Soil Moisture Deficit Calculator
Calculate soil moisture deficit and MAD-based irrigation triggers from sensor data, field capacity, and crop ET.
About this calculator
This calculator converts raw soil-moisture sensor percentages into the actionable numbers growers actually need: how much plant-available water is left, and exactly when to turn the irrigation back on. Plant Available Water (PAW) capacity is the water held between field capacity and the wilting point — the range roots can actually draw from — computed as (field capacity − wilting point) × root depth × 12 to convert feet to inches. Current plant-available water uses the same formula but with today's sensor reading in place of field capacity, and the soil moisture deficit is simply how much water is missing to get back to field capacity: (field capacity − current moisture) × root depth × 12. The Management Allowable Depletion (MAD) percentage — typically 50% for most row crops — sets the trigger point: multiply it by total PAW capacity to get the depletion level (in inches) at which irrigation should begin, well before plants reach the wilting point and experience stress.
The calculator compares current depletion to that trigger, flags whether irrigation is needed right now, and projects days remaining until the trigger is reached at the current crop ET rate, plus the specific sensor reading (in %VWC) at which an alarm should be set. The core assumption is a uniform, well-mixed root zone with one field capacity and one wilting point value — real soil profiles are often layered (a sandy topsoil over a clay subsoil, for instance) with moisture-holding characteristics that change with depth, which a single-layer model like this one doesn't capture. For stratified soils, treat this as a first approximation and consider a layered water-balance model instead.
Inputs
Results
Plant available water capacity (in)
5.4
Soil moisture deficit (in)
2.88
How to Use This Calculator
- Enter Field capacity (%vol), Wilting point (%vol), and Current soil moisture (%vol).
- Set Root zone depth (ft), Allowable depletion (%), and Crop ET (in/day).
- Adjust Sensor depth (in) as needed.
- Review Plant available water capacity (in) and Soil moisture deficit (in).
- Use Current plant available water (in) and Current depletion (%) to inform your decision.
How the result changes with Field capacity (%vol)
| Field capacity (%vol) | Plant available water capacity (in) | Soil moisture deficit (in) |
|---|---|---|
| 15 | 0.36 | 0 |
| 23 | 2.88 | 0.36 |
| 45 | 10.8 | 8.28 |
| 50 | 12.6 | 10.08 |
What each input means
- Field capacity (%vol)
- Volumetric water content at field capacity for your soil type.
- Wilting point (%vol)
- Permanent wilting point — below this, plants cannot extract water.
- Current soil moisture (%vol)
- Current volumetric water content from sensor reading.
- Root zone depth (ft)
- Effective rooting depth of the crop.
- Allowable depletion (%)
- Management Allowable Depletion threshold for triggering irrigation.
- Crop ET (in/day)
- Daily evapotranspiration rate for the crop.
- Sensor depth (in)
- Depth of soil moisture sensor placement.
What each result means
- Plant available water capacity (in)
- Total plant available water in the root zone at field capacity.
- Current plant available water (in)
- Current amount of plant-available water in the root zone.
- Soil moisture deficit (in)
- Water needed to refill soil to field capacity.
- Current depletion (%)
- Current depletion as percentage of total PAW.
- MAD trigger depth (in)
- Depletion depth that triggers irrigation.
- Irrigation needed now? (1=Yes)
- 1 if current depletion exceeds MAD threshold, 0 otherwise.
- Days until trigger
- Estimated days before depletion reaches MAD at current ET.
- Recommended application (in)
- Application depth to refill to field capacity.
- Trigger sensor reading (%vol)
- Set sensor alarm at this volumetric water content.
- Relative water content (%)
- Current moisture as % of plant-available range (0=wilting, 100=FC).
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersField capacity (%vol) = 30, Wilting point (%vol) = 15, Current soil moisture (%vol) = 22, Root zone depth (ft) = 3 = 7 input(s) provided
- Calculate Plant available water capacityPlant available water capacity = (fc - wp) * rootDepthFt * 125.4 = 5.4
- Calculate Soil moisture deficit2.88 = 2.88
- Calculate Current plant available water2.52 = 2.52
- Calculate Current depletion53.3 = 53.3
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What's the difference between the soil moisture deficit and the MAD trigger depth?
Soil moisture deficit is simply how much water is currently missing to get all the way back to field capacity: (field capacity − current moisture) × root depth × 12. The MAD trigger depth is a fixed threshold — your chosen Management Allowable Depletion percentage times total plant-available water capacity — that marks the depletion level at which irrigation should start, well before the deficit reaches its maximum. Irrigation is flagged as needed once current depletion reaches or exceeds that trigger, not necessarily when the deficit is fully maxed out.
Why is the wilting point value capped just below field capacity in the calculation?
The formula sets wp = min(fieldCapacity − 0.01, wiltingPoint), which guards against a user-entered wilting point that's equal to or above field capacity — a physically impossible soil condition that would make plant-available water zero or negative and break the rest of the math (division and multiplication by a negative range). The 0.01 buffer keeps a tiny positive available-water range even in that edge case, avoiding a divide-by-zero error or an impossible negative deficit.
How is 'days until trigger' calculated, and what does it assume?
It takes the remaining depletion room before hitting the MAD trigger (madTrigger minus current depletion in inches) and divides by your entered crop ET rate in inches per day. That assumes ET stays constant at today's rate for every day until the trigger — in reality ET rises and falls with weather and crop growth stage, so treat the days-remaining figure as a snapshot estimate that needs rechecking as conditions change, not a fixed countdown.
What does the trigger sensor reading (%VWC) output mean, and how do I use it?
It converts the MAD trigger depth back into the actual volumetric water content a soil moisture sensor would read at that depletion level, computed as field capacity minus (trigger depth ÷ (root depth × 12)). Rather than watching the deficit-in-inches math yourself, you can set your sensor's irrigation alarm directly at this percentage — when the sensor reading drops to or below it, you're at the point this calculator says irrigation should begin.
Does the sensor depth I enter change any of the results?
No — Sensor depth (in) is collected as a record of where the sensor is physically placed, but it does not feed into any of the calculations here. All of the depletion, trigger, and days-until-trigger math uses Root zone depth (ft) instead.
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