Soil moisture monitoring and variable rate irrigation (VRI) on the farm

By Smart-Irrigation.com · Last reviewed October 2026 · Sources linked inline and listed at the end

Knowing when to irrigate and how much to apply is as important as the hardware. This guide covers the scheduling approach CSU Extension recommends, the soil moisture sensors researchers are testing in Colorado fields, and what variable rate irrigation (VRI) on a center pivot involves and costs.

Start with the water balance

CSU Extension's fact sheet 4.707 describes the water balance approach: tracking the soil water deficit in the root zone by accounting for every addition (precipitation and irrigation) and subtraction, mainly crop evapotranspiration (ET) (CSU Extension 4.707). Key pieces:

Crop ET can be obtained from the Colorado Agricultural Meteorological Network (CoAgMet) or by using atmometers.
Management allowed depletion (MAD) is the share of the root zone's available water capacity you let the crop use before irrigating, because plants start to experience stress before soil reaches the wilting point. CSU gives the equation d_MAD = (MAD/100) × AWC × D_rz, where AWC is available water capacity and D_rz is root-zone depth.
When to irrigate: typically when the soil water deficit approaches d_MAD. If system capacity is limited, CSU advises irrigating more often rather than waiting, so the deficit doesn't exceed d_MAD.
Ground-truth it. CSU says it's good practice to occasionally check, for example once a week, whether the calculated deficit matches actual soil water readings from soil moisture sensors.

CSU fact sheet 0.514 adds practical steps: use a soil probe during and after irrigation to see how deep water penetrates, determine root-zone moisture and water-holding capacity by measurement or the feel method, and use flumes and water meters to know how much you apply (CSU Extension 0.514).

Soil moisture sensors in Colorado fields

Colorado State University's Agricultural Experiment Station describes soil moisture measurement as "an important component of irrigation water management to better inform the timing and depth of irrigation applications." A CSU field study in irrigated corn during the 2021 growing season, across the Arkansas, Colorado, and South Platte river basins, installed these sensors side by side at 8, 12, and 20 inches (CSU AES):

Sentek TriSCAN probes
Acclima TDR 315 probes
Campbell Scientific SoilVUE and CS655 probes
Meter Group TEROS 12 sensors
Irrometer Watermark granular matrix sensors

The project compares factory and field calibrations in three irrigated Colorado soils, with neutron-probe and lab soil measurements as references. That points to a practical lesson for any operation: sensor readings depend on calibration to your soil. Look for CSU's published results, and ask your dealer how a sensor is calibrated for your soil texture.

Two broad sensor families

Type	What it measures	Examples from the CSU study
Volumetric water content sensors (capacitance, TDR, and similar)	The share of soil volume that is water	Sentek TriSCAN, Acclima TDR 315, Campbell SoilVUE and CS655, Meter TEROS 12
Soil water tension sensors (granular matrix, tensiometers)	How tightly water is held by soil, which relates to how hard the crop must work to extract it	Irrometer Watermark

CSU's 4.707 also mentions gypsum blocks (a resistance method) among the tools for reading soil water (CSU 4.707).

Placement basics

Install sensors at several depths through the managed root zone. CSU's 4.707 lists irrigation management depths by crop and growth stage.
Choose representative locations by soil type and away from field edges, wheel tracks, and pivot towers.
Log readings and compare them with your water-balance deficit weekly.

Variable rate irrigation on center pivots

Utah State University Extension's peer-reviewed Precision Irrigation Guide for Center Pivots (2023) describes VRI as technology that varies the application rate and/or speed of irrigation to match the needs of different areas of a field. It says VRI "can improve water use efficiency and produce water savings in some conditions" (USU Extension).

Speed control vs. zone control

	Speed control VRI	Zone control VRI
How it works	Pivot changes travel speed, creating pie-shaped zones; flow rate unchanged	Individual sprinklers or banks are pulsed to vary application along the pivot as well as around it
Upfront hardware (USU example)	About $1,800	About $20,000
Total annual cost (USU example)	$880 per pivot, or $7.33 per acre per year	$3,163 per pivot, or $26.36 per acre per year
Best fit	Problem areas such as slopes and infiltration issues, or multiple crops planted in sectors	Fields with complex variability in soils, topography, or yield

USU's figures use its Irrigation Investment Calculator defaults: a 120-acre pivot irrigating 20 weeks a year with a 20-year life, no yield increase, and a 0.7-inch annual reduction in water applied. Your costs will differ (USU Extension).

USU gives a speed-control example: if runoff always occurs on a slope between 20 and 40 degrees of a pivot's rotation, the pivot can speed up and pass back and forth across that sector. It applies the same total water, but in smaller doses with more time between them, which improves infiltration and reduces runoff (USU). USU notes that speed control usually needs only control-panel changes and possibly a GPS unit at the end tower, while zone control requires more hardware, labor, technical skill, and maintenance.

Building a prescription map

USU describes data sources ranging from simple and accessible to more costly, such as soil maps, apparent electrical conductivity (ECa) surveys, elevation, yield maps, imagery, and ET data (USU). Combining a prescription map with in-field soil moisture sensors lets you check whether zones are behaving as predicted.

Paying for monitoring and VRI

NRCS conservation practice 449, Irrigation Water Management, covers the management side of scheduling and measuring water (NRCS). Colorado lists irrigation water management among its EQIP priorities (NRCS Colorado). Ask your field office what Colorado's current payment schedule covers. See our EQIP guide; the FY2027 cutoff is Nov. 20, 2026.

Getting started checklist

Pull daily ET for your nearest station from CoAgMet.
Look up rooting depth and MAD for your crop in CSU fact sheets 4.707 and 4.715.
Install soil moisture sensors at multiple depths in representative spots, and calibrate them to your soil where possible.
Measure applied water with a flume or meter.
Run a water balance through one season before investing in VRI. It will show you whether your field's variability justifies it.
If you pursue VRI, start by modeling it in USU's Irrigation Investment Calculator with your own numbers.

Sources

CSU Extension, Fact Sheet 4.707, Irrigation Scheduling: The Water Balance Approach: https://extension.colostate.edu/topic-areas/agriculture/irrigation-scheduling-the-water-balance-approach-4-707/
CSU Extension, Fact Sheet 0.514, Nitrogen and Irrigation Management: https://extension.colostate.edu/docs/pubs/crops/00514.pdf
Colorado Agricultural Meteorological Network (CoAgMet): https://coagmet.colostate.edu/
CSU Agricultural Experiment Station, Irrigated soil moisture sensors: https://aes.colostate.edu/wcrc/grand-valley/irrigation-and-water-resources/irrigated-soil-moisture-sensors/
Utah State University Extension, Precision Irrigation Guide for Center Pivots (2023): https://extension.usu.edu/crops/research/precision-irrigation-guide-for-center-pivots
USDA NRCS, Conservation Practice Standards: https://www.nrcs.usda.gov/resources/guides-and-instructions/conservation-practice-standards
USDA NRCS, EQIP – Colorado: https://www.nrcs.usda.gov/programs-initiatives/environmental-quality-incentives-program/colorado/environmental-quality