Agriculture Reference
In-Depth Information
Fig. 10.14
Two eddy covari-
ance systems installed in a
large banana screenhouse,
located at the Western Galilee
of northern Israel. The sys-
tems measure whole canopy
turbulent fluxes of water
vapor, heat and CO
2
. Their
deployment above the canopy
does not interfere with the
crop. (Source: Tanny 2007,
private collection)
using specific expressions for the different parameters, based on the greenhouse
ventilation rate, specific stomatal resistance and leaf boundary layer resistance. Ex-
periments conducted in a greenhouse with a tomato crop resulted with very good
agreement between estimated and measured (by lysimeters) ETc.
The use of evaporative cooling devices like pad and fan or fogging (see Sect. 3.1)
increase the greenhouse air water vapor concentration and hence should be tak-
en into account when crop water requirement is considered. For example, Fuchs
et al. (
2006b
) demonstrated in a rose crop that operating the wet pad cooling sys-
tem reduced transpiration. Such result may have implications on irrigation needs
of greenhouse crops. Although crop transpiration is reduced, the additional water
is supplied through the wet pad, such that the total amount of water required is
not changed much. The operation of the wet pad also cools down non-transpiring
organs of the plants like flowers or fruits, which is an advantage.
In fan ventilated greenhouses, and provided there is no water condensation, the
total amount of crop transpiration can be measured through a mass balance of the
water vapor (Teitel et al.
2010
), implemented between air inlet and outlet. In screen-
houses this approach cannot be implemented. Therefore, Tanny et al. (
2006
,
2010
)
and Dicken et al. (2013) have examined using the Eddy Covariance technique to
measure whole canopy ET in large screenhouses. In this technique, vertical air ve-
locity and water vapor concentration are measured at a high sampling rate, usually
10 Hz; the covariance of these two variables is the net vertical turbulent flux of
water vapor, i.e., ET.
Usually air temperature is also measured at high frequency such that sensible
heat flux can be obtained as well. If CO
2
concentration is also measured at high
frequency, its vertical flux can also be determined by this approach. This latter mea-
surement also facilitates the calculation of the water use efficiency as the ratio be-
tween CO
2
and water vapor fluxes. Eddy covariance sensors are installed within the
air boundary layer above the canopy, so this approach has the advantage that there
is no interference with the crop or the soil (Fig.
10.14
).
