What it does
Think of the root zone as a tank. It is full at field capacity and empty, for the plant, at the wilting point. Each day the crop draws water out (its evapotranspiration), and rain and irrigation put water back. This page keeps that account day by day, the “checkbook” method of FAO-56, and tells you when the crop has used the water it can take easily and how much to put back.
- Total available water: TAW = 1000 × (field capacity − wilting point) × root depth, in mm.
- Readily available water: RAW = p × TAW. While the depletion stays below RAW the crop is not stressed.
- Crop water use: ETc = Kc × ET0. Once depletion passes RAW, use falls by the stress factor Ks = (TAW − depletion) ÷ ((1 − p) × TAW).
- Rain or irrigation that would fill the tank above field capacity drains below the roots (deep percolation).
How to use it
- Type the soil’s field capacity and wilting point (% water by volume), the root depth, the crop’s p and Kc, and how dry the soil is on the first day (0 mm = at field capacity).
- Say how much rain counts, the trigger (100 % of RAW irrigates as soon as RAW is used up), any room to leave for rain, and your system’s application efficiency.
- Paste or open the daily table: date, ET0 and rain in mm. Optional columns named Kc (for a crop that grows through the season) and irrigation (what you actually applied) are used if present.
Choose No irrigation to see how dry the soil would get from rain alone, with the days of water stress.
Worked example
The page opens with a made-up 61-day summer starting 1 June: a soil of 30 % and 14 % water by volume with roots to 0.6 m gives TAW = 96 mm and RAW = 48 mm with p = 0.5. With Kc 1.1, 90 % of rain counting above 2 mm, and 80 % efficiency, the field needs 6 irrigations: 305.6 mm net, 382.0 mm applied, the first on 6 June (52.2 mm net). A 38 mm storm on 21 June overfills the root zone and 10.6 mm drains below the roots. For 10 ha that is 38,195 m³ of water to pump.
With No irrigation, the crop would be stressed on 55 of the 61 days and the depletion would end at 93.7 of 96 mm.
Tips
- Leave room for rain in a wet season: refilling to, say, 10 mm below field capacity lets the next shower soak in instead of draining.
- Make ET0 with the evapotranspiration calculator from your weather data.
- The single Kc folds soil evaporation into one number; after frequent wetting of bare soil, real use can be higher.
FAQ
Is this an irrigation recommendation? It is a water balance from the numbers you type. Check soil moisture in the field and follow your local adviser.
Is my data uploaded? No. Everything runs in your browser and works offline.
Where the numbers come from
- Daily root-zone water balance with a single crop coefficient: total and readily available water, the water stress coefficient Ks, deep percolation and the depletion-fraction adjustment p + 0.04 (5 − ETc): Allen, R. G., Pereira, L. S., Raes, D. and Smith, M. (1998), Crop evapotranspiration, FAO Irrigation and Drainage Paper 56, chapter 8 (equations only; none of its tables or example data are reproduced).
- No soil, crop coefficient or depletion fraction tables are bundled: users type their own values.
- Checked against an independent Python implementation on 200 random seasons (with Kc and irrigation columns, adjusted p and both plans): 67,700 daily values agree to 2 × 10⁻¹⁴ mm. The example season, soil and crop are made up, the weather drawn at random by fibub.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.