
Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
Table 1. Names, symbols and units of all quantities used in calculations of hourly ET
o
values.
Symbol Unit
Meteorological Data
Mean hourly temperature Ta
o
C
Mean hourly RH RH percent
Mean hourly wind speed at 2 meter height u
2
m/sec
Total hourly solar radiation Rs MJ/m
2
/hour
Data Related to Station's Location
Elevation Z meters
Weather station's longitude (user-entered) Lm degrees
Always west of Greenwich line minutes
(with Greenwich line as 0 degrees) seconds
Weather station's latitude (user-entered) phi degrees
minutes
seconds
North or south of the Equator (user-entered) N or S No Units
Standard meridian (midpoint of the time zone) Lz degrees
Anemometer position above the ground level (user-entered) Ht meters
Data Related to Time
Day of the year J or DOY integer
Begin time hour (military time, hour; e.g., 1400) tBegin hour
End time hour (military time, hour; e.g., 1500) tEnd hour
Standard (military) time at step's midpoint (e.g.; 14.5) time hour
Calculated Values
Slope of the vapor pressure curve
Δ
kPa/
o
C
Specific heat capacity at constant pressure C
p
=0.001103 MJ/kg/
o
C
Ratio of mol weights of water to dry air
ε=0.622
no units
A-10
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