Campbell Scientific VisualWeather Software - Manual (Page 56)

Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Over the years the Food and Agriculture Organization of the United Nations
(FAO) has published several documents related to evapotranspiration and other
agriculture related issues. We allow the option of switching between three ET
o
methods:
1. Standardized Reference ET
o
for Short Crops (~0.12 meter in height),
Grass Reference
2. Standardized Reference ET
o
for Tall Crops (~0.50 meter in height),
Alfalfa Reference
3. FAO56 Penman-Monteith Method for Estimation of ET
o
The only difference between these methods are the values used for C
n
, and C
d
in the ET
o
equation. These values are described in section 7 below.
The ET
o
equation is as follows:
) u C + (1 +
) e - (e u 273)) + /(T (C + G) - (R 0.408
= ET
2 d
a s 2 n n
o γ
γ
Δ
Δ
(1)
where, ET
o
= reference evapotranspiration (mm/day);
Δ = slope of the vapor pressure versus temperature curve (kPa/°C)
R
n
= net radiation at the crop surface (MJ/m²/hour)
G = soil heat flux density (MJ/m²/hour)
γ = psychrometric constant (kPa/°C)
T = air temperature at 2 m height (°C)
u
2
= avg wind speed at 2 m height (m/s)
e
s
= saturation vapor pressure (kPa)
e
a
= actual vapor pressure (kPa)
e
s
- e
a
= saturation vapor pressure deficit (kPa)
calculating ET
o
:
Because the main climatic factors which determine ET values (solar radiation,
temperature, relative humidity, and wind speed) vary throughout the day,
VisualWeather uses hourly average data values rather than their daily averages
in calculating ET
o
Equations used to calculate the value of each variable in equation (1), related
quantities, their units, etc., are presented below.
Please refer to Table 1 at the end of this Appendix for listings of
all quantities and their symbols and units used in the ET
o
calculations)
NOTE
1. Calculations for Δ [2, 3] (slope of the vapor pressure curve).
Δ =
4098[0.6108 e ]
(T + 237.3)
(17.27T)/(T+237.3)
2
(2)
T is temperature in °C.
A-2