Similar estimates were made by the engineering advisory committee of net depletions at State boundary lines and at other key locations in the upper drainage basin. Since the estimates were based on annual averages for a 32-year period, they are not applicable to any particular year. Depletions above Lee Ferry are increasing gradually as water uses expand in the upper basin although annual variations result from fluctuations in runoff. Using basic data of the committee, the Bureau of Reclamation derived a sequence of past annual depletions at Lee Ferry for the period 1896-1947 and at potential project storage sites for the period 1906-47.

Virgin flow

As defined by the Upper Colorado River Basin compact, the term "virgin flow" means "the flow of any stream undepleted by the activities of man." The estimated virgin flow at Lee Ferry for the 18961947 period was determined by adding the estimated consumptive use in the upper basin in terms of manmade depletions at Lee Ferry to the computed historical flow at Lee Ferry. This flow and the factors considered in its computation are shown in the following table.

1 Flows of 1896-1923 period for Paria River and 1896-1921 period for Colorado River were estimated by correlation with upstream gaged records.

Annual virgin flow

2 Correction factor = 1⁄2 (1+:

1914-45 mean annual virgin flow

. Successive approximations of virgin

flow were made in order to compute the figures in column 3. First approximation was made using annual mean and historical flow.

3 Depletion rate: Column 3X1.287.

(1.287=

1914–45 mean annual depletion exclusive of transbasin exports). 1914-45 mean annual irrigated acreage

4 Depletions from past domestic and industrial use in upper basin are considered negligible.

Present modified flow

The present modified flow is the flow expected at any point with all upstream existing and authorized projects in operation. It was estimated at various sites by assuming a recurrence of past water supply