Still Waters - Mathematics and Statistics for the Environment - page 578

20.3.4 p enman e quation
The Penman equation (Winter et al., 1995) estimates potential evapotranspiration:

= (

)

γ HE

e

+

a

E (20.13)

( ) +

0

γ 1

where
E 0 = Evapotranspiration.
∆ = Slope of the saturation absolute humidity curve at the air temperature.
γ = Psychrometric constant.
H e = Evaporation equivalent of the net radiation.
E a = Aerodynamic expression for evaporation.

20.3.5 d e b ruin -K eijman e quation
The DeBruin-Keijman equation (Winter et al., 1995) determines evaporation rates as a function of
the moisture content of the air above the water body, the heat stored in the still water body, and the
psychrometric constant, which is a function of atmospheric pressure and latent heat of vaporization:

×−

SVP
SVP

( )

PET = QQ

n

(20.14)

x

095 . + 0 63 . γ

where SVP is the saturated vapor pressure at mean air temperature (millibars/K). All other terms
have been defined previously.

20.3.6 p apadaKis e quation
The Papadakis equation (Winger et al., 1995) does not account for the heat flux that occurs in the
still water body to determine evaporation. Instead, the equation depends on the difference in the
saturated vapor pressure above the water body at maximum and minimum air temperatures, and
evaporation is defined by the equation

[

( )

]

PET = 0 5625 . e max − e min − 2 (20.15)

0 0

where all terms have been defined previously.

REFERENCES AND RECOMMENDED READING
Cole, G.A. (1994). Textbook of Limnology , 4th ed. Waveland Press, Prospect Heights, IL.
Lee, T.M. and Swancar, A. (1996). Influence of Evaporation, Ground Water, and Uncertainty in the Hydrologic
Budget of Lake Lucerne, a Seepage Lake in Polk County, Florida . U.S. Geological Survey, Atlanta, GA.
Mosner, M.S. and Aulenbach, B.T. (2003). Comparison of Methods Used to Estimate Lake Evaporation for
a Water Budget of Lake Seminole, Southwestern Georgia and Northwestern Florida . U.S. Geological
Survey, Atlanta, GA.
Rosenberry, D.O., Sturrock, A.M., and Winter, T.C. (1993). Evaluation of the energy budget method of deter-
mining evaporation at Williams Lake, Minnesota, using alternative instrumentation and study approaches.
Water Resources Research , 29(8), 2473-2483.
Spellman, F.R. and Drinan, J. (2001). Stream Ecology and Self-Purification, , 2nd ed. Technomic, Lancaster, PA.
Wetzel, R.G. (1975). Limnology . W.B. Saunders, Philadelphia, PA.
Winter, T.C., Rosenberry, D.O., and Sturrock, A.M. (1995). Evaluation of eleven equations for determining
evaporation for a small lake in the north central United States. Water Resources Research , 31(4), 983-993.