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Calcimator

Soil Moisture Sensor Calculator

Calculate sensor placement depths, irrigation trigger points, and scheduling intervals from soil water-holding capacity.

Inputs

%
%
%

Results

Readily available water (in)

1.8

Irrigation trigger (vol %)

22.5

AWC (in/in)0.15
Total available water (in)3.6
Primary sensor depth (in)8
Secondary sensor depth (in)16
Irrigation interval (days)7.2
Sensors recommended2
Refill VWC30
Deep Check Depth (in)24
Gross Irrigation Depth (in)2.25
How to Use This Calculator
  1. Enter field capacity and wilting point as volumetric percentages for your soil type.
  2. Set root zone depth in inches appropriate to your crop.
  3. Enter Management Allowed Depletion (%) — typically 50% for most crops.
  4. Input daily crop ET (in/day) and field area in acres.
  5. Read Irrigation trigger (vol%) to program sensor alerts and Irrigation interval (days) for scheduling.

How the result changes with Field capacity (vol %)

Field capacity (vol %)Readily available water (in)Irrigation trigger (vol %)
100.129.5
230.9619
382.7626.5
504.232.5

What each input means

Field capacity (vol %)
Soil volumetric water content at field capacity. Sandy=10-15%, loam=25-35%, clay=35-45%.
Wilting point (vol %)
Permanent wilting point. Sandy=5-8%, loam=12-18%, clay=20-30%.
Root zone depth (in)
Effective root zone depth. Turf=6-8, vegetables=12-18, corn=36-48, trees=48-72.
Allowed depletion (%)
Management Allowed Depletion. Shallow roots/sandy=30-40%, deep roots=50-65%.
Daily crop ET (in/day)
Current or peak daily crop evapotranspiration.
Field area (acres)
Total field area for sensor count recommendation.

What each result means

AWC (in/in)
Available water capacity: inches of water per inch of soil.
Total available water (in)
Total available water in the root zone.
Readily available water (in)
Water available before stress = TAW × MAD fraction.
Irrigation trigger (vol %)
Start irrigating when sensor reads below this volumetric water content.
Primary sensor depth (in)
Install primary sensor at 1/3 of root zone depth.
Secondary sensor depth (in)
Install secondary sensor at 2/3 of root zone depth.
Irrigation interval (days)
Maximum days between irrigations before crop stress.
Sensors recommended
Total sensors needed (2 per management zone, 1 zone per 40 acres).

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Field capacity (vol %) = 30, Wilting point (vol %) = 15, Root zone depth (in) = 24, Allowed depletion (%) = 50 = 6 input(s) provided
  2. Calculate Readily available water
    Readily available water = tawIn * madFraction
    1.8 = 1.8
  3. Calculate Irrigation trigger
    Irrigation trigger = fieldCapacityPct - (awcSafe * 100 * madFraction)
    22.5 = 22.5
  4. Calculate AWC
    AWC = max(0.01, awcInPerIn)
    0.15 = 0.15
  5. Calculate Total available water
    Total available water = awcSafe * rootDepthIn
    3.6 = 3.6

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