How to use this calculator
- Enter or choose moisture readings are.
- Enter or choose field capacity.
- Enter or choose current moisture.
- Enter or choose wilting point.
- Enter or choose root depth.
- The water deficit updates as you type. Use Copy result or Share to keep it.
Method
Deficit (mm) = 1000 (θFC − θ) Zr; TAW = 1000 (θFC − θWP) Zr; Depletion = Deficit ÷ TAW; θ (vol) = θ (grav) × Bulk density
- θ
- volumetric water content (m³/m³)
- Zr
- root depth (m)
- Bulk density
- g/cm³, to convert gravimetric readings
Worked example
- Deficit = 1000 × (0.32 − 0.24) × 0.6 = 48 mm.
- TAW = 1000 × (0.32 − 0.14) × 0.6 = 108 mm, so 48 ÷ 108 = 44.44% is used.
- Refill volume = 48 mm × 10 = 480 m³ per hectare.
The root zone is 48 mm short of field capacity (480 m³/ha), with 44% of its available water used.
What the result means
Compare the depletion with your crop’s allowable depletion (often 30–60%). When it is reached, apply the deficit divided by your application efficiency.
Assumptions and limits
- The moisture reading represents the whole root zone.
- Water density is 1 g/cm³ when converting gravimetric readings.
- Single-layer estimate; layered soils need a layer-by-layer sum.
Questions
Volumetric or gravimetric?
Most soil moisture probes read volumetric %. Oven-dry samples give gravimetric %; multiply by bulk density to convert.
Should I apply the whole deficit?
Apply the deficit plus losses. Applying more than the deficit drains below the roots and can leach nutrients.