Soil Moisture Deficit & Refill Exactly What's Lost
Tells You the deficit
Enter field capacity, current moisture and root depth to get the soil-moisture deficit in mm, the deficit percentage and a clear irrigate-or-hold call.
Soil moisture deficit
Next: apply about 72 mm to bring the 60 cm root zone back to field capacity.
Volumetric % values come from a soil moisture sensor or gravimetric sampling; deficit assumes the whole root depth dries uniformly.
Soil-moisture deficit — key facts
- Deficit
- (FC − moisture) × root depth
- Refill to
- Field capacity
- Allowable depletion
- ≈ 40–60% of available water
- Refill exactly
- Avoids waste & waterlogging
- 1 mm over 1 ha
- = 10 m³ (10,000 L)
- Heavy soils
- Hold more; longer intervals
- Use
- When & how much to irrigate
- Privacy
- Runs in your browser; nothing uploaded
Refill what the root zone has lost — no more, no less
As your crop transpires and the soil dries, the root zone falls below field capacity. The gap between the two is the soil-moisture deficit: the exact depth of water you would need to add to bring the soil back to full. Irrigate to that depth and you refill the store perfectly. Add less and the crop goes short; add more and the surplus drains below the roots, wasting water and nutrients and risking waterlogging.
This tool computes the deficit in mm, the deficit as a percentage of available water, an irrigate-or-hold recommendation, and the root depth in use from field capacity, current moisture and root depth. It answers both scheduling questions at once — when to water and how much. Pair it with the Irrigation Scheduling, Tensiometer and Soil Water Holding Capacity tools for a complete soil-based programme.
Refill, don't overshoot
Add exactly the deficit back to field capacity.
Avoid waterlogging
No surplus draining below the root zone.
Know the depletion
See how close you are to the stress point.
Clear call
Irrigate or hold, with the depth to apply.
Frequently Asked Questions
What is the soil-moisture deficit (SMD)?+
The soil-moisture deficit is how much water the root zone has lost below field capacity — in other words, the depth of water you would need to add to bring the soil back to full. It rises as the crop transpires and the soil dries, and irrigating to exactly that depth refills the store without waste.
How is the soil-moisture deficit calculated?+
Deficit (mm) = (field capacity − current moisture) × root depth, with moisture as a volumetric fraction and depth in mm. For example, if field capacity is 0.30, current moisture is 0.22 and the root zone is 600 mm deep, the deficit is (0.30 − 0.22) × 600 = 48 mm.
Why should I refill exactly the deficit?+
Adding exactly the deficit brings the root zone back to field capacity — the most it can hold against gravity. Adding less leaves the crop short; adding more pushes water below the roots where it drains away, wasting water and nutrients and risking waterlogging. Matching the refill to the deficit is the heart of efficient irrigation.
What is field capacity?+
Field capacity is the amount of water a soil holds after free drainage has stopped, a day or two after saturation. It is the upper limit of plant-available water and the target you refill to. Clay soils have a higher field capacity than sandy soils, which is why heavy soils can go longer between irrigations.
What does the deficit percentage tell me?+
The deficit percentage expresses how depleted the root zone is relative to its total available water. As it approaches the crop's allowable depletion (often 40–60% of available water), the crop starts to feel stress and it is time to irrigate. The tool flags this so you do not let the soil dry past the comfortable zone.
How do I measure current soil moisture?+
You can read it directly with a soil-moisture probe or capacitance sensor, infer it from a tensiometer reading, or estimate it by running a water balance from the last irrigation using daily ETc. Whatever the source, enter it as a volumetric fraction (or percent) and the tool converts it to a depth deficit.
What root depth should I use?+
Use the effective root depth — the layer from which the crop actively draws water, not the deepest stray root. It grows through the season, from a shallow seedling zone to the full mature depth, so update it as the crop develops. A deeper root zone holds more water and tolerates a larger deficit before stress.
When does the tool say irrigate versus hold?+
It compares the deficit against the refill point you set. If the deficit has reached or exceeded the threshold, it advises irrigating and tells you the depth to apply; if the soil is still comfortably wet, it advises holding. This keeps you from irrigating too early (wasting water) or too late (stressing the crop).
Does refilling the deficit account for irrigation losses?+
The deficit is the net water the root zone needs. Because not all applied water reaches the roots, the gross amount to pump is larger — divide by your system efficiency (drip ~90%, sprinkler ~75%, surface ~55%). Use the Irrigation Water tool to convert the net deficit into the gross depth and volume to apply.
How does this fit irrigation scheduling?+
The deficit answers both scheduling questions at once: when to irrigate (when the deficit reaches the refill point) and how much (the depth equal to the deficit). Track it alongside daily ETc and you have a complete, soil-based schedule. See the Irrigation Scheduling and Tensiometer tools to automate the trigger.