Sensor Trigger & Refill Depth
Reads the trigger point
From your soil texture, root depth, allowed depletion and crop water use, get the exact sensor reading to start irrigation and the refill depth in mm to return the root zone to field capacity.
Sensor & soil setup
Next: set the sensor alarm at 59 kPa and plan to apply 60 mm gross each cycle — roughly every 10 days at this ET. Watch a hot spell, which shortens the interval.
Trigger VWC = field capacity − MAD × (FC − wilting); tension read off the texture water-release curve. Refill (net) = MAD × available water × root depth; gross = net ÷ efficiency. Sources: NRCS NEH Part 623, FAO-56.
Sensor trigger & refill — key facts
- Trigger point
- field capacity − MAD × (FC − wilting)
- Net refill
- MAD × available water × root depth
- Gross refill
- net ÷ application efficiency
- Interval
- allowable depletion ÷ daily ET
- Typical MAD
- 30–50% of available water
- Field capacity
- ≈ −33 kPa loams, −10 kPa sands
- Source
- NRCS NEH 623, FAO-56
- Privacy
- Runs in your browser; nothing uploaded
A sensor reading only helps if you know the number to act on
A soil-moisture sensor tells you how wet the ground is, but two questions decide whether it saves water or wastes it: at what reading do you turn the water on, and how much do you put back? Both answers live on the soil's water-release curve — the relationship between how tightly the soil holds water (tension, in kPa) and how much water is actually there (volumetric content, in percent). The trigger point is where your allowed depletion lands on that curve; the refill depth is the gap back up to field capacity.
This calculator reads both off the curve for your texture, root depth and management. It gives the trigger tension and the equivalent percent VWC so you can program any sensor, the net and gross refill depth in millimetres, the allowable depletion buffer and the likely interval — then judges whether your schedule is comfortable, tight or heading for stress. Pair it with the Sprinkler Catch-Can Uniformity and Center-Pivot Application Rate tools to make sure that refill depth lands evenly.
Soil water-holding reference (NRCS / FAO-56)
| Soil texture | Field capacity (% VWC) | Wilting point (% VWC) | Available water (mm/m) | FC tension (kPa) |
|---|---|---|---|---|
| Sand | 10 | 4 | 60 | 10 |
| Loamy sand | 12 | 5 | 90 | 10 |
| Sandy loam | 18 | 8 | 120 | 15 |
| Loam | 27 | 12 | 170 | 33 |
| Silt loam | 31 | 13 | 200 | 33 |
| Clay loam | 36 | 18 | 180 | 33 |
| Clay | 42 | 24 | 150 | 33 |
Representative midpoints from USDA-NRCS National Engineering Handbook Part 623 and FAO Irrigation & Drainage Paper 56. Permanent wilting point taken at −1500 kPa for every texture. Measure your own field where possible.
How to set your trigger in five steps
- 1. Pick the soil. Choose the texture closest to your field; it sets the water-release curve and available water.
- 2. Set the root zone. Enter the active root and sensor depth at this growth stage.
- 3. Choose the depletion. Set MAD — 30–50% for most crops; lower for shallow high-value crops.
- 4. Add water use. Enter daily ET and your application efficiency for the gross refill depth.
- 5. Program the sensor. Set the alarm at the trigger tension and apply the gross refill depth each cycle.
Frequently Asked Questions
At what sensor reading should I trigger irrigation?+
Irrigate when the sensor reaches the trigger tension the tool reports — the point at which your chosen Management Allowed Depletion (MAD) of the available water has been used. On a loam managing 50% MAD, that lands around 40–60 kPa; on sand it is lower (the curve rises fast), on clay higher. The tool reads this point off your soil's water-release curve and also gives the equivalent percent volumetric water content for capacitance probes.
How much water should I put back when I irrigate?+
Refill the depth that was depleted — net refill = MAD × available water × root depth. For a loam holding 170 mm/m over a 60 cm root zone at 50% MAD, that is 51 mm net. Divide by your application efficiency for the gross depth to apply (51 ÷ 0.85 ≈ 60 mm). Applying that much returns the root zone to field capacity without pushing water past the roots.
What is Management Allowed Depletion (MAD)?+
MAD is the fraction of the soil's plant-available water you are willing to let the crop use before refilling. Most crops are managed at 40–50% to avoid stress; shallow-rooted, high-value crops use 30–40%, while deficit-irrigation strategies may push to 60% or more. A higher MAD means longer intervals but a drier trigger and more stress risk — which is why the tool flags comfortable, tight or stress.
What is the difference between field capacity and wilting point?+
Field capacity is the water a soil holds after free drainage (about −33 kPa for loams, −10 kPa for sands); permanent wilting point is the water held so tightly (−1500 kPa) that the plant can no longer extract it. The water between them is the plant-available water. Your trigger point sits between the two — closer to field capacity for low MAD, nearer wilting for high MAD.
Why does the trigger tension depend on soil texture?+
Each texture has its own water-release curve. Sands give up water at low tension and then dry abruptly, so their trigger tension is low (20–40 kPa) and intervals are short. Clays hold water at higher tension and release it gradually, so the same percent depletion corresponds to a higher kPa (60–100 kPa). The tool uses a texture-specific curve shape, so the trigger reflects your actual soil.
How do I convert between kPa tension and percent VWC?+
They are two readings of the same soil-water state. Tensiometers and gypsum/Watermark sensors read tension in kPa (how hard the soil holds water); capacitance and TDR probes read volumetric water content as a percentage. The tool gives both at the trigger point so you can program whichever sensor you own — the kPa for tension devices, the % VWC for probes.
How often will I need to irrigate?+
The interval is the allowable depletion divided by daily crop water use (ET). A loam allowing 51 mm of depletion under 5 mm/day ET gives about a 10-day interval; sandy soil with only 7 mm allowable under the same ET needs water in under two days. The interval shortens in hot, windy weather as ET rises, so treat it as a guide and let the sensor confirm.
What is available water capacity and why does it matter?+
Available water capacity (AWC) is the millimetres of plant-usable water a soil holds per metre of depth — roughly 60 mm/m for sand up to 200 mm/m for silt loam. It sets how much buffer the root zone has: high-AWC soils store more, so they tolerate longer intervals; low-AWC sands hold little and demand frequent, light irrigation. The tool uses NRCS representative values by texture.
Does root depth change the answer?+
Strongly. Deeper roots reach more stored water, so the allowable depletion (mm) and refill depth scale directly with root depth. A 30 cm root zone on loam allows about 25 mm; a 90 cm zone allows about 76 mm at the same MAD — a much longer interval. Use the active rooting depth at the current growth stage, not the maximum the crop can eventually reach.
Where should I place the sensor?+
Put the main sensor at about the mid-point of the active root zone where most water uptake happens, and ideally a second deeper sensor near the bottom to confirm you are not over-irrigating past the roots. Trigger on the upper sensor reaching the tension this tool reports; the deep sensor staying steady tells you the refill depth is right.
What if the result says 'stress risk'?+
That verdict means your MAD is high enough that the trigger tension lets the soil dry into the stress range before you refill. Reduce the MAD toward 40–50% so irrigation starts sooner, or split into lighter, more frequent applications. The goal is to refill before the crop hits the steep part of the water-release curve where uptake becomes difficult.
Are these figures exact for my field?+
They are sound planning figures built on NRCS available-water values and FAO-56 depletion theory, but real soils vary — measure your own field capacity and wilting point where you can, and calibrate the trigger to crop response over a season. Use the tool to set the starting threshold and refill depth, then let your sensors fine-tune it.