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<br />u~ <br />...-j <br />o <br />~ 640,000 ton pick up come from upland drainage. The balance of about 500,000 <br />.~~ tons is estimated to be added by the irrigation return flows of the Grand <br />c.' Valley and is relatively uniform from year to year. Prel iminary analysis <br />shows that the siqnificant variation in salt loading from year to year <br />appears to be a function primarily of surface and subsurface runoff from <br />the upland drainage. <br /> <br />Irrigation water budgets of selected canal systems in the valley are shown <br />in Table 4. <br /> <br />TABLE 4 <br /> <br /> Spills and .: Crop Surface <br />Canal and Total I rri - :Seepage from :: Irri- Runoff <br />Year Water gated Canals and Farm ':gation and Deep <br />Record :Oiverted: Area Latera15 :Del ivery: Use :Percolation <br /> (A/F) : (ac) (percent of total water di verted) <br />Government <br />Highline Canal <br />1965-71 272,000 21 ,600 43 57 20 37 <br />Mesa Canal <br />1965-71 21 ,000 1,000 13 87 13 74 <br /> <br />p. detailed analysis (by the Colorado Water Conservation Board) of several <br />--canal systems for the 1965-71 irrigation seasons indicates that 13 to 20 <br />percent of the water diverted is consumptively used by irr~gated crops. <br />Similar results are reported from Colorado State University research stud- <br />ies. Therefore, aQout 80 to 87 percent is spilled into surface ditches or <br />percolates into underlying saline aquifers. This amounts to about 10 to 20 <br />feet diverted for each irrigated acre. The maximum consumptive use by crops <br />is 2.5 feet leaving the balance to return to the Colorado River each year <br />or to be used by phreatophytes in the valley. Table 5 shows the monthly <br />operation for one canal system. <br /> <br />Based on analysis of available data for the major canal systems in the Grand <br />Valley, an annual irrigation water budget has been developed and shown in <br />Figure 2. The correlary annual salt budget for this irrigated area is also <br />shown in the fiqure. <br /> <br />The salt budget shows that while the gross outflow is 980,000 tons per year, <br />tht: p-ick up from the irrigatc.! ored is 510,000 tons. In addition, the up- <br />land drainage contributes 140,000 tons per year on the average. <br /> <br />This information provides a basis for a general appraisal of the types of <br />system, on-farm and lipl and m'2'SL:r'es which can be most effective in redUCing <br />salt loading. <br /> <br />- 5 - <br />