Calculator Inputs
Professional Notes
Why allowed depletion matters
Allowed depletion defines how much root‑zone water can be used before irrigation should refill the profile. Managing this threshold reduces stress cycles that lower yield, cause blossom drop, or slow vegetative growth. It also prevents unnecessary watering that increases leaching and disease pressure.
Interpreting total available water
Total available water (TAW) represents water held between field capacity and wilting point within the active root depth. Deeper roots and finer soils generally increase TAW. When using AWC in mm/m, you can compare soils quickly without requiring moisture limits.
Selecting a depletion fraction
The depletion fraction p reflects crop sensitivity and atmospheric demand. Leafy greens, seedlings, and shallow beds often benefit from lower p values, while established shrubs may tolerate higher p. The safety buffer reduces p to provide operational margin during heat spikes, windy days, or uneven distribution.
Turning depletion into an irrigation target
Allowed depletion is expressed as millimeters of water depth in the root zone. Net refill depth equals the allowed depletion, while gross applied depth adjusts for efficiency losses. Converting millimeters to liters is direct: one millimeter over one square meter equals one liter, enabling fast hose or tank planning.
Example data and scheduling insight
Example: FC 30%, PWP 15%, root depth 0.35 m gives TAW 52.5 mm. With p 0.45 and a 10% buffer, effective p becomes 0.405 and allowed depletion is about 21.3 mm. With ETc 4.0 mm/day, the interval is roughly 5.3 days, before efficiency adjustments and operational constraints.
Example Data Table
| Scenario | Root depth (m) | Soil data | p | TAW (mm) | Allowed depletion (mm) | ETc (mm/day) | Interval (days) |
|---|---|---|---|---|---|---|---|
| Vegetable bed | 0.35 | FC 30%, PWP 15% | 0.45 | 52.5 | 23.6 | 4.0 | 5.9 |
| Turf area | 0.25 | AWC 120 mm/m | 0.55 | 30.0 | 16.5 | 5.5 | 3.0 |
| Shrub row | 0.60 | FC 28%, PWP 14% | 0.50 | 84.0 | 42.0 | 3.5 | 12.0 |
Formula Used
TAW(mm) = (FC − PWP) × Zr × 1000
If using available water capacity:
TAW(mm) = AWC(mm/m) × Zr(m)
Allowed depletion(mm) = TAW × p_eff
How to Use This Calculator
- Select a method: moisture limits or available water capacity.
- Enter root zone depth and a depletion fraction p.
- Set a safety buffer if you prefer conservative irrigation.
- Add efficiency and area to estimate gross liters to apply.
- Enter ETc to estimate days between irrigations.
- Optionally add current moisture or current deficit to see timing.
FAQs
1) What does allowed depletion tell me?
It estimates how much water the root zone can lose before irrigation should refill the profile. Staying within this limit helps reduce plant stress and supports steadier growth and yield.
2) Which method should I choose?
Use moisture limits when you know field capacity and wilting point. Use AWC when you have a soil water holding value in mm per meter. Both methods produce comparable depletion targets.
3) What is a good p value for gardens?
Many vegetable beds use 0.30–0.50, while established shrubs may tolerate 0.50–0.65. Choose lower values during high heat, shallow rooting, or when uniformity is uncertain.
4) Why add a safety buffer?
A buffer reduces the effective depletion fraction, creating margin for hot days, wind, uneven application, or delays. It helps prevent overshooting the threshold when conditions change quickly.
5) How does efficiency affect my watering amount?
Efficiency converts the net refill depth to a gross applied depth. Lower efficiency means you must apply more to deliver the same water to roots. It does not change the root‑zone depletion itself.
6) Can I use this with sensor readings?
Yes. Enter current soil moisture when using moisture limits, or enter current depletion percent when using AWC. The tool estimates remaining capacity before the allowed limit is reached.
7) Does rainfall change the results?
Rainfall reduces depletion by refilling the root zone. After meaningful rain, recheck current moisture or deficit. If the profile refills above your target, the next irrigation can be delayed.