Agriculture Reference
In-Depth Information
0.5
0.4
Clay
0.3
Clay loam
Loam
Sandy loam
0.2
0.1
Sand
0.0
1
10
100
1000
10,000
100,000
Water potential, h (cm of water)
FIGURE 11.4
Water retention of five different textured soils. (Data from Carsel, R.F.,
Parrish, R.S.,
Water Resour. Res.,
24, 755-769, 1988. With permission.)
This is attributable to both relatively higher field capacity (at about -100 cm of water
potential) and lower permanent wilting point (PWP) water content (at about -10,000
cm of water potential) of medium texture soils than the coarse- or the fine-textured
soils. In coarse-textured soil, water content at PWP is low but so is the water content
at field capacity. In fine-textured soils, water content at field capacity is higher but
so is the water content at PWP. Soil texture may affect sensor performance by way
of range of soil water contents or potentials that are sensitive to sensor response and
crop root water update.
Soil variability usually refers to spatial variability in a given field that receives
the same irrigation scheduling. Multiple sensors are needed to cover large texture
or structure changes in the field in order to obtain a more representative assessment
of the soil water status. For automated systems, there is a potential for within-field
site-specific water application. Spatially distributed soil sensors will be needed for
site-specific real time measurements.
With any point-source measurements, continuous hydraulic contact between the sen-
sor body and the surrounding soil is very important for making reliable measurements.
Care is needed when installing these electronic sensors, especially in the case of smaller
sensors, for the best possible soil contact. In some cases, sensors or parts of sensors can
be installed in undisturbed soil, such as the waveguides for TDR or frequency domain
probes. This not only helps to enhance contact but also can preserve soil structure. In
most other instances, such as with tensiometers and granular matrix blocks, soil distur-
bances are unavoidable. Installation before crop root zone development is needed. In
soils that may be subjected to cracks, periodic repacking is required to maintain adequate
hydraulic contact between the sensor body and surrounding soil.
11.4
AUTOMATION WITH CANOPY SENSING
11.4.1 F
IELD
-B
ASED
M
EASUREMENT
S
YSTEMS
Common sensors that are integrated into irrigation management include geographi-
cal positioning systems (GPS) for identifying location, infrared thermometers (IRTs)
with band pass filters in the long wave infrared region (8-14
μ
m) for remote surface
