Variable Rate Irrigation Calculator

Calculate VRI prescription rates by management zone based on soil AWC, crop ET demand, and pivot flow capacity.

Field size (acres)

Number of VRI zones

Soil AWC (in/ft)

Effective root depth (ft)

Allowable depletion (%)

Current depletion (%)

Crop ET (in/day)

Pivot flow rate (GPM)

Application efficiency (%)

Tab to move between fields

Total AWC in root zone (in)

5.4

Gross application depth (in)

1.91

MAD trigger point (in)2.7

Current depletion (in)1.62

Days until irrigation trigger4.3

Average zone size (acres)20.8

Volume per zone (gal)1,078,174

Total field volume (gal)6,469,041

System run time (hr)134.8

VRI water savings (%)8

Water saved (gal)517,523

How to Use This Calculator

Enter Field size (acres), Number of VRI zones, and Soil AWC (in/ft).
Set Effective root depth (ft), Allowable depletion (%), and Current depletion (%).
Adjust Crop ET (in/day), Pivot flow rate (GPM) as needed.
Review Total AWC in root zone (in) and Gross application depth (in).
Use MAD trigger point (in) and Current depletion (in) to inform your decision.

How the result changes with Soil AWC (in/ft)

Field size (acres): Total irrigated area under the pivot or lateral system.
Number of VRI zones: Management zones defined by soil type, topography, or yield map.
Soil AWC (in/ft): Available water-holding capacity of soil in inches per foot of depth.
Effective root depth (ft): Active root zone depth for the crop being irrigated.
Allowable depletion (%): Management Allowable Depletion — percentage of AWC that can be used before irrigating.
Current depletion (%): Current soil moisture depletion as percentage of total AWC.
Crop ET (in/day): Daily crop evapotranspiration rate.
Pivot flow rate (GPM): Total system flow rate in gallons per minute.
Application efficiency (%): Irrigation application efficiency accounting for wind, evaporation, and runoff losses.

Total AWC in root zone (in): Total available water capacity in the effective root zone.
MAD trigger point (in): Depletion level that triggers irrigation.
Current depletion (in): Current soil moisture depletion in inches.
Gross application depth (in): Total water to apply accounting for system efficiency losses.
Days until irrigation trigger: Estimated days before soil moisture reaches MAD threshold.
Average zone size (acres): Average area per VRI management zone.
Volume per zone (gal): Water volume needed per average zone.
Total field volume (gal): Total water volume for the entire field application.
System run time (hr): Hours of operation needed to complete the irrigation event.
VRI water savings (%): Estimated water savings from variable rate vs. uniform application.
Water saved (gal): Gallons saved per irrigation event using VRI.

Identify Input Parameters
4 parameters
Field size (acres) = 125, Number of VRI zones = 6, Soil AWC (in/ft) = 1.8, Effective root depth (ft) = 3 = 9 input(s) provided
Calculate Total AWC in root zone
Total AWC in root zone = soilAwcInPerFt * rootDepthFt
5.4 = 5.4
Calculate Gross application depth
Gross application depth = netApplicationIn / (systemEfficiency / 100)
1.91 = 1.91
Calculate MAD trigger point
MAD trigger point = totalAwc * (madPct / 100)
2.7 = 2.7
Calculate Current depletion
Current depletion = totalAwc * (currentDepletionPct / 100)
1.62 = 1.62