Irrigation Scheduling Calculator: Days Between Watering
Good irrigation scheduling refills the root zone just before plants start to struggle. How long that takes depends on how much water your soil holds, how deep the roots go and how fast the weather dries things out.
Runs in your browser. Nothing you enter is stored.
What this calculator does
Enter your soil, root depth and daily water use. The calculator gives the interval between waterings and the depth to apply each time.
How to use the irrigation scheduling calculator
- Choose your soil texture.
- Pick a crop or enter the root depth.
- Keep 50% allowable depletion unless the crop is sensitive.
- Enter daily crop water use (ET) from a local weather service or the crop water calculator.
- Apply the gross depth at the interval shown; adjust after rain.
How the calculation works
root zone water = available water % × root depth net depth = root zone water × allowable depletion interval (days) = net depth ÷ daily ET gross depth = net depth ÷ efficiency
Available water capacity is the water a soil holds between field capacity and wilting point, from about 7% of soil volume for sand to 20% for silt loam.
Worked example
Loam (17% available water) with 30 cm roots holds 51 mm. At 50% depletion you can use 25.5 mm, which lasts about 5 days at 5 mm a day. Apply 31.9 mm each time with an 80%-efficient system.
Assumptions and limitations
- Available water values are typical for each texture.
- Subtract effective rainfall from the next irrigation.
- Young plants with small root systems need shorter intervals.
Frequently asked questions
How do I know when to irrigate?
When plants have used about half the available water in their root zone, which this calculator turns into a number of days.
What is allowable depletion?
The share of available soil water plants can use before stress affects growth, usually 30–60% depending on the crop.
Where do I find ET for my area?
Many weather services and agricultural networks publish daily reference ET; multiply by a crop coefficient for your crop.
Why water less often on clay?
Clay holds more water, so it lasts longer between waterings, but it absorbs slowly, so apply it gently.