Agriculture Reference
In-Depth Information
in open fields around the world (Goldberg et al.
1971
; Hanson et al.
1997
; Lamont
2005
). Drip irrigation alone or in combination with plasticulture has significant-
ly contributed to water savings and in many cases improved WUE by reducing
runoff and evapotranspiration losses (Stanghellini et al.
2003
; Jones
2004
; Kirnak
and Demirtas
2006
) (Table
3.1
) For large-scale production, water savings can be
achieved with center pivot systems using drops converted to low-energy precision
application (LEPA) heads placed at about 30 cm above the ground (Piccinni et al.
2009
). Deficit irrigation implies that water is supplied to the crop at levels below
crop evapotransiration (ET) levels, deliberately allowing crops to sustain some de-
gree of water deficit without significant yield reduction but with important savings
in irrigation water. Deficit irrigation strategies applied through drip systems have
been shown to optimize water savings and productivity in several vegetable crops
(Table
3.1
). One environmentally-friendly approach to increase the use of marginal
water (saline water) is grafting of salt-sensitive plants onto salt-tolerant rootstocks
(Colla et al.
2010
; Edelstein et al.
2011
).
Application of Crop Coefficients
Irrigation can be improved by the application of on-site microclimatological data
and crop coefficients (K
C
), which are calculated as the ratio of the crop evapotrans-
piration (ET
C
) to a reference crop (ET
O
) (Allen et al.
1998
). ET
O
may be measured
directly from a reference crop such as a perennial grass (Watson and Burnett
1995
)
or computed from weather data using either temperature models (Thornthwate
1948
; Doorenbos and Pruitt
1977
), radiation models (Doorenbos and Pruitt
1977
;
Hargreaves and Samani
1985
), or combination models (Allen et al.
1998
). Weighing
lysimeters are employed to measure ET
O
and ET
C
directly by detecting simultane-
ous changes in the weight of the soil/crop unit (Schneider et al.
1998
; Marek et al.
2006
). Once K
C
values are determined, growers can calculate real time irrigation
recommendations (ET
C
) which can be obtained by local weather stations that de-
termine ET
O
and therefore solve the equation: ET
C
= K
C
× ET
O
. Current K
C
values
published for vegetable crops are given based on three growth stages: initial, K
Ci
;
middle K
Cm
; and late development K
Ce
(Allen et al.
1998
). Some examples include
cabbage (0.7
i
, 1.05
m
and 0.95
e
), tomato (0.6
i
, 1.15
m
and 0.7-0.9
e
), cantaloupe (0.5
i
,
0.85
m
and 0.6
e
), potato (0.5
i
, 1.10
m
and 0.65
e
), peas (fresh) (0.5
i
, 1.15
m
and 0.30
e
),
artichoke (0.5
i
, 1.0
m
and 0.95
e
), spinach (0.7
i
, 1.00
m
and 0.95
e
) and onions (0.7
i
,
1.05
m
and 0.75
e
). Recent studies have confirmed that K
C
recommendations need to
be more precise both in terms of time and space. Piccinni et al. (
2009
) developed
growth-stage specific K
C
's for onions and spinach based on leaf developmental
stages in order to further assist growers in maximizing irrigation management. The
values obtained for onion in the semi-arid Wintergarden region of Texas under silty-
clay soils were: 0.40 (emergence), 0.55 (two leaf), 0.75 (3-4 leaf), 0.85 (5-6 leaves),
0.90 (7-9 leaves), 0.85 (fully developed bulb) and 0.70 (dry leaf). The application
of newly-developed K
C
's for irrigation management has shown improvements in
water use efficiency, yield and quality of short-day onions (Leskovar et al.
2011
).
