conditions and by deducting from historical flows for each year the depletions that would have resulted from the operation of all upstream projects constructed or authorized since that year.

Average annual present modified flows at key points in the upper drainage basin for the period 1914-47 and for the dry decade 1931-40 are shown in the following table. The next table shows a derivation of present modified flows of the Colorado River at Lee Ferry for each year from 1906-47. The chart following that table compares the present modified flow at Lee Ferry with the virgin and historical flows. Average present modified flow at key points

Annual present modified flow-Colorado River at Lee Ferry

WATER AVAILABLE FOR FUTURE DEVELOPMENT

With completion of authorized projects, water-consuming uses would deplete the flow of the Colorado River at Lee Ferry by an estimated 2,548,000 acre-feet annually. Thus, there would remain for future development 4,952,000 acre-feet of the 7,500,000 acre-feet apportioned the upper basin by the Colorado River compact. Present and authorized consumptive use and the use remaining for future development are shown by States in the following table. Consumptive use is estimated in terms of long-time average manmade depletions in the river flow at Lee Ferry.

Without long-time regulatory storage, the upper basin could plan to utilize only a part of the 7,500,000 acre-feet apportioned it by the Colorado River compact since in protracted dry periods less than the compact-apportioned water would be available after the required Lee Ferry flows were provided. In the period 1931-40, the most severe drought decade of record, the estimated total virgin flow of the Colorado River amounted to 118,320,000 acre-feet. Thus, after meeting the 75 million acre-feet Lee Ferry flow obligation provided in the compact, the upper basin would have had available for use in this 10-year period a total of only 43,320,000 acre-feet or an average of 4,332,000 acre-feet annually. Permissible consumptive use in the upper basin would have been less than this theoretical average. The quantity of water available in any 10-year period would not be known until the end of the period. In the meantime, safe operation would require that deliveries at Lee Ferry be maintained ahead of the average schedule to insure against drought-caused shortages in the last years of the period. Upper basin use also would be curtailed in dry years by water shortages at points of diversion on tributary streams. Particularly severe shortages would have occurred in the extremely low runoff years of 1931, 1934, and 1940, as indicated graphically on the following chart. If the upper basin is required to release water to Mexico during dry periods, corresponding reductions in upper basin use would be required.

Unless future droughts are much more severe than that of 1931-40 no curtailment of present water uses in the upper basin will be re

quired to meet the 10-year Lee Ferry flow obligation even without regulatory storage. As upper basin use increases, however, the need for regulatory storage will arise and become progressively greater. The point in upper basin development at which regulatory storage will become necessary will be determined by future runoff quantities. Through the upper Colorado River Basin compact, approved April 6, 1949, the States of the upper basin agreed on a division of their apportioned water and on their respective obligations with respect to the delivery of water at Lee Ferry. This made possible the formulation of a plan for basinwide development in which all potential projects are correlated. Therefore, in the analysis of this report all projects that would consume water of the upper Colorado River system, authorized subsequent to approval of the upper basin compact, are considered to be dependent on system regulatory storage for an assured water supply.

Evaporation

Increased evaporation resulting from manmade reservoirs is recognized by the upper basin States as consumptive use chargeable against the basin's water apportionment. Evaporation losses will vary from year to year, depending largely on the amount of storage provided in the basin. Estimated evaporation losses from reservoirs of the storage project are shown in the tables on reservoir operation in Chapter VI, Project Operations.

Only a few evaporation measuring pans are located in the upper Colorado River Basin and these do not provide data directly applicable to the remote sites at which reservoirs of the Colorado River storage project would be located. In estimating evaporation losses the Bureau used the available pan data as a base and established relationships with recorded meteorologic data. From these relationships curves were projected to indicate the gross evaporation to be expected from free water surfaces at various elevations on the Colorado River and its tributaries in the upper basin. The engineering advisory committee to the upper Colorado River Compact Commission adopted the Bureau's analysis of gross reservoir water surface evaporation. The Bureau, in computing net evaporation losses, allowed a credit for present river channel losses and other natural consumptive uses in areas to be inundated. For example, the gross evaporation rate from the free water surface of Glen Canyon Reservoir is expected to be 63 inches a year. With adjustments for present channel losses and natural consumptive uses within the reservoir area the net rate of evaporation at the maximum water surface level is estimated at 54 inches a year.

STORAGE REQUIREMENTS-COLORADO RIVER STORAGE PROJECT

The location within the upper basin of present and potential waterconsuming uses should be considered in planning project reservoirs and electric-power facilities. Therefore, a distribution of apportioned consumptive use in various areas of each upper-basin State was assumed. Potential uses were estimated from data on individual projects where available, and otherwise from general studies of tributaries or sub-basins. Adjustments in the estimates may become necessary as future detailed project investigations are made. No substantial variation is anticipated, however, in the assumed effect at Lee Ferry or in the overall operation of the regulatory reservoir system.

An assumption was made as to the rate at which the remaining unused apportioned water in the upper basin would be developed. Projects now under construction or authorized for construction were considered to be in full operation by 1956, the year preceding operation year 1, resulting in stream-flow depletions previously described under present modified flow conditions. It was assumed that 80 percent of the remaining apportioned water use would be developed by 2006 (operation year 50), and that 100 percent would be developed by 2031 (operation year 75).

Storage capacity required for river regulation

If the upper basin were to attain ultimate development of its apportioned water without regard to its compact obligations to the lower basin, the resulting flow at Lee Ferry would vary widely from year to year and the 10-year average would frequently be less than 75 million acre-feet. An analysis of the flow at Lee Ferry under this condition indicates the magnitude of the storage capacity required to regulate the river for compact fulfillment. Such an analysis is shown by the table following.

The estimated ultimate use of apportioned water in the upper basin, shown in column 2 of the table, averages 7,500,000 acre-feet annually over the period 1914-45. The use would vary considerably from year to year, however, being influenced by variations in available flows at points of diversion. Column 4 shows the average flow at Lee Ferry for each 10-year period ending in the year indicated and column 5 compares the 10-year flow with the compact-required 75 million acrefeet.

The greatest 10-year flow deficiency at Lee Ferry is shown in the table as 20,800,000 acre-feet for the period ending in 1940. More detailed studies based on monthly data rather than on the annual data used in the table indicate a deficiency of 23 million acre-feet for the 1931-40 decade, showing a need for that amount of storage. capacity for river regulation.