Variable rate application of many crop inputs is a cornerstone of precision agriculture. There has been much work investigating the effects of variable rate application of nutrients, herbicides, defoliants, etc. However the ability to variably apply irrigation water has been a challenge which is gradually being overcome by researchers in the USA.
The advantages of the adoption of variable rate irrigation practices to broad acre grain and cotton irrigators in Australia could be significant. This is because variably applying water not only provides the opportunity for increased profitability, but also has potential for water savings allowing the opportunity for additional production.
Variable Rate Irrigation (VRI) Systems
Precision application of irrigation water has long been the target of many in the irrigation industry. We have been able to install precision application systems such as drip and Centre Pivot & Lateral Move (CPLM) for a number of decades, and recent progress in surface irrigation management allows for much improved precision for this method also.
But a move to variable rate irrigation requires much more than the ability to apply water precisely. In a variable rate irrigation system, different amounts of water need to be precisely applied in different parts of a field. This places a new level of complexity on existing irrigation systems.
Currently there is no technological solution for variably applying determined amounts of water through a surface irrigation system. Similarly, whilst it is theoretically possible to implement drip irrigation systems with emitters of different discharge characteristics, individual emitters would need to be separately controlled in order to achieve variable applications across a field.
Hence work on variable rate irrigation systems in the USA has focused on CPLM machines as not only can emitters be controlled separately, but because the machine travels across the field, altering the patterns in which these emitters apply water allows for a truly variable application of irrigation water.
Application for VRI systems
The application of variable rate irrigation in the USA has concentrated around two major issues. Firstly, many farms, particularly in South-Eastern States, do not have fields that cover a single continuous cropping area. For this reason, it is not uncommon to see Centre Pivot irrigators crossing roads, ponds, swamps, creeks, rocky outcrops or other non-cropped areas. Providing a mechanism for reducing or preventing water application on these areas results in reduced water use.
Secondly, accurate measures of crop water requirements can provide the opportunity for variably applying irrigation water to precisely meet the needs of plants in different areas of a field or indeed, ultimately, apply the precise requirements for individual plants.
The first step for researchers in the USA to address either of these issues was to design a method for variably applying the water. This has been approached in two distinct ways, as researchers in different parts of the country tried to find a solution.
One of the methods employed by researchers such as Dr. John Sadler and Dr. Carl Camp from USDA-ARS in South Carolina utilized a number of underslung pipes along a machine to deliver different flow rates. In this system each span is divided into a series of separate manifolds, so that the smallest area that can be irrigated individually is approximately 6 meters wide. This setup results in a system that is constantly applying water, although the amount of water coming out of each set of emitters may be different.
In contrast, the system perfected at the University of Georgia in Tifton, utilizes solenoids on each outlet so that the amount applied by each emitter is controlled by reducing the time that water is applied rather than reducing the rate. To apply 50% of the regular flow you would shut the solenoid for 50% of the time, maybe closed for 1 minute out of every 2.
Whilst the manifold system has proven to be quite complex and is currently only utilized in a number of areas for research, the system pioneered in Georgia has been licensed for use commercially and has been installed on 23 center pivots in Georgia, with approximately 15 more machines to be installed in neighboring states in the near future.
All of the commercial installations in this area to date have been aimed at reducing water use on non-cropped areas. In order to achieve this, the installations are quite simple as the control method involves a pre-programmed application map which reduces or prevents water application on those parts of the field that don’t require it.
However researchers in Texas, Colorado and Missouri are taking the system one step further. They are now concentrating on sensing the crop water requirements so that the VRI system automatically applies water as it is required by the crop, rather than in a pre-programmed sequence.
Optimizing Water Inputs
Researches Dr. Troy Peters and Dr. Paul Colaizzi of the USDA-ARS at Bushland, Texas, have been using Infra-red thermocouples (IRT) mounted on their VRI pivot to measure crop canopy temperature in order to estimate crop water stress. Sections of their pivot have now been fully automated so that the temperature information is collected and the amount of irrigation for each plot is automatically determined and applied. Figure 2 illustrates this variation in canopy temperature across their field. Each of the colored dots indicates a separate control zone which means that the irrigation can be very specifically tailored to the requirements of quite small zones.
How it works ?
You can defi ne custom irrigation zones with the easy-to-use mapping software. Zones may be defined by soil type, topography, crop type or field obstacles. The variable rate program is then loaded into the Precision VRI controller that directs individual sprinklers through wireless nodes. Sprinklers will turn on or off for zone control or pulse at the precise speed to achieve full variable rate application.
Individual sprinkler control
Each sprinkler is programmed to turn on/off or pulse at a customized rate depending on crop, terrain or obstacle.
Benefits for VRI systems
• Saves water. sprinklers are used more efficiently and can be turned off over tracks, drains, buildings, etc. Application rates over wet areas can be decreased.
• Decreases power consumption. it only runs when and where it’s programmed, so no power is wasted (with use of a variable frequency drive).
• Changes application rate over different crops or soil types. rather than farming to the limitations of an existing irrigation system, crops that require varying amounts of water can be grown under that system, achieving total flexibility.
• Reduces over-watering on laterals and part circles by running different forward and reverse plans.
• Less track maintenance. in areas where there are tracks, Precision VRI can be programmed to shut off, which means less erosion and decreased water use.
• Reduces runoff and leaching. lowering application rates in certain areas decreases overwatering.
• Saves on fertigation and chemigation costs. chemicals can be efficiently applied.
• Decreases and eliminates watering in low or flooded areas.
• Provides Web-based irrigation recording and reporting. all data is securely stored. View current status and position, and historical graphs.