10
T air temperature [K]
Kfo, Kn,, Kw turbulent transfer coefficients for heat, momentum
and water vapor, respectively [m2*s_1]
Combining the equations 2-2 and 2-3 and rearranging the following
expression for evaporation is obtained.
k _g_
Si dz
K 3D-
"1 dz
(2-5)
The profile methods determine the rate of evaporation by analysis of
the vertical profile of various atmospheric variables such as specific
humidity or vapor pressure and wind velocity. King (1966) described
the procedures for determining the evaporation rate for adiabatic wind
profiles (neutral stability) and stratified conditions. One of the
basic assumptions used in the profile methods is that the turbulent
transfer coefficients of water vapor and momentum are equal. This
implies that the momentum and mass displacement thicknesses are the
same. The Richardson number (Ri) or the ratio of height (z) to the
Monin-Obukov length (L) are used as a measure of atmospheric
instability and can be determined by equations 2-6 and 2-7,
respectively.
where:
Ri
L
au
dz
+ r )
u*
(
k g H
Cpa^a^
(2-6)
(2-7)
9
acceleration due to gravity [m*s-2]