. <br />pmpa aaier allmbneamgoi„axeler <br />Pa <br />LOW <br />HIGH <br />RN4r Ylw Uprlrerr19r4af CN4ny14tAeMS WYSLM <br />The continuous use and reuse of water for irrigation in the lower <br />Arkansas River Valley concentrates salts In the surface water <br />and ground water. Theelevated salinity concentrations cause <br />reduced crops yields. <br />4AM <br />ALLUVIAL AQIAFER <br />94frh 17 <br />1) <br />-T <br />3,810 <br />(m9y 6 ?y <br />6aee Cana, 2,160 40145 <br />1010 6.3 <br />3.00o <br />25 %Redman 2,170 4,014.6 <br />AVERAGE 9ALNRY GONLIEN1RA710M9 <br />N TM ARKANSAS iMVER <br />E <br />1,610 6.4 <br />100%Rediaon 2,030 4,014.7 <br />1 X00 72 <br />a <br />63 <br />3 <br />Meal Dry W <br />2XW <br />4,014.6 <br />1,610 <br />52 <br />Mee 2 Dry ip <br />2,010 <br />4AI4.6 <br />1,760 <br />SA <br />Met 3Dry,p <br />ipw <br />4PI42 <br />1,790 <br />3.1 <br />100%D,yip <br />1,630 <br />4X742 <br />�Lau9en of <br />1,000 <br />A <br />S <br />iuyy Arr <br />600 <br />� <br />4m <br />3w <br />3m >® 1m tit <br />® 9 <br />SIMULATED EFFECTS OF CHARGES IN IRRIGATION ON SALINITY <br />IN AN 11 MILE REACH OF THE LOWER ARKANSAS RIVER VALLEY, <br />SOUTHEAST COLORADO <br />by <br />KM. ao9, 11nNea9yof M —.1)Ppe -m ape=Y ( phyW- <br />Mc 1 E. leafs, US. Dey. gd S41ey. Pl O <br />Mak Pelson,Ur1W1ly Mmawie Ogw"m "60b9yan9 Omphye <br />i—,,1 F.Kk ar ,, US. Ge A4W SKVeY, I },yon Nrgia <br />MODEL CALIBRATION <br />- ---------- • -t. -- ( al...e.e+.. - ------ <br />- - -- --- ----- <br />' -- - -` - - --- --- ------- --- <br />-- -- - -- --' <br />`tee °�' �� <br />Measured Salinity Concentrations -- February 1972 Simulated Salinity Concentrations -- February 1972 <br />q <br />- ----- <br />Measured Salinity Concentrations -- February 1982 Simulated Salinity Concentrations -- February 1982 <br />YE AN <br />The model was calibrated toaqui'erwater -1 eve l measurements th at we re made <br />at 12 wells at least once a year we th e 24 year study period. A comparison of <br />average monthly simulated water levels to the observed water levels indicates <br />that the model is able to reasonably predict the water -level trends. <br />f <br />S� <br />MODEL RESULTS <br />Due to stricter administration of Colorado <br />ground -water pumping regulations, irrigation <br />from ground -water sources could be sub- <br />stantially reduced in the future. <br />*Three scenarios of reduced pumpage were <br />modeled; (1) a 25% reduction; (2) a 50% red- <br />uction; and (3) a 100% reduction. <br />*Thousands of acres of historically irrigated <br />land in the Arkansas River Valley have been <br />subject to dry up following the transfer of <br />agricultural water rights to municipal use. <br />• Four scenarios of dry up were modeled; <br />(1) area 1 dry up (20% of area); (2) area 2 <br />dryup (33% of area); and (4) complete dryup <br />(100% of area). <br />REDUCED GROUND -WATER <br />PUMPAGE <br />MODEL RUN <br />�E <br />CESSATION OF IRRIGATION <br />ALLU AQUIFER <br />ALLUVIAL AQIAFER <br />94frh 17 <br />1) <br />np ARK ,e A NN SASNVEA <br />91rlq Canes +m %.rills, o�V, <br />­ IwwRimr. <br />OnYl1 . I•e0 <br />MODEL H.rd <br />(m9y 6 ?y <br />6aee Cana, 2,160 40145 <br />1010 6.3 <br />Sh* -Ar.e VA6 P—pp ReAleam <br />25 %Redman 2,170 4,014.6 <br />1,610 62 <br />50%R.& ton 2,160 4,0145 <br />1,610 6.4 <br />100%Rediaon 2,030 4,014.7 <br />1 X00 72 <br />E191LANATON <br />— 269t PUaP.P RMiWm <br />fiVk MiPpsRrkctlm <br />lmllft.M .RMh . <br />IM 1916 1990 1996 1➢0 1996 <br />0 Salinity concentrations in the aquifer decreased in <br />proportion to the magnitude of irrigation reductions. <br />The salinity concentration of the Arkansas River <br />decreased in response to reduced irrigation. <br />Aquifer salinity concentrations decreased for the <br />first 10 -12 years of the study period. This is reflective <br />of the aquifer residence time. <br />YEAR YEAR <br />Simulated changes in ground -water salinity concentrations relative to the base condition <br />Conclusions <br />1971 1976 19" ins 1999 1996 <br />YEAR <br />Model predictions of stream Iowaiid stream flow gains and losses are consistent <br />with the conce ptual u nice rs tan dirrg of the stream- aquifer system. St ream flow gains <br />and losses vary seasonally and ever longer time periods, depending on the stream- <br />flow regime. No streamflow data was available at the downstream end of the study <br />area to verify this relation. <br />MODEL RUN <br />M9 LAM TION <br />— —1 O,yup <br />N� 2Dr p <br />30ryup <br />. - lilaxfkyup <br />1071 1976 19® 196 1— 1916 <br />Decreased ground -water pumpage resulted in more ground -water <br />return flow to the river. <br />•Cessation of irngation resulted in less ground -water return flow to <br />the river. <br />�Aquiferwater levels were relatively insensitive to reduced irrigation <br />9uA,ybn ens empire Deayr,:3 1,r M. E-. S9S WAD <br />DDO0JU0a <br />1971 1976 11" 1996 1900 1995 <br />ALLUVIAL AQIAFER <br />ARKANSAS AVER <br />swrq�a c.r. <br />m wyriwa <br />sr,gg <br />i <br />MODEL r9.91 <br />p <br />M <br />B —Cai*m <br />2,160 <br />4A14.6 <br />1,610 <br />63 <br />e�naY' <br />Meal Dry W <br />2XW <br />4,014.6 <br />1,610 <br />52 <br />Mee 2 Dry ip <br />2,010 <br />4AI4.6 <br />1,760 <br />SA <br />Met 3Dry,p <br />ipw <br />4PI42 <br />1,790 <br />3.1 <br />100%D,yip <br />1,630 <br />4X742 <br />1,730 <br />23 <br />M9 LAM TION <br />— —1 O,yup <br />N� 2Dr p <br />30ryup <br />. - lilaxfkyup <br />1071 1976 19® 196 1— 1916 <br />Decreased ground -water pumpage resulted in more ground -water <br />return flow to the river. <br />•Cessation of irngation resulted in less ground -water return flow to <br />the river. <br />�Aquiferwater levels were relatively insensitive to reduced irrigation <br />9uA,ybn ens empire Deayr,:3 1,r M. E-. S9S WAD <br />DDO0JU0a <br />1971 1976 11" 1996 1900 1995 <br />