11
r = adiabatic lapse rate [^nr1]
= 9.86 x 10-3
k von Karman's constant [dimensionless]
= 0.428
U* shear velocity [nrs"1]
For neutral conditions the familiar logarithmic velocity profile was
used. For unstable or diabatic conditions a variation of the
logarithmic profile was used or the KEYPS function (Panofsky, 1963) was
used to determine the velocity profile. King stressed that in using
profile methods to determine evaporation, one must take extreme care in
measuring wind velocity. He suggested that spatial averaging be used
for wind speed measurements near ground level and that 30- to 60-minute
averages be used considering the steady state assumptions made in
developing the equations for the profile equations. Corrections for
lower atmospheric instability caused by density gradients in the air
near the earth's surface must also be used.
Another method requiring only measurements of atmospheric
parameters is the eddy flux method (Goddard and Pruitt, 1966). The
method is based upon the turbulent transport equations of motion in
the atmosphere. The equations of turbulent motion in a fluid include
the transient random fluctuations in velocity, temperature, and mass
concentration of water vapor of the air. The following equations
describe the shear stress (r), sensible heat flux (H) and latent heat
flux (LE) due to the turbulent motion of the atmosphere
* K 3U
^a^m
- paw'u'
(2-8)
cpaPaKh fj"
+ CpaPaW''
(2-9)