Irrigation Interval
Enter root-zone available water, management allowable depletion MAD and daily crop evapotranspiration ETc to compute usable water and the days between irrigations for scheduling.
Input Data
Results
At a glance:Irrigation interval is how many days can pass between two irrigations without stressing the crop. It is set by three quantities: root-zone available water (between field capacity and wilting point, usable by the crop, mm), management allowable depletion MAD (the fraction of available water allowed to be used before refill), and daily crop evapotranspiration ETc (water used per day, mm/day). In practice you do not wait until the soil dries to the wilting point — the crop would already be stressed and yield-reduced — but set a buffer: refill after using a certain fraction (often 50%, 30–40% for drought-sensitive crops); that fraction is MAD. So usable water = available water × MAD, the water depth allowed to be consumed in one irrigation cycle; dividing by daily ETc gives how many days it lasts — the irrigation interval. A longer interval means more soil storage or slower use, easier management; conversely shallow-rooted crops, sandy soil or hot midsummer with large ETc shorten the interval, needing frequent light irrigations. Combining available water, MAD and ETc schedules irrigation to field reality, avoiding drought stress without over-irrigation that wastes water and leaches fertilizer.
Formula
Usable water: usable(mm) = available water(mm) × (MAD% ÷ 100).
Irrigation interval: interval(days) = usable water(mm) ÷ daily ETc(mm/day).
Each irrigation depth refills the consumed usable water.
$$W_{usable} = AW \times \frac{MAD\%}{100}$$$$T_{interval} = \frac{W_{usable}}{ET_c}$$How to Use
- Enter root-zone available water (mm; use the soil water storage calculator first).
- Enter MAD (%) — usually 50% — and daily ETc (mm/day).
- The tool returns usable water and interval days; larger ETc or smaller MAD shortens the interval.
Scenario effect on irrigation interval (example: available water 108 mm, MAD 50%)
| Scenario | Daily ETc | Approx. interval |
|---|---|---|
| Cool / early growth | 3 mm/day | ~18 days |
| Mild season | 4.5 mm/day | ~12 days |
| Midsummer / canopy closed | 6 mm/day | ~9 days |
| Hot drought | 9 mm/day | ~6 days |
Example fixes available water and MAD, varying only daily ETc; real interval also depends on soil storage, root depth and MAD. Hotter weather raises ETc and shortens the interval.
Case Studies
Irrigation interval of a loam vegetable field
Loam field, root-zone available water 108 mm, crop MAD 50%, midsummer ETc = 6 mm/day.
Usable water = 108 × 50% = 54 mm; interval = 54 ÷ 6 = 9 days.
Irrigate about every 9 days, refilling ~54 mm each time, keeping stable supply without drought stress.
Tighten MAD for a sensitive crop
Same field, drought-sensitive leafy greens; tighten MAD from 50% to 40% for quality, ETc still 6 mm/day.
Usable water = 108 × 40% = 43.2 mm; interval = 43.2 ÷ 6 = 7.2 days.
Interval shortens ~2 days — irrigate more often, slightly less each time, to keep root-zone moisture stable and avoid quality drop.
FAQ
What MAD value should I use?
MAD is the fraction of available water allowed to be used before refill, a buffer against drought stress. General field crops use 50%; drought-sensitive, quality-demanding crops (e.g. leafy greens, some fruit-tree flowering) use 30–40%; drought-tolerant crops or late growth can be looser. Smaller MAD keeps soil wetter, shortens the interval and irrigates more often but safer; larger MAD saves labor but raises drought risk.
How do I get available water?
Root-zone available water = (field capacity − wilting point) ÷ 100 × root depth(cm) × 10, computable directly with this site's soil water storage calculator. It depends on soil texture (more in loam, less in sand) and root depth (deeper roots use a thicker layer and more storage). Enter that value here to estimate the interval.
Where does daily ETc come from?
ETc = crop coefficient Kc × reference evapotranspiration ET₀, the crop's daily water use (mm/day), from this site's crop water need calculator. It varies with weather (larger when hotter/drier) and growth stage (peak at mid-season canopy closure). Use the current stage's average daily ETc for scheduling; when it heats up, ETc rises and the interval automatically shortens.
How much water per irrigation?
Each irrigation depth roughly equals the usable water consumed in the interval, i.e. available water × MAD (54 mm here), refilling the root zone back near field capacity. Divide this net depth by the irrigation system efficiency for the actual gross volume to supply (see the Irrigation Water calculator). Interval and per-event volume must match to keep stable soil moisture.
Is the computed interval fixed?
No. It is an estimate for current conditions and changes dynamically with weather and stage: higher temperature and canopy closure raise ETc and shorten the interval; cool or early/late stages lengthen it. Effective rainfall during the period can also postpone irrigation. Treat it as a starting schedule and adjust with soil-moisture observation or sensors.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.