Water Vapor Transfer from Wetted Circular Plane Surface under Field Conditions

Abstract

A basic equation for transpiration rate from a plant leaf may be written as follows
W_T=εDΔC_L, where W_T is transpiration rate, ε: physiological factor, D: transfer coefficient of water vapor from a wetted leaf-shaped plate surface to the bulk air, and ΔC_L: water vapor concentration departure of the leaf surface from the bulk air. The vapor transfer coefficient is given by the boundary layer theory and laboratory-experiments. But, there is not much information on the coefficients under the field condition in which the air flow is turbulent and solar radiation is strong.
This paper represents some information as to the evaporation of water from a watted plane surface under the field condition.
A circular plane has no effect of wind direction. Moreover, a wetted surface made of very smooth cotton cloth colored like a citrus leaf surface is not disturbed by wind like as free water surface. Then, the horizontal circular plane whose surface was always wetted, being supplied from a water reservoir under-placed was used in the field.
The evaporation rate from the upward-facing horizontal circular plane surface, 20cm in diameter, was measured simultaneously with the temperatures of the wetted surface and the bulk air, water vapor pressure of the air, wind velocity, solar radiation, etc.
Thirteen-day data obtained in summer clear days, 1959, were analyzed and the following results were obtained,
1) As long as the wind velocity is constant, the temperature departure of the wetted surface from the air increases approximately linearly with insolation and the lower wind velocity is, the larger the increasing rate.
On the other hand, solar radiation being constant, the temperature decreases with increasing wind velocity, and the decreasing rate is larger under lower wind velocity.
2) When wind velocity is constant, there is a linear relation between the evaporation rate and the vapor pressure departure of the evaporating surface from the bulk air. The proportional constant is larger under higher wind velocity than that under lower wind velocity.
3) As long as the vapor pressure difference is constant, the evaporation rate increases with increasing wind velocity, and the increasing rate is larger under lower wind velocity.
4) When the wind velocity is relatively large, the evaporation is mainly caused by forced convection. But, in the case of relatively larger vapor pressure departure in the daytime, the transport by free convection is considerably effective to the vapor transfer. Even though at around 4m/s in wind speed, the transport by free convection is about 10 percent of the transport by forced convection.
5) Under the field condition, transfer coefficient of water vapor by forced convection increases with four-fifths power of wind velocity. This numerical value of the power shows that the boundary layer over the surface is turbulent.
Then, in the range of normal air temperature and solar radiation, average water vapor transfer coefficients D of an upward-facing horizontal circular plane surface, 20cm in diameter, are given by the following equations,
(1) for upward free convection,
D=0.81×|ρ_A/ρ_S-1|^1/4+2.02+10^-2u^4/5(cm/sec),
(2) for downward free convection,
D=0.41×|ρ_A/ρ_S-1|^1/4+2.02×10^-2u^4/5 (cm/sec),
where ρ_A and ρ_S are the densities of the air outside the boundary layer and at the surface, respectively, and u is the velocity of the outside wind.