Irrigation Scheduling Calculator & How Often & How Much
Schedules water for sandy soils
“Ask in public — the answer helps everyone who lands here after you.
Stop guessing — from soil, roots, allowable depletion and crop ET, get how often to irrigate and how much to apply (net & gross depth, m³/ha), with a live soil-moisture view of the root zone.
Enter your field & water
Allowable depletion ~50% for most crops (less for sensitive ones). Drip ~90%, sprinkler ~75%, flood ~60%.
Next: shorten the interval in hot, windy weather (higher ET) and on sandy soils, and lengthen it in cool weather or deep, heavy soils. Adjust ET to the crop's stage — peak demand is at flowering/fruiting.
FAO-style estimate from soil water-holding capacity. Local soils and weather vary — use a soil-moisture sensor or evaporation pan to fine-tune.
Irrigation scheduling — key facts
- TAW
- AWC (mm/m) × root depth (m)
- RAW
- TAW × allowable depletion
- Interval
- RAW ÷ daily ET
- Gross depth
- net ÷ efficiency
- 1 mm over 1 ha
- = 10 m³
- Typical MAD
- ≈ 50%
- Loam AWC
- ≈ 170 mm/m
- Privacy
- Runs in your browser; nothing uploaded
Irrigate to the soil-water balance
The soil is a reservoir. It fills to field capacity after rain or irrigation, the crop draws it down day by day, and you refill before the plant is stressed. The usable part of that reservoir is the readily available water — the total available water in the root zone multiplied by the allowable depletion. Divide RAW by the crop's daily water use and you get the interval; the depth to apply is RAW itself, grossed up for your system's efficiency. This is the FAO water-balance method extension services teach, and the soil-moisture bar above shows the root zone, the usable band and the stress line.
Scheduling this way avoids the two costly mistakes: over-watering (which leaches nutrients, wastes pumping energy and can drown roots) and under-watering (which stresses the crop and cuts yield, especially at flowering and fruiting). Sandy soils and hot, windy weather shorten the interval; deep, heavy soils and cool weather lengthen it. Treat the figures as a strong starting schedule and fine-tune with a moisture sensor or an auger feel test.
Know the interval
See how many days the root zone lasts before it needs refilling.
Apply the right depth
Get net and gross mm (and m³/ha) so you neither over- nor under-water.
Match the soil
Sandy little-and-often or heavy slow-and-deep — the soil sets the rhythm.
Save water & energy
Avoid leaching and wasted pumping by refilling only what the crop used.
Frequently Asked Questions
How do I know when to irrigate?+
Irrigate when the crop has used the 'readily available water' (RAW) in its root zone — the share of soil water it can take before stress. RAW = total available water × the allowable depletion (often 50%). Dividing RAW by the daily crop water use (ET) gives the days between irrigations. This tool computes all of it.
What is total available water (TAW)?+
TAW is the water the root zone can hold between field capacity and wilting point. It equals the soil's available water capacity (mm per metre of depth) multiplied by the rooting depth in metres. A loam holding 170 mm/m with 0.6 m roots has a TAW of about 102 mm.
What is allowable depletion (MAD)?+
The maximum allowable depletion is how much of the available water you let the crop use before refilling — commonly about 50% for most field crops, less for shallow-rooted or stress-sensitive ones. Lower MAD means more frequent, lighter irrigations.
How much water should I apply per irrigation?+
Apply the net depth (equal to RAW) to refill the root zone, then divide by your system efficiency for the gross depth actually pumped — for example 51 mm net at 90% drip efficiency is about 57 mm gross. One millimetre over a hectare is 10 m³, so the tool also shows m³/ha.
How does soil texture change irrigation?+
Sandy soils hold little water (≈70 mm/m), so they need small, frequent irrigations; clays and silt loams hold much more (≈190–200 mm/m) and can go longer between waterings. The tool carries an available-water capacity for each texture.
What is crop ET and where do I get it?+
Crop evapotranspiration (ETc) is the daily water the crop loses to the air and uses for growth, in millimetres. It rises with heat, wind, sun and crop canopy, peaking around flowering/fruiting. Get reference ET from a local weather station or evaporation pan and adjust by the crop coefficient for its stage.
Why divide by irrigation efficiency?+
Not all pumped water reaches the root zone — some is lost to evaporation, runoff and deep percolation. Dividing the net depth by efficiency (≈90% drip, 75% sprinkler, 60% flood) gives the gross depth you must actually apply to refill the soil.
Should I irrigate more in hot weather?+
Yes — higher temperatures, wind and sun raise ET, so the root zone empties faster and the interval shortens. Re-run the calculation with the season's peak ET, and lengthen the interval again in cool or cloudy spells.
What is deficit irrigation?+
Deliberately allowing more depletion (a higher MAD) or applying less than full ET to save water, accepting some yield trade-off. It can be efficient for hardy crops or at less sensitive growth stages, but risks yield loss if pushed too far at flowering or grain fill.
How accurate is this schedule?+
It's a sound FAO-style water-balance estimate and a strong starting point. Real soils, rooting and weather vary, so confirm with a soil-moisture sensor, an auger feel test, or by tracking daily ET and rainfall, and adjust the interval to what you observe.
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