12
C. M. Flores-Cayuela et al.
where DOY is the Julian Day and Rs in is the solar radiation inside the greenhouse (mm/day) that can be estimated from external radiation data, using the cover
transmissivity value (τ) which depends on the material (Eq. 2).
Rs inv = Rs out · τ
(2)
Rs out is the outdoor solar radiation (mm/day); since climate predictions do not
provide this information directly, it has been calculated through Angstrom’s method
[3], which links solar radiation to extraterrestrial radiation and the relative insolation
duration. To estimate this last one, cloudiness predictions (%) are used, which allow
an approximation of the real daily insolation that will occur with respect to the
maximum possible.
Weather precipitation forecast is considered in the irrigation scheduling of outdoor
crops.
2.2.3 Soil Water Content
In outdoor crops, it is necessary to consider the amount of water stored in the soil to
determine whether irrigation is necessary or not (when the amount of water stored
in the soil is enough to satisfy their ETC). On the contrary, for greenhouse crops
with drip irrigation systems and highly frequent irrigation and no rainfall input, it is
reasonable to ignore the role of the soil as a water store and consider that the water
content in the soil does not change over time [16].
Total water available in the soil (TAW) for the crop is determined by the crop’s
depth of roots (zr) and by the upper limit θ FC (field capacity) and lower limit θ PWP
(permanent wilting point) of soil water storage Eq. (3), which depend on the soil’s
hydrophilic characteristics, n being the day index.
TWA n = 1000 · (θ FC − θ PWP ) · zr
(3)
However, not all the water stored in the soil can be used by the crop, only a
fraction of it is really available for the crop (RAW) that varies according to the crop
[3]. Once the soil moisture deficit (Dr) reaches the RAW value, water extraction is
more difficult for the plant and leads to a reduction of ETc. To avoid this situation,
20% of TAW has been fixed as the threshold of allowable depletion level (ADL) in
the soil to start irrigation. To determine the Dr n (mm) in the soil at the end of each
day (soil water balance), the Dr theorical,n and Dr real,n are previously calculated using
Eqs. (4) and (5), respectively, adapted from Allen et al. [3]:
Dr theorical,n = Dr n−1 − ER n − I n + (ET c n · K s n )
(4)
Equation (4) estimates theoretical water deficit from the balance of inputs (irrigation and effective rainfall) and outputs (evapotranspiration) at the end of the day,
C. M. Flores-Cayuela et al.
where DOY is the Julian Day and Rs in is the solar radiation inside the greenhouse (mm/day) that can be estimated from external radiation data, using the cover
transmissivity value (τ) which depends on the material (Eq. 2).
Rs inv = Rs out · τ
(2)
Rs out is the outdoor solar radiation (mm/day); since climate predictions do not
provide this information directly, it has been calculated through Angstrom’s method
[3], which links solar radiation to extraterrestrial radiation and the relative insolation
duration. To estimate this last one, cloudiness predictions (%) are used, which allow
an approximation of the real daily insolation that will occur with respect to the
maximum possible.
Weather precipitation forecast is considered in the irrigation scheduling of outdoor
crops.
2.2.3 Soil Water Content
In outdoor crops, it is necessary to consider the amount of water stored in the soil to
determine whether irrigation is necessary or not (when the amount of water stored
in the soil is enough to satisfy their ETC). On the contrary, for greenhouse crops
with drip irrigation systems and highly frequent irrigation and no rainfall input, it is
reasonable to ignore the role of the soil as a water store and consider that the water
content in the soil does not change over time [16].
Total water available in the soil (TAW) for the crop is determined by the crop’s
depth of roots (zr) and by the upper limit θ FC (field capacity) and lower limit θ PWP
(permanent wilting point) of soil water storage Eq. (3), which depend on the soil’s
hydrophilic characteristics, n being the day index.
TWA n = 1000 · (θ FC − θ PWP ) · zr
(3)
However, not all the water stored in the soil can be used by the crop, only a
fraction of it is really available for the crop (RAW) that varies according to the crop
[3]. Once the soil moisture deficit (Dr) reaches the RAW value, water extraction is
more difficult for the plant and leads to a reduction of ETc. To avoid this situation,
20% of TAW has been fixed as the threshold of allowable depletion level (ADL) in
the soil to start irrigation. To determine the Dr n (mm) in the soil at the end of each
day (soil water balance), the Dr theorical,n and Dr real,n are previously calculated using
Eqs. (4) and (5), respectively, adapted from Allen et al. [3]:
Dr theorical,n = Dr n−1 − ER n − I n + (ET c n · K s n )
(4)
Equation (4) estimates theoretical water deficit from the balance of inputs (irrigation and effective rainfall) and outputs (evapotranspiration) at the end of the day,