Soil Moisture Deficit & Refill Exactly What's Lost

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.