Agriculture Reference
In-Depth Information
providing a measurement of the soil water content. Because of the complexity and
cost, TDR, capacitance or frequency domain sensors are commonly used for research
and development and not commonly used for on-farm irrigation management.
Unlike soil water content sensors, devices that can measure soil water potential
are more widely used for making soil water status assessment for on-farm irrigation
management (Phene et al. 1973). Tensiometers and gypsum blocks are traditionally
used as the water potential sensors for agricultural crop production. Tensiometers,
when connected to pressure transducers and calibrated, can provide electronic out-
puts for driving automated irrigation management decisions. Limitations with tensi-
ometers are the narrow range of soil water tension of less than about -70 kPa before
exceeding air entry values for the porous ceramic or stainless-steel cups. Water loss
and the need for periodic refilling is another drawback with tensiometers (Cassell
and Klute 1986). Gypsum blocks are much simpler in design and operation but suffer
from lack of accuracy for absolute measurements and may lose integrity over time
(Shull and Dylla 1980). To overcome the limitations of a particular type of sensor or
to achieve a set of measurements of different soil properties, different sensors may
be used simultaneously in a single irrigation or infiltration event (Wang et al. 1998).
Since the late 1980s, granular matrix sensors such as the Watermark
®
(Irrometer
Co., Riverside, CA) are becoming more widely used not only for research and develop-
ment but also for on-farm irrigation scheduling purposes (Thomson and Armstrong
1987; Wang and McCann 1988; McCann et al. 1992; Shock et al. 1998; Intrigliolo and
Castel 2004). The granular matrix sensors are relatively maintenance-free, low cost,
and can provide reasonable accuracy and reliability in field applications. The sensors
consist of a pair of electrodes embedded in a porous matrix of a composite of gypsum
and other granules that can absorb and release soil water in responding to soil water
potential changes. Electrical resistance is measured from the electrodes and calibrated
against soil water potential readings. Like in any porous medium, thermal and hydrau-
lic hysteresis have been reported for the granular matrix sensors (McCann et al. 1992)
and calibrations should include the temperature effect (Wang and McCann 1988).
11. 3.2 F
ACTORS
A
FFECTING
S
OIL
W
ATER
S
ENSING
Soil water sensors have long been used for on-farm irrigation management, and some
of the common factors to consider when deploying these types of sensors include
crop type, soil texture, soil variability, and sensor contact with the soil.
Crop type can dictate the sensor's suitability for making irrigation manage-
ment decisions (Greenwood et al. 2010). Crops with shallower rooting depths and
lower tolerance to water stress will need to be irrigated frequently and need sensors
that can respond quickly to water content or potential changes. Deep-rooted crops,
in most cases, can tolerate some water stress, and less frequent irrigation may be
needed (Huguet et al. 1992). Therefore, sensor response time is not as critical for
deep-rooted crops as for shallow-rooted crops.
Soil texture is an important factor when selecting soil sensors. Not only is water
retention distinctively different between different textures (Figure 11.4), available
water capacity of soil also varies with soil texture. Generally, medium texture soils
have higher available water capacity than either the fine- or the coarse-textured soils.
