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~ N C7:l (.oJ (,Q . . provides a some"hat different perspective on the definition of "present hydrologic conditions," The first approach could be labeled "flow conditons under present (1975-1976) levels of development." In this instance, one " depicts the flo"5 which would occur in the long run future if the present level of development were to remain constant (recall, how- ever, that depletions would vary some from year to year, even though the level of development were unchanged). Put another way, this approach depicts the future equilibrium condition to which the hydrologic regime would tend over time under present levels of development, , Estimated long-run flow condtions under present levels of depletion are given in table 3.4 in the form of estimated per- centile exceedence 'flows at the outflow point of each WAUl. They are based upon the estimates of natural flows for the period 1906- 1974, as discussed above. By way of comparison with natural flows, the annual means in this table can be checked against those in table 3.1. In all instances, of course, the average annual natural flow for a WAU exceeds the mean annual flow under present levels of depletion by the average annual amount of the depletions which occur above the outflow of any given WAU (see table 3.2). I Another comparison between natural flows and flow conditions under present levels of depletion can be derived from table 3.5. As can be seen, reservoir regulation would, over the long run, uni- formly reduce the percentage of flows under present levels of devel- opment that would occur in the months of May - July, while the percentage of 001;S in the winter months would increase to some extent. This effect is, of course, most pronounced at Lees Ferry due .to the cumulative impact of Lake Powell's operation and that of upstream reservoirs. The second approach to characterizing "present hydrologic conditions" estimates the flows which will occur during the present year (calendar year 1979) given the level of development which now exists ("now" being as of 1975-1976). In this instance, present flow conditions are depicted as the flo'.s which one may, at this moment in time, and under present day reservoir conditions, expect to observe. EA~ressed as percentile exceedence flows at the outflow of each WAU, table 3.6 represents the estimated probability of certain flo1;s occurring in calendar year 1979, This second approach, which could be labeled "present day flows," differs from the first in that it represents an estimate of the pro- bability of encountering a given flow at the present time, as opposed 1. A percentile exceedence flow is defined as that flow which will be equalled or exceeded a specified percentage of the time. 3-12