Agriculture Reference
In-Depth Information
Crop Water Requirements and Evapotranspiration
Water scarcity has become a significant limitation in agricultural production world-
wide. Hence it is necessary to accurately estimate the crop water needs, or whole
canopy ET, under different conditions in order to optimize irrigation and increase the
water saving.
Evapotranspiration can be either measured or estimated by models, the common
measurement techniques being the lysimeter and sap flow gauges. One lysimeter
application is based on installing several planted pots on load cells and continu-
ously monitoring their weight. From this data, and knowledge of the irrigation,
the amount of water consumed by the plant during a certain time period can be ex-
tracted which can then be used to guide the next irrigation. The lysimeter technique
is useful for small and moderate size plants but for large plants or trees it is rarely
used due to obvious technical limitations (Ghavami
1973
; Israeli and Nimri
1986
).
Sap flow gauges are based on measuring the sap flow rate in the stem using stem
temperature variations induced by sap flow. Common sap flow techniques are the
heat-pulse (Cohen
1994
) and thermal dissipation (Granier
1985
). In recent years the
use of the eddy covariance technique to measure whole canopy ET was examined
in screenhouses, and is discussed below.
Crop water requirements for protected crops can be estimated using the well-
known Penman-Monteith (PM) equation, which is derived from principles of en-
ergy balance and transport processes. The general expression for the PM equation
is (Allen
1998
):
∆−+ −
(
R G Ce e r
)
ρ
(
)/
N
aps a a
(10.1)
ET
=
∆+
γ
*
in which
γγ
r
r
*
=+
1
c
.
a
In Eq. 10.1, ET is the evapotranspiration (W m
−2
),
Δ
is the slope of the saturation
vapor pressure-temperature curve (kPa K
−1
),
R
N
is the canopy net radiation (W m
−2
),
G
is the soil heat-flux density (W m
−2
),
ρ
a
is air density (kg m
−3
),
C
p
is air specific
heat at constant pressure (J kg
−1
K
−1
),
e
s
and
e
a
are the saturated and actual vapor
pressure (kPa),
γ
is the psychrometric constant (kPa K
−1
),
r
a
is the aerodynamic
resistance (s m
−1
), and
r
c
is the canopy resistance (s m
−1
).
A common method for estimating crop water requirements for canopies in open
field conditions is to use the concept of reference evapotranspiration, ET0, and then
apply a crop coefficient, Kc, which is an empirical parameter, specific for each crop
and growth stage (Allen et al.
1998
). The ET0 is calculated for a well irrigated and
uniform reference grass crop at a height of 0.12 m, conditions which dictate certain
values for the resistance terms,
r
a
and
r
c
, in Eq. (10.1). Substituting these values in
Eq. (10.1) results with the equation of daily ET0 (Allen et al.
1998
):
