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(. '::~~~}) PROJECT OPERATIONS FROM AN IWl'RASTA'l'E STANDPOINT Tributary Irrigation Units On the tributary 'Units, the irrigation systems are to be rehabilitated. a.nd enlarged to se:t"V'e both presently irrigated la.nd.s and new lands. On ll.ll of the units, except the IJ..ano, new or additional storage will also be provided. Com;plete record.s of divers:\.ol'l and retum flows are not available for any of the tributary 'Units. As stated in the previous section concerning administration Ul'lder the Rio Grande COlIg;lact, the adjustments to otowi flows for Com;pact purposes are to be made on an !l.IIl11J.al basis to reflect the increased consum;ptive use caused by project operatio1l.s. The consum;ptive use under present conditions is to be estimated from a. sU:t"V'ey of present land utilization in the tribu"l;ary units and an;r a.:r-eas affected by them. The consum;ptive use under project conditions is to be s1mi1arly estimated and the difference will be used in making the annual correction to the recorded flow at Otowi. This appraisal is adequate for Rio Grande Com;paat administra- tion. However, for intrastate operation it is neoessary to acc01mt for the water on a daily or short-term basis. The daily acoOUl'lting for intrastate operation lIIl1st be done in a. manner com;pletely consistent with the annual accounting required for administration of th.e Rio Grande Com;paot. Two alternative methods of water accounting were considered for these units. The first was the aorrelation method, w'hiah was discarded fer the reasons discussed below. The second mathod, which is reeollllllended, is the more direet and sim;plified method of determining the on-61te depletion for each tributary unit and translating sueh depletion "~O the replacemeXl.t point at the otowi. gaging station. Under the correla:tion mathed, relationships of t0ta.l :i.nf'low to outflow under present conditions would be developed for each tributary unit, and for each of the river reaches between gaging stations on the Rio Grande between the Colorado-New Mexico State line ,aXl.d otowi, and between HeroXl. No.4 Reservoir and otow1., The effect of project opera- tions would be assessed by the departure from these re1a.ticm.ships under project operation. The major disadvantage of this mathod is the difficulty in defining the relatively small V01'U.llle6 of depletions and channel losses within the normal deviations of eorrelations among large vol'U.llles of flow. Millre practical results can be . achieved by more directly appraising the effect the uni;;s would hB.ve on flows of the Rio Grande, and h.enee. the requiremant for replacemant releases from Heron No.4 Rese:t"V'oir. The difference in Rio Grande flows, eau.sed by proJec'c condi-tions, will be as a result of (1) withholding water from the river while it is being stored, (2) additional divers;l.Ons of natural flow for use on new 18