RAW, ETc and the irrigation interval
Every irrigation schedule answers two questions: how much water to apply, and how often. This calculator answers the second one. It compares two depths that describe your crop's water budget: readily available water (RAW), the water your crop can draw from the root zone before stress sets in, and ETc, how fast the crop uses that water each day. Divide one by the other and the result is the number of days the crop can go before it needs its next irrigation.
This idea comes directly from FAO Irrigation and Drainage Paper 56, the standard reference for crop water requirements. RAW is defined there as RAW = p × TAW, where TAW is the total available water in the root zone and p is a depletion fraction specific to the crop. Daily ETc is defined as ETc = ETo × Kc, where ETo is the reference evapotranspiration for the local climate and Kc is a crop coefficient for the current growth stage.
If you already have both RAW and ETc as depths, this calculator does the one division for you. If you only have the building blocks, RAW's ingredients (soil available water capacity, root depth and depletion fraction) and ETc's ingredients (ETo and Kc) each have their own calculators linked throughout this page.
Check local delivery rules first. Where irrigation districts run on a fixed rotation or allocation schedule, the days water is actually available can override any calculated interval.
How this irrigation interval calculator works
The calculator takes RAW and daily ETc, in the same depth units, and divides one by the other. If you don't have ETc directly, it can build it from ETo and Kc first.
1. Daily crop water use (only if using ETo × Kc)
Crop evapotranspiration
ETc = ETo × Kc
ETo is the reference evapotranspiration for grass or a similar reference surface. Kc adjusts it for your specific crop and growth stage.
2. Irrigation interval
Days between irrigations
Interval (days) = RAW ÷ ETc
RAW and ETc must be in the same depth units (inches or millimeters). The result is how many days it takes ETc to use up the full RAW.
3. Whole-day interval and water used
Because irrigation happens on whole days, the calculator also rounds the interval down and shows the depth actually used by that point.
Practical scheduling values
Whole-day interval = FLOOR(RAW ÷ ETc)
Depth used by then = ETc × Whole-day interval
Rounding down keeps a small margin, since the exact interval is the point RAW reaches zero, not before.
Unit conversions
US and metric depths convert directly: 1 inch = 25.4 millimeters. Since RAW and ETc are both depths (ETc is a depth per day), the same factor converts either one, and the day count itself doesn't change with units.
Displayed depths use 2 decimal places in US units and 1 decimal place in metric units, matching the precision used in the calculation. The interval is shown to 1 decimal place with "about," since it depends on today's ETc holding steady, which real weather rarely does exactly.
When this calculator isn't the right tool
- You need RAW or ETc themselves. Use the management allowed depletion calculator for RAW, or the crop evapotranspiration calculator for ETc.
- You want rainfall included and a running daily balance, not a single division. Use the soil water balance calculator.
- You want a full schedule with system run times and application depths. Use the irrigation scheduling calculator.
- You're watering a home lawn by a weekly depth rather than RAW and ETc. Use the lawn watering calculator.
Irrigation interval calculation example
A vegetable field has a readily available water of 1.4 inches, and the grower already has a daily ETc figure of 0.30 inches per day from a nearby weather station's crop water use report.
- Interval: 1.4 in ÷ 0.30 in/day = 4.667, shown as about 4.7 days.
- Whole-day interval: FLOOR(4.667) = 4 days.
- Depth used by day 4: 0.30 in/day × 4 days = 1.20 in, leaving a small margin under the 1.4 in RAW.
The same calculation in metric, from ETo × Kc
A turf area has 30 mm of RAW. The local reference ET is 5.0 mm/day and the crop coefficient for this turf is 0.85.
- ETc: 5.0 mm/day × 0.85 = 4.25 mm/day.
- Interval: 30 mm ÷ 4.25 mm/day = 7.059, shown as about 7.1 days.
- Whole-day interval: FLOOR(7.059) = 7 days.
- Depth used by day 7: 4.25 mm/day × 7 days = 29.8 mm, just under the 30 mm RAW.
Using the interval in practice
The number this calculator returns is the outer limit, the point where readily available water is fully used up and the crop is expected to start drawing down harder-to-extract water. Two practical adjustments turn that number into a workable schedule.
Round down, not up. An interval of 6.7 days means 6 full days pass with the crop comfortably inside its RAW; on day 7 the crop runs past that point before the day is over. Scheduling irrigation on day 6 keeps a buffer against a hotter day, a delayed irrigation, or a malfunctioning valve.
Recheck ETc as conditions change. ETc responds to weather and to the crop's growth stage, both of which shift over a season. A single ETc entered today describes today's conditions, not the whole season, so recalculating every few days, especially through a heat wave or after a big change in canopy cover, keeps the interval current.
Watch for very short intervals. An interval under 1 day means the crop's water use outpaces its RAW within a single day, which is common on shallow-rooted seedlings, very sandy soils, or in extreme heat. In that case, more frequent, lighter irrigation, such as with drip or micro-spray systems, usually fits better than trying to force a once-a-day schedule. The drip run time calculator and drip emitter calculator can help size that kind of system.
Factors that change your irrigation interval
Both RAW and ETc respond to real, measurable conditions. This table maps the common ones to what to adjust before you rely on the result.
| Factor | Effect | What to adjust |
|---|---|---|
| Root zone depth | A deeper root zone holds more total available water, raising RAW. | Re-check RAW if roots have grown deeper, or use a shallower depth for young plants. |
| Soil texture | Finer soils (loam, clay) generally hold more available water per foot than coarse sandy soils. | Recalculate RAW from your actual soil's available water capacity. |
| Crop growth stage | Kc rises through early growth to a peak at mid-season, then falls toward harvest. | Update Kc as the crop moves between growth stages. |
| Heat, wind and low humidity | All three raise ETo, and therefore ETc, shortening the interval. | Re-run the calculation with current-week ETo rather than an average. |
| Cool or cloudy weather | Lowers ETo and ETc, lengthening the interval. | Same as above; ETc entered a week ago can be out of date. |
| Depletion fraction (p) | A lower p (more sensitive crop, or high ETc conditions) reduces RAW for the same soil and root depth. | Confirm p on the management allowed depletion calculator rather than assuming a single value fits every crop. |
Uneven irrigation coverage doesn't change the interval math directly, but it does mean some areas reach their RAW limit sooner than the average suggests. The distribution uniformity calculator and scheduling coefficient calculator quantify that gap so you can shorten the interval to protect the driest spots.
Frequently asked questions
What is readily available water (RAW), and how is it different from total available water (TAW)?
Total available water (TAW) is all the water a soil holds between field capacity and the wilting point within the root zone. Readily available water (RAW) is the portion of that TAW a crop can extract before it starts showing water stress, calculated as RAW = p × TAW, where p is a depletion fraction that depends on the crop and its growth stage. This calculator uses RAW, not TAW, because RAW marks the point where irrigation is actually needed.
How do I find my crop's daily water use (ETc)?
Crop evapotranspiration (ETc) is usually calculated as ETc = ETo × Kc, where ETo is the reference evapotranspiration for your area and Kc is a crop coefficient for the current growth stage. Many regional weather networks and agricultural extension services publish daily or weekly ETo. If you already have a daily ETc figure, enter it directly; otherwise use the ETo × Kc option, or work it out first on the crop evapotranspiration calculator.
See the crop evapotranspiration calculator and reference ET calculator.
What does the calculated "interval" actually represent?
It's the number of days it takes your crop's daily water use to draw the soil's readily available water down to the refill point, assuming today's water use rate stays constant and no rain or irrigation happens in between. It answers "how long can I wait?", not "exactly which calendar day should I irrigate?", since real weather changes ETc from one day to the next.
Does this calculator account for rainfall?
No. It only divides readily available water by daily water use. If rain is expected or has fallen, it effectively adds to your RAW and extends the interval, but this tool doesn't do that math for you. For a running, day-by-day balance that includes rainfall, use the soil water balance calculator instead.
See the soil water balance calculator and effective rainfall calculator.
What crop coefficient (Kc) should I use?
Kc depends on the crop and its growth stage, and FAO-56 lists values for most crops between about 0.15, for bare or newly planted soil, and 1.2, for a full-cover, well-watered crop at mid-season. The crop coefficient table has typical starting values, but local extension guidance for your specific crop and climate is usually more precise.
Should I round the interval up or down in practice?
Round down. If the calculation gives 6.7 days, planning around 7 days means the crop spends part of a day past its readily available water before you irrigate. Scheduling on the 6th day keeps a small buffer for a hotter-than-expected day or a delayed irrigation.
Why does my calculated interval look too short or too long?
The result is only as good as the RAW and ETc you enter. A short interval usually means a shallow root zone, a coarse sandy soil, or high ETc from hot, dry, or windy weather. A long interval usually means a deep root zone, a fine-textured soil, or low ETc during cool or cloudy conditions. Double-check both values before trusting an unusual result.
Can I use this for a home lawn instead of a farm crop?
Yes, the RAW ÷ ETc math is the same for turf as for row crops; you just need turf-scale RAW and ETc values rather than field-crop ones. Most homeowners find it easier to start from the lawn watering calculator, which uses a simple weekly depth target instead of RAW and ETc.
See the lawn watering calculator.
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
- The interval formula and both worked examples were verified by hand against the calculator's output.
- Formulas were checked against FAO-56 and USDA NRCS irrigation scheduling guidance.
- Unit conversions between inches and millimeters were checked to round-trip without changing the result.
Sources
- Allen, R.G., Pereira, L.S., Raes, D., Smith, M. (1998). Food and Agriculture Organization of the United Nations. Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements (FAO Irrigation and Drainage Paper 56).
- USDA Natural Resources Conservation Service. Conservation Practice Standard: Irrigation Water Management (Code 449).
- Agriculture Victoria. Using Readily Available Water to Help Schedule Irrigation.
- Oregon State University Extension Service. Irrigation Water Scheduling (EM 9717).