WSP07539

• File Number: 8281.300
• Description: Colorado River Studies and Investigations - Colorado River Consumptive Uses and Losses Report
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 10/1/1991
• Title: Consumptive Uses and Losses in the Arizona Portion of the Upper Colorado River Basin for the Water Year 1990
• Water Supply Pro - Doc Type: Report/Study

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Consumptive Uses and Losses

16

RESERVOIR AND STOCK POND EVAPORATION

Reservoir evaporation losses are considerable and make up about 8 percent of
the total water uses and losses in the study area. A listing of the reservoirs in
the study area, including their locations and normal water surface areas, was
obtained from a previous BIA report entitled "1985 Survey of Irrigated Land in
the Arizona Portion of the Upper Colorado River" and are shown in table 6.
Under normal conditions, the lakes are divided into two classes of evaporative
surface area:

· Those used primarily for irrigation.
. Those used primarily for fisheries and recreational values.

Efforts are made to maintain a nearly full condition and to minimize water
surface fluctuations in reservoirs managed for a fishery. Reservoirs used
primarily for irrigation experience large fluctuations in water levels and,
correspondingly, large variations in surface area. For an "average" year
.operating condition, the evaporative surface area of a reservoir used primarily
for irrigation was estimated as one-half the water surface area of a full
reservoir. The water surface area of a reservoir used primarily for fishing is
assumed to be equal to 'the normal water surface area. The stock ponds are
assumed to have water surface areas equal to one-third the water surface area
that occurs when the ponds are full. For a "dry" year condition like that of
year 1990, it was estimated that the water supply was about 50 percent of
normal. Therefore, for purposes of this report, the "average" year assumptions
were reduced by one-half..

This method of evaporation estimation was used because continuous useful
reservoir level records for all reservoirs from which surface area could be
accurately computed were not available. Another factor complicating the
estimating procedure is that evaporation rates are not known at the individual
reservoirs. For this report, the NOAA Technical Report NWSA 33 "Evaporation
Atlas for the Contiguous 48 United States," June 1982, was used to determine
the amount of evaporation. It was assumed that any consumptive use by
natural vegetation prior to construction of the reservoirs and stock ponds is
negligible and, therefore, was not subtracted from reservoir evaporation losses.

The computed values for reservoir and stock pond evaporation for water year
1990 are shown in table 6. The total evaporative losses in 1990 are 2,920
(:1:740) with the reservoir and stockpond evaporation uncertainty being
estimated as 30 percent each.

· In a conversation with Jerry Gonzales of ses during September 1991, it was estimated that
the reservoirs filled to half their nonnal capacity during the 1990 season. It was suggested that
a study be done on actual capacities of reservoirs as they have silted in considerably in recent
years.