Agriculture Reference
In-Depth Information
2
water stressed crop, no transpiration, upper baseline
0
-2
-4
-6
-8
-10
difference A - B
difference A - C
=
crop water stress index
-12
vapor pressure deficit in kPa
humid climate
arid climate
Fig. 6.18 Graphical interpretation of the crop water stress index
Sensing by the crop water stress index has distinct advantages.
The first advantage is that the sensing criterion - the transpiration - is a result of
the plants own internal moisture control system. The transpiration reflects
response of the crop to the existing water supply. This response is probably
more pinpointed than other human estimation of the needed moisture.
The second advantage has to do with the physical quantity that is involved.
Recording temperatures is easily possible in a non-contact, site-specific mode,
either by proximal on-the-go sensing or as well by remote recording (Sadler
et al. 2002 ).
However, fact is also that sensing of the canopy- as well as of the air temperature
plus the vapor pressure deficit alone does not suffice for the recording of the crop
water stress index. The positions of the upper and lower baseline (Fig. 6.18 ) must be
precisely defined. Theoretically, these positions depend on several meteorological
factors - among others wind speed - and on crop properties. Rather complicated
approaches have been proposed for determining the positions of the baselines
(Jackson et al. 1988 ; Payero and Irmak 2006 ). A rather simple recommendation
(Alchanatis et al. 2010 ; Moeller et al. 2007 ) that is empirically based is:
to define the upper baseline by adding 5 °C to the sensed air temperature and
subtracting this sum from the respective leaf temperature (see ordinate in
Fig. 6.18 )
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