Sustaining Water Resources Variable Rate Irrigation

By Charles Hillyer

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 at 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. An easier way to understand VRI is by seeing it at

100× speed. A YouTube video called A Time-lapse Video of Variable Rate Irrigation shows VRI in action.

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 prescription: 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.

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 is 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, ranging from $3,000 to $8,000, makes it an appealing alternative to the complexity of full VRI.

The following are items to consider related to the use of VRI.

How will you create prescriptions?

All of 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.

in Agriculture

Catch-can testing of a variable rate irrigation system near Horse Heaven Hills, Washington. The area within the white square is set to full irrigation depth (100 percent) while the area immediately surrounding this box should receive no water (0 percent). The amount of overspray will depend on sprinkler throw, nozzle height and drop spacing.

14 Irrigation TODAY | October 2016

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