Stimulated Effects of Irrigation on Salinity in the Arkansas River Valley in CO

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rn aO co c U) ra 0 16 c �a E2 0a I J tf -100 -200 -300 -400 -500 -600 -700 r r T T T r •- r r T T r Year - — - - 25% Decrease in pumping 50% Decrease in pumping 100% Decrease in pumping Figure 11. Simulated change in ground water salinity for scenarios of decreased pumpage, relative to baseline conditions. m 0 , 0 -100 N. ` 200 "•----- -- - - - - .._ �� -300 c U � 400 m -500 > > -600 Eo .700 rn rn w rn�� 4� rn rn' Year - - - - - Cessation of Irrigation -Area 1 . — - - Cessation of Irrigation -Area 2 Cessation of Irrigation -Area 3 Cessation of Irrigation-All Areas Figure 12. Simulated change in ground water salinity for scenarios of decreased irrigated acreage, relative to baseline conditions. through time (Figure 11). The variability is a function of the rela- tive proportion of ground water to surface water that was histori- cally applied for irrigation. In the relatively dry years of 1971 -82, surface water diversions were relatively small (0.6 m/yr), and irri- gation from ground water (0.6 m/yr) represented 50% of the total applied water. In the relatively wet years of 1983 -95 when surface water was more plentiful (0.8 m/yr), ground water represented about 33% (0.4 m/yr) of the total applied water. Therefore, a fixed percentage decrease in withdrawals had a slightly more pronounced effect on salinity in the drier period, 1971 -82 (Figure 11). Decreased pumping had a minim effect on river salinity; in the three decreased pumping scenarios, salinity in the river essentially remained unchanged (Table 2). As expected, streamflow gains generally increased in response to decreased ground water withdrawals. The complete cessation of pumping increased the gains from 0.18 m /s to 0.20 m /s (Table 2). Simulated ground water levels relative to the baseline simulations, however, were relatively insensitive to changes in pumping. Decreased Irrigated Acreage Water transfers between agricultural and urban users in and to semiarid regions of the United States are being used increasingly because of competition for scarce water resources. During the 1987 -93 drought, for example, 1 billion m of water was transferred between different entities with the California water banking system (Bouwer 1994). However, the impacts of these transfers on the qual- ity of surface and ground water remains largely unknown. In response to increased demands for municipal water supplies in Colorado, agricultural water rights have been sold and transferred for municipal use along the Colorado Front Range. As of 1990, about 19,440 hectares of historically irrigated land in the lower Arkansas River valley has ceased to be irrigated following the transfer of water rights (Howe et al. 1990). In accordance with the legal stipulations of these transfers, it was mandated that these historically irrigated lands could no longer be irrigated. Therefore, the calibrated model was used to estimate the effects of decreased irrigated acreage in the study area. Four scenarios in the management category of irrigation ces- sation were simulated: (1 to 3) individually ceasing irrigation on sub- areas 1, 2, and 3; and (4) ceasing irrigation on all three sub - areas. 83 Table 3 Model Results for Scenarios of Decreased Irrigated Acreage Alluvial Aquifer Arkansas River Average Monthly Average Monthly Water Average Monthly Average Monthly Model Run Salinity (mg/L) Level (m above mean sea level) Salinity (mg/L) Streamflow Gains (rn %) Base Condition 2180 1224.42 1810 0.176 Study Area -Wide Decreases in Irrigated Acreage Area 1 (20% of irrigated study area) 2080 (- 4.6 %) 1224.42 1810(0.0 0.162 ( -7.9 %) Area 2 (33% of irrigated study area) 2010 ( -7.8 %) 1224.42 1780 ( -1.7 %) 0.151 ( -14 %) Area 3 (47% of irrigated study area) 1960(-10 1224.33 1780 ( -1.7 %) 0.087 (-51 Areas 1, 2, and 3 (100% of irrigated study area) 1630(-25 1224.33 1730 (-4.4 0.064 (-64 [ Numbers in parentheses indicate the percent difference between the modeled base condition and the indicated scenario. 83