Campbell Scientific VisualWeather Software - Manual (Page 55)

Appendix A. Evapotranspiration, Vapor
Pressure Deficit, and Crop Water Needs
A.1 Evapotranspiration
This appendix explains the process of evapotranspiration (ET) and the methods
VisualWeather uses to calculate hourly ET values. It also explains the
calculations for vapor pressure deficit of the air and crop irrigation based on
ET.
Evapotranspiration is the process of water loss in vapor form from a unit
surface of land both directly (evaporation) and from leaf surfaces
(transpiration).
The energy required for vaporization comes mainly from the solar radiation
and the ambient air temperature. Higher values of solar radiation and ambient
air temperature increase ET. As the air near the soil surface saturates with
water vapor, the rate of water loss due to evaporation decreases. Therefore,
with increasing humidity ET decreases. The wind displaces saturated air with
relatively drier air. Thus, at higher wind speeds ET increases. Other factors,
such as soil composition, irrigation frequency, rain, and crop type also play
important roles that affect the process.
Given the diversity of crops, their heights, ability to absorb solar radiation,
foliage resistance to wind, and variability in wind speeds at various altitudes
some standards have to be predefined as a basis for comparisons of ET under
variable climatic conditions.
For the purpose of standardization the following definition has been adopted [1]
for a reference crop surface.
'A hypothetical reference crop with an assumed crop height of 0.12
meters, fixed surface resistance of 70 S/m and an albedo of 0.23'.
This hypothetical reference crop surface closely resembles an adequately
watered, extensive surface of green grass of uniform height, completely
shading the ground.
The (bulk) surface resistance, rs, consists of resistance to vapor flow through
the soil surface and crop leaves. The latter is called stomatal resistance.
Stomata are small openings on leaves through which the vapor is lost. For the
reference crop the value of rs is fixed at 70 S/m.
Albedo is a crop's ability to reflect the incident radiation. Also known as
canopy reflection coefficient, its value for the reference crop is 0.23 (unitless
number). Reference crop surface albedo = (radiation energy reflected by
reference crop surface) / (solar radiation incident on the reference crop surface)
= 0.23.
A-1