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<br />w:::.. <br />o <br />c:.n <br />w <br /> <br />Evaporation from Lake Mead <br /> <br />Annual evaporation rates from Lake Mead, as reported by <br />the U.S. Geological Survey, averaged 85.2 inches (216 cm)/yr <br />prior to the construction of Glen Canyon Dam (Figure 4), <br />Evaporation rates decreased significantly after 1964 when <br />Lake Powell was filled to operating levels and discharges <br />were increased to normal. In the period from 1965-1970, <br />evaporation rates decreased to about 74 inches (188 cm)/yr <br />which reflects the changes in energy advection caused by <br />cold-water discharges from Glen Canyon Dam. Advection was <br />especially pronounced during this period because of low lake <br />elevations in Lake Mead and relatively high discharges from <br />Glen Canyon Dam (Figure 5). Annual discharges were 65% of <br />the Lake Mead volume in 1965 and averaged nearly 50% <br />j;hrou/',hout the period. <br /> <br />Annual Evaporation Rate <br /> <br />,- 95 <br />~ <br />. <br />~ 85 <br />~ <br />u <br />.= <br /> 75 <br />1100 <br />'" <br />0 <br />"900 <br />- <br />~ <br />. <br />"- 700 <br />~ <br />- <br />u <br />... 500 54 56 <br /> <br /> <br /> <br />Annual Water Loss <br /> <br /> <br /> <br />58 <br /> <br />60 <br /> <br />62 <br /> <br />64 66 <br />W 0 t er <br /> <br />68 70 <br />Ve 0 r <br /> <br />72 <br /> <br />74 <br /> <br />76 <br /> <br />78 <br /> <br />Figure 4. Historical Rates of Annual Evaporation and <br />Total Evaporative Water Losses from Lake <br />Mead [USGS Data]. <br /> <br />The volume of Lake Mead rose steadily from 1964 through <br />1974, but river discharges remained fairly constant after <br />1965 (Figure 5). This lessened the influence of advection on <br />the reservoir heat budget, and evaporation rates increased <br />somewhat during 1970-1974. The abrupt increases in evapora- <br />tion rates in 1975-76 and subsequent decreases in 1977-78 <br /> <br />8 <br />