Agriculture Reference
In-Depth Information
depending on the particular irrigation methods, for example, large center-pivot
versus small solid set or drip systems. An automated pressurized irrigation system
rarely functions just for irrigation purposes, and chemigation is often times built into
the system. Chemical injection can be achieved with positive displacement pumps
or the Venturi effect using a pressure differential created with a valve or pressure
regulator installed between the chemical tank and the irrigation pipe.
11.2. 3 E
VAPOTRANSPIRATION
C
ONTROLS
Scheduling irrigation using crop evapotranspiration (ET
c
) as the control has long
been used in agricultural applications (Jensen et al. 1970; Allen et al. 1998, 2011). To
determine ambient atmospheric conditions for ET estimation, a weather station, with
sensors to measure solar radiation, wind speed, air temperature, and relative humid-
ity, is needed. Based on measured weather parameters, hourly ET
o
can be calculated
from latent heat flux using an energy balance approach such as the modified Penman-
Monteith equation as adopted by the California Irrigation Management Information
Systems method (California Department of Water Resources, Sacramento, CA):
⎛
⎜
⎞
⎟
37
273 16
γ
γ
uD
v
T
+
.
sR
−
++
(
G
)
a
ET
o
=
n
+
(11.1)
λγ
[
s
(
1
C u
)
s
+
(
1
+
Cu
d
)
d
where
s
is the slope of the saturation vapor pressure curve,
R
n
is the net radiation,
G
is the soil heat flux,
is the psychometric
constant,
T
a
is the air temperature,
C
d
is the bulk surface and aerodynamic resistance
coefficient,
u
is wind speed, and
D
v
is the vapor pressure deficit.
Potential ET or ET
o
computed from the weather parameters is just a theoretical
value referring to evaporative water loss to the limit of energy available to convert
liquid to vapor phase water. Corrections are needed to reflect actual crop conditions
in determining a more realistic ET
c
value. This is typically done by generating a crop
coefficient (
K
c
) curve, which tends to be crop specific and is sometimes subject to
management practices such as row spacing or whether water stress is applied. Some
benchmark
K
c
values for various crop types are provided in FAO-56 (Allen et al.
1998). Efforts are also being made to refine the quantification of
K
c
using remote
sensing or other plant sensing techniques at real time or near real-time so as to more
precisely determine crop water needs (Moran et al. 1997; Bastiaanssen et al. 1998;
Wu and Wang 2005; Tasumi and Allen 2007; Trout et al. 2010).
Irrigation automation using weather-based ET estimation has been adopted by the
agricultural sector but with varying degrees of sophistication. Several types of irri-
gation controllers are commercially available that can be automatically updated by
either a simple weather sensor (humidity or air temperature) or via a wireless elec-
tronic device (cellular phones) that receives a daily update on ET from a network of
local weather stations. However, a majority of growers or farm managers make their
irrigation decisions by considering the ET demand but not using a fully automated
λ
is the latent heat of vaporization of water,
γ
