7012

• UCREFRP Catalog Number: 7012
• Author: Cross, J. N.
• Title: Ecological Distribution of the Fishes of the Virgin River ‎(Utah, Arizona, Nevada)‎.
• USFW Year: 1975.
• USFW - Doc Type: University of Nevada,
• Copyright Material: NO

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the average temperature is 6]° F (16° C)(Utah Water Research
Laboratory 1974). A diurnal temperature fluctuation of 50° F (10° C}
is not uncommon at St. George (Cook 1960}. The average time between
killing frosts in the Littlefield-t4esquite area is about 250 days
(Glancy and Van Denburgh 1969), about 200 days in the central basin
(U.S. Bureau of Reclamation 1963} and less than 100 days above
2,100 m (Bagley et al. 1958). '
The mean annual precipitation above St. George is approximate]y
3.26 x 109 m3 per year yet the mean annual river discharge at
St. George is only 2.04 x 108 m3 per year or about 6~ of the
precipitation falling. Slightly more than 1~ of total precipitation
is consumed by irrigated agriculture; unmeasured flow accounts for
some also; the remainder is returned to the atmosphere by evaporation
and transpiration from the land where it falls (Bagley et al. 1958).
Precipitation at St. George and Zion National Park is far below the'
evapo-transpiration potential. Table 3 presents estimates of pan
evaporation at St. George and Zion National Park.