What is management allowed depletion?
A crop draws water from the soil between two limits: field capacity, the wettest the soil stays after draining, and wilting point, where no more water is available to the roots. The depth of water held between those two limits over the root zone is the total available water (TAW). In practice, a crop shows stress well before the soil actually reaches wilting point, because water becomes harder to extract as the soil dries.
Management allowed depletion (MAD) is the share of TAW you choose to let the crop use before you irrigate again, usually written as a percentage. Multiplying MAD by TAW gives the readily available water (RAW): the depth of water the crop can draw down without yield-limiting stress. FAO Irrigation and Drainage Paper No. 56 (FAO-56) uses a closely related figure, the depletion fraction for no stress (p), and lists a starting value for dozens of crops. This calculator uses those values by default and lets you override them.
| Term | What it means | Source |
|---|---|---|
| TAW | Total available water: the depth held between field capacity and wilting point over the root zone. | FAO-56 Eq. 82 |
| p | FAO-56's depletion fraction for no stress: a physically based typical value, by crop, for ETc ≈ 5 mm/day. | FAO-56 Table 22 |
| RAW | Readily available water: p (or your MAD) × TAW. The depth the crop can safely use before irrigating. | FAO-56 Eq. 83 |
| MAD | Management allowed depletion: the depletion percentage you actually choose, which may match p or differ from it based on risk and cost. | Merriam & Keller, 1978 |
In practice, growers and this calculator use MAD and RAW as the working numbers, filled in with FAO-56's p as a sourced starting point. If you're scheduling a full water balance rather than a single depletion depth, the soil water balance calculator tracks depletion day by day.
How this calculator works
The calculator multiplies your total available water by a depletion percentage to get a depth of readily available water. If you add your crop's daily water use, it also adjusts that percentage for the weather and estimates how many days that water is likely to last.
1. Total available water (TAW)
Direct entry
TAW = the value you enter
From AWC and root depth
TAW = Available water capacity × Root depth
For example, 2.0 in/ft × 2.5 ft = 5.0 in. Get AWC from the available water capacity calculator and root depth from the root zone depth calculator.
2. Readily available water (RAW)
Core formula
RAW = MAD × TAW
FAO-56 Equation 83, where MAD is the depletion fraction expressed as a decimal (40% = 0.40).
3. Optional: adjust for crop water use (ETc)
Weather-adjusted depletion fraction
p (adjusted) = p (table) + 0.04 × (5 − ETc)
FAO-56 Table 22, footnote 2. ETc is in mm/day; the result is limited to between 0.10 and 0.80. Hotter, higher-ETc days lower the safe depletion fraction; cooler days raise it.
4. Optional: estimated days between irrigations
This calculator's own estimate, not a FAO-56 formula
Days ≈ RAW ÷ ETc
A simple average, assuming steady crop water use with no rain, runoff or capillary rise. For a day-by-day forecast, use the soil water balance calculator or the irrigation interval calculator.
When this calculator isn't the right tool
It calculates a single depletion depth from TAW and a percentage. Use a different tool for:
- Finding TAW itself from soil texture. See the available water capacity calculator.
- Flooded rice. FAO-56 defines its depletion fraction for rice as a share of saturation, not TAW, so this tool doesn't include it.
- A day-by-day irrigation schedule. See the soil water balance calculator and the irrigation scheduling calculator.
- Converting the resulting depth to a sprinkler or drip run time. See the sprinkler run time calculator or the drip run time calculator.
Worked example
A grower already knows their tomato bed's TAW is 4.00 in. They select "Tomato," which fills in FAO-56's depletion fraction of 40%. Their crop is using an average of 0.12 in/day of water this week.
- TAW: 4.00 in (entered directly).
- Standard RAW: 0.40 × 4.00 in = 1.60 in, using FAO-56's table value.
- ETc in mm/day: 0.12 in/day × 25.4 = 3.05 mm/day.
- Adjusted depletion fraction: 0.40 + 0.04 × (5 − 3.05) = 0.40 + 0.078 = 0.478 (about 48%).
- Adjusted RAW: 0.478 × 4.00 in = about 1.91 in.
- Days between irrigations: 1.91 in ÷ 0.12 in/day ≈ about 16 days.
The same calculation in metric
A field corn (maize) crop has a TAW of 130 mm over its root zone, no ETc entered. Selecting "Maize / field corn (grain)" fills in FAO-56's depletion fraction of 55%.
- TAW: 130 mm.
- RAW: 0.55 × 130 mm = 71.5 mm.
Because no ETc was entered, the calculator shows only the standard FAO-56 value, with no weather adjustment or day count.
Choosing a MAD percentage
FAO-56 Table 22 gives a physically grounded starting point: how much depletion a crop typically tolerates before its evapotranspiration begins to drop below potential. Selecting a crop above fills that value in, from as low as 20% for shallow, sensitive crops like strawberries and lettuce to 65-80% for deep-rooted, drought-tolerant crops like cotton, avocado and olives.
Colorado State University Extension frames the practical range a little differently, in terms of risk and crop value: growers commonly run drought-sensitive or high-value crops at 25-40% depletion, most row crops around 50%, and tolerant crops such as some wine grapes as high as 80-90%. That's the distinction FAO-56 itself draws between p, a physical threshold, and MAD as Merriam and Keller originally defined it: MAD also reflects how much yield risk you're willing to accept and how expensive water or irrigation labor is for you.
A MAD above the FAO-56 value is a deliberate trade. Running depletion higher than the table's p means accepting some reduction in crop water use, and potentially yield, in exchange for irrigating less often. That can be the right call for water-limited or lower-value plantings, but it should be a choice, not an accident.
Whichever percentage you use, check local watering restrictions and any water district allocation rules before finalizing a schedule around it.
What changes the safe depletion level
| Factor | Effect | What to adjust |
|---|---|---|
| Crop water use (ETc) | Table values apply at about 5 mm/day. Hotter, higher-ETc days push the safe fraction down; cooler days push it up. | Enter your ETc for the adjusted value and day estimate. |
| Soil texture | FAO-56 notes fine (clay) soils generally tolerate 5-10% less depletion than the table value; coarse (sandy) soils tolerate 5-10% more. | Nudge MAD down for heavy clay, up for sandy soil. |
| Rooting depth | TAW scales directly with root depth, and depth changes through the season as roots develop. | Use the crop's current effective rooting depth, not its mature depth, early in the season. |
| Crop value and risk tolerance | Higher-value or drought-sensitive crops are usually managed at a lower MAD than the physical threshold to avoid any stress; tolerant or lower-value crops can run closer to, or above, it. | Pick "Other crop" and enter your own percentage. |
| Growth stage | Crops are typically most sensitive to depletion around flowering and yield formation. | Consider a lower MAD during those windows even if using a higher one elsewhere in the season. |
None of these factors change the arithmetic; they change which MAD percentage is the right one to type in. The soil infiltration rate calculator and soil infiltration rate chart can help characterize how fast a given soil accepts water once you do irrigate.
Frequently asked questions
What is management allowed depletion (MAD)?
Management allowed depletion is the share of a crop's total available soil water that you choose to let it use up before you irrigate again. It is usually written as a percentage of TAW, so a MAD of 40% means you irrigate once 40% of the water held between field capacity and wilting point has been used. FAO-56 uses a closely related figure called the depletion fraction for no stress (p) to estimate the point where yield-limiting stress begins, and lists typical p values by crop. The term MAD itself comes from Merriam and Keller's 1978 irrigation system evaluation guide, and unlike p it can also reflect how much risk you are willing to accept.
How is MAD different from readily available water (RAW)?
RAW is a calculated depth of water, in inches or millimeters, equal to MAD (or FAO-56's p) multiplied by TAW. MAD is the percentage; RAW is the volume of water that percentage represents in your specific soil and root zone. This calculator's main result is RAW: how much water your crop can draw down before you need to irrigate again, given the MAD you enter.
What MAD percentage should I use for my crop?
FAO-56 Table 22 lists a depletion fraction for dozens of crops, generally 20% for shallow, sensitive crops like strawberries up to 65-80% for deep-rooted, stress-tolerant crops like cotton or avocado. Selecting a crop from the list fills in that value, and you can edit it. Colorado State University Extension notes growers often run drought-sensitive or high-value crops nearer 25-40% and more tolerant crops as high as 80-90%, so treat the FAO-56 figure as a starting point rather than a fixed rule.
How do I find my soil's total available water (TAW)?
TAW equals the difference between field capacity and wilting point, multiplied by your root depth, expressed as a water depth. If you already have a TAW figure, such as from a soil test or a Web Soil Survey report, enter it directly. Otherwise, use the available water capacity calculator and the root zone depth calculator to build the two numbers this calculator's second TAW option needs.
See the available water capacity calculator and the root zone depth calculator.
Does MAD change with the weather or crop water use (ETc)?
Yes. FAO-56 states that the tabulated depletion fraction applies to a crop evapotranspiration rate of about 5 millimeters a day, and gives a formula to adjust it for other rates: hotter, higher-ET days allow less depletion before stress, while cooler days allow more. Enter your crop's daily water use in the optional field to see this adjusted value alongside the standard one.
What happens if I let the soil deplete beyond RAW?
Beyond RAW, FAO-56 describes crop water uptake as no longer able to keep pace with evapotranspiration demand, so actual crop water use starts to fall below its potential rate. This does not mean the crop is at wilting point; the soil still holds water down to TAW, but the plant can no longer extract it fast enough to avoid some reduction in transpiration and, eventually, yield. Irrigation scheduled at or before this point is the practical goal of using MAD or RAW.
Does soil texture affect the safe depletion percentage?
Somewhat. FAO-56 notes that its table values can generally be reduced by 5-10% for fine-textured soils such as clay and increased by 5-10% for coarser soils such as sand, because water becomes harder to extract at the same fraction of depletion in fine soils. This calculator does not apply that adjustment automatically; if your soil is notably heavy or light, nudge the MAD percentage down or up accordingly.
Can I use a higher MAD than the FAO table to save water?
You can enter any percentage as a custom value, and some growers deliberately do run above the FAO-56 figure. Per FAO-56, this is the distinction between p (a physical stress threshold) and MAD as originally defined by Merriam and Keller, which also folds in management and economic judgment. Running above the physical threshold means accepting some yield-limiting stress in exchange for using less water, so it should be a deliberate choice, not a default.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formula, unit conversion and both worked examples verified by hand against FAO-56 Chapter 8.
- Every crop depletion fraction (p) and root depth range checked against FAO-56 Table 22.
- The MAD-vs-p distinction and typical grower ranges checked against Merriam & Keller (1978) and Colorado State University Extension.
Sources
- Allen, R.G., Pereira, L.S., Raes, D., Smith, M. (1998). FAO Irrigation and Drainage Paper No. 56: Crop Evapotranspiration, Chapter 8, ETc under soil water stress conditions.
- Merriam, J.L. and Keller, J. (1978). Farm Irrigation System Evaluation: A Guide for Management, Utah State University.
- Card, A. and Bauder, T. Colorado State University Extension. Determining Irrigation Run Times with Drip Tape on Specialty Crops (Fact Sheet 5.623).
- University of Florida IFAS Extension. Crop Water Use and Irrigation Scheduling Guide.