Calculator
Example data table
| Soil | Field Capacity (% v/v) | Wilting Point (% v/v) | Depth (cm) | AWC (mm) | Total Water (L) |
|---|---|---|---|---|---|
| Sand | 10 | 5 | 30 | 15.00 | 150.00 |
| Sandy Loam | 20 | 10 | 30 | 30.00 | 300.00 |
| Loam | 27 | 13 | 30 | 42.00 | 420.00 |
| Silt Loam | 33 | 15 | 30 | 54.00 | 540.00 |
| Clay Loam | 36 | 20 | 30 | 48.00 | 480.00 |
Formula used
Available Water Capacity (AWC) is the plant-available water stored in a root zone. Using volumetric water contents:
θFC= field capacity (% v/v) ÷ 100θWP= wilting point (% v/v) ÷ 100AWC fraction=θFC − θWPAWC depth=(θFC − θWP) × root depth
Conversions used:
- 1 mm water over 1 m² equals 1 liter.
- 1 inch equals 25.4 mm, and 1 L equals 0.264172 gal.
How to use this calculator
- Select your unit system, then choose a soil preset if desired.
- Enter Field Capacity and Wilting Point from a lab test, or rely on the preset.
- Set the Root zone depth to match the crop rooting depth.
- Optionally enter Area to get liters and gallons.
- Adjust p, depletion, and efficiency to estimate how much to apply now.
- Press Calculate. Use the download buttons in the results card.
Root Zone Depth Drives Storage
Available water capacity is reported as a water depth stored in the root zone. A 20 cm turf root zone stores less water than a 45 cm vegetable root zone, even with identical moisture limits. Use realistic rooting depths for each bed or lawn to avoid overwatering.
Interpreting Field Capacity and Wilting Point
Field capacity (θFC) approximates water held after free drainage, while wilting point (θWP) marks water too tightly held for plants. The calculator converts both to fractions and subtracts them to obtain plant-available water. Small errors matter: a 3% change in θFC−θWP across 30 cm changes storage by 9 mm.
Texture Presets and When to Override
Presets provide typical θFC and θWP values for common textures, helpful when lab data is unavailable. Override presets when you have measured values, high organic matter, raised-bed mixes, or amended soils. Compost and biochar can shift θFC upward, while compaction can reduce storage and infiltration.
Using RAW and Depletion for Scheduling
Readily available water (RAW) equals AWC multiplied by depletion fraction p. Sensitive crops and hot conditions often use smaller p values to limit stress. If you estimate current depletion (for example, 60% of AWC), the tool returns net refill needed now and an applied depth based on efficiency, supporting irrigation intervals.
Efficiency and Practical Application Amounts
Efficiency accounts for losses from wind drift, runoff, deep percolation, and uneven coverage. With area entered, the calculator converts millimeters to liters and gallons for hose timers or drip zones. Compare applied depth against emitter rates and run time to apply water slowly enough for infiltration, especially in clay and sloped beds.
FAQs
What does available water capacity represent?
It is the water held between field capacity and wilting point within the root zone. The result is expressed as a water depth (mm or inches) and, if area is entered, as a volume in liters or gallons.
Where do field capacity and wilting point values come from?
Best sources are soil lab reports or published soil-water curves for your soil. If you lack measurements, use the texture presets as starting points and refine after observing drainage, wilting, and irrigation response.
Why does the calculator ask for depletion fraction p?
p estimates how much of AWC plants can use before stress becomes likely. Lower p suits sensitive crops or hot, windy weather. Higher p may fit hardy plants or cool seasons with lower evaporative demand.
How should I choose root zone depth?
Use typical rooting depth for the crop and site conditions. Shallow-rooted turf may be 15–25 cm, many vegetables 30–60 cm, and shrubs deeper. Restricted beds or compacted layers can reduce effective depth.
What does application efficiency change?
Efficiency converts net refill (what the root zone needs) to gross applied depth (what you must deliver). Losses include runoff, wind drift, and deep percolation. Drip systems usually have higher efficiency than sprinklers.
Do I need to enter area?
No. Area is optional and only affects volume outputs. Without area, you still get AWC, RAW, and refill depths. Enter area when you want liters or gallons for irrigation scheduling and estimating run times.