Available Water Capacity Calculator
Enter field capacity, permanent wilting point, and root-zone depth to compute available water capacity AWC (%vol) = FC − PWP and the usable stored water depth (mm) for irrigation planning and soil water interpretation.
Input Data
Results
At a glance:Enter field capacity, permanent wilting point, and root-zone depth to compute available water capacity AWC (%vol) = FC − PWP and the usable stored water depth (mm) for irrigation planning and soil water interpretation.
Formula
AWC(%vol) = θ_FC − θ_PWP.
Stored depth (mm) = AWC(%) ÷ 100 × root depth (mm).
$$AWC = \theta_{FC} - \theta_{PWP}$$$$D_{aw} = \frac{AWC}{100} \times d_{root}$$How to Use
- Enter field capacity FC and permanent wilting point PWP (%vol).
- Enter the effective root-zone depth (mm).
- The calculator returns AWC and the usable stored water depth.
FAQ
How do I obtain field capacity and permanent wilting point?
Most accurate is measurement: saturate the soil in the field, cover to prevent evaporation, and sample after free drainage stops (about 1 day for sand, 2–3 for clay) to get FC; or equilibrate in a pressure chamber at −0.033 MPa (or −0.01 MPa). PWP is measured after equilibration at −1.5 MPa. Without lab access, use empirical texture values (see the table on this page) or pedotransfer functions estimating from texture and organic matter.
Should AWC use volumetric or gravimetric water content?
This calculator uses volumetric water content (%vol), because the stored-depth conversion needs a volume basis: depth = AWC (vol fraction) × root depth. If your FC/PWP are gravimetric (%mass), convert to volumetric first: θv = θg × bulk density, then enter. Mixing the two bases gives wrong results.
Why not use up all of AWC before irrigating?
Water within AWC is not equally available — the closer to the wilting point, the tighter the soil holds it and the harder roots extract it, so the crop enters water stress early, hurting growth and yield. In practice a Management Allowable Depletion (MAD, often 40–60%) is set, letting the crop consume only part of AWC before refilling, keeping it in the easy-extraction range. Irrigation trigger = MAD × AWC × root depth.
How should root-zone depth be determined?
Use the effective root-zone depth — where most absorbing roots (typically ~70–80% of root mass) are, not the deepest root. It changes with crop and growth stage: shallow vegetables ~300–600 mm, mature field crops 600–1200 mm, trees deeper. Early growth has shallow roots and small storage, needing more frequent irrigation; as roots deepen, single refill volume and interval both increase.
Clay holds a lot of water, so why isn't its available water always highest?
Clay has many tiny pores and very high field capacity, but because its pores are fine and surface area large, it binds water very tightly, so its permanent wilting point is also high — much water sits beyond what plants can pull. AWC is the gap FC − PWP; clay is high at both ends, so the net gap is compressed. Loam has high enough FC and not-too-high PWP, giving the largest gap, so its available water often beats clay.
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References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.