This article appears in the February 2017 issue of Potato Grower.
Precision agriculture has been around for a while. Technologies like RTK GPS, auto-steer tractors and precision spraying have revolutionized how modern producers operate. These technologies, along with big advances in information technology, have made it possible to move away from “farming on the average” and move toward applying the right amount at the right place. Variable rate irrigation promises to do for pivots what auto-steer and precision spraying have done for tractors: bring irrigation into the precision ag age.
There are two types of variable rate irrigation. The first, variable speed irrigation, is a system where the pivot’s speed changes as it moves around the field. The second, sometimes called zone-control VRI, site-specific VRI or simply VRI, is a system where valves are attached to each sprinkler and those valves regulate how much water the pivot disperses. The system pulses groups of valves on and off at different rates and varies the application depth by changing the ratio of time spent open or closed.
The pivot uses a map, usually called a prescription, to determine how much water should go at each location. There are two types of VRI prescriptions: grid and vector. With a vector approach, the user can draw small polygons on a map and set the application depth for each polygon. The grid approach uses a map that looks like a radar screen, and each grid cell can have its own application depth.
Variable speed irrigation (VSI) is a low-cost alternative to VRI. There has not been much peer-reviewed research on the efficacy of VSI, but a study conducted by the Northwest Energy Efficiency Alliance found that VSI did increase water use efficiency in all their test cases. VSI does have some additional practical benefits, the most obvious being its simplicity. VSI prescriptions are easy to modify, which means an irrigator can make adjustments to compensate for runoff, ponding or significant soil variations. VSI doesn’t have the spatial precision of VRI, but its lower cost to implement, ranging from $3,000 to $8,000, makes it an appealing alternative to the complexity of full VRI.
The following are items related to VRI use for growers to consider:
How will you create prescriptions?
All the VRI system vendors include some type of software for writing prescriptions, but these tools will not help determine how much water to apply. For areas that have no return on investment from irrigation, the solution is easy: apply zero on those areas. If soil variations are complicated, then creating a prescription will be more complicated. This is where a professional service can be valuable. Companies like CropMetrics and Precision Water Works will create the prescription based on a variety of high-resolution field data. This kind of service can be immensely useful for someone who has never built a prescription or has limited experience working with geospatial data.
Does your local service provider have experience installing VRI systems?
I have been involved with the setup of four different VRI systems as part of several research projects. Three of the four fields had some significant problems with the installation. After installation, insist that the service provider demonstrate that the system works and provide documentation on the sprinkler bank locations. Verifying both of these things will prevent many headaches later on.
Do you have a variable frequency drive?
VRI changes a pivot’s flow rate as it moves around the field, and these dips in flow are an opportunity to save energy. A variable frequency drive (VFD) will cut back the motor’s speed to take advantage of the lower flow requirement. If pumping with electricity, install a VFD to realize the most energy savings.
Does your field have enough variability to justify VRI?
This is probably the most difficult question to answer. Bob Evans, formerly with the USDA Agricultural Research Service, did a survey of VRI literature in 2013. He found that water savings from VRI typically ranged from 5 to 15 percent. These savings are relatively modest when considering that the cost of a VRI system can range from $15,000 to $30,000, depending on the pivot size. However, 5 to 15 percent can add up quickly when considering fertigation.
Most VRI research to date has focused on hardware aspects, but recently more management-focused experiments have emerged. Some of these studies examined the savings potential of dynamically controlled VRI systems, and the results look promising. The biggest obstacle to using dynamic VRI is the burden of managing all that information. As agricultural information technology develops, VRI has the potential to become a central component in precision irrigation management.
This article originally appeared in the Irrigation Association’s Irrigation Today publication in October 2016.