Environmental Engineering Reference
In-Depth Information
Island than at Overton Arm or Water Barge Cove. Daily evaporation was greatest
from late June through early July and was least from mid-December through late
Ja nua r y.
Monthly Rates
To evaluate the temporal (size of) variation in monthly evaporation at each station,
monthly evaporation rates for each station were averaged and compared to the total
evaporation rate for each month of data collection. Temporal data were available
however, monthly data were insufficient for the Overton Arm station to compute
average monthly evaporation. Total monthly evaporation rates at three stations, with
some exceptions, generally were within 10% of average monthly rates; consequently,
annual variation in monthly evaporation typically was minimal between 1997 and
1999. Some months, however, exhibited significant differences between average
and total evaporation. These differences were as great as 31% at Water Barge Cove
(February 1997), 14% at Sentinel Island (January 1998 and 1999), and 22% at Virgin
Basin (December 1998 and 1999). Some of the difference in monthly evaporation
rates from year to year may be due to errors in meteorological and water-temperature
data collected at each station, but year-to-year differences for most months likely are
due to actual differences in evaporation.
To evaluate the spatial variation in evaporation, total monthly evaporation rates
at all four stations were averaged for every month of data collection, and the average
was compared to the total monthly evaporation rate at each station. For each station,
total monthly evaporation rates compared well to average monthly rates with cor-
relation coefficients of 0.96 or higher. However, total monthly evaporation at Water
Barge Cove generally was less than total evaporation rates at the open-water stations
when rates were less the 6.5 in. Total monthly evaporation rates for all three open-
water stations were nearly equal and compared well to average evaporation rates;
the correlation coefficient for Sentinel Island and for Overton Arm was 0.98, and
for Virgin Basin it was 0.96. This evaluation suggests that the spatial variation in
evaporation is minimal for open-water areas of Lake Mead.
The monthly volume of evaporated water was computed using the average
monthly open-water evaporation rate, in feet, and the average monthly surface area
of Lake Mead for July 1997 through December 1999, in acres. The volume of water
evaporated in 1 month ranged from 46,000 acre-ft in February 1998 to 126,000
acre-ft in July 1998.
Annual Rates
The average monthly rates for the Lake Mead open-water evaporation stations
were computed for 1998 and 1999. For open-water stations, the sum of the aver-
age monthly rates for 1998 was 88.9 in. (7.4 ft), and the sum for 1999 was 90.7 in.
(7.6 ft). For these 2 years, the average annual Lake Mead evaporation rate was 89.9
in. (7.5 ft). Monthly evaporation rates were available for only the Sentinel Island
station for January to March 1998, and those rates are used instead of an average
rate. Evaporation rates at the Sentinel Island station are generally representative of
evaporation of the lake as a whole. For April 1998 through November 1999, the total
