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of Table 1 gives these values for the subbasins in question. It was found that the long-term crop irrigation requirement in the four P&rts of the main stem South Platte Bas~n was as follows I 1.41 acre-feet per acre for Subbasin AI 1.42 acre-feet per acre for Subbasin B; 1.30 acre-feet per acre for Subbasin 0; and 1.35 acre-fe",t per acre for Prospect Valley. The crop irrigation requirement in acre-feet per acre goes only part way in giving us the answer for water requirements in the basin; the total :require- ment cannot be determined without knowing the land acreage under irrigation. Land Use In the three-year period from 1954 to 1956 land classifiers of the Bureau of Reclamation classified the lands in the basin from the Narrows dam site to the State line. When these studies, were expanded to include effect of upstream development, it was necessary to know the acreage of irrigated land in the basin between Denver and the Narrows dam site. A study to determine this acreage was completed in 1961. The "study was of a reconnais- sance nature, and separated irrigated lands into three categories; those served entirely from the river, those served entirely by pump, and those served by a combination of the two. The results of these studies are listed in Table 1 under Ristoric Rydrologic Conditions 1952-l957, on line 1. The entire area is brqken down into three reaches; the first extending from Denver to Weld County; the second from Weld Oounty to Narrows, and the third from Narrows to the State line. The acreage presently irrigated in the main stem South Platte Valley by diversions from the river or pumping under ditches diverting from the river was found to be 362,000 acres. Ground Water The use of ground water in the South Platte River Basin is becoming an increasingly important part of the crop irrigation supply. In the years 1954 to 1956 when surface supplies were low, ground water contributed from 33 to 38 percent of the total water supply; furthermore, these computations did not take into account the surface water losses experienced in the canals. Use of ground water reduces the surface water irrigation requirement. Thus, g:roun~ water must be considered in determining irrigation requirements. A study was made to determine the amount of water pumped to lands under each ditch or ditch system in the main stem South Platte River l3s.sin from Denver to the State line during the 1952 to 1957 study period. In this study use Was made of such factors 17