Agriculture Reference
In-Depth Information
suffer water stress and yield losses when
the deficit is large.
The quantification of the crop eva-
potranspiration (
ET
c
) or maximum ET of the
crop, which would involve the maximum
yield under non-limiting water supply con-
ditions, is:
The evaporation pan acts by integrating
the climate conditions (radiation, wind,
temperature and humidity). It consists of a
cylindrical pan, made of galvanized steel,
diameter 121 cm and height 25.5 cm, sup-
ported by a wooden platform that stands
15 cm over the soil. It is filled with water
that must be clear and its level must always
be maintained between 5 and 7.5 cm below
the edge of the pan. Measurement of the
water level is done by means of a limnimeter
provided with a micrometric screw and the
difference between two consecutive meas-
urements is the evaporation in the period
between the measurements. There are also
other models.
Daily readings are recommended, in
the early hours of the day. For the previous
calculations the average of the daily evapo-
ration readings must be used, over the
period of at least 1 week.
The value of the
K
p
depends on the
general climate conditions and the sur-
rounding environment (Doorenbos and
Pruitt, 1976).
Therefore, the
ET
c
in the pan method
would be:
ET
c
=
K
c
×
ET
0
(11.4)
where
K
c
is the crop coefficient, whose
value depends on the crop (size and devel-
opment stage, transplant or sowing date)
and
ET
0
is the reference evapotranspiration
which is taken as standard and depends on
the existing climate conditions.
In order to estimate the
ET
0
several
methods have been proposed by the Food
and Agriculture Organization of the United
Nations (FAO) (Doorenbos and Pruitt, 1976).
In greenhouses, the Class A evaporation pan
method (Photo 11.4) is easy to apply. It esti-
mates the value of
ET
0
as a function of the
evaporation from a water surface (
E
0
):
ET
0
= K
p
×
E
0
(11.5)
where:
ET
0
= Reference ET
K
p
= Pan coefficient
E
0
= Evaporation from a water surface
=××
(11.6)
ET
K
K
E
c
c p
0
Photo 11.4.
Class A evaporation pan.
