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pre-project conditions obviously will depend tlpon a determ:tnat;l.oIl of the amount of water that could have been diverted from n.a.~1' e.Qd retu:rn flows under historic conditions. To re:f'J.ect shortages that' would have occurred under historic conditions, the pre';project con- sum;ptive use shall be assWlled to bear the Same relationship to the pre-project theoretioal consum;ptive use requirement thS.t the water supply that would have been available under pre-project condi-tions bears to -the pre-projeot theoretical diversion req1ilireme:nt. This oom;putation mt'lSt be consistent on an annual basis wi-th the calcula- tion of' Clonsum;ptive use required. for adjustment of the otowi index supply previously explained. An appraisal of the physical characteristics of each tributary unit will be made to derive the normal IOOnthly pattern of return flow leaving the project area which would occur under both pre-project aild project conditions with a :full water supply and normal olimatic conditions. Such monthly patterns can be cGm;pt1ted for each unit either by distributing deep percolation, using the Glover adaptation of the. Dupuit-ForshheiDl$r idealization, or by adoption of a typical pattern from other irrigation projeots frOlll which return flow data ars'.availab1.e. The pre-project and project return flow patterns will be adjusted to reflect actual water supply and cHmat1c conditions. Daily values of return flow for each un1 t will be derived by dividing the IIWnthly value by the number of days in thellWll.th. The difference between the project and pre-project return flows thus determined, adjusted for conveyanoe losses to otowi, will be credited on a daily bll.sis to each appropriate unit in determining the required releases from Heron No.4 Rese:t"V'oir. It will be practicable te adjust the return flow patterns for wa.ter supply and climatic conditions before determining a return flow credit, since the diversions and consum;ptive use affecting the return flows on a given day will have oQc'l1rred a IIIOnth or more before the return flows reach the river. During periods when return flow credit exceed.s the effect of storage and.additiorial natural flow diversions, the return flow will be oori.sidElred'as a delivery of San J1i18.!1-Chama water at. GtWifort1le. mane Rio Grande Conse:t"V'ancy Distriot or lIlUnic:l.pal and industrial users. ., ~ - ~ '- The return flow estimates described above are required for proper accounting of the wa.ter from an intrastate standpoint, and. are not subject to exaot calculation or measurement. However, the quantities involved in the estimates are small in relation to the qu.antities. of. iJi1ported water for the tributary units, and the probable errors would not Significantly affect the overall resut. 20