Campbell Scientific VisualWeather Software - Manual (Page 63)

Appendix A. Evapotranspiration, Vapor Pressure Deficit, and Crop Water Needs
This difference is calculated by calculating the saturated vapor pressure of the
air
VP deficit = (e°(T
hr
)-e
a
))
Where,
e°(T) saturated vapor pressure (kPa) of the air at temperature T
(Ref. Equation (19) above)
e
a
actual vapor pressure (kPa) of the air
(Ref. Equation (20) above)
A.3 Crop Water Needs, Crop Coefficients
A crop's life cycle consists of four stages, initial period, development stage,
mid-season stage, and the late-season stage.
• The initial stage is the length of time (days) between the planting date and
the date at which approximately 10% of the ground surface is covered
with green vegetation.
• The development stage is the number of days between the 10% ground
cover to nearly full ground cover. The full ground cover often coincides
with flowering.
• The mid-season starts at full ground cover and ends when a crop shows
signs of maturity, e.g. yellowing of leaves.
• The late season lasts between crop's maturity to its harvest.
During each stage a crop's water need will vary. This requires adjusting the
amount of water supply to the crop.
The amount of water (in mm/day or inches per day) needed by a crop is
calculated by multiplying the reference crop evaporation (ET
o
) described
earlier by the crop coefficient (Kc) applicable to that crop in a given stage of
its growth.
ETc = ET
o
x Kc (22)
These topics, along with useful data applicable to several crops can be
obtained by accessing the following web site:
http://www.fao.org/docrep/X0490E/x0490e00.htm#Contents
Information available at this web site should serve only as a guide. It is the
responsibility of users to provide the correct value of Kc applicable to their
situation. Contacting local agricultural agencies may provide information
about crops grown in that region.
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