Agriculture Reference
In-Depth Information
system because they also need to consider many other factors such as on-farm field
operations that are not easily integrated into an automated system.
11.3 AUTOMATION WITH SOIL SENSING
Soil water sensing has been widely used for irrigation scheduling for research and
development and for commercial applications in many sectors including agricultural
crop production. The basic objective is straightforward—to periodically replenish
the soil water depleted by plant root water update. Soil water sensing is a critical step
for determining the rate and amount of water depletion before deciding if an irriga-
tion is needed, and if so, how much to irrigate. The decision process requires the
determination of a preselected range of soil water content to be maintained in order
to fully meet total crop water demands (consumptive use and soils evaporation). The
upper bound is usually the field capacity of the soil and the lower bound a threshold
soil water content, which can vary depending on soil and plant type, and stages of
plant development. Soil water held within these upper and lower bounds is consid-
ered plant available water or readily available water for plant update (James 1988).
11. 3.1 S
OIL
W
ATER
S
ENSORS
Although soil water content can be accurately measured with gravimetric or neutron
gauge methods, automating irrigation management using soil water sensing requires
rapid
in situ
quantification of soil water status and a signal output in an electronic
form. Therefore, only electronic sensors or sensors that generate electronic signals
can be used in irrigation automation (Abraham et al. 2000). Moreover, the ability or
degree of difficulty of root water extraction responds directly to soil water potential.
Therefore, soil water potential sensors are also readily applicable in sensing soil water
status for irrigation scheduling purposes. If electronic sensors measure soil water
content, conversion is needed to infer water potential values using soil water retention
characteristic curves, which can be described with various mathematical functions
such as that reported by Brooks and Corey (1966) and van Genuchten (1980):
Brooks and Corey equation:
−
b
=
⎛
⎞
⎟
θθ
θθ
−
−
h
h
Θ=
r
e
(11.2)
⎜
s
r
van Genuchten equation:
Θ
= [1 +
α
(-
h
)
n
]
-
m
(11.3)
m
= (1 −
n
−1
)
(11.4)
where
Θ
is the normalized water content or degree of water saturation,
θ
is the
apparent soil water content,
θ
r
is the residual water content,
θ
s
is the saturated water
