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,"!: ~~~.. r':. Table 2 'Model Results for Scenarios of Decreased Ground Water Pumpage for Irrigation Alluvial Aquifer Arkansas Rh:er Average Monthly Average Monthl}. Water Average Monthly Average Monthly Model Run Salinity (mgIL) level (m above mean sea level) SalinilY (mgIL) Streamflow Gains (mJ/s) Base Condition (annual 3Vg. of 1.6 aflacre pumped) 2180 1224.42 1810 0.176 Study Area-Wide Decreases in Ground Water Pumpage 25% Reduction (annual avg. of 1.2 af/acre pumped) 2170(-0.5%)1 1224.42 1810 (0.0%) 0.176(0.00/0) 50% Reducti(ln (annual avg. of 0.8 af/acre pumped) 2160 (-0.9%) 1224.42 1810 (0.0%) 0.179(1.6%) 100% Reduction (a~rlUal avg. of 0.0 aflacre pumped) 2030 (-6.9%) 1224.48 1800 (- 0.6%) 0.196(11%) lNumbers in patentheses indicate [he percent difference between the modeled [I:J.se condition and the reduced-pumpage scenanos aquifer are largest during seasonal high flows in otherv...ise rela- tively dry years. The temporal nature of the gain-loss relation is evident in the model simulations for Ihe dry period from 1974-78 (Figure 10). During wel years. more streamflow is available for diversion into the canal and the hydraulic gradient between the aquifer and thc river increases because recharge increases due to larger surface water applications and canal leakage. Therefore, losses from the river to the aquifer are not as large during the seasonally high flows in relatively wet years. The temporal nature of the gain-loss relation is evident in the model simulations for a relatively wet period in 1982- 87 (Figure 10). A streamflow gauge at the downstream end of the study area would improve the understanding of stream-aquifc:r interac- tions, which might improve the model calibration. The streamflow gauge thai was in place at the downstream end of the study site dur- ing the original 1971-72 sludy (Konilcow and Bredehoeft 1974a and 1974b) had been removed at the end of that study. Several potential sources of uncertainty in the model might account for errors in model simulations of water levels, salinity, and stream-aquifer interaction. In addition to errors inherent to the selection of model parameters, these sources include the estimations of canal flow, evapotranspiralion, and ground waler recharge. Dash (1994) estimaled that discharge in the Fort Lyon Canal, which is measured using a 12 m Parshall flume, was accurate within 6% of actual discharge during freeflow conditions in the flume. During periods of backwater conditions in the flume, measured discharge was only accurate within 40% of Ihe aClual discharge. Backwater conditions at flows larger than 8.4 m'ls Iypically occur during lale spring and early summer (Dash 1994). Bccause applied surface water was estimated to equal 5.5% of tOlal canal diversions, there might be substantial error in these estimates during periods of backwater conditions. Estimates of evapotranspiration are another source of model error. Although the Blaney-Criddle method of evapotranspiration estimation is well accepted and widely used, it is a rather simplistic model of a complicated process and is diffi- cult to verify. The same is true for the detennination of aquifer recharge. The amount of water recharged to the aquifer is a critical component of the model; however, it is difficult 10 verify because it varies in space and time and is a complex function of many physical and c1imalological variables. The model recharge parameler used in this study was estimated by comparing simulated and mea- sured ground waler levels and salinity. 82 Potential Impact of Water Use Changes As previously discussed, two water supply issues in the ATl-~no;:~o;: RivPT vllllpy rllrn>ntl.}L.a.re.()fintPTP<;:t. (1) rhe...pQtPntial v::al. ley.wide decrease in ground water withdrawals due to stricter enforcement of state pumping regulations; and (2) the current and future decrease in irrigated acreage resulting from the transfer of water from agricultural to municipal use. Decreased ground water withdrawals or decreases in irrigated acreage might result in changes in ground water recharge, a decrease in the evapoconcen- tration of salts, and a change in the gain-loss relation between the stream and aquifer. The calibrated model was used to estimate the effects of these changes in the study area. Two categories of irri- gation management were simulated: (I) a decrease in ground water withdrawals for irrigation, and (2) thc cessation of all irrigation from ground water and surface water sources. Individual simulation scenarios in the two management categories were perfonned with the three sub-areas (Figure 2) affected individually and collec- tively by the potential changes in irrigation practices. We assume Ihat crop types, farming praclices, and irrigation methodology remain essentially unchanged, as it is beyond the scope of this study 10 examine the effects of such potential changes. Decreased Ground Water Withdrawals for Irrigation During the 24 year study period, the average total applied irrigation water was 1.2 mlyr, of which about 0.5 mlyr was applied from ground water sources. In the modeled scenarios of decreased pumping, the total amounl of applied water was decreased by decreasing ground water withdrawals. Surface water applications remained unchanged; they were not increased to offset the decrease in lOW applications. Thee scenarios of decreased historical ground water with- drawals were simulated: (1) 25% decrease in pumping; (2) 50% decrcase in pumping; and (3) 100% decrease in pumping. The sirnulaled decrease in ground waler salinity that resulted from decreased ground water withdrawals ranged from 0.5 to 6.9% (Table 2) and was related 10 the decrease in ground water with- drawals. The largest change in ground waler salinity tesulted from the complete cessation of pumping. In this scenario, the average monthly ground water salinity decreased from 2180 to 2030 mgll... The relative difference between average ground water salinity for the 24 year baseline and the decrea,ed pumping scenarios varied