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most of these lands lying in the intermediate and lower zones of the basin. Most of the readily available sources of irrigation water were developed between 1850 and 1900 by private individuals and small irrigation companies. The practice of diverting water for crops began in the high, fertile valleys fed by small streams and rivers. Flow was easily managed, and diversion dams were built on many small streams. The water was transported via man-made canals to adjacent pasture and farm land. In recent years, larger and more elaborate irrigation pro- jects have been constructed, and much larger quantities of water are now diverted from the streams in the basin. The average annual irrigation consumptive use for the period 1914-57 was 1,769,100 acre-feet (Iorns et al. 1965). Average use in the five-year period from 1971 to 1975 increased to 2,188,000 acre- feet (Bureau of Reclamation 1977). Several times this amount is di- verted from the streams, applied to the land, and, except for the amount used consumptively, is returned to the stream. Approximately 43 percent of the irrigation water consumed in the upper basin is used in the Upper Main Stem Subregion. A nearly equal amount is used in the Green River Subregion (39 percent) and only 18 percent is used in the San Juan-Colorado Subregion. The importance of water for irrigation is further demonstrated when water use in each subregion is considered (Table 1.2). Seventy- three percent of the water depleted by man from the Green River Sub- region and 62 percent of that depleted from the Upper Main Stem is used for irrigation. The proportion of water depleted by irrigation practices in the San Juan-Colorado Subregion (33 percent) is surpassed by reservoir evaporation (52 percent). Transmountain Exports Water exported from the upper basin is carried by tunnels, canals, and aquaducts through or across watershed divides. These diversions usually originate in the high mountain streams and small rivers where water quality is high. Evaporative losses from small 12