WSP00346

• File Number: 8220.101.17
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 8/1/1986
• Author: USDOI - BOR
• Title: Lake Powell Evaporation
• Water Supply Pro - Doc Type: Report/Study

MASS TRANSFER METHOD

The following is a brief description of the mass-transfer method. A simple
example of the same type as the Lake Hefner quasi-empirical equation is

E=Nu(eo-ea),

(1 )

in which E=evaporation, in inches per day;
N=a coefficient of proportionality, hereafter called the mass-
transfer coefficient;
u=wind speed, in miles per hour, at some height above the water
surface; a numerical subscript, if used, indicates the height in
meters;
eo=saturation vapor pressure in millibars, corresponding to the
temperature of the water surface;
ea=vapor pressure of the air, in millibars; a numerical subscript,
if used, indicates the height in meters.

Nearly all the mass-transfer equations to be found in the literature have
one thing in common: evaporation is considered to be proportional to the
product of the wind speed, u, and the vapor-pressure difference, eo-ea.
In a few equations, the wind speed, u, has an exponent, usually less than
unity.

The mass-transfer coefficient, N, represents a combination of many
variables in the published mass-transfer equations. Among these are the
manner of the variation of wind with height, the size of the lake, the
roughness of the water surface, atmospheric stability, barometric pressure,
and density and kinematic viscosity of the air.

The following mass-transfer equation was developed for Lake Mead and used
to make monthly determinations of evaporation rates. These determinations
have been checked on an annual basis by the energy-budget and pan-
evaporation methods.

E = 2.65 x 10-3 U2 (eo-e2)

(2)

E = evaporation in inches per day.

U2= is wind speed in knots.

eo= is saturation vapor pressure at the temperature of the water
surface.

e2= is the vapor pressure of the air computed in millibars at 2
meters above the water surface.

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