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,..- , . temperature of Lake Mead outflow has remained the same. Less energy is thus available for the process of evaporation resulting in a substantial water savings. The temperature of inflow to Lake Mead is based on daily observations at the Grand Canyon gaging station (Phantom Ranch) and corrected by a factor developed during extensive evaporation studies at Lake Mead in 1952 and 1953. Outflow temperatures are from daily observations below Hoover Dam. The tem- perature data are weighted according to the flow of the river in deriving mean annual values. Corresponding evaporation shown in Table 5 is that published by the U.S. Geological Survey in its annual Water Supply Papers and consists of. the mean from two methods of computing water surface evaporation; namely, the Mass-Transfer Method and a Weather Bureau Pan Method, the latter being modified to account for wind measurements. solar radiation. dew point, air temperature, advective energy computed from inflow and outflow data, change of energy stored in the reservoir and water surface temperatures, which is quite a departure from the simple application of the usual Weather Bureau pan coefficient. An Energy-Budget Method is used also for comparative purposes. :~ ~, ;-" ~ ' Roughly, Table 5 indicates an average reduction of over 8 inches in the annual evaporation rate subsequent to 1.963. Annual water savings thus approximate 75,000 acre feet at the present low surface levels of Lake Mead, whereas, when Lake Mead is full, the annual savings would approximate 100,000 acre feet. There would also be an indeterminable, but small re- duction in evaporation from the river surface. -,. ,~ (Table 5 can be found on the next page, page 12.) CONCLUSION !." This presentation is an attempt to bring attention to positive bene- fits often overlooked by critics of large dams and reservoirs in carrying out their environmental concerns. The author recognizes the unusual surround- ings of this particular reach containing the Grand Canyon of the Colorado River and Lake Mead. Yet there are items herein of common interest to many river regulatory situations which might offer encouragement to the future planners and operators of large dams and reservoirs. ;.: J...:: ~.: 11