Environmental Engineering Reference
In-Depth Information
Meteorological Data
Meteorological and water temperature data were used to compute evaporation rates
for Lake Mead. Measurements were made every 10 to 30 seconds and were aver-
aged for 20-minute periods. Some of the missing or incorrect 20-minute data were
estimated or computed to maximize the amount of data available for the evaporation
computation. Where data were missing for short periods, they were estimated from
trends of the data before and after the periods of missing or incorrect data. Where
data were missing or incorrect for longer periods, they were computed from other
available data at that station, data from another station, or data for another year.
Typically, a regression was developed with two sets of measurements for a different
period, but for similar environmental conditions, using a complete set of data. The
regression was then used to estimate the missing incorrect data from a complete set
of data.
Monthly values of meteorological data and water temperatures were computed
from 20-minute averaged data collected at each station for 1997, 1998, and 1999.
Average daily air temperature was about the same at all four platforms, whereas
relative humidity and water temperature were similar at the open-water platforms.
Generally, water temperature was higher and relative humidity was lower at the shel-
tered cove platform (Water Barge Cove) than at the open-water platforms.
Daily air temperature, water temperature, and relative humidity at the Sentinel
Island station were compared for 1997, 1998, and 1999. Air temperature varied from
year to year, but the seasonal pattern was consistent. The maximum daily air temper-
ature occurred about mid-July, with some daily average air temperatures exceeding
95°F, and the minimum daily air temperature occurred from late December through
February. Water temperature did not vary much from year to year. Maximum daily
water temperature occurred in late July to early August, and the minimum daily
water temperature occurred in late February. Relative humidity fluctuated from day
to day and differed greatly from year to year; however, there was a seasonal pattern
of high relative humidity in January that gradually decreased to a low at the end of
June, followed by a gradual increase to higher relative humidity in December.
Daily average wind speed for 1999 was 5 mph. Wind direction was predominately
from the southeast to the southwest, occasionally from the northwest, and rarely
from the northeast. Most daily wind speeds were less than 10 mph. The Virgin Basin
location experienced more daily wind greater than 10 mph than other locations.
Evaporation Rates
Evaporation rates were computed at 20-minute intervals to evaluate diurnal fluctua-
tions of lake evaporation. The 20-minute period evaporation rates were also used to
identify periods of poor or missing energy-budget data. Daily evaporation rates are
the sum of 20-minute periods, monthly rates are the sum of daily evaporation, and
annual rates are the sum of monthly evaporation.
Daily Rates
The daily evaporation rates at the four evaporation stations were compared for
calendar year 1999, and they showed similar daily fluctuations. However, the mag-
nitude of fluctuations in daily evaporation was greater at Virgin basin and Sentinel
