where:
E = evaporation rate [m-s-1]
Cpa = specific heat of air [J-kg-l'o-1]
Pa = density of dry air [kg'm-3]
hfg = latent heat of vaporization [J'kg-1]
= psychrometric constant [kPa*'K-1]
e(T) = saturated vapor pressure [kPa] at temperature T
Ts = soil surface temperature [OK]
Td = ambient dew point temperature [OK]
R = resistance to vapor movement from the soil to air
[kg-s-m-4]
The resistance term for the models presented by Conaway and Van Bavel
(1967), Tanner and Fuchs (1968), and Novak and Black (1985) represents
the resistance due to the laminar boundary layer. The boundary layer
resistance is a function of the wind speed, atmospheric instability,
and the surface roughness height. Their models are used to predict
evaporation from a well-watered bare soil surface. Jagtap and Jones
(1986) and Camillo and Gurney (1986) developed resistance terms to
include the boundary layer resistance in series with the resistance of
vapor flow in the soil. The soil resistance term is included since
after the soil surface dries, the water must change to vapor in the
soil below the surface then diffuse to the soil-atmosphere interface.
The soil resistance term developed by Jagtap and Jones was determined
by regression analysis as a function of cumulative evaporation, water
in the soil profile available for evaporation, and a daily running
average of the net radiation. The net radiation empirically accounted
for the heat flux into the soil, while the ratio of the cumulative