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e) , '} ) i . It is extremely doubtful whether man will ever be able to measure, on a large scal~, with any degree of certainty, the actual losses to the immediate strearn flo\V due to irrigation. It is likewise very doubtful, except in experi- mental plots, whether the increased runoff due to overgrazing and other ac- tions. of man can be measured on a large scale. Therefore, the second con- oept, the indestructibility of water, has been considered as a sound basis for estimating the water available for synthetic liquid fuel plants. SOME EXAMl?LES OF CONSUMPTIVE USE DETERMINATIONS Man has yet to devise a method for determining transpiration from growing vegetation ,that is reliable. but there are some areas where the inflow-outflow measurements give some idea of consumptive use. One of these is the St. Vrain valley, a tributary of the South Platte. The year 1926 was considered for the studies on the Colorado Big Thomps6n , Project as an ideal year for water supply in that itwas ample for all declleed rights. On the St. Vrain for that year. the headgate diversion from Table 1, page 10, Vol. l,Big Thompson Report. was 1.3$ acre feet per acre for 81. 806 adres /l-ct/ 'c,;,... or 16.6 inches. Their big crops are sugar beets, alfalfa-corn and small grains. vegetables.. This 16.6 inches includes evaporation, transpiration from vegetation, seepage and surface returl:l flow. The measured, return flow was 5',623 acre feet J e aving only 59,169 acre feet for evaporation, transpiration, seepage, surface waste and other losses. or 8.67 inches, only a portion of which would be actual consumptive use. Drawing NQ.ll. Vol I. Colorado Big Thompson Project report. shows the average supply on the South Platte for Districts I, (Denver to Kersey~ Z (Kersey to 'Balzac, 64 (Balzac to Nebraska State line. for years 1925-'35, to be 849. 000 acre feet and the average return flow for the sarneperiod 86a, 000 acre feet. The reason for this being higher than supply is undoubtedly from underground return flow appearing on the surface from the tributaries in these districts. A pOl'tion of that 8.67 inches loss in the S4 Vl'ain undoubt- edly appears in this wat.er. It might be well to compare the Engineering Committee's estimate of oon- sumptive use on the Uncompahgre Project in the Colorado River Basin with this.as the two areas are silnilar in most all respects: Acres IrI'igated Elevation Latitude Average Temperature Average Wlnd Precipitation Frost free period Uncompahgre 12,345 5,600 38045' N 48.40 F 5 mi. hr. a inches 152 days St. vrain 81, 806 4.800 400 10' 47..40 F 15 mi. hr. 14 inches 140 days -13-