NESPAL > Water Use & Quality > Research Projects > Variable Rate Irrigation
Variable Rate Irrigation
by Calvin Perry
View the Enhancing Irrigation Efficiencies presentation or Visit the Variable Rate Irrigation Website.
Variable-Rate Irrigation (VRI), also called site specific or precision irrigation, is a relatively new concept in agriculture. Variable-rate irrigation is a tool of Precision Agriculture that involves the delivery of irrigation water in optimum amounts over an entire field. Very few fields are uniform - most have variable topographic and soil conditions with corresponding soil water variations. Most center-pivot irrigation systems currently in use apply a constant rate of water and the ability to vary application rates over an entire field has not been possible with current systems.
In Georgia, there are approximately 9000 center pivot irrigation systems. Many of these systems do not make complete circles and could benefit from fail-safe shut-off and reverse controls. Similarly, many of these systems have areas that are not cropped (often due to drainage problems) and could benefit from the ability to apply varying amounts of irrigation water to specific areas in a field. In addition, VRI irrigation systems provide an outstanding platform on which to mount sensors for real time monitoring of plant and soil conditions which would interact with a control system for optimal environmental benefits.
NESPAL is at the forefront in precision agriculture research, including VRI systems, in the Southeast and has installed a 4 tower 600 foot center pivot (CP) irrigation system and equipped it with a variable-rate irrigation control system. The basic water control method was developed by Greg Harting. The VRI controls have been developed by the FarmScan group (Bentley, Western Australia). The beta version of this system acts as an add-on to a standard CP system. The manufacturer's control panel functions as the CP normally would and the VRI system manages the sprinklers and end gun. Desktop software allows the definitions of the pivot and control areas or management zones to be made prior to going to the CP and the variable-rate controller. Once at the CP, the controller reads the desired water application control map from a standard PCMCIA data card. The FarmScan system uses one inexpensive proprietary GPS receiver (non-differential) for determining CP position/angle. Water application is varied by using Harting's method of cycling control valves on and off based on percent of time required to achieve desired application rate.
The major components of the NESPAL VRI system include
• FarmScan Canlink3000 controller with accompanying desktop software
• FarmScan proprietary control "nodes"
• electrical air control valves (solenoids)
• air-actuated water control valves
• pressure regulators
• air compressor & reservoir tanks
• GPS receiver (for end-tower position/angle).
The basic function of the VRI system is as follows:
• pivot information is entered into the desktop software
• desired application rates are defined in the desktop software
• a control map is transferred from desktop computer to controller via data card
• the controller determines pivot angle via GPS
• based on the control map, the controller cycles air valves to set application rate.
The FarmScan development team is currently implementing CP speed control as an option for applying less or more water in certain areas of a field. If the CP travel speed could be increased in "wetter" areas (which would apply less water), this would enable the pivot to return more quickly to other areas of a field requiring more water (where the pivot speed would decrease to apply more water).
