Agriculture Reference
In-Depth Information
but this needs to be validated by field experimentation. King et al. (2005a) used
chemical injection proportional to time-averaged flow rate for nitrogen application
with a variable rate sprinkler. Measured nitrogen applications were within 4% of
target applications.
Site-specific chemigation achieved by varying the depth of water applied results
in spatially varied soil water content that may not correspond to optimum soil water
regimes. Subsequent spatially variable water application must address any imbal-
ances in soil moisture resulting from site-specific chemigation. To avoid this issue,
King et al. (2009) added a separate reduced volume chemical application system
capable of site-specific chemigation to an SSI-equipped center pivot. The indepen-
dent site-specific chemigation system used pulsing to vary chemical application.
Constant chemical concentration in the applied water was maintained by on-demand
mixing of the chemical solution before pressurization in the chemical application
lateral.
11.6 SUMM ARY
Automation in irrigation water management requires a systems approach with real-
time or near-real-time assessment of soil water status, crop water status, and an auto-
mated water delivery system, controlled with programmable electronics, either over
an entire crop field as a whole or capable of applying variable rates to meet site-
specific water needs. Instrumentation and hardware typically deployed in an auto-
mated system are discussed in the paper. In-depth reviews are made on techniques
for estimating soil and plant water status and state of the art for SSI.
Irrigation scheduling decisions have often relied on direct or indirect measure-
ment of soil water content or water potential. Traditional sensors such as neutron
gauges and tensiometers are still in use in some commercial irrigation management.
Relatively newer sensors such as TDR or capacitance probes and granular matrix
sensors (e.g., Watermark
®
) are more applicable for near-real-time measurement of
soil water content, thus for automating irrigation. Any deployment or selection of
soil sensors should take into consideration crop type (rooting depth), soil type (sand
versus clay), soil variability, and the mechanism of the sensors (contact-based or
volume-based measurements).
Techniques for assessment of plant water status for irrigation management have
been developed and tested more recently. Spectral and thermal ground-based remote
sensors mounted on self-propelled irrigation systems are capable of providing infor-
mation to farmers in a timelier manner than aircraft or satellite sources. Infrared
thermocouple thermometers mounted on a moving center pivot lateral can provide
radiometric temperature measurements of in-field crop canopy. Software to control
drip and moving sprinkler systems has been integrated with plant-feedback infor-
mation, and IRT measurements made from a moving sprinkler can then be used
to provide spatial and temporal temperature maps that correspond to in-field water
stress levels of crops.
The concept of SSI has been under development for more than 20 years.
Automation in SSI is mostly designed for center pivot and lateral move sprinkler irri-
gation systems. Recent advances and cost reduction in electronics (PLCs) and sensor
