Variable Rate Irrigation

VRI: how it works and how growers can benefit

Most fields are more variable than they appear. Differences in soil types, textures, organic matter, elevation, and drainage strongly influence how quickly soil dries after irrigation or rainfall. When a uniform irrigation rate is applied across these variable conditions, some areas receive more water than needed, while others experience water stress. 

Variable Rate Irrigation (VRI) is a precision irrigation approach designed to address this problem. By adjusting water application depth across different parts of the field, VRI allows growers to better match water application to local soil and crop conditions. 

This article provides an overview of VRI, how it works with center-pivot systems, the types of VRI available, and how prescription maps are developed. The goal is to provide a practical, accessible guide for anyone interested in improving irrigation management with VRI.

Why VRI matters

VRI is becoming increasingly relevant as weather patterns become more variable and irrigation input costs continue to rise. Fields with noticeable differences in soil texture, slope, or drainage benefit most because a single uniform rate often results in uneven water distribution. 

In these fields, VRI can:

• Reduce overwatering and waterlogging.
• Protect sandy or elevated areas from crop stress.
• Improve irrigation efficiency and reduce energy use.
• Reduce nutrient leaching risk in sensitive zones.
• Support more uniform crop development across the season.

The value of VRI is highest where field variability is persistent and well-defined.

When VRI may not be needed

Not every irrigated field will benefit from Variable Rate Irrigation. Fields with relatively uniform soils, minimal elevation change, and consistent crop performance often see limited improvement from VRI compared to well-managed uniform-rate irrigation.

In these situations, improving irrigation scheduling, system maintenance, and application timing may provide greater returns than investing in VRI hardware or software. VRI is most effective in fields where spatial variability is persistent, well-defined, and repeatable across seasons.

What you need to get started with VRI

Basic requirements:

• A center pivot equipped with VRI capability.
• Field variability information (soil EC maps, NDVI imagery, soil data, elevation).
• A basic understanding of Management zones.
• A prescription map software (e.g., Reinke, Valley VRI Commander, Agsense).
• Ability to upload a prescription file to the pivot panel.
• Irrigation scheduling tools such as the Irrigation Management Assistant (IMA) tool, checkbooks, or soil moisture sensors.

Types of Variable Rate Irrigation (VRI) Systems

Variable Rate Irrigation systems differ in how precisely they control water application along the center pivot. The level of control determines how finely irrigation depth can be adjusted across the field, as well as system cost and complexity. The three most common VRI approaches are described below.

Speed-Control VRI

Speed-control VRI adjusts irrigation depth by varying the pivot’s travel speed. When the pivot moves more slowly, more water is applied; when it moves faster, less water is applied. The irrigation rate is uniform across the entire pivot span at any given time.

This approach is the simplest and lowest-cost form of VRI. However, because the entire pivot speeds up or slows down together, speed control cannot address variability across the field width at the same pivot position. As a result, it is best suited for fields with limited spatial variability or situations where only broad changes in irrigation depth are needed.

Zone-Control VRI

Zone-control VRI divides the pivot into management zones, typically by grouping multiple sprinklers together along the pivot span. Each zone is controlled by electronic valves that turn water on or off as the pivot moves through different parts of the field.

This approach allows different irrigation depths to be applied simultaneously across zones of the field, making it well-suited for fields with distinct soil- or landscape-driven variability. Zone-control VRI is widely available and represents a balance between precision, cost, and management complexity. Most commercial VRI systems installed today fall into this category.

Sprinkler-level (individual nozzle) VRI

Sprinkler-level VRI provides the highest precision by controlling individual sprinklers or small groups along the pivot. Each nozzle can be turned on or off independently, allowing irrigation depth to be adjusted at a very fine spatial resolution.

This level of control is especially useful in fields with highly variable soils, irregular field shapes, or specialty crops where precise water management is critical. However, sprinkler-level VRI requires more complex hardware, additional wiring or control components, and a higher upfront investment. It also demands careful system design and maintenance.

How Variable Rate Irrigation works in the field

VRI allows different irrigation depths to be applied to different parts of a field during the same irrigation event.

Unlike conventional center-pivot irrigation, which applies at a uniform depth across the entire pivot circle, VRI uses field-specific information to adjust application rates spatially as the pivot moves.

The VRI process begins with identifying spatial variability within a field. Many irrigated fields contain a mix of sandy areas that dry quickly and heavier or lower-lying areas that retain water longer.

This variability is commonly characterized by using soil electrical conductivity (EC), elevation, soil texture information, historical yield data, and crop imagery such as NDVI. These data layers are used to delineate management zones, the areas of the field that are expected to behave similarly with respect to soil water storage and crop response.

Once management zones are established, irrigation depths are assigned to each zone through a prescription map. The prescription specifies the amount of water to be applied to each zone during an irrigation event.

This prescription file is uploaded to the pivot controller, which uses GPS-based position tracking to determine the pivot’s location in the field and applies the assigned irrigation depth as it moves through each zone.

Water applications are controlled by pivot hardware, such as electronic valves or solenoids, which regulate flow to sprinkler groups or individual nozzles, depending on the VRI system design. In zone-control systems, groups of sprinklers are turned on or off as the pivot enters different zones. In more advanced systems, individual sprinklers can be controlled independently, allowing finer spatial control of irrigation depth.

Throughout the season, prescription maps may remain static or be updated dynamically. Static prescriptions are based on stable field characteristics such as soil texture, EC, and elevation, and are often used as a baseline irrigation strategy. 

On the other hand, dynamic prescriptions incorporate information that varies over the season, such as NDVI imagery or soil moisture measurements, enabling prescription maps to be adjusted in response to evolving crop conditions. 

Regardless of the software used, VRI systems apply irrigation according to the provided prescription map. The technology determines where water is applied differently, but it does not determine how much water the crop needs. That decision requires an understanding of soil water-holding capacity, rooting depth, crop water demand, and current soil moisture conditions.

Zones define where, not how much

Most VRI software platforms are effective at delineating the field into management zones using spatial data. However, zone delineation alone does not determine appropriate irrigation depth. Assigning irrigation amounts requires field knowledge and supporting tools such as soil moisture sensors or irrigation scheduling models that track soil water balance over time.

Successful VRI implementation depends on integrating spatial zone maps with irrigation scheduling decisions. Without this integration, even well-defined zones can result in over-irrigation in some areas and water stress in others.